page_title: Extraterrestrial atmosphere
section_title: Methane
hierarchical_section_title: Extraterrestrial atmosphere / Methane
caption_attribution_description: English: NASA Identifies Ice Cloud Above Cruising Altitude on Titan - This cloud in the stratosphere over Titan’s north pole (left) is similar to Earth’s polar stratospheric clouds (right). NASA scientists found that Titan’s cloud contains methane ice, which was not previously thought to form in that part of the atmosphere. Cassini first spotted the cloud in 2006. Description: NASA scientists have identified an unexpected high-altitude methane ice cloud on Saturn's moon Titan that is similar to exotic clouds found far above Earth's poles. This lofty cloud, imaged by NASA's Cassini spacecraft, was part of the winter cap of condensation over Titan's north pole. Now, eight years after spotting this mysterious bit of atmospheric fluff, researchers have determined that it contains methane ice, which produces a much denser cloud than the ethane ice previously identified there. "The idea that methane clouds could form this high on Titan is completely new," said Carrie Anderson, a Cassini participating scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study. "Nobody considered that possible before." Methane clouds were already known to exist in Titan's troposphere, the lowest layer of the atmosphere. Like rain and snow clouds on Earth, those clouds form through a cycle of evaporation and condensation, with vapor rising from the surface, encountering cooler and cooler temperatures and falling back down as precipitation. On Titan, however, the vapor at work is methane instead of water. The newly identified cloud instead developed in the stratosphere, the layer above the troposphere. Earth has its own polar stratospheric clouds, which typically form above the North Pole and South Pole between 49,000 and 82,000 feet (15 to 25 kilometers) -- well above cruising altitude for airplanes. These rare clouds don't form until the temperature drops to minus 108 degrees Fahrenheit (minus 78 degrees Celsius). Other stratospheric clouds had been identified on Titan already, including a very thin, diffuse cloud of ethane, a chemical formed after methane breaks down. Delicate clouds made from cyanoacetylene and hydrogen cyanide, which form from reactions of methane byproducts with nitrogen molecules, also have been found there. But methane clouds were thought unlikely in Titan's stratosphere. Because the troposphere traps most of the moisture, stratospheric clouds require extreme cold. Even the stratosphere temperature of minus 333 degrees Fahrenheit (minus 203 degrees Celsius), observed by Cassini just south of the equator, was not frigid enough to allow the scant methane in this region of the atmosphere to condense into ice. What Anderson and her Goddard co-author, Robert Samuelson, noted is that temperatures in Titan's lower stratosphere are not the same at all latitudes. Data from Cassini's Composite Infrared Spectrometer and the spacecraft's radio science instrument showed that the high-altitude temperature near the north pole was much colder than that just south of the equator. It turns out that this temperature difference -- as much as 11 degrees Fahrenheit (6 degrees Celsius) -- is more than enough to yield methane ice. Other factors support the methane identification. Initial observations of the cloud system were consistent with small particles composed of ethane ice. Later observations revealed some regions to be clumpier and denser, suggesting that more than one ice could be present. The team confirmed that the larger particles are the right size for methane ice and that the expected amount of methane -- one-and-a-half percent, which is enough to form ice particles -- is present in the lower polar stratosphere. The mechanism for forming these high-altitude clouds appears to be different from what happens in the troposphere. Titan has a global circulation pattern in which warm air in the summer hemisphere wells up from the surface and enters the stratosphere, slowly making its way to the winter pole. There, the air mass sinks back down, cooling as it descends, which allows the stratospheric methane clouds to form. "Cassini has been steadily gathering evidence of this global circulation pattern, and the identification of this new methane cloud is another strong indicator that the process works the way we think it does," said Michael Flasar, Goddard scientist and principal investigator for Cassini's Composite Infrared Spectrometer (CIRS). Like Earth's stratospheric clouds, this methane cloud was located near the winter pole, above 65 degrees north latitude. Anderson and Samuelson estimate that this type of cloud system -- which they call subsidence-induced methane clouds, or SIMCs for short -- could develop between 98,000 to 164,000 feet (30 to 50 kilometers) in altitude above Titan's surface. "Titan continues to amaze with natural processes similar to those on the Earth, yet involving materials different from our familiar water," said Scott Edgington, Cassini deputy project scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "As we approach southern winter solstice on Titan, we will further explore how these cloud formation processes might vary with season." The results of this study are available online in the journal Icarus.
