泰坦(土星的卫星)Titan (moon)

蛋黄土卫六Titan

Titan is the largest moon of Saturn and the second-largest natural satellite in the Solar System. It is the only moon known to have a dense atmosphere, and the only known body in space, other than Earth, where clear evidence of stable bodies of surface liquid has been found.

土卫六Titan是土星最大的卫星,也是太阳系中第二大自然卫星。 它是唯一已知的大气层密集的卫星,也是太空中除地球外,唯一已知有明显稳定的表面液体的天体。

Titan is the sixth gravitationally rounded moon from Saturn. Frequently described as a planet-like moon, Titan is 50% larger than Earth’s moon and 80% more massive. It is the second-largest moon in the Solar System after Jupiter’s moon Ganymede, and is larger than the planet Mercury, but only 40% as massive. Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the sixth known planetary satellite (after Earth’s moon and the four Galilean moons of Jupiter). Titan orbits Saturn at 20 Saturn radii. From Titan’s surface, Saturn subtends an arc of 5.09 degrees and would appear 11.4 times larger in the sky than the Moon from Earth.

Titan是土星轨道往外的第六颗卫星。 Titan通常被描述为类似行星的卫星,比地球的卫星大50%,比地球的质量大80%。 它是太阳系中第二大的卫星,仅次于木星的卫星“木卫三”,比水星大,但质量只有水星的40%。 Titan是由荷兰天文学家克里斯蒂安·惠更斯(Christiaan Huygens)在1655年发现的,它是土星的第一个已知卫星,也是已知的第六颗行星状卫星(仅次于地球的月亮和木星的四个伽利略卫星)。 土卫六以土星半径20杯的轨道绕土星运行。 从土卫六的表面看,土星对着5.09度的弧线,在天空中看起来比从地球上的月球大11.4倍。

Titan is primarily composed of ice and rocky material, which is likely differentiated into a rocky core surrounded by various layers of ice, including a crust of ice Ih and a subsurface layer of ammonia-rich liquid water.[11] Much as with Venus before the Space Age, the dense opaque atmosphere prevented understanding of Titan’s surface until the Cassini–Huygens mission in 2004 provided new information, including the discovery of liquid hydrocarbon lakes in Titan’s polar regions. The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found.

土卫六主要由冰和岩石材料组成,很可能会分化成被各种冰层包围的岩心,包括冰壳Ih和地下富含氨的液态水。[11] 就像太空时代之前的金星一样,浓密的不透明大气阻止了人们对泰坦表面的了解,直到2004年的卡西尼-惠更斯飞行任务提供了新的信息,包括在泰坦极地地区发现了液态烃湖。 尽管已经发现了山脉和几种可能的低温冰晶,但地质上年轻的表面通常是光滑的,几乎没有撞击坑。

The atmosphere of Titan is largely nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas (probably of liquid methane and ethane), and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan’s methane cycle is analogous to Earth’s water cycle, at the much lower temperature of about 94 K (−179.2 °C; −290.5 °F).

土卫六的大气层主要是氮气。 微量成分会形成甲烷和乙烷云以及富含氮的有机烟雾。 气候-包括风和雨-形成了类似于地球的表面特征,例如沙丘,河流,湖泊,海洋(可能是液态甲烷和乙烷)和三角洲,并且就像地球那样受季节性气候的影响。 凭借其液体(地表和地下)以及强大的氮气气氛,泰坦的甲烷循环类似于地球的水循环,其温度低得多,约为94 K(−179.2°C; −290.5°F)。

历史History

探索Discovery

Christiaan Huygens discovered Titan in 1655.

Titan was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens.[12][13] Huygens was inspired by Galileo‘s discovery of Jupiter’s four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his older brother Constantijn Huygens, Jr., began building telescopes around 1650 and discovered the first observed moon orbiting Saturn with one of the telescopes they built.[14] It was the sixth moon ever discovered, after Earth’s Moon and the Galilean moons of Jupiter.[15]

泰坦是1655年3月25日由荷兰天文学家克里斯蒂安·惠更斯发现的[12] [13]。 惠更斯的灵感来自伽利略在1610年发现木星的四个最大卫星以及他对望远镜技术的改进。 克里斯蒂安(Christiaan)在他的哥哥小康斯坦丁·惠更斯(Constantijn Huygens,Jr.)的帮助下,于1650年左右开始制造望远镜,并发现了他们用其中一个望远镜观测到的第一个绕土星运行的月球。[14] 这是有史以来第六个发现的卫星,仅次于地球的月球和木星的伽利略卫星。[15]

命名Naming

Huygens named his discovery Saturni Luna (or Luna Saturni, Latin for “Saturn’s moon”), publishing in the 1655 tract De Saturni Luna Observatio Nova (A New Observation of Saturn’s Moon).[16] After Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V (with Titan then in fourth position). Other early epithets for Titan include “Saturn’s ordinary satellite”.[17] Titan is officially numbered Saturn VI because after the 1789 discoveries the numbering scheme was frozen to avoid causing any more confusion (Titan having borne the numbers II and IV as well as VI). Numerous small moons have been discovered closer to Saturn since then.

惠更斯将他的发现命名为Saturni Luna(拉丁语为Luna Saturni,意为“土星的月亮”),发表于1655年的《 De Saturni Luna观测新星》(土星月球的新观测)[16]。 乔瓦尼·多梅尼科·卡西尼(Giovanni Domenico Cassini)发表他在1673年至1686年之间又发现了四个土星卫星的发现后,天文学家习惯于将这些卫星和土卫六称为土星一号至五号(土卫六处于第四位置)。 泰坦的其他早期称呼还包括“土星的普通卫星”。[17] 土卫六被正式编号为土星六号,因为1789年发现后,该编号方案被冻结以避免引起更多的混乱(Titan曾经的名称有土卫二,土卫五和土卫六)。 从那以后,在土星附近发现了许多小卫星。

The name Titan, and the names of all seven satellites of Saturn then known, came from John Herschel (son of William Herschel, discoverer of two other Saturnian moons, Mimas and Enceladus), in his 1847 publication Results of Astronomical Observations Made during the Years 1834, 5, 6, 7, 8, at the Cape of Good Hope.[18][19] He suggested the names of the mythological Titans (Ancient GreekΤῑτᾶνες), brothers and sisters of Cronus, the Greek Saturn. In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age.

泰坦(Titan)的名字以及当时所有土星的所有七颗卫星的名字都来自约翰·赫歇尔(John Herschel)(威廉·赫歇尔的儿子,另外两个土星卫星Mimas和Enceladus的发现者),他在1847年出版的《多年来的天文观测结果》中 1834、5、6、7、8,位于好望角。[18] [19] 他提出了神话般的土卫六(古希腊语:ῑῑτνες)的名字,土星克罗努斯的兄弟姐妹。 在希腊神话中,泰坦是由传说中的黄金时代统治的盖亚和天王星的后代组成的强大神灵的种族。

轨道和旋转Orbit and rotation

Titan’s orbit (highlighted in red) among the other large inner moons of Saturn. The moons outside its orbit are (from the outside to the inside) Iapetus and Hyperion; those inside are Rhea, Dione, Tethys, Enceladus, and Mimas.

土星的轨道(以红色突出显示)在土星的其他大型内部卫星之间。 轨道外的卫星是(从外到内)伊帕特斯和海波龙; 内部是Rhea,Dione,Tethys,Enceladus和Mimas。

Titan orbits Saturn once every 15 days and 22 hours. Like the Moon and many of the satellites of the giant planets, its rotational period (its day) is identical to its orbital period; Titan is tidally locked in synchronous rotation with Saturn, and permanently shows one face to the planet, so Titan’s “day” is equal to its orbit period. Because of this, there is a sub-Saturnian point on its surface, from which the planet would always appear to hang directly overhead. Longitudes on Titan are measured westward, starting from the meridian passing through this point.[20] Its orbital eccentricity is 0.0288, and the orbital plane is inclined 0.348 degrees relative to the Saturnian equator.[2] Viewed from Earth, Titan reaches an angular distance of about 20 Saturn radii (just over 1,200,000 kilometers (750,000 mi)) from Saturn and subtends a disk 0.8 arcseconds in diameter.

土卫六每15天22小时绕土星运行一次。 就像月亮和巨型行星的许多卫星一样,它的自转周期(它的一天)与它的轨道周期是相同的。 土卫六与土星同步旋转时处于潮汐锁定状态,并永久只向土星露出一边,因此土卫六的“日”等于其轨道周期。 因此,在其表面上存在一个土星下点,从该点看来,行星总是似乎直接悬挂在上方。 从经过这一点的子午线开始,土卫六的经度是向西测量的。[20] 它的轨道偏心率为0.0288,并且轨道平面相对于土星赤道倾斜0.348度。[2] 从地球上看,土卫六与土星的夹角约为20土星半径(刚好超过1,200,000公里(750,000英里)),其直径为0.8弧秒。

The small, irregularly shaped satellite Hyperion is locked in a 3:4 orbital resonance with Titan. A “slow and smooth” evolution of the resonance—in which Hyperion migrated from a chaotic orbit—is considered unlikely, based on models. Hyperion probably formed in a stable orbital island, whereas the massive Titan absorbed or ejected bodies that made close approaches.[21]

形状不规则的小型卫星Hyperion与Titan锁定在3:4的轨道共振中。 根据模型,共振的“缓慢而平滑”的演化(Hyperion从混沌轨道迁移出来)是不可能的。 Hyperion可能在一个稳定的轨道岛中形成,而庞大的Titan吸收或弹出了紧密接近的物体。[21]

体积特征Bulk characteristics

Size comparison: Titan (lower left) with the Moon and Earth (top and right)

A model of Titan’s internal structure showing ice-six layer

Titan is 5,149.46 kilometers (3,199.73 mi) in diameter,[3] 1.06 times that of the planet Mercury, 1.48 that of the Moon, and 0.40 that of Earth. Before the arrival of Voyager 1 in 1980, Titan was thought to be slightly larger than Ganymede (diameter 5,262 kilometers (3,270 mi)) and thus the largest moon in the Solar System; this was an overestimation caused by Titan’s dense, opaque atmosphere, with a haze layer 100-200 kilometres above its surface. This increases its apparent diameter.[22] 

泰坦的直径为5,149.46公里(3,199.73英里),[3]是水星行星的1.06倍,月球的1.48倍,地球的0.40倍。在1980年旅行者1号到达之前,土卫六被认为比木卫三稍大(直径5,262公里(3,270英里)),因此是太阳系中最大的卫星。这是土卫六浓密,不透明的大气所造成的,高估了其表面上方100-200公里的薄雾层。这增加了它的表观直径。[22]

Titan’s diameter and mass (and thus its density) are similar to those of the Jovian moons Ganymede and Callisto.[23] Based on its bulk density of 1.88 g/cm3, Titan’s composition is half water ice and half rocky material. Though similar in composition to Dione and Enceladus, it is denser due to gravitational compression. It has a mass 1/4226 that of Saturn, making it the largest moon of the gas giants relative to the mass of its primary. It is second in terms of relative diameter of moons to a gas giant; Titan being 1/22.609 of Saturn’s diameter, Triton is larger in diameter relative to Neptune at 1/18.092.

