The climate of Titan , the largest moon of Saturn, is similar in many respects to that of Earth, despite having a far lower surface temperature. Its thick atmosphere, methane rain, and possible cryovolcanism create an analogue, though with different materials, to the climatic changes undergone by Earth during its far shorter year.
Titan receives just about 1% of the amount of sunlight Earth does. [1] The average surface temperature is about 90.6 K (-182.55 °C, or -296.59 °F). [2] At this temperature water ice has an extremely low vapor pressure, so the atmosphere is nearly free of water vapor. However the methane in the atmosphere causes a substantial greenhouse effect which keeps the surface of Titan at a much higher temperature than what would otherwise be the thermal equilibrium. [3] [4] [5]
Haze in Titan's atmosphere contributes to an anti-greenhouse effect by reflecting sunlight back into space, making its surface significantly colder than its upper atmosphere. [3] This partially compensates for the greenhouse warming, and keeps the surface somewhat cooler than would otherwise be expected from the greenhouse effect alone. [6] According to McKay et al., "the anti-greenhouse effect on Titan reduces the surface temperature by 9 K whereas the greenhouse effect increases it by 21 K. The net effect is that the surface temperature (94 K) is 12 K warmer than the effective temperature 82 K. [i.e., the equilibrium that would be reached in the absence of any atmosphere]" [3]
Titan's orbital tilt with respect to the Sun is very close to Saturn's axial tilt (about 27°), and its axial tilt with respect to its orbit is zero. This means that the direction of incoming sunlight is driven almost entirely by Titan's day-night cycle and Saturn's year cycle. The day cycle on Titan lasts 15.9 Earth days, which is how long it takes Titan to orbit Saturn. Titan is tidally locked, so the same part of Titan always faces Saturn, and there is no separate "month" cycle.
Seasonal change is driven by Saturn's year: it takes Saturn about 29.5 Earth years to orbit the Sun, exposing different amounts of sunlight to Titan's northern and southern hemispheres during different parts of the Saturnian year. Seasonal weather changes include larger hydrocarbon lakes in the northern hemisphere during the winter, decreased haze around the equinoxes due to changing atmospheric circulation, and associated ice clouds in the South Polar regions. [7] [8] The last equinox occurred on August 11, 2009; this was the spring equinox for the northern hemisphere, meaning the southern hemisphere is getting less sunlight and moving into winter. [9]
Surface winds are normally low (<1 meter per second). Recent computer simulations indicate that the huge dunes of soot like material raining down from the atmosphere in the equatorial regions may instead be shaped by rare storm winds that happen only every fifteen years when Titan is in equinox. [10] The storms produce strong downdrafts, flowing eastward at up to 10 meters per second when they reach the surface. In late 2010, the equivalent of early Spring in Titan's northern hemisphere, a series of methane storms were observed in Titan's equatorial desert regions. [11]
Due to the eccentricity of Saturn's orbit, Titan is about 12% closer to the Sun during the southern hemisphere summer, making southern summers shorter but hotter than northern summers. This asymmetry may contribute to topological differences between the hemispheres - the northern hemisphere has many more hydrocarbon lakes. [12] Titan's lakes are largely placid, with few waves or ripples; however, Cassini has found evidence of increasing turbulence during the northern hemisphere summer, suggesting that surface winds may strengthen during certain times of the Titanian year. [13] Waves and ripples have also been seen by Cassini. [14]
The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto the moon's surface. [15] In October 2007, observers noted an increase in apparent opacity in the clouds above the equatorial Xanadu region, suggestive of "methane drizzle", though this was not direct evidence for rain. [16] However, subsequent images of lakes in Titan's southern hemisphere taken over one year show that they are enlarged and filled by seasonal hydrocarbon rainfall. [5] [17] It is possible that areas of Titan's surface may be coated in a layer of tholins, but this has not been confirmed. [18] The presence of rain indicates that Titan may be the only Solar System body besides Earth upon which rainbows could form. However, given the extreme opacity of the atmosphere to visible light, the vast majority of any rainbows would be visible only in the infrared. [19]
The number of methane lakes visible near Titan's south pole is decidedly smaller than the number observed near the north pole. As the south pole is currently in summer and the north pole in winter, an emerging hypothesis is that methane rains onto the poles in winter and evaporates in summer. [20] According to a paper by Tetsuya Tokano of the University of Cologne, cyclones driven by this evaporation and involving rain as well as gale-force winds of up to 20 m/s (45 mph) are expected to form over the large northern seas (Kraken Mare, Ligeia Mare, Punga Mare) only in the northern summer, lasting up to ten days. [21] Calculations suggest that, as the northern hemisphere, where most of the lakes reside, enters the long Titanean summer, wind speeds might increase to 3 km/h, levels sufficient to produce waves. [22] Waves have been observed on several occasions by Cassini RADAR and the Visual and Infrared Mapping Spectrometer since 2014, which were likely generated from summer winds [23] [24] or tidal currents. [25] [26]
