Please analyze URL: https://www.britannica.com/science/extraterrestrial-life/Life-in-the-solar-system, Title: Extraterrestrial life - Solar System, Planets, Astrobiology, Content: Extraterrestrial life - Solar System, Planets, Astrobiology | Britannica
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[extraterrestrial life](https://www.britannica.com/science/extraterrestrial-life)

Table of Contents

*   [Introduction](https://www.britannica.com/science/extraterrestrial-life)
    
*   [Universal criteria](https://www.britannica.com/science/extraterrestrial-life#ref279235)
    
*   [The search for extraterrestrial life](https://www.britannica.com/science/extraterrestrial-life/The-search-for-extraterrestrial-life)
    
*   [Life in the solar system](https://www.britannica.com/science/extraterrestrial-life/Life-in-the-solar-system)
    
    *   [The Moon and Mercury](https://www.britannica.com/science/extraterrestrial-life/Life-in-the-solar-system#ref279238)
    
    *   [Martian “vegetation” and “canals”](https://www.britannica.com/science/extraterrestrial-life/Life-in-the-solar-system#ref279239)
    
    *   [Venus](https://www.britannica.com/science/extraterrestrial-life/Venus)
    
    *   [Jovian planets](https://www.britannica.com/science/extraterrestrial-life/Venus#ref279241)
    
    *   [Europa, other Jovian moons, comets, and asteroids](https://www.britannica.com/science/extraterrestrial-life/Venus#ref279242)
    
*   [Life beyond the solar system](https://www.britannica.com/science/extraterrestrial-life/Life-beyond-the-solar-system)
    
    *   [The Drake equation and extrasolar life](https://www.britannica.com/science/extraterrestrial-life/Life-beyond-the-solar-system#ref279244)
    
    *   [Searching for technical