土卫六的直径和质量(因而密度)与木卫六的木卫三和木卫四的相似。[23]基于1.88 g / cm3的堆积密度,Titan的成分是一半的水冰和一半的岩石材料。尽管其成分与Dione和Enceladus相似,但由于重力压缩其密度更高。它的质量是土星的1/4226,相对于其主要质量而言,它是天然气巨人中最大的卫星。就卫星的相对直径而言,它仅次于天然气巨人。土卫六是土星直径的1 / 22.609,特里顿的直径相对于海王星大,为1 / 18.092。

Titan is probably partially differentiated into distinct layers with a 3,400-kilometer (2,100 mi) rocky center.[24] This rocky center is surrounded by several layers composed of different crystalline forms of ice.[25] Its interior may still be hot enough for a liquid layer consisting of a “magma” composed of water and ammonia between the ice Ih crust and deeper ice layers made of high-pressure forms of ice. The presence of ammonia allows water to remain liquid even at a temperature as low as 176 K (−97 °C) (for eutectic mixture with water).[26] 

土卫六可能以3,400公里(2,100英里)的岩石中心部分地分为不同的层。[24] 这个多岩石的中心被几层由不同晶体形式的冰所包围。[25] 其内部可能仍然足够热,以用于由在冰壳之间的由水和氨组成的“岩浆”组成的液体层和由高压形式的冰制成的更深的冰层。 氨的存在即使在低至176 K(-97°C)的温度下(对于与水的共晶混合物),水仍然保持液态。[26]

The Cassini probe discovered the evidence for the layered structure in the form of natural extremely-low-frequency radio waves in Titan’s atmosphere. Titan’s surface is thought to be a poor reflector of extremely-low-frequency radio waves, so they may instead be reflecting off the liquid–ice boundary of a subsurface ocean.[27] Surface features were observed by the Cassini spacecraft to systematically shift by up to 30 kilometers (19 mi) between October 2005 and May 2007, which suggests that the crust is decoupled from the interior, and provides additional evidence for an interior liquid layer.[28] Further supporting evidence for a liquid layer and ice shell decoupled from the solid core comes from the way the gravity field varies as Titan orbits Saturn.[29] Comparison of the gravity field with the RADAR-based topography observations[30] also suggests that the ice shell may be substantially rigid.[31][32]

卡西尼号探测器发现了土卫六大气层中天然超低频无线电波形式的分层结构的证据。 土卫六的表面被认为是极低频无线电波的不良反射器,因此它们可能会从地下海洋的液冰边界反射出去。[27] 卡西尼号飞船观测到其表面特征在2005年10月至2007年5月之间系统地移动了30公里(19英里),这表明地壳与内部是分离的,并为内部液体层提供了额外的证据。[28] ] 液层和冰壳与实心解耦的进一步支持证据来自重力场随土卫六绕土星运行而变化的方式。[29] 将重力场与基于RADAR的地形观测结果进行比较[30]也表明,冰壳可能基本是刚性的。[31] [32]

形成Formation

The moons of Jupiter and Saturn are thought to have formed through co-accretion, a similar process to that believed to have formed the planets in the Solar System. As the young gas giants formed, they were surrounded by discs of material that gradually coalesced into moons. Whereas Jupiter possesses four large satellites in highly regular, planet-like orbits, Titan overwhelmingly dominates Saturn’s system and possesses a high orbital eccentricity not immediately explained by co-accretion alone. A proposed model for the formation of Titan is that Saturn’s system began with a group of moons similar to Jupiter’s Galilean satellites, but that they were disrupted by a series of giant impacts, which would go on to form Titan. Saturn’s mid-sized moons, such as Iapetus and Rhea, were formed from the debris of these collisions. Such a violent beginning would also explain Titan’s orbital eccentricity.[33]

据认为,木星和土星的卫星是通过共吸积聚形成的,这一过程与据信在太阳系中形成行星的过程相似。 随着年轻的天然气巨行星的形成,它们被逐渐聚结成卫星的圆盘状物质所包围。 木星拥有高度规则的类似行星轨道的四颗大型卫星,而土卫六则压倒了土星的系统,并且拥有很高的轨道偏心率,仅靠共积就无法立即解释。 一个提出的模型认为土卫六的形成是土星系统以类似于木星伽利略卫星开始形成,但是它们受到一系列巨大撞击的干扰,这些撞击最终形成了土卫六。 土星的中型卫星,例如Iapetus和Rhea,是由这些碰撞的碎片形成的。 这样的暴力开始也可以解释泰坦的轨道偏心率。[33]

A 2014 analysis of Titan’s atmospheric nitrogen suggested that it has possibly been sourced from material similar to that found in the Oort cloud and not from sources present during co-accretion of materials around Saturn.[34]

2014年对土卫六大气氮的分析表明,它可能源自类似于奥尔特云中发现的物质,而不是土星周围物质共积期间存在的来源。[34]

大气Atmosphere

Main article: Atmosphere of Titan

True-color image of layers of haze in Titan’s atmosphere

泰坦大气层中霾层的真彩色图像

Titan is the only known moon with a significant atmosphere,[35] and its atmosphere is the only nitrogen-rich dense atmosphere in the Solar System aside from Earth’s. Observations of it made in 2004 by Cassini suggest that Titan is a “super rotator”, like Venus, with an atmosphere that rotates much faster than its surface.[36] Observations from the Voyager space probes have shown that Titan’s atmosphere is denser than Earth’s, with a surface pressure about 1.45 atm. It is also about 1.19 times as massive as Earth’s overall,[37] or about 7.3 times more massive on a per surface area basis. Opaque haze layers block most visible light from the Sun and other sources and obscures Titan’s surface features.[38] Titan’s lower gravity means that its atmosphere is far more extended than Earth’s.[39] The atmosphere of Titan is opaque at many wavelengths and as a result, a complete reflectance spectrum of the surface is impossible to acquire from orbit.[40] It was not until the arrival of the Cassini–Huygens spacecraft in 2004 that the first direct images of Titan’s surface were obtained.[41]

土卫六是已知的唯一具有明显大气层的卫星,[35]它的大气层是除地球地球外太阳系中唯一富氮的稠密大气层。卡西尼号在2004年对它的观测表明,泰坦像金星一样是“超级旋转器”,其大气旋转的速度比其表面快得多。[36] Voyager太空探测器的观测结果显示,土卫六的大气层比地球的大气层密度大,表面压力约为1.45个大气压。它的质量也大约是地球整体的1.19倍,[37]或每单位表面积的质量大约是7.3倍。不透明的雾层阻挡了来自太阳和其他光源的大部分可见光,并遮盖了泰坦的表面特征。[38]土卫六的重力较低,意味着其大气层比地球的大气层扩展得多。[39]泰坦的大气层在许多波长下都是不透明的,因此无法从轨道上获得表面的完整反射光谱。[40]直到2004年卡西尼-惠更斯号飞船到达时,才获得了泰坦表面的第一张直接图像。[41]

Titan’s South Pole Vortex—a swirling HCN gas cloud (November 29, 2012).

Titan’s atmospheric composition is nitrogen (97%), methane (2.7±0.1%), hydrogen (0.1–0.2%) with trace amounts of other gases.[10] There are trace amounts of other hydrocarbons, such as ethanediacetylenemethylacetyleneacetylene and propane, and of other gases, such as cyanoacetylenehydrogen cyanidecarbon dioxidecarbon monoxidecyanogenargon and helium.[9] The hydrocarbons are thought to form in Titan’s upper atmosphere in reactions resulting from the breakup of methane by the Sun’s ultraviolet light, producing a thick orange smog.[42] Titan spends 95% of its time within Saturn’s magnetosphere, which may help shield it from the solar wind.[43]

土卫六的大气成分为氮气(97%),甲烷(2.7±0.1%),氢气(0.1–0.2%)和微量其他气体。[10] [9]还有痕量的其他碳氢化合物,例如乙烷,二乙炔,甲基乙炔,乙炔和丙烷,以及其他气体,例如氰基乙炔,氰化氢,二氧化碳,一氧化碳,氰,氩和氦。[9] 据认为,碳氢化合物是在土卫六上层大气中形成的,其反应是由于太阳的紫外线分解甲烷而产生的浓橙色烟雾。[42] 土卫六将95%的时间都用在土星的磁层中,这可能有助于使其免受太阳风的伤害。[43]

Energy from the Sun should have converted all traces of methane in Titan’s atmosphere into more complex hydrocarbons within 50 million years—a short time compared to the age of the Solar System. This suggests that methane must be replenished by a reservoir on or within Titan itself.[44] The ultimate origin of the methane in its atmosphere may be its interior, released via eruptions from cryovolcanoes.[45][46][47][48][49]

在五千万年之内,来自太阳的能量本应将土卫六大气中的所有甲烷痕迹转化为更复杂的碳氢化合物,这与太阳系的寿命相比是很短的时间。 这表明甲烷必须通过土卫六自身内部或内部的储层补充。[44] 甲烷在大气中的最终起源可能是其内部,是通过低温烷的爆发而释放出来的。[45] [46] [47] [48] [49]

Sunset studies on Titan by Cassini help to better understand exoplanetatmospheres (artist’s concept).

卡西尼号对泰坦的日落研究有助于更好地了解系外大气层(艺术想象图)。

Traceorganic gases in Titan’s atmosphereHNC (left) and HC3N (right).

泰坦大气中的微量有机气体-HNC(左)和HC3N(右)。

On April 3, 2013, NASA reported that complex organic chemicals, collectively called tholins, likely arise on Titan, based on studies simulating the atmosphere of Titan.[50]

2013年4月3日,美国国家航空航天局(NASA)报告称,基于模拟泰坦大气的研究,复杂的有机化学物质(统称为“tholins”)可能会在泰坦上生成。[50]

On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan.[51]

2013年6月6日,IAA-CSIC的科学家报告了在泰坦高层大气中发现多环芳烃的情况。[51]

On September 30, 2013, propene was detected in the atmosphere of Titan by NASA‘s Cassini spacecraft, using its composite infrared spectrometer (CIRS).[52] This is the first time propene has been found on any moon or planet other than Earth and is the first chemical found by the CIRS. The detection of propene fills a mysterious gap in observations that date back to NASA’s Voyager 1 spacecraft’s first close planetary flyby of Titan in 1980, during which it was discovered that many of the gases that make up Titan’s brown haze were hydrocarbons, theoretically formed via the recombination of radicals created by the Sun’s ultraviolet photolysis of methane.[42]

2013年9月30日,美国宇航局卡西尼号航天器使用其复合红外光谱仪(CIRS)在泰坦大气中检测到丙烯。[52] 这是在地球以外的任何月亮或行星上首次发现丙烯,并且是CIRS首次发现的化学物质。 丙烯的检测填补了可追溯到1980年美国宇航局1号旅行者首次太空飞行的泰坦行星绕行观测中的一个神秘缺口,在此期间,人们发现构成泰坦棕雾的许多气体都是碳氢化合物,理论上是通过碳氢化合物形成的。 太阳紫外线分解甲烷产生的自由基的重组[42]。

On October 24, 2014, methane was found in polar clouds on Titan.[53][54]

2014年10月24日,在泰坦极地云中发现了甲烷。[53] [54]

Polar clouds, made of methane, on Titan (left) compared with polar clouds on Earth (right), which are made of water or water ice.