Simulations of global wind patterns based on wind speed data taken by Huygens during its descent have suggested that Titan's atmosphere circulates in a single enormous Hadley cell. Warm gas rises in Titan's southern hemisphere—which was experiencing summer during Huygens' descent—and sinks in the northern hemisphere, resulting in high-altitude gas flow from south to north and low-altitude gas flow from north to south. Such a large Hadley cell is only possible on a slowly rotating world such as Titan. [27] The pole-to-pole wind circulation cell appears to be centered on the stratosphere; simulations suggest it ought to change every twelve years, with a three-year transition period, over the course of Titan's year (30 terrestrial years). [28] This cell creates a global band of low pressure—what is in effect a variation of Earth's Intertropical Convergence Zone (ITCZ). Unlike on Earth, however, where the oceans confine the ITCZ to the tropics, on Titan, the zone wanders from one pole to the other, taking methane rainclouds with it. This means that Titan, despite its frigid temperatures, can be said to have a tropical climate. [29]
In June 2012, Cassini imaged a rotating polar vortex on Titan's southern pole, which the imaging team believe is related to a "polar hood"—an area of dense, high altitude haze seen over the northern pole since the probe's arrival in 2004. As the hemispheres are now switching seasons since the 2009 equinox, with the southern pole entering winter and the north entering summer, it is hypothesised that this vortex could mark the formation of a new, southern polar hood. [30] [31]
Titan's clouds, probably composed of methane, ethane, or other simple organics, are scattered and variable, punctuating the overall haze. [32]
In September 2006, Cassini imaged a large cloud at a height of 40 km over Titan's north pole. Although methane is known to condense in Titan's atmosphere, the cloud was more likely to be ethane, as the detected size of the particles was only 1–3 micrometers and ethane can also freeze at these altitudes. In December, Cassini again observed cloud cover and detected methane, ethane and other organics. The cloud was over 2400 km in diameter and was still visible during a following flyby a month later. One hypothesis is that it is currently raining (or, if cool enough, snowing) on the north pole; the downdrafts at high northern latitudes are strong enough to drive organic particles towards the surface. These were the strongest evidence yet for the long-hypothesized "methanological" cycle (analogous to Earth's hydrological cycle) on Titan. [33]
Clouds have also been found over the south polar region. While typically covering 1% of Titan's disk, 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 Titanean summer generate uplift in the atmosphere, resulting in convection. This explanation is complicated by the fact that cloud formation has been observed not only post–summer solstice but also at mid-spring. Increased methane humidity at the south pole possibly contributes to the rapid increases in cloud size. [34] There had been summer in Titan's southern hemisphere until 2010, when Saturn's orbit, which governs the moon's motion, tilted the northern hemisphere towards the Sun. [27] When the seasons switch, it is expected that ethane will begin to condense over the south pole. [35]
Research models that match well with observations suggest that clouds on Titan cluster at preferred coordinates and that cloud cover varies by distance from the surface on different parts of the satellite. In the polar regions (above 60 degrees latitude), widespread and permanent ethane clouds appear in and above the troposphere; at lower latitudes, mainly methane clouds are found between 15 and 18 km, and are more sporadic and localized. In the summer hemisphere, frequent, thick but sporadic methane clouds seem to cluster around 40°. [28]
Ground-based observations also reveal seasonal variations in cloud cover. Over the course of Saturn's 30-year orbit, Titan's cloud systems appear to manifest for 25 years, and then fade for four to five years before reappearing again. [33]
Cassini has also detected high-altitude, white, cirrus-type clouds in Titan's upper atmosphere, likely formed of methane. [37]
Although no evidence of lightning activity has yet been observed on Titan, computer models suggest that clouds in the moon's lower troposphere can accumulate enough charge to generate lightning from an altitude of roughly 20 km. [38] The presence of lightning in Titan's atmosphere would favour the production of organic materials. Cassini did not detect any lightning in Titan's atmosphere, [39] though lightning could still be present if it was too weak to be detected. [40] Recent computer simulations have shown that under certain circumstances streamer discharges, the early stages of lightning discharges, may be formable on Titan. [41]
Triton is the largest natural satellite of the planet Neptune. It is the only moon of Neptune massive enough to be rounded under its own gravity, and orbits Neptune with a retrograde orbit—an orbit in the direction opposite to its planet's rotation—the only large moon in the Solar System to do so. Because of its retrograde orbit and composition similar to Pluto, Triton is thought to have been a dwarf planet, captured from the Kuiper belt. Triton hosts a thin but well-structured atmosphere, composed primarily of nitrogen and hosting a layer of clouds.