context_page_description: 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 Pluto. 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.
context_section_description: Detection of methane in astronomical bodies is of interest to science and technology, as it may be evidence of extraterrestrial life (biosignature), it may help provide organic ingredients for life to form, and also, methane could be used as a fuel or rocket propellant for future robotic and crewed missions in the Solar System.
  Mercury – the tenuous atmosphere contains trace amounts of methane.
  Venus – the atmosphere contains a large amount of methane from 60 km (37 mi) to the surface according to data collected by the Pioneer Venus Large Probe Neutral Mass Spectrometer
  Moon – traces are outgassed from the surface
  Mars – the Martian atmosphere contains 10 nmol/mol methane. The source of methane on Mars has not been determined. Research suggests that methane may come from volcanoes, fault lines, or methanogens, that it may be a byproduct of electrical discharges from dust devils and dust storms, or that it may be the result of UV radiation. In January 2009, NASA scientists announced that they had discovered that the planet often vents methane into the atmosphere in specific areas, leading some to speculate this may be a sign of biological activity below the surface. The Curiosity rover, which landed on Mars in August 2012, can distinguish between different isotopologues of methane; but even if the mission determines that microscopic Martian life is the source of the methane, it probably resides far below the surface, beyond the rover's reach. The first measurements with the Tunable Laser Spectrometer (TLS) indicated that there is less than 5 ppb of methane at the landing site. On 16 December 2014, NASA reported the Curiosity rover detected a "tenfold spike", likely localized, in the amount of methane in the Martian atmosphere. Sample measurements taken "a dozen times over 20 months" showed increases in late 2013 and early 2014, averaging "7 parts of methane per billion in the atmosphere." Before and after that, readings averaged around one-tenth that level. The spikes in concentration suggest that Mars is episodically producing or releasing methane from an unknown source. The ExoMars Trace Gas Orbiter will perform measurements of methane starting in April 2018, as well as its decomposition products such as formaldehyde and methanol.
  Jupiter – the atmosphere contains 3000 ± 1000 ppm methane
  Saturn – the atmosphere contains 4500 ± 2000 ppm methane
  Enceladus – the atmosphere contains 1.7% methane
  Iapetus
  Titan –  the atmosphere contains 1.6% methane and thousands of methane lakes have been detected on the surface. In the upper atmosphere, methane is converted into more complex molecules including acetylene, a process that also produces molecular hydrogen. There is evidence that acetylene and hydrogen are recycled into methane near the surface. This suggests the presence either of an exotic catalyst, possibly an unknown form of methanogenic life. Methane showers, probably prompted by changing seasons, have also been observed. On October 24, 2014, methane was found in polar clouds on Titan.
  Uranus – the atmosphere contains 2.3% methane
  Ariel – methane is believed to be a constituent of Ariel's surface ice
  Miranda
  Oberon – about 20% of Oberon's surface ice is composed of methane-related carbon/nitrogen compounds
  Titania – about 20% of Titania's surface ice is composed of methane-related organic compounds
  Umbriel – methane is a constituent of Umbriel's surface ice
  Neptune – the atmosphere contains 1.5 ± 0.5% methane
  Triton – Triton has a tenuous nitrogen atmosphere with small amounts of methane near the surface.
  Pluto – spectroscopic analysis of Pluto's surface reveals it to contain traces of methane
  Charon – methane is believed present on Charon, but it is not completely confirmed
  Eris – infrared light from the object revealed the presence of methane ice
  Halley's Comet
  Comet Hyakutake – terrestrial observations found ethane and methane in the comet
  Extrasolar planets – methane was detected on extrasolar planet HD 189733b; this is the first detection of an organic compound on a planet outside the solar system. Its origin is unkn
caption: Polar clouds, made of methane, on Titan (left) compared with polar clouds on Earth (right).
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