Titan上由甲烷形成的极地云在左侧,而地球上由水和水冰形成的极地云在右侧。

气候Climate

Main article: Climate of Titan

Atmospheric polar vortex over Titan’s south pole

Titan’s surface temperature is about 94 K (−179.2 °C). At this temperature, water ice has an extremely low vapor pressure, so the little water vapor present appears limited to the stratosphere.[55] Titan receives about 1% as much sunlight as Earth.[56] Before sunlight reaches the surface, about 90% has been absorbed by the thick atmosphere, leaving only 0.1% of the amount of light Earth receives.[57]

土卫六的表面温度约为94 K(−179.2°C)。 在此温度下,水冰的蒸气压极低,因此出现的少量水蒸气似乎仅限于平流层。[55] 泰坦所吸收的阳光大约是地球的1%。[56] 在阳光到达地面之前,大约90%的气体已被浓厚的大气层吸收,剩下的只是地球接收到的光量的0.1%。[57]

Atmospheric methane creates a greenhouse effect on Titan’s surface, without which Titan would be far colder.[58] Conversely, haze in Titan’s atmosphere contributes to an anti-greenhouse effect by reflecting sunlight back into space, cancelling a portion of the greenhouse effect and making its surface significantly colder than its upper atmosphere.[59]

大气中的甲烷会在土卫六的表面上产生温室效应,否则土卫六会变得更冷。[58] 相反,土卫六大气中的雾霾通过将太阳光反射回太空,抵消了一部分温室效应并使其表面明显比其上层大气更冷,从而起到了抗温室效应的作用。[59]

Methane clouds (animated; July 2014).[60]

Titan’s clouds, probably composed of methane, ethane or other simple organics, are scattered and variable, punctuating the overall haze.[22] The findings of the Huygens probe indicate that Titan’s atmosphere periodically rains liquid methane and other organic compounds onto its surface.[61]

泰坦的云很可能是由甲烷,乙烷或其他简单的有机物组成的,散乱且多变,使整体雾霾破裂。[22] 惠更斯探测器的发现表明,土卫六的大气层会定期向其表面下垂液态甲烷和其他有机化合物。[61]

Clouds typically cover 1% of Titan’s disk, though outburst events have been observed in which the cloud cover rapidly expands to as much as 8%. One hypothesis asserts that the southern clouds are formed when heightened levels of sunlight during the southern summer generate uplift in the atmosphere, resulting in convection. This explanation is complicated by the fact that cloud formation has been observed not only after the southern summer solstice but also during mid-spring. Increased methane humidity at the south pole possibly contributes to the rapid increases in cloud size.[62] It was summer in Titan’s southern hemisphere until 2010, when Saturn’s orbit, which governs Titan’s motion, moved Titan’s northern hemisphere into the sunlight.[63] When the seasons switch, it is expected that ethane will begin to condense over the south pole.[64]

云层通常覆盖Titan盘的1%,尽管已经观察到爆发事件,其中云层迅速扩展到8%。 一种假设认为,南极云是在南极夏季日照强度升高时产生的,从而形成对流。 由于不仅在南部夏季,而且在春季中期也观察到了云的形成,这一解释变得复杂了。 南极甲烷湿度的增加可能导致云的大小迅速增加。[62] 那是土卫六南半球的夏天,直到2010年,控制土卫六运动的土星轨道将土卫六的北半球带入了阳光。[63] 当季节改变时,预计乙烷将在南极开始凝结。[64]

地面特征Surface features

See also: List of geological features on Titan

Titan − the surface under the haze (December 2018)

Titan表面被雾霾覆盖

Global map of Titan – with IAU labels (August 2016).

泰坦全球地图-具有IAU标签(2016年8月)。

Titan – infrared views (2004–2017)

土卫六–红外视野(2004–2017)

Titan’s North Pole (2014)

泰坦的北极(2014)

Titan’s South Pole (2014)

泰坦的南极(2014)

The surface of Titan has been described as “complex, fluid-processed, [and] geologically young”.[65] Titan has been around since the Solar System’s formation, but its surface is much younger, between 100 million and 1 billion years old. Geological processes may have reshaped Titan’s surface.[66] Titan’s atmosphere is twice as thick as Earth’s, making it difficult for astronomical instruments to image its surface in the visible light spectrum.[67] The Cassini spacecraft used infrared instruments, radar altimetry and synthetic aperture radar (SAR) imaging to map portions of Titan during its close fly-bys.

土卫六的表面被描述为“复杂的,由流体形成,并且在地质上还很年轻”。[65] 自太阳系形成以来,土卫六一直存在,但其表面要年轻得多,大约有1亿到10亿年的历史。 地质过程可能重塑了泰坦的表面。[66] 土卫六的大气层厚度是地球的两倍,因此天文学仪器很难在可见光谱中对其表面成像。[67] 卡西尼号飞船使用红外仪器,雷达测高仪和合成孔径雷达(SAR)成像来绘制“泰坦”近距离飞掠期间的部分地图。

The first images revealed a diverse geology, with both rough and smooth areas. There are features that may be volcanic in origin, disgorging water mixed with ammonia onto the surface. There is also evidence that Titan’s ice shell may be substantially rigid,[31][32] which would suggest little geologic activity.[68] There are also streaky features, some of them hundreds of kilometers in length, that appear to be caused by windblown particles.[69][70] Examination has also shown the surface to be relatively smooth; the few objects that seem to be impact craters appeared to have been filled in, perhaps by raining hydrocarbons or volcanoes. Radar altimetry suggests height variation is low, typically no more than 150 meters. Occasional elevation changes of 500 meters have been discovered and Titan has mountains that sometimes reach several hundred meters to more than 1 kilometer in height.[71]

最初的图像显示出多样的地质,包括粗糙和光滑的区域。 有些特征可能起源于火山,将与氨混合的水散布到地表。 也有证据表明,泰坦的冰壳可能基本上是刚性的,[31] [32]这表明地质活动很少。[68] 还有一些斑纹特征,其中一些长数百公里,似乎是由风吹颗粒引起的。[69] [70] 检查还显示表面相对光滑; 似乎是陨石坑的一些物体似乎已经被填满了,也许是由于下雨的碳氢化合物或火山。 雷达测高仪表明高度变化很低,通常不超过150米。 人们发现海拔偶尔会发生500米的变化,泰坦山脉的山脉有时会达到几百米,甚至超过一公里。[71]

Titan’s surface is marked by broad regions of bright and dark terrain. These include Xanadu, a large, reflective equatorial area about the size of Australia. It was first identified in infrared images from the Hubble Space Telescope in 1994, and later viewed by the Cassini spacecraft. The convoluted region is filled with hills and cut by valleys and chasms.[72] It is criss-crossed in places by dark lineaments—sinuous topographical features resembling ridges or crevices. These may represent tectonic activity, which would indicate that Xanadu is geologically young. Alternatively, the lineaments may be liquid-formed channels, suggesting old terrain that has been cut through by stream systems.[73] There are dark areas of similar size elsewhere on Titan, observed from the ground and by Cassini; at least one of these, Ligeia Mare, Titan’s second-largest sea, is almost a pure methane sea.[74][75]

土卫六的表面以明亮和黑暗的地形为特征。 其中包括Xanadu,这是一个与澳大利亚相当的反射性大赤道区。 它最早是在1994年从哈勃太空望远镜的红外图像中识别出来的,后来被卡西尼号飞船观测到。 曲折的地区到处都是丘陵,被山谷和峡谷所割断。[72] 它的位置被深色线条纵横交错—蜿蜒的地形特征类似于山脊或缝隙。 这些可能代表了构造活动,这表明世外桃源在地质上还很年轻。 或者,这些构造可能是液体形成的通道,表明河流系统已经割破了旧地形。[73] 从地面和卡西尼号观测到的土卫六其他地方都有大小相似的暗区。 其中至少一个是土卫六的第二大海利吉亚马雷,几乎是纯甲烷海。[74] [75]

湖Lakes

Main article: Lakes of Titan

Titan lakes (September 11, 2017)

False-color Cassini radar mosaic of Titan’s north polar region. Blue coloring indicates low radar reflectivity, caused by hydrocarbon seas, lakes and tributary networks filled with liquid ethane, methane and dissolved N
2.[10] About half of the large body at lower left, Kraken Mare, is shown. Ligeia Mare is at lower right.

泰坦北极地区的伪色卡西尼号雷达马赛克。 蓝色表示雷达反射率低,这是由充有液态乙烷,甲烷和溶解氮的烃类海洋,湖泊和支流网络引起的[10] 显示了左下角的大天体大约一半,即Kraken Mare。 Ligeia Mare在右下角。

Mosaic of three Huygens images of channel system on Titan

泰坦河道系统的三个惠更斯图像的镶嵌图

Rimmed lakes of Titan(artist concept)

The possibility of hydrocarbon seas on Titan was first suggested based on Voyager 1 and 2 data that showed Titan to have a thick atmosphere of approximately the correct temperature and composition to support them, but direct evidence was not obtained until 1995 when data from Hubble and other observations suggested the existence of liquid methane on Titan, either in disconnected pockets or on the scale of satellite-wide oceans, similar to water on Earth.[76]

最初是根据旅行者1和旅行者2的数据提出的在泰坦上存在碳海的可能性,该数据表明泰坦大气层厚厚,温度和组成近似正确,足以支撑它们,但是直到1995年哈勃等人的数据才获得直接证据。 观测表明,土卫六上存在液态甲烷,存在于不连贯的地区中,或者存在于整个卫星范围的海洋中,类似于地球上的水。[76]

The Cassini mission confirmed the former hypothesis. When the probe arrived in the Saturnian system in 2004, it was hoped that hydrocarbon lakes or oceans would be detected from the sunlight reflected off their surface, but no specular reflections were initially observed.[77] Near Titan’s south pole, an enigmatic dark feature named Ontario Lacus was identified[78] (and later confirmed to be a lake).[79] A possible shoreline was also identified near the pole via radar imagery.[80] Following a flyby on July 22, 2006, in which the Cassini spacecraft’s radar imaged the northern latitudes (that were then in winter), several large, smooth (and thus dark to radar) patches were seen dotting the surface near the pole.[81] Based on the observations, scientists announced “definitive evidence of lakes filled with methane on Saturn’s moon Titan” in January 2007.[82][83] 

卡西尼号飞行任务证实了以前的假设。 当探针于2004年到达土星系统时,希望能从其表面反射的阳光中检测出烃类湖泊或海洋,但最初并未观察到镜面反射。[77] 在土卫六的南极附近,发现了一个神秘的黑暗特征,名为安大略湖,[78](后来确认是湖泊)。[79] 还通过雷达图像在极点附近发现了一条可能的海岸线。[80] 2006年7月22日,卡西尼号飞船的雷达对北纬(当时是冬天)的影像进行了掠过之后,看到几个大而光滑的斑块(因此对雷达来说是暗的)点附近的极点表面。[81 ] 基于这些观察,科学家于2007年1月宣布“土星月球土卫六上充满甲烷的湖泊的确凿证据”。[82] [83]

The Cassini–Huygens team concluded that the imaged features are almost certainly the long-sought hydrocarbon lakes, the first stable bodies of surface liquid found outside Earth.[82] Some appear to have channels associated with liquid and lie in topographical depressions.[82] The liquid erosion features appear to be a very recent occurrence: channels in some regions have created surprisingly little erosion, suggesting erosion on Titan is extremely slow, or some other recent phenomena may have wiped out older riverbeds and landforms.[66] 

卡西尼-惠更斯队得出的结论是,成像特征几乎可以肯定是人们长期以来寻求的烃类湖泊,这是在地球以外发现的第一个稳定的地表液体。[82] 有些似乎具有与液体相关的通道,并且位于地形凹陷处。[82] 液体侵蚀特征似乎是最近才发生的:一些地区的河道几乎没有产生侵蚀,这表明土卫六上的侵蚀非常缓慢,或者其他一些最近的现象可能已经清除了较旧的河床和地形。[66]

Overall, the Cassini radar observations have shown that lakes cover only a small percentage of the surface, making Titan much drier than Earth.[84] Most of the lakes are concentrated near the poles (where the relative lack of sunlight prevents evaporation), but several long-standing hydrocarbon lakes in the equatorial desert regions have also been discovered, including one near the Huygens landing site in the Shangri-La region, which is about half the size of the Great Salt Lake in Utah, USA. The equatorial lakes are probably “oases“, i.e. the likely supplier is underground aquifers.[85]

总体而言,卡西尼号雷达的观测表明,湖泊仅覆盖了很小一部分地表,使泰坦比地球干燥得多。[84] 大多数湖泊都集中在两极附近(相对缺乏阳光阻止了蒸发),但是在赤道沙漠地区也发现了几个长期存在的碳氢化合物湖,其中一个在香格里拉地区的惠更斯登陆点附近。 ,面积约为美国犹他州大盐湖的一半。 赤道湖泊可能是“绿洲”,即可能的供应者是地下蓄水层。[85]