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine-and-a-half times that of Earth. It has only one-eighth the average density of Earth, but is over 95 times more massive. Even though Saturn is nearly the size of Jupiter, Saturn has less than one-third of Jupiter's mass. Saturn orbits the Sun at a distance of 9.59 AU (1,434 million km) with an orbital period of 29.45 years.
Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a supercritical phase of matter, which in astronomy is called 'ice' or volatiles. The planet's atmosphere has a complex layered cloud structure and has the lowest minimum temperature of 49 K out of all the Solar System's planets. It has a marked axial tilt of 82.23° with a retrograde rotation period of 17 hours and 14 minutes. This means that in an 84-Earth-year orbital period around the Sun, its poles get around 42 years of continuous sunlight, followed by 42 years of continuous darkness.
Titan is the largest moon of Saturn and the second-largest in the Solar System. It is the only moon known to have an atmosphere denser than the Earth's, and is the only known object in space other than Earth on which clear evidence of stable bodies of surface liquid has been found. Titan is one of the seven gravitationally rounded moons of Saturn and the second-most distant among them. 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 Mercury, but only 40% as massive due to Mercury being made of mostly dense iron and rock, while a large portion of Titan is made of less-dense ice.
Cassini–Huygens, commonly called Cassini, was a space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, including its rings and natural satellites. The Flagship-class robotic spacecraft comprised both NASA's Cassini space probe and ESA's Huygens lander, which landed on Saturn's largest moon, Titan. Cassini was the fourth space probe to visit Saturn and the first to enter its orbit, where it stayed from 2004 to 2017. The two craft took their names from the astronomers Giovanni Cassini and Christiaan Huygens.
Ontario Lacus is a lake composed of methane, ethane and propane near the south pole of Saturn's moon Titan. Its character as a hydrocarbon lake was confirmed by observations from the Cassini spacecraft, published in the 31 July 2008 edition of Nature. Ontario Lacus has a surface area of about 15,000 square kilometers (5,800 sq mi), about 20% smaller than its terrestrial namesake, Lake Ontario in North America. In April 2012, it was announced that it may be more like a mudflat or salt pan.
An extraterrestrial vortex is a vortex that occurs on planets and natural satellites other than Earth that have sufficient atmospheres. Most observed extraterrestrial vortices have been seen in large cyclones, or anticyclones. However, occasional dust storms have been known to produce vortices on Mars and Titan. Various spacecraft missions have recorded evidence of past and present extraterrestrial vortices. The largest extraterrestrial vortices are found on the gas giants, Jupiter and Saturn, and the ice giants, Uranus and Neptune.
The climate of Mars has been a topic of scientific curiosity for centuries, in part because it is the only terrestrial planet whose surface can be easily directly observed in detail from the Earth with help from a telescope.
The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earth's atmosphere. In addition to Earth, many of the other astronomical objects in the Solar System have atmospheres. These include all the gas giants, as well as Mars, Venus and Titan. Several moons and other bodies also have atmospheres, as do comets and the Sun. There is evidence that extrasolar planets can have an atmosphere. Comparisons of these atmospheres to one another and to Earth's atmosphere broaden our basic understanding of atmospheric processes such as the greenhouse effect, aerosol and cloud physics, and atmospheric chemistry and dynamics.
Whether there is life on Titan, the largest moon of Saturn, is currently an open question and a topic of scientific assessment and research. Titan is far colder than Earth, but of all the places in the Solar System, Titan is the only place besides Earth known to have liquids in the form of rivers, lakes, and seas on its surface. Its thick atmosphere is chemically active and rich in carbon compounds. On the surface there are small and large bodies of both liquid methane and ethane, and it is likely that there is a layer of liquid water under its ice shell. Some scientists speculate that these liquid mixes may provide prebiotic chemistry for living cells different from those on Earth.
The atmosphere of Uranus is composed primarily of hydrogen and helium. At depth it is significantly enriched in volatiles such as water, ammonia and methane. The opposite is true for the upper atmosphere, which contains very few gases heavier than hydrogen and helium due to its low temperature. Uranus's atmosphere is the coldest of all the planets, with its temperature reaching as low as 49 K.