Evolving feature in Ligeia Mare

In June 2008, the Visual and Infrared Mapping Spectrometer on Cassini confirmed the presence of liquid ethane beyond doubt in Ontario Lacus.[86] On December 21, 2008, Cassini passed directly over Ontario Lacus and observed specular reflection in radar. The strength of the reflection saturated the probe’s receiver, indicating that the lake level did not vary by more than 3 mm (implying either that surface winds were minimal, or the lake’s hydrocarbon fluid is viscous).[87][88]

2008年6月,卡西尼号上的可见光和红外光谱仪证实了安大略省的Lacus无疑存在液态乙烷。[86] 2008年12月21日,卡西尼号直接越过安大略省拉库斯,并在雷达中观测到镜面反射。 反射的强度使探头的接收器饱和,表明湖泊水位的变化不超过3毫米(这意味着表面风很小,或者湖泊的碳氢化合物流体很粘稠)。[87] [88]

Near-infrared radiation from the Sun reflecting off Titan’s hydrocarbon seas

太阳从泰坦的碳氢化合物海反射回来的近红外辐射

On July 8, 2009, Cassini’s VIMS observed a specular reflection indicative of a smooth, mirror-like surface, off what today is called Jingpo Lacus, a lake in the north polar region shortly after the area emerged from 15 years of winter darkness. Specular reflections are indicative of a smooth, mirror-like surface, so the observation corroborated the inference of the presence of a large liquid body drawn from radar imaging.[89][90]

2009年7月8日,卡西尼号(Cassini)的VIMS观测到镜面反射指示出光滑的镜面状表面,该区域现今被称为景颇拉库斯(Jingpo Lacus),这是北极地区在经历15年冬季黑暗之后不久出现的湖泊。 镜面反射表示光滑的镜面状表面,因此该观察结果证实了从雷达成像中提取的大液体存在的推断。[89] [90]

Early radar measurements made in July 2009 and January 2010 indicated that Ontario Lacus was extremely shallow, with an average depth of 0.4–3 m, and a maximum depth of 3 to 7 m (9.8 to 23.0 ft).[91] In contrast, the northern hemisphere’s Ligeia Mare was initially mapped to depths exceeding 8 m, the maximum discernable by the radar instrument and the analysis techniques of the time.[91] Later science analysis, released in 2014, more fully mapped the depths of Titan’s three methane seas and showed depths of more than 200 meters (660 ft). Ligeia Mare averages from 20 to 40 m (66 to 131 ft) in depth, while other parts of Ligeia did not register any radar reflection at all, indicating a depth of more than 200 m (660 ft). While only the second largest of Titan’s methane seas, Ligeia “contains enough liquid methane to fill three Lake Michigans“.[92]

2009年7月和2010年1月进行的早期雷达测量表明,安大略省拉库斯非常浅,平均深度为0.4–3 m,最大深度为3至7 m(9.8至23.0 ft)。[91] 相比之下,北半球的Ligeia Mare最初绘制的深度超过8 m,这是雷达仪器和当时的分析技术可识别的最大深度。[91] 2014年发布的后来的科学分析更全面地绘制了泰坦的三个甲烷海的深度,并显示了200多米(660英尺)的深度。 Ligeia Mare平均深度为20到40 m(66到131 ft),而Ligeia的其他部分根本没有记录到任何雷达反射,表明深度超过200 m(660 ft)。 利吉亚虽然仅是泰坦的第二大甲烷海,但“含有足够的液态甲烷以填满三个密歇根湖”。[92]

In May 2013, Cassini’s radar altimeter observed Titan’s Vid Flumina channels, defined as a drainage network connected to Titan’s second largest hydrocarbon sea, Ligeia Mare. Analysis of the received altimeter echoes showed that the channels are located in deep (up to ~570 m), steep-sided, canyons and have strong specular surface reflections that indicate they are currently liquid filled. Elevations of the liquid in these channels are at the same level as Ligeia Mare to within a vertical precision of about 0.7 m, consistent with the interpretation of drowned river valleys.

2013年5月,卡西尼号的雷达高度计观测到了泰坦的Vid Flumina通道,该通道定义为与泰坦第二大油气海利吉亚·马雷相连的排水网络。 对接收到的高度计回波的分析表明,这些通道位于深处(约570μm),陡峭的侧面的峡谷中,并具有强烈的镜面反射,表明它们当前已充满液体。 这些通道中的液体高度与Ligeia Mare处于同一水平,其垂直精度约为0.7µm,这与淹没河谷的解释一致。

Specular reflections are also observed in lower order tributaries elevated above the level of Ligeia Mare, consistent with drainage feeding into the main channel system. This is likely the first direct evidence of the presence of liquid channels on Titan and the first observation of hundred-meter deep canyons on Titan. Vid Flumina canyons are thus drowned by the sea but there are a few isolated observations to attest to the presence of surface liquids standing at higher elevations.[93]

在高于Ligeia Mare水平面的低阶支流中也观察到镜面反射,这与排水进入主河道系统一致。 这很可能是第一个在土卫六上存在液体通道的直接证据,也是对在土卫六上百米深峡谷的首次观察。 因此,Vid Flumina峡谷被海水淹没了,但有一些孤立的观察结果证明存在着海拔较高的地表液体。[93]

During six flybys of Titan from 2006 to 2011, Cassini gathered radiometric tracking and optical navigation data from which investigators could roughly infer Titan’s changing shape. The density of Titan is consistent with a body that is about 60% rock and 40% water. The team’s analyses suggest that Titan’s surface can rise and fall by up to 10 metres during each orbit. That degree of warping suggests that Titan’s interior is relatively deformable, and that the most likely model of Titan is one in which an icy shell dozens of kilometres thick floats atop a global ocean.[94] The team’s findings, together with the results of previous studies, hint that Titan’s ocean may lie no more than 100 kilometers (62 mi) below its surface.[94][95] On July 2, 2014, NASA reported the ocean inside Titan may be as salty as the Dead Sea.[96][97] On September 3, 2014, NASA reported studies suggesting methane rainfall on Titan may interact with a layer of icy materials underground, called an “alkanofer”, to produce ethane and propane that may eventually feed into rivers and lakes.[98]

在2006年至2011年的六次泰坦飞越期间,卡西尼号收集了辐射跟踪和光学导航数据,调查人员可以从中大致推断出泰坦的形状变化。土卫六的密度与大约60%的岩石和40%的水的物体一致。研究小组的分析表明,泰坦的表面在每个轨道上最多可以上升和下降10米。这种变形程度表明,土卫六的内部是相对易变形的,土卫六最可能的模型是其中数十公里厚的冰壳漂浮在全球海洋上的模型。[94]研究小组的发现以及先前的研究结果表明,土卫六的海洋可能位于其表面以下不超过100公里(62英里)的地方。[94] [95] 2014年7月2日,美国国家航空航天局(NASA)报告说,泰坦内部的海洋可能像死海一样咸。[96] [97] 2014年9月3日,美国国家航空航天局(NASA)报告称,土卫六上的甲烷降雨可能与地下的一层结冰物质“链烷烃”相互作用,产生乙烷和丙烷,并最终将其注入河流和湖泊。[98]

In 2016, Cassini found the first evidence of fluid-filled channels on Titan, in a series of deep, steep-sided canyons flowing into Ligeia Mare. This network of canyons, dubbed Vid Flumina, range in depth from 240 to 570 m and have sides as steep as 40°. They are believed to have formed either by crustal uplifting, like Earth’s Grand Canyon, or a lowering of sea level, or perhaps a combination of the two. The depth of erosion suggests that liquid flows in this part of Titan are long-term features that persist for thousands of years.[99]

2016年,卡西尼号在一系列流入利吉亚马雷的深深陡峭峡谷中发现了土卫六上充满流体通道的第一个证据。 这个被称为Vid Flumina的峡谷网络的深度范围从240到570 m,且边坡高达40°。 据信,它们是由地壳隆起形成的,例如地球的大峡谷,或者是海平面下降,或者两者结合。 侵蚀的深度表明,土卫六这一部分的液体流动是长期的特征,可以持续数千年。[99]

撞击坑Impact craters

Radar image of a 139 km-diameter[100] impact crater on Titan’s surface, showing a smooth floor, rugged rim, and possibly a central peak.

土卫六表面上直径为139公里[100]的撞击坑的雷达图像,显示出光滑的地面,崎的边缘以及可能出现的中央峰。

Radar, SAR and imaging data from Cassini have revealed few impact craters on Titan’s surface.[66] These impacts appear to be relatively young, compared to Titan’s age.[66] The few impact craters discovered include a 440-kilometer-wide (270 mi) two-ring impact basin named Menrva seen by Cassini’s ISS as a bright-dark concentric pattern.[101] A smaller, 60-kilometer-wide (37 mi), flat-floored crater named Sinlap[102] and a 30 km (19 mi) crater with a central peak and dark floor named Ksa have also been observed.[103] 

来自卡西尼号的雷达,SAR和成像数据显示,土卫六表面几乎没有撞击坑。[66] 与土卫六的年龄相比,这些影响似乎还比较年轻。[66] 被发现的几个撞击坑包括一个440公里宽(270英里)的两环撞击盆地,名为卡森尼(Cassini’s ISS)将其视为明暗同心圆图案,称为Menrva。[101] 还观察到了一个较小的,宽60公里(37英里)的平底火山口,名为Sinlap [102];还有一个30公里(19英里)的火山口,其中心峰和深色地面称为Ksa。[103]

Radar and Cassini imaging have also revealed “crateriforms”, circular features on the surface of Titan that may be impact related, but lack certain features that would make identification certain. For example, a 90-kilometer-wide (56 mi) ring of bright, rough material known as Guabonito has been observed by Cassini.[104] This feature is thought to be an impact crater filled in by dark, windblown sediment. Several other similar features have been observed in the dark Shangri-la and Aaru regions. Radar observed several circular features that may be craters in the bright region Xanadu during Cassini’s April 30, 2006 flyby of Titan.[105]

雷达和卡西尼号成像还显示了“陨石形”,土卫六表面上的圆形特征可能与撞击有关,但缺乏某些可以确定身份的特征。 例如,卡西尼号观测到90公里宽(56英里)的明亮粗糙的环,称为瓜博尼托,[104] 人们认为该特征是一个撞击坑,里面充满了深色的风沙。 在黑暗的香格里拉和亚鲁地区也观察到了其他一些相似的特征。 雷达在卡西尼号(Cassini)2006年4月30日飞越泰坦(Titan)的过程中观察到了几个圆形特征,这些特征可能是在明亮的地区Xanadu的环形山。[105]

Ligeia Mare – SAR and clearer despeckled views.[106]

Many of Titan’s craters or probable craters display evidence of extensive erosion, and all show some indication of modification.[100] Most large craters have breached or incomplete rims, despite the fact that some craters on Titan have relatively more massive rims than those anywhere else in the Solar System. There is little evidence of formation of palimpsests through viscoelastic crustal relaxation, unlike on other large icy moons.[100] Most craters lack central peaks and have smooth floors, possibly due to impact-generation or later eruption of cryovolcanic lava. Infill from various geological processes is one reason for Titan’s relative deficiency of craters; atmospheric shielding also plays a role. It is estimated that Titan’s atmosphere reduces the number of craters on its surface by a factor of two.[107]

土卫六的许多陨石坑或可能的陨石坑显示出广泛腐蚀的迹象,并且都显示出一些变质的迹象。[100] 尽管事实上泰坦上的某些陨石坑比太阳系其他地方的陨石坑质量更大,但大多数大型陨石坑都破裂或不完整。 与其他大型冰卫星不同,几乎没有证据表明通过粘弹性地壳松弛形成了最苍白的现象。[100] 大多数陨石坑缺乏中心峰,地面平坦,这可能是由于产生冲击力或后来形成的低温火山熔岩喷发。 来自各种地质过程的填充是泰坦相对缺乏陨石坑的原因之一。 大气屏蔽也起着作用。 据估计,土卫六的大气层将其表面的撞击坑数量减少了二分之一。[107]

The limited high-resolution radar coverage of Titan obtained through 2007 (22%) suggested the existence of nonuniformities in its crater distribution. Xanadu has 2–9 times more craters than elsewhere. The leading hemisphere has a 30% higher density than the trailing hemisphere. There are lower crater densities in areas of equatorial dunes and in the north polar region (where hydrocarbon lakes and seas are most common).[100]

到2007年为止获得的有限的泰坦高分辨率雷达覆盖率(22%)表明其撞击坑分布中存在不均匀性。 Xanadu的陨石坑比其他地方多出2–9倍。 前半球的密度比后半球高30%。 在赤道沙丘地区和北极地区(最常见的是碳氢化合物湖泊和海洋),撞击坑密度较低。[100]

Pre-Cassini models of impact trajectories and angles suggest that where the impactor strikes the water ice crust, a small amount of ejecta remains as liquid water within the crater. It may persist as liquid for centuries or longer, sufficient for “the synthesis of simple precursor molecules to the origin of life”.[108]

卡西尼之前的撞击轨迹和角度模型表明,撞击器撞击水冰壳时,少量的喷射水会留在火山口内作为液态水。 它可能以液体形式存在数百年或更长时间,足以“合成简单的前体分子到生命的起源”。[108]

低温火山作用和山脉Cryovolcanism and mountains

See also: Cryovolcano

Near-infrared image of Tortola Facula, thought to be a possible cryovolcano

Tortola Facula的近红外图像,被认为是可能的低温晶体

Scientists have long speculated that conditions on Titan resemble those of early Earth, though at a much lower temperature. The detection of argon-40 in the atmosphere in 2004 indicated that volcanoes had spawned plumes of “lava” composed of water and ammonia.[109] Global maps of the lake distribution on Titan’s surface revealed that there is not enough surface methane to account for its continued presence in its atmosphere, and thus that a significant portion must be added through volcanic processes.[110]

长期以来,科学家一直在猜测泰坦的情况类似于地球早期的情况,尽管温度要低得多。 [109] 2004年在大气中检测到的氩40表明火山喷出了由水和氨组成的“熔岩”羽。[109] 关于土卫六表面湖泊分布的全球地图显示,没有足够的表层甲烷来解释其在大气中的持续存在,因此,必须通过火山过程添加大量甲烷。[110]

Still, there is a paucity of surface features that can be unambiguously interpreted as cryovolcanoes.[111] One of the first of such features revealed by Cassini radar observations in 2004, called Ganesa Macula, resembles the geographic features called “pancake domes” found on Venus, and was thus initially thought to be cryovolcanic in origin, until Kirk et al. refuted this hypothesis at the American Geophysical Union annual meeting in December 2008. The feature was found to be not a dome at all, but appeared to result from accidental combination of light and dark patches.[112][113] In 2004 Cassini also detected an unusually bright feature (called Tortola Facula), which was interpreted as a cryovolcanic dome.[114] No similar features have been identified as of 2010.[115] In December 2008, astronomers announced the discovery of two transient but unusually long-lived “bright spots” in Titan’s atmosphere, which appear too persistent to be explained by mere weather patterns, suggesting they were the result of extended cryovolcanic episodes.[26]

尽管如此,可以毫无疑问地解释为低温火山的表面特征仍然很少。[111]卡西尼号(Cassini)在2004年的雷达观测所揭示的首批此类特征之一,即加内萨·马库拉(Ganesa Macula),类似于金星上发现的“薄饼穹顶”地理特征,因此最初被认为是冰山蜡质的,直到柯克等人才提出。 [112] [113]在2008年12月的美国地球物理联合会年度会议上驳斥了这一假设。该特征根本不是圆顶,而似乎是由明暗区域的偶然组合造成的。[112] [113] 2004年,卡西尼号还检测到一个异常明亮的特征(称为Tortola Facula),该特征被解释为低温火山圆顶。[114]截至2010年,没有发现类似的特征。[115] 2008年12月,天文学家宣布在土卫六大气层中发现了两个短暂但异常长寿的“亮点”,这些亮点似乎过于持久,无法用单纯的天气模式来解释,这表明它们是低温火山爆发的结果。[26]

In March 2009, structures resembling lava flows were announced in a region of Titan called Hotei Arcus, which appears to fluctuate in brightness over several months. Though many phenomena were suggested to explain this fluctuation, the lava flows were found to rise 200 meters (660 ft) above Titan’s surface, consistent with it having been erupted from beneath the surface.[116]

2009年3月,在名为Hotei Arcus的土卫六地区宣布了类似熔岩流的结构,该区域的亮度似乎在几个月内波动。 尽管提出了许多现象来解释这种波动,但发现熔岩流在土卫六的表面上方上升了200米(660英尺),这与从表面下方喷出的熔岩一致。[116]

A mountain range measuring 150 kilometers (93 mi) long, 30 kilometers (19 mi) wide and 1.5 kilometers (0.93 mi) high was also discovered by Cassini in 2006. This range lies in the southern hemisphere and is thought to be composed of icy material and covered in methane snow. The movement of tectonic plates, perhaps influenced by a nearby impact basin, could have opened a gap through which the mountain’s material upwelled.[117] Prior to Cassini, scientists assumed that most of the topography on Titan would be impact structures, yet these findings reveal that similar to Earth, the mountains were formed through geological processes.[118] In December 2010, the Cassini mission team announced the most compelling possible cryovolcano yet found. Named Sotra Patera, it is one in a chain of at least three mountains, each between 1000 and 1500 m in height, several of which are topped by large craters. The ground around their bases appears to be overlaid by frozen lava flows.[119]

卡西尼号(Cassini)在2006年还发现了一个长150公里(93英里),宽30公里(19英里),高1.5公里(0.93英里)的山脉。该山脉位于南半球,被认为是冰冷的 物质并被甲烷雪覆盖。 构造板块的运动可能受到附近冲积盆的影响,可能会造成一个缝隙,使山体的物质通过该缝隙上升。[117] 在卡西尼号之前,科学家们认为土卫六上的大部分地形都是冲击构造,但这些发现表明,与地球相似,山脉是通过地质过程形成的。[118] 2010年12月,卡西尼号任务小组宣布了迄今发现的最引人注目的低温陶瓷。 它被命名为Sotra Patera,是至少三座山的链条之一,每座山的高度在1000到1500 m之间,其中有几座被大火山口覆盖。 他们基地周围的地面似乎被冻结的熔岩流覆盖。[119]

Most of Titan’s highest peaks occur near its equator in so-called “ridge belts”. They are believed to be analogous to Earth’s fold mountains such as the Rockies or the Himalayas, formed by the collision and buckling of tectonic plates, or to subduction zones like the Andes, where upwelling lava (or cryolava) from a melting descending plate rises to the surface. One possible mechanism for their formation is tidal forces from Saturn. Because Titan’s icy mantle is less viscous than Earth’s magma mantle, and because its icy bedrock is softer than Earth’s granite bedrock, mountains are unlikely to reach heights as great as those on Earth. In 2016, the Cassini team announced what they believe to be the tallest mountain on Titan. Located in the Mithrim Montes range, it is 3,337 m tall.[120]

土卫六的最高峰大部分出现在赤道附近的所谓“山脊带”中。 它们被认为类似于由构造板块的碰撞和屈曲形成的洛矶山脉或喜马拉雅山等地球褶皱山峰,或类似于安第斯山脉的俯冲带,从融化的下降板块上升的熔岩(或低温熔岩)上升到 表面。 一种可能的形成机制是土星的潮汐力。 由于土卫六的冰冷地幔不如地球的岩浆地幔粘稠,并且由于其冰冷的基岩比地球的花岗岩基岩更软,因此山脉的高度不可能达到与地球上的高度相同的高度。 2016年,卡西尼号团队宣布了他们认为是土卫六最高的山峰,位于Mithrim Montes山脉,高3,337 m。[120]

False-color VIMS image of the possible cryovolcano Sotra Patera, combined with a 3D map based on radar data, showing 1000-meter-high peaks and a 1500-meter-deep crater.

可能的冷冻硫磺草木Sotra Patera的伪彩色VIMS图像与基于雷达数据的3D地图相结合,显示出1000米高的峰和1500米深的火山口。

If volcanism on Titan really exists, the hypothesis is that it is driven by energy released from the decay of radioactive elements within the mantle, as it is on Earth.[26] Magma on Earth is made of liquid rock, which is less dense than the solid rocky crust through which it erupts. Because ice is less dense than water, Titan’s watery magma would be denser than its solid icy crust. This means that cryovolcanism on Titan would require a large amount of additional energy to operate, possibly via tidal flexing from nearby Saturn.[26] The low-pressure ice, overlaying a liquid layer of ammonium sulfate, ascends buoyantly, and the unstable system can produce dramatic plume events. Titan is resurfaced through the process by grain-sized ice and ammonium sulfate ash, which helps produce a wind-shaped landscape and sand dune features.[121]

如果真的存在着土卫六上的火山,则假说是它是由地幔中放射性元素衰变释放的能量驱动的,就像地球上一样。[26] 地球上的岩浆是由液态岩石制成的,它的密度低于通过其喷发的固态岩石外壳。 由于冰的密度小于水的密度,因此土卫六的含水岩浆比其结冰的硬壳更致密。 这意味着土卫六上的冰山火山活动可能需要大量额外的能量才能运作,可能是通过附近土星的潮汐弯曲来实现的。[26] 覆盖在硫酸铵液体层上的低压冰漂浮上升,不稳定的系统会产生剧烈的羽流事件。 在整个过程中,泰坦通过粒度大小的冰和硫酸铵灰分重新铺贴,这有助于产生风状景观和沙丘特征。[121]

In 2008 Jeffrey Moore (planetary geologist of Ames Research Center) proposed an alternate view of Titan’s geology. Noting that no volcanic features had been unambiguously identified on Titan so far, he asserted that Titan is a geologically dead world, whose surface is shaped only by impact cratering, fluvial and eolian erosion, mass wasting and other exogenic processes. According to this hypothesis, methane is not emitted by volcanoes but slowly diffuses out of Titan’s cold and stiff interior. Ganesa Macula may be an eroded impact crater with a dark dune in the center. The mountainous ridges observed in some regions can be explained as heavily degraded scarps of large multi-ring impact structures or as a result of the global contraction due to the slow cooling of the interior.