The atmosphere of Titan is the dense layer of gases surrounding Titan, the largest moon of Saturn. Titan is the only natural satellite in the Solar System with an atmosphere that is denser than the atmosphere of Earth and is one of two moons with an atmosphere significant enough to drive weather. Titan's lower atmosphere is primarily composed of nitrogen (94.2%), methane (5.65%), and hydrogen (0.099%). There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene, propane, PAHs and of other gases, such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, acetonitrile, argon and helium. The isotopic study of nitrogen isotopes ratio also suggests acetonitrile may be present in quantities exceeding hydrogen cyanide and cyanoacetylene. The surface pressure is about 50% higher than on Earth at 1.5 bars which is near the triple point of methane and allows there to be gaseous methane in the atmosphere and liquid methane on the surface. The orange color as seen from space is produced by other more complex chemicals in small quantities, possibly tholins, tar-like organic precipitates.
Lakes of liquid ethane and methane exist on the surface of Titan, Saturn's largest moon. This was confirmed by the Cassini–Huygens space probe, as had been suspected since the 1980s. The large bodies of liquid are known as maria (seas) and the small ones as lacūs (lakes).
The climate of Uranus is heavily influenced by both its lack of internal heat, which limits atmospheric activity, and by its extreme axial tilt, which induces intense seasonal variation. Uranus's atmosphere is remarkably bland in comparison to the other giant planets which it otherwise closely resembles. When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet. Later observations from the ground or by the Hubble Space Telescope made in the 1990s and the 2000s revealed bright clouds in the northern (winter) hemisphere. In 2006 a dark spot similar to the Great Dark Spot on Neptune was detected.
Ligeia Mare is a lake in the north polar region of Titan, the planet Saturn's largest moon. It is the second largest body of liquid on the surface of Titan, after Kraken Mare. Larger than Lake Superior on Earth, it is mostly composed of liquid methane, with unknown but lesser components of dissolved nitrogen and ethane, as well as other organic compounds. It is located at 78° N, 249° W, and has been fully imaged by the Cassini spacecraft. Measuring roughly 420 km (260 mi) by 350 km (217 mi) across, it has a surface area of about 126,000 km2, and a shoreline over 2,000 km (1,240 mi) in length. The lake may be hydrologically connected to the larger Kraken Mare. Its namesake is Ligeia, one of the sirens in Greek mythology.
Titan Saturn System Mission (TSSM) was a joint NASA–ESA proposal for an exploration of Saturn and its moons Titan and Enceladus, where many complex phenomena were revealed by Cassini. TSSM was proposed to launch in 2020, get gravity assists from Earth and Venus, and arrive at the Saturn system in 2029. The 4-year prime mission would include a two-year Saturn tour, a 2-month Titan aero-sampling phase, and a 20-month Titan orbit phase.
The atmosphere of Pluto is the layer of gasses that surround the dwarf planet Pluto. It consists mainly of nitrogen (N2), with minor amounts of methane (CH4) and carbon monoxide (CO), all of which are vaporized from surface ices on Pluto's surface. It contains layered haze, probably consisting of heavier compounds which form from these gases due to high-energy radiation. The atmosphere of Pluto is notable for its strong and not completely understood seasonal changes caused by peculiarities of the orbital and axial rotation of Pluto.
Titan Mare Explorer (TiME) is a proposed design for a lander for Saturn's moon Titan. TiME is a relatively low-cost, outer-planet mission designed to measure the organic constituents on Titan and would have performed the first nautical exploration of an extraterrestrial sea, analyze its nature and, possibly, observe its shoreline. As a Discovery-class mission it was designed to be cost-capped at US$425 million, not counting launch vehicle funding. It was proposed to NASA in 2009 by Proxemy Research as a scout-like pioneering mission, originally as part of NASA's Discovery Program. The TiME mission design reached the finalist stage during that Discovery mission selection, but was not selected, and despite attempts in the U.S. Senate failed to get earmark funding in 2013. A related Titan Submarine has also been proposed.
The planet Mars has two permanent polar ice caps. During a pole's winter, it lies in continuous darkness, chilling the surface and causing the deposition of 25–30% of the atmosphere into slabs of CO2 ice (dry ice). When the poles are again exposed to sunlight, the frozen CO2 sublimes. These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth-like frost and large cirrus clouds.
Carrie Anderson is an American planetary scientist at NASA's Goddard Space Flight Center.