2008年,埃姆斯研究中心的行星地质学家杰弗里·摩尔(Jeffrey Moore)提出了泰坦地质学的另一种观点。 他指出,到目前为止,在土卫六上还没有明确地鉴定出火山特征,他断言土卫六是一个地质死亡的世界,其表面仅由撞击坑,河流和风蚀,大量浪费和其他外生过程形成。 根据这个假设,甲烷不是由火山释放的,而是从泰坦寒冷而僵硬的内部缓慢扩散出来的。 Ganesa Macula可能是一个受侵蚀的撞击坑,中心处有一个黑暗的沙丘。 在某些地区观察到的山脊可以解释为大型多环撞击结构的严重退化的山崖,或者是由于内部缓慢冷却导致整体收缩的结果。

Even in this case, Titan may still have an internal ocean made of the eutectic water–ammonia mixture with a temperature of 176 K (−97 °C), which is low enough to be explained by the decay of radioactive elements in the core. The bright Xanadu terrain may be a degraded heavily cratered terrain similar to that observed on the surface of Callisto. Indeed, were it not for its lack of an atmosphere, Callisto could serve as a model for Titan’s geology in this scenario. Jeffrey Moore even called Titan Callisto with weather.[111][122]

即使在这种情况下,土卫六仍可能有一个由温度为176 K(-97°C)的共晶水-氨混合物构成的内部海洋,这个温度足够低,可以用堆芯中放射性元素的衰减来解释。 明亮的Xanadu地形可能是退化的,坑坑洼洼的地形,类似于Callisto表面上观察到的那样。 确实,如果不是因为缺乏气氛,Callisto可以在这种情况下充当泰坦地质学的模型。 杰弗里·摩尔(Jeffrey Moore)甚至把Titan叫做“有天气的Callisto”。[111] [122]

Many of the more prominent mountains and hills have been given official names by the International Astronomical Union. According to JPL, “By convention, mountains on Titan are named for mountains from Middle-earth, the fictional setting in fantasy novels by J. R. R. Tolkien.” Colles (collections of hills) are named for characters from the same Tolkien works.[123]

国际天文学联合会已将许多较著名的山丘命名为官方名称。 根据JPL的说法,“按照惯例,土卫六上的山脉是以J. R. R. Tolkien的幻想小说中的虚构背景中土山脉命名的。” Colles (山丘的集合)是根据同一托尔金作品中的人物命名的。[123]

暗赤道地形Dark equatorial terrain

Sand dunes in the Namib Desert on Earth (top), compared with dunes in Belet on Titan

地球上纳米比沙漠中的沙丘(上),泰坦上的贝莱特中的沙丘

In the first images of Titan’s surface taken by Earth-based telescopes in the early 2000s, large regions of dark terrain were revealed straddling Titan’s equator.[124] Prior to the arrival of Cassini, these regions were thought to be seas of liquid hydrocarbons.[125] Radar images captured by the Cassini spacecraft have instead revealed some of these regions to be extensive plains covered in longitudinal dunes, up to 330 ft (100 m) high[126] about a kilometer wide, and tens to hundreds of kilometers long.[127] 

在2000年代初期,由天基望远镜拍摄的第一幅泰坦表面图像显示,横跨泰坦赤道的大片黑暗地形被发现。[124] 在卡西尼号到达之前,这些地区被认为是液态烃海洋。[125] 卡西尼号航天器捕获的雷达图像反而显示出其中一些区域是被纵向沙丘覆盖的广阔平原,高达330英尺(100 m)高[126]约一公里,长数十至数百公里。[127 ]

Dunes of this type are always aligned with average wind direction. In the case of Titan, steady zonal (eastward) winds combine with variable tidal winds (approximately 0.5 meters per second).[128] The tidal winds are the result of tidal forces from Saturn on Titan’s atmosphere, which are 400 times stronger than the tidal forces of the Moon on Earth and tend to drive wind toward the equator. This wind pattern, it was hypothesized, causes granular material on the surface to gradually build up in long parallel dunes aligned west-to-east. The dunes break up around mountains, where the wind direction shifts.

这种沙丘总是与平均风向对齐。 就泰坦而言,稳定的纬向风(东风)与潮汐风的可变风相结合(每秒约0.5米)。[128] 潮汐风是土星在土卫六大气层上产生的潮汐力的结果,它比月球在地球上的潮汐力强400倍,并且倾向于将风吹向赤道。 据推测,这种风型使表面上的颗粒状物质逐渐堆积成西向东排列的长平行沙丘。 沙丘在山周围破裂,风向发生变化。

The longitudinal (or linear) dunes were initially presumed to be formed by moderately variable winds that either follow one mean direction or alternate between two different directions. Subsequent observations indicate that the dunes point to the east although climate simulations indicate Titan’s surface winds blow toward the west. At less than 1 meter per second, they are not powerful enough to lift and transport surface material. Recent computer simulations indicate that the dunes may be the result of rare storm winds that happen only every fifteen years when Titan is in equinox.[129] These storms produce strong downdrafts, flowing eastward at up to 10 meters per second when they reach the surface.

最初假定纵向(或线性)沙丘是由适度变化的风形成的,该风要么遵循一个平均方向,要么在两个不同方向之间交替。 随后的观察表明,沙丘指向东方,尽管气候模拟表明土卫六的地表风向西吹。 它们的速度不足每秒1米,因此不足以提升和运输表面材料。 最近的计算机模拟表明,沙丘可能是罕见的暴风雨的结果,这种暴风雨仅在泰坦每15年发生一次的春分中产生。[129] 这些风暴产生强烈的向下气流,到达地面后以每秒10米的速度向东流动。

The “sand” on Titan is likely not made up of small grains of silicates like the sand on Earth,[130] but rather might have formed when liquid methane rained and eroded the water-ice bedrock, possibly in the form of flash floods. Alternatively, the sand could also have come from organic solids called tholins, produced by photochemical reactions in Titan’s atmosphere.[126][128][131] Studies of dunes’ composition in May 2008 revealed that they possessed less water than the rest of Titan, and are thus most likely derived from organic soot like hydrocarbon polymers clumping together after raining onto the surface.[132] 

土卫六上的“沙”可能不是像地球上的沙子一样由硅酸盐的小颗粒组成的,[130]而是当液态甲烷降雨并侵蚀了水冰基岩时形成的,可能是洪水泛滥的形式。 另外,沙子也可能来自称为tholins的有机固体,是通过泰坦大气中的光化学反应产生的。[126] [128] [131] 2008年5月对沙丘的组成进行的研究表明,沙丘的水分比泰坦其余部分少,因此很可能源自雨后表面结成团块的有机烟灰状烃类聚合物。[132]

Calculations indicate the sand on Titan has a density of one-third that of terrestrial sand.[133] The low density combined with the dryness of Titan’s atmosphere might cause the grains to clump together because of static electricity buildup. The “stickiness” might make it difficult for the generally mild breeze close to Titan’s surface to move the dunes although more powerful winds from seasonal storms could still blow them eastward.[134]

计算表明,土卫六上的沙的密度是地球沙的三分之一。[133] 低密度加上土卫六大气的干燥可能会导致颗粒由于静电积聚而结块。 “粘性”可能会使靠近土卫六表面的一般微风难以移动沙丘,尽管来自季节性风暴的更强风仍会将它们吹向东。[134]

Around equinox, strong downburst winds can lift micron-sized solid organic particles up from the dunes to create Titanian dust storms, observed as intense and short-lived brightenings in the infrared.[135]

Titan – three dust storms detected in 2009-2010.[136]

Titan——三个在2009-2010探测到的沙尘暴

观察探索Observation and exploration

Voyager 1 view of haze on Titan’s limb (1980)

Titan is never visible to the naked eye, but can be observed through small telescopes or strong binoculars. Amateur observation is difficult because of the proximity of Titan to Saturn’s brilliant globe and ring system; an occulting bar, covering part of the eyepiece and used to block the bright planet, greatly improves viewing.[137] Titan has a maximum apparent magnitude of +8.2,[8] and mean opposition magnitude 8.4.[138] This compares to +4.6[138] for the similarly sized Ganymede, in the Jovian system.

泰坦用肉眼不可见,但是可以通过小型望远镜或强大的双筒望远镜观察到。 由于土卫六靠近土星的明亮的土星环,因此业余观察很难。 遮盖住目镜一部分并用来遮挡明亮星球的遮挡条大大改善了视线。[137] 土卫六的最大视在震级为+8.2,[8],平均相对震级为8.4。[138] 相比之下,木星系统中大小相同的木卫三为+4.6 [138]。

Observations of Titan prior to the space age were limited. In 1907 Spanish astronomer Josep Comas i Solà observed limb darkening of Titan, the first evidence that the body has an atmosphere. In 1944 Gerard P. Kuiper used a spectroscopic technique to detect an atmosphere of methane.[139]

在太空时代之前对泰坦的观测是有限的。 1907年,西班牙天文学家Josep Comas iSolà观测到泰坦limb变黑,这是天体具有大气的第一个证据。 1944年,杰拉德·P·柯伊珀(Gerard P. Kuiper)使用光谱技术来检测甲烷气氛。[139]

Cassini‘s Titan flyby radio signal studies (artist’s concept)

卡西尼号的土卫六飞越无线电信号研究(艺术想象图)

The first probe to visit the Saturnian system was Pioneer 11 in 1979, which revealed that Titan was probably too cold to support life.[140] It took images of Titan, including Titan and Saturn together in mid to late 1979.[141] The quality was soon surpassed by the two Voyagers.

进入土星系统的第一个探测器是1979年的先锋11号,它显示土卫六可能太冷而无法维持生命。[140] 它在1979年中后期拍摄了泰坦的影像,包括泰坦和土星。[141] 很快,两个旅行者就接着开始探测

Titan was examined by both Voyager 1 and 2 in 1980 and 1981, respectively. Voyager 1‘s trajectory was designed to provide an optimized Titan flyby, during which the spacecraft was able to determine the density, composition, and temperature of the atmosphere, and obtain a precise measurement of Titan’s mass.[142] Atmospheric haze prevented direct imaging of the surface, though in 2004 intensive digital processing of images taken through Voyager 1‘s orange filter did reveal hints of the light and dark features now known as Xanadu and Shangri-la,[143] which had been observed in the infrared by the Hubble Space Telescope. Voyager 2, which would have been diverted to perform the Titan flyby if Voyager 1 had been unable to, did not pass near Titan and continued on to Uranus and Neptune.[142]:94

泰坦号分别在1980年和1981年受到旅行者1号和旅行者2号的检查。 旅行者1号的航迹旨在提供优化的泰坦飞越,在此期间,航天器能够确定大气层的密度,组成和温度,并获得对泰坦质量的精确测量。[142] 大气雾霾阻止了表面的直接成像,尽管在2004年,通过旅行者1号橙色滤镜对图像进行了密集的数字处理,确实揭示了现今称为Xanadu和Shangri-la的明暗特征的暗示[143]。 哈勃太空望远镜拍摄的红外线。如果旅行者1号没有完成飞掠Titan的任务,旅行者2号将接任。不过旅行者1号最终has完成了任务,于是旅行者2号就开始朝天王星和海王星飞去。

卡西尼-惠更斯Cassini–Huygens

Main articles: Cassini–Huygens and Huygens (spacecraft)

Cassini image of Titan in front of the rings of Saturn

Cassini image of Titan, behind Epimetheus and the rings

Even with the data provided by the Voyagers, Titan remained a body of mystery—a large satellite shrouded in an atmosphere that makes detailed observation difficult. The mystery that had surrounded Titan since the 17th-century observations of Christiaan Huygens and Giovanni Cassini was revealed by a spacecraft named in their honor.

即使有了旅行者号提供的数据,土卫六仍然是一个谜,一个巨大的卫星笼罩在大气中,很难进行详细观察。 自从17世纪对克里斯蒂安·惠更斯和乔瓦尼·卡西尼的观测以来,围绕泰坦的谜团被一个以他们的名字命名的航天器揭示出来。

The Cassini–Huygens spacecraft reached Saturn on July 1, 2004, and began the process of mapping Titan’s surface by radar. A joint project of the European Space Agency (ESA) and NASACassini–Huygens proved a very successful mission. The Cassini probe flew by Titan on October 26, 2004, and took the highest-resolution images ever of Titan’s surface, at only 1,200 kilometers (750 mi), discerning patches of light and dark that would be invisible to the human eye.

卡西尼-惠更斯号航天器于2004年7月1日到达土星,并开始通过雷达对土卫六的表面进行制图。 卡西尼-惠更斯号是欧洲航天局(ESA)和美国宇航局的一项联合项目,这项任务非常成功。 卡西尼号探测器于2004年10月26日由土卫六飞行,并在仅1,200公里(750英里)的高度拍摄了土卫六表面的最高分辨率图像,辨别出肉眼和肉眼看不见的斑点。

On July 22, 2006, Cassini made its first targeted, close fly-by at 950 kilometers (590 mi) from Titan; the closest flyby was at 880 kilometers (550 mi) on June 21, 2010.[144] Liquid has been found in abundance on the surface in the north polar region, in the form of many lakes and seas discovered by Cassini.[81]

2006年7月22日,卡西尼号在距泰坦950公里(590英里)处首次定向近距离飞行; 2010年6月21日,最近的飞越距离为880公里(550英里)。[144] 在卡西尼发现的许多湖泊和海洋中,在北极地区的表面大量发现了液体。[81]

惠更斯着陆Huygens landing

Huygensin situ image from Titan’s surface—the only image from the surface of a body farther away than Mars

惠更斯号拍摄的Titan地表——唯一比火星更远的天体地表图像

Same image with contrast enhanced

提升对比度

Huygens was an atmospheric probe that touched down on Titan on January 14, 2005,[145] discovering that many of its surface features seem to have been formed by fluids at some point in the past.[146] Titan is the most distant body from Earth to have a space probe land on its surface.[147]

惠更斯是一个大气探测器,于2005年1月14日降落在土卫六,[145]发现其表面的许多特征似乎是在过去某个时刻由流体形成的。[146] Titan是其表面有一个太空探测器着陆的离地球最远的天体。[147]

The Huygens probe landed just off the easternmost tip of a bright region now called Adiri. The probe photographed pale hills with dark “rivers” running down to a dark plain. Current understanding is that the hills (also referred to as highlands) are composed mainly of water ice. Dark organic compounds, created in the upper atmosphere by the ultraviolet radiation of the Sun, may rain from Titan’s atmosphere. They are washed down the hills with the methane rain and are deposited on the plains over geological time scales.[148]

惠更斯号探测器降落在现称为阿迪里(Adiri)的明亮区域的最东端。 探测器拍摄了苍白的山丘,上面有黑色的“河流”,一直流到一片黑暗的平原。 当前的理解是,丘陵(也称为高地)主要由水冰组成。 太阳的紫外线辐射在高层大气中形成的深色有机化合物,可能会在泰坦的大气下雨时落下,然后它们被甲烷雨冲下山坡,并在地质时间尺度上沉积在平原上。[148]

After landing, Huygens photographed a dark plain covered in small rocks and pebbles, which are composed of water ice.[148] The two rocks just below the middle of the image on the right are smaller than they may appear: the left-hand one is 15 centimeters across, and the one in the center is 4 centimeters across, at a distance of about 85 centimeters from Huygens. There is evidence of erosion at the base of the rocks, indicating possible fluvial activity. The surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice. The “soil” visible in the images is interpreted to be precipitation from the hydrocarbon haze above.

降落后,惠更斯拍摄了一片黑暗的平原,上面覆盖着由水冰组成的小岩石和小卵石。[148] 右侧图像中间正下方的两块岩石比它们看起来的要小:左侧的一块宽15厘米,中间的一块宽4厘米,与惠更斯相距约85厘米 。 岩石底部有侵蚀迹象,表明可能有河流活动。 该表面比最初预期的要暗,由水和碳氢化合物的混合物组成。 图像中可见的“土壤”被解释为来自上方烃雾的沉淀。

In March 2007, NASA, ESA, and COSPAR decided to name the Huygens landing site the Hubert Curien Memorial Station in memory of the former president of the ESA.[149]

[149]在2007年3月,NASA,ESA和COSPAR决定将惠更斯登陆点命名为Hubert Curien纪念站,以纪念ESA的前任总统。[149]

计划中:蜻蜓探测器Planned: Dragonfly

Main article: Dragonfly (spacecraft)

The Dragonfly mission will launch in 2026 consisting of a large drone powered by an RTG to fly in the atmosphere of Titan as the New Frontiers program mission #4.[150][151] Its instruments will study how far prebiotic chemistry may have progressed.[152]

[150] [151],蜻蜓任务将于2026年发射,其中包括一架由RTG供电的大型无人机,在泰坦大气中飞行。[150] [151] 它的仪器将研究益生元化学的进展程度。[152]

拟议任务或概念任务Proposed or conceptual missions

The balloon proposed for the Titan Saturn System Mission (artistic rendition)

泰坦土星系统任务建议的气球(艺术再现)

There have been several conceptual missions proposed in recent years for returning a robotic space probe to Titan. Initial conceptual work has been completed for such missions by NASA, the ESA and JPL. At present, none of these proposals have become funded missions.

近年来,有人提出了一些概念性任务,要求将机器人太空探测器返回泰坦。 NASA,ESA和JPL已为此类任务完成了初步的概念性工作。 目前,这些建议都还未获得资助。

The Titan Saturn System Mission (TSSM) was a joint NASA/ESA proposal for exploration of Saturn‘s moons.[153] It envisions a hot-air balloon floating in Titan’s atmosphere for six months. It was competing against the Europa Jupiter System Mission (EJSM) proposal for funding. In February 2009 it was announced that ESA/NASA had given the EJSM mission priority ahead of the TSSM.[154]

土卫六土星系统任务(TSSM)是NASA / ESA提出的一项有关探索土星卫星的建议。[153] 它设想了一个热气球在泰坦的大气中漂浮六个月。 它与欧罗巴木星系统任务(EJSM)的提案竞争。 2009年2月,宣布ESA / NASA在TSSM之前将EJSM任务列为优先任务。[154]

The proposed Titan Mare Explorer (TiME) was a low-cost lander that would splash down in a lake in Titan’s northern hemisphere and float on the surface of the lake for three to six months.[155][156][157] It was selected for a Phase-A design study in 2011 as a candidate mission for the 12th NASA Discovery Program opportunity,[158] but was not selected for flight.[159]

拟议中的泰坦·马雷探险者号(TiME)是一种低成本着陆器,可以降落在泰坦北半球的一个湖中,并在湖面漂浮三到六个月。[155] [156] [157] 它被选为2011年A期设计研究的对象,以作为第12项NASA探索计划机会的候选任务,[158]但未被选中进行飞行。[159]

Another mission to Titan proposed in early 2012 by Jason Barnes, a scientist at the University of Idaho, is the Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR): an unmanned plane (or drone) that would fly through Titan’s atmosphere and take high-definition images of the surface of Titan. NASA did not approve the requested $715 million, and the future of the project is uncertain.[160][161]

爱达荷大学的科学家杰森·巴恩斯(Jason Barnes)于2012年初提出了另一项对土卫六的任务,它是用于实地和机载土卫六侦察飞行的航空器(AVIATR):一种可通过土卫六大气层飞行的无人机(或无人驾驶飞机), 拍摄土卫六表面的高清图像。 美国航空航天局未批准所要求的7.15亿美元,并且该项目的未来不确定。[160] [161]

A conceptual design for another lake lander was proposed in late 2012 by the Spanish-based private engineering firm SENER and the Centro de Astrobiología in Madrid. The concept probe is called Titan Lake In-situ Sampling Propelled Explorer (TALISE).[162][163] The major difference compared to the TiME probe would be that TALISE is envisioned with its own propulsion system and would therefore not be limited to simply drifting on the lake when it splashes down.

西班牙私人工程公司SENER和马德里的天文生物中心在2012年末提出了另一座湖着陆器的概念设计。 该概念探测器称为“泰坦湖原位采样推进探测器”(TALISE)。[162] [163] 与TiME探测器相比,主要区别在于TALISE带有自己的推进系统,因此它的运动不再限于降落在湖面上后靠飘动。

Discovery Program contestant for its mission #13 is Journey to Enceladus and Titan (JET), an astrobiology Saturn orbiter that would assess the habitability potential of Enceladus and Titan.[164][165][166]

探索计划的第13个任务参赛者是土卫六土星轨道飞行器土卫六和土卫六之旅(JET),它将评估土卫二和土卫六的可居住性潜力。[164] [165] [166]

In 2015, the NASA Innovative Advanced Concepts program (NIAC) awarded a Phase II grant[167] to a design study of a submarine to explore the seas of Titan.[168][169][170]

[168] [169] [170]在2015年,美国国家航空航天局(NASA)创新高级概念计划(NIAC)向一艘潜艇的设计研究提供了第二阶段赠款[167] [169] [169] [170]。

可能的生命Prebiotic conditions and life

Main article: Life on TitanSee also: Planetary habitability

Titan is thought to be a prebiotic environment rich in complex organic compounds,[50][171] but its surface is in a deep freeze at −179 °C (−290.2 °F; 94.1 K) so life as we know it cannot exist on the moon’s frigid surface.[172] However, Titan seems to contain a global ocean beneath its ice shell, and within this ocean, conditions are potentially suitable for microbial life.[173][174][175]

土卫六被认为是富含复杂有机化合物的益生元环境,[50] [171],但其表面处于−179°C(−290.2°F; 94.1 K)的深度冻结中,因此我们所知的生命不可能存在于Titan的冰冷表面上。[172] 然而,土卫六似乎在其冰壳之下包含着一个全球海洋,在这个海洋中,条件可能适合微生物生活。[173] [174] [175]

The Cassini–Huygens mission was not equipped to provide evidence for biosignatures or complex organic compounds; it showed an environment on Titan that is similar, in some ways, to ones hypothesized for the primordial Earth.[176] Scientists surmise that the atmosphere of early Earth was similar in composition to the current atmosphere on Titan, with the important exception of a lack of water vapor on Titan.[177][171]

卡西尼号–惠更斯号飞行任务无法为生物特征或复杂的有机化合物提供证据; 它显示了土卫六上的环境,在某些方面与原始地球的假设相似。[176] 科学家推测,早期地球的大气成分与土卫六上的当前大气相似,但重要的例外是土卫六上缺乏水蒸气。[177] [171]

复杂分子的形成Formation of complex molecules

The Miller–Urey experiment and several following experiments have shown that with an atmosphere similar to that of Titan and the addition of UV radiation, complex molecules and polymer substances like tholins can be generated. The reaction starts with dissociation of nitrogen and methane, forming hydrogen cyanide and acetylene. Further reactions have been studied extensively.[178]

Miller-Urey实验和以下几个实验表明,在类似于泰坦的气氛和紫外线辐射的作用下,可以生成复杂的分子和高分子物质,如tholins。 该反应从氮和甲烷的离解开始,形成氰化氢和乙炔。 进一步的反应已被广泛研究。[178]

It has been reported that when energy was applied to a combination of gases like those in Titan’s atmosphere, five nucleotide bases, the building blocks of DNA and RNA, were among the many compounds produced. In addition, amino acids, the building blocks of protein were found. It was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present.[179]

据报道,当将能量施加到诸如泰坦大气中的气体的混合物中时,产生的许多化合物中包括五个核苷酸碱基,DNA和RNA的构成部分。 此外,还发现了氨基酸,蛋白质的组成部分。 这是在这种实验中首次发现不存在液态水的核苷酸碱基和氨基酸。[179]

On April 3, 2013, NASA reported that complex organic chemicals could arise on Titan based on studies simulating the atmosphere of Titan.[50]

2013年4月3日,美国国家航空航天局(NASA)报告称,基于模拟泰坦大气的研究,泰坦上可能会生成复杂的有机化学物。[50]

On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons (PAH) in the upper atmosphere of Titan.[51]

2013年6月6日,IAA-CSIC的科学家报告了在泰坦高层大气中检测到多环芳烃(PAH)的情况。[51]

On July 26, 2017, Cassini scientists positively identified the presence of carbon chain anions in Titan’s upper atmosphere which appeared to be involved in the production of large complex organics.[180] These highly reactive molecules were previously known to contribute to building complex organics in the Interstellar Medium, therefore highlighting a possibly universal stepping stone to producing complex organic material.[181]

2017年7月26日,卡西尼号的科学家们积极地确定了土卫六高空大气层中存在碳链阴离子,这似乎与大型复杂有机物的生产有关。[180] 先前已知这些高反应性分子有助于在星际介质中构建复杂的有机物,因此突显了生产通用有机材料的可能的通用垫脚石。[181]

On July 28, 2017, scientists reported that acrylonitrile, or vinyl cyanide, (C2H3CN), possibly essential for life by being related to cell membrane and vesicle structure formation, had been found on Titan.[182][183][184]

2017年7月28日,科学家报告称,在Titan上发现了可能与生命有关的丙烯腈或乙烯基氰(C2H3CN),与细胞膜和囊泡的结构形成有关。[182] [183] [184]

In October 2018, researchers reported low-temperature chemical pathways from simple organic compounds to complex polycyclic aromatic hydrocarbon (PAH) chemicals. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere of Titan, and may be significant pathways, in terms of the PAH world hypothesis, in producing precursors to biochemicals related to life as we know it.[185][186]

2018年10月,研究人员报告了从简单的有机化合物到复杂的多环芳烃(PAH)化学品的低温化学途径。 这样的化学途径可能有助于解释在泰坦的低温大气中PAH的存在,并且就PAH世界假设而言,在生产与生命有关的生物化学物质的前体方面,这可能是重要的途径。

可能的地下栖息地Possible subsurface habitats

Laboratory simulations have led to the suggestion that enough organic material exists on Titan to start a chemical evolution analogous to what is thought to have started life on Earth. The analogy assumes the presence of liquid water for longer periods than is currently observable; several hypotheses postulate that liquid water from an impact could be preserved under a frozen isolation layer.[187] It has also been hypothesized that liquid-ammonia oceans could exist deep below the surface.[173][188] Another model suggests an ammonia–water solution as much as 200 kilometers (120 mi) deep beneath a water-ice crust with conditions that, although extreme by terrestrial standards, are such that life could survive.[174] Heat transfer between the interior and upper layers would be critical in sustaining any subsurface oceanic life.[173] Detection of microbial life on Titan would depend on its biogenic effects, with the atmospheric methane and nitrogen examined.[174]

实验室模拟得出这样的建议:土卫六上存在足够的有机物质,可以开始化学演化,类似于人们认为已经开始在地球上生命的物质。 这个比喻假设液态水的存在时间比目前可观察到的时间更长。 一些假设假设撞击产生的液态水可以保存在冷冻的隔离层下。[187] 还假设液氨海洋可能存在于地表深处。[173] [188] 另一个模型表明,氨水水溶液在水冰壳的下方深达200公里(120英里),其条件虽然按地面标准是极端的,但可以生存。[174] 内层和上层之间的热传递对于维持任何地下海洋生物至关重要。[173] 在土卫六上检测微生物生命将取决于其生物成因,并检查了大气中的甲烷和氮。[174]

地表上的甲烷与生命Methane and life at the surface

See also: Hypothetical types of biochemistry

It has been speculated that life could exist in the lakes of liquid methane on Titan, just as organisms on Earth live in water.[189] Such organisms would inhale H2 in place of O2, metabolize it with acetylene instead of glucose, and exhale methane instead of carbon dioxide.[175][189] However, such hypothetical organisms would be required to metabolize at a deep freeze temperature of −179 °C (−290.2 °F; 94.1 K).[172]

据推测,就像地球上的生物生活在水中一样,泰坦岛上的液态甲烷湖中可能存在生命。[189] 这类生物会吸入H2代替O2,并用乙炔代替葡萄糖进行代谢,并呼出甲烷而不是二氧化碳。[175] [189] 但是,将需要这种假设的生物在−179°C(−290.2°F; 94.1 K)的深度冷冻温度下代谢。[172]

All life forms on Earth (including methanogens) use liquid water as a solvent; it is speculated that life on Titan might instead use a liquid hydrocarbon, such as methane or ethane,[190] although water is a stronger solvent than methane.[191] Water is also more chemically reactive, and can break down large organic molecules through hydrolysis.[190] A life form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way.[190]

地球上所有生命形式(包括产甲烷菌)都使用液态水作为溶剂; 据推测,土卫六上的生命可能会改用液态碳氢化合物,例如甲烷或乙烷,[190]尽管水是比甲烷强的溶剂。[191] 水还具有更高的化学反应性,并且可以通过水解分解大的有机分子。[190] [190]以溶剂为碳氢化合物的生命形式不会面临其生物分子被破坏的风险。[190]

In 2005, astrobiologist Chris McKay argued that if methanogenic life did exist on the surface of Titan, it would likely have a measurable effect on the mixing ratio in the Titan troposphere: levels of hydrogen and acetylene would be measurably lower than otherwise expected.[189]

2005年,天体生物学家克里斯·麦凯(Chris McKay)认为,如果土卫六表面确实存在产甲烷生命,那么它可能会对土卫六对流层中的混合比产生可测量的影响:氢和乙炔的含量将明显低于其他预期。[189] ]

In 2010, Darrell Strobel, from Johns Hopkins University, identified a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward flow at a rate of roughly 1028 molecules per second and disappearance of hydrogen near Titan’s surface; as Strobel noted, his findings were in line with the effects McKay had predicted if methanogenic life-forms were present.[189][191][192] The same year, another study showed low levels of acetylene on Titan’s surface, which were interpreted by McKay as consistent with the hypothesis of organisms consuming hydrocarbons.[191] 

2010年,约翰·霍普金斯大学(Johns Hopkins University)的达雷尔·斯特罗贝尔(Darrell Strobel)发现,土卫六高层大气层中的氢分子比下层大气层中的氢分子丰富得多,他们认为向下流动的速度约为每秒1028个分子,并且氢的消失率接近 泰坦的表面; 正如Strobel所指出的那样,他的发现与麦凯(McKay)所预测的是否存在产甲烷生命形式是一致的。[189] [191] [192] 同年,另一项研究表明土卫六表面的乙炔含量低,这被麦凯解释为与食用碳氢化合物的生物假说相符。[191]

Although restating the biological hypothesis, he cautioned that other explanations for the hydrogen and acetylene findings are more likely: the possibilities of yet unidentified physical or chemical processes (e.g. a surface catalyst accepting hydrocarbons or hydrogen), or flaws in the current models of material flow.[175] Composition data and transport models need to be substantiated, etc. Even so, despite saying that a non-biological catalytic explanation would be less startling than a biological one, McKay noted that the discovery of a catalyst effective at 95 K (−180 °C) would still be significant.[175]

尽管重申了生物学假设,但他警告说,对于氢和乙炔发现的其他解释更有可能:尚未确定的物理或化学过程(例如,表面催化剂接受烃或氢)的可能性,或当前物质流模型中的缺陷 。[175] 成分数据和传输模型需要得到证实,依此类推。尽管如此,尽管说非生物催化解释比生物解释令人吃惊,但麦凯指出,发现了一种在95 K(-180°C)有效的催化剂 仍然很重要。[175]

As NASA notes in its news article on the June 2010 findings: “To date, methane-based life forms are only hypothetical. Scientists have not yet detected this form of life anywhere.”[191] As the NASA statement also says: “some scientists believe these chemical signatures bolster the argument for a primitive, exotic form of life or precursor to life on Titan’s surface.”[191]

正如NASA在其关于2010年6月调查结果的新闻文章中指出的那样:“迄今为止,基于甲烷的生命形式只是假设的。科学家尚未在任何地方发现这种生命形式。” [191]正如NASA的声明还说:“ 科学家们相信,这些化学特征支持了关于泰坦表面上原始的,奇异的生命形式或生命先兆的观点。” [191]

In February 2015, a hypothetical cell membrane capable of functioning in liquid methane at cryogenic temperatures (deep freeze) conditions was modeled. Composed of small molecules containing carbon, hydrogen, and nitrogen, it would have the same stability and flexibility as cell membranes on Earth, which are composed of phospholipids, compounds of carbon, hydrogen, oxygen, and phosphorus. This hypothetical cell membrane was termed an “azotosome“, a combination of “azote”, French for nitrogen, and “liposome“.[193][194]

2015年2月,对一种能够在低温(深度冷冻)条件下在液态甲烷中起作用的假想细胞膜进行了建模。 它由含有碳,氢和氮的小分子组成,具有与地球上由磷脂,碳,氢,氧和磷的化合物组成的细胞膜相同的稳定性和柔韧性。 这种假设的细胞膜被称为“ azotosome ”,即法语中的氮“ azote ”,和“ liposome ”的组合。[193] [194]

阻碍Obstacles

Despite these biological possibilities, there are formidable obstacles to life on Titan, and any analogy to Earth is inexact. At a vast distance from the Sun, Titan is frigid, and its atmosphere lacks CO2. At Titan’s surface, water exists only in solid form. Because of these difficulties, scientists such as Jonathan Lunine have viewed Titan less as a likely habitat for life, than as an experiment for examining hypotheses on the conditions that prevailed prior to the appearance of life on Earth.[195] Although life itself may not exist, the prebiotic conditions on Titan and the associated organic chemistry remain of great interest in understanding the early history of the terrestrial biosphere.[176] Using Titan as a prebiotic experiment involves not only observation through spacecraft, but laboratory experiments, and chemical and photochemical modeling on Earth.[178]

尽管有这些生物学上的可能性,但土卫六上仍然存在巨大的生命障碍,与地球的任何比喻都不精确。 泰坦(Titan)在距离太阳很远的地方十分寒冷,并且其大气层中缺乏二氧化碳。 在土卫六的表面,水仅以固体形式存在。 由于这些困难,乔纳森·卢尼(Jonathan Lunine)等科学家将土卫六视为生命的可能栖息地,而不是将其视作在生命出现在地球上之前普遍存在的条件下进行检验的实验。[195] 尽管生命本身可能并不存在,但对于了解地球生物圈的早期历史,泰坦上的益生元条件和相关的有机化学仍然引起人们极大的兴趣。[176] 将土卫六用作益生元实验不仅涉及通过航天器的观测,还涉及实验室实验以及地球上的化学和光化学模型。[178]

Panspermia hypothesis

It is hypothesized that large asteroid and cometary impacts on Earth’s surface may have caused fragments of microbe-laden rock to escape Earth’s gravity, suggesting the possibility of transpermia. Calculations indicate that these would encounter many of the bodies in the Solar System, including Titan.[196][197] On the other hand, Jonathan Lunine has argued that any living things in Titan’s cryogenic hydrocarbon lakes would need to be so different chemically from Earth life that it would not be possible for one to be the ancestor of the other.[198]

据推测,大的小行星和彗星撞击地球表面可能导致载有微生物的岩石碎片逃脱地球引力,这表明有可能发生 transpermia 现象。 计算表明,它们会遇到太阳系中的许多物体,包括土卫六。[196] [197] 另一方面,乔纳森·卢尼(Jonathan Lunine)辩称,泰坦(Titan)低温碳氢化合物湖泊中的任何生物在化学上都必须与地球生命相去甚远,以至于地球上的生物不可能成为Titan上生物的祖先。[198]

未来的状况Future conditions

Conditions on Titan could become far more habitable in the far future. Five billion years from now, as the Sun becomes a red giant, its surface temperature could rise enough for Titan to support liquid water on its surface, making it habitable.[199] As the Sun’s ultraviolet output decreases, the haze in Titan’s upper atmosphere will be depleted, lessening the anti-greenhouse effect on the surface and enabling the greenhouse created by atmospheric methane to play a far greater role. These conditions together could create a habitable environment, and could persist for several hundred million years. This is proposed to have been sufficient time for simple life to spawn on Earth, though the presence of ammonia on Titan would cause chemical reactions to proceed more slowly.[200]

在不久的将来,泰坦岛上的条件可能变得更加适宜。 从现在起五十亿年后,随着太阳成为红巨星,其地表温度可能上升到足以使泰坦在其表面上支撑液态水的位置,使其可居住。[199] 随着太阳紫外线输出量的减少,土卫六高层大气中的阴霾将被耗尽,从而减轻表面的抗温室效应,并让大气甲烷产生的温室发挥更大的作用。 这些条件共同可以创造一个宜居的环境,并且可以持续数亿年。 尽管在土卫六上存在氨会导致化学反应进行得更慢,但据提议,这已经足够使它们在Titan上产生简单的生命。[200]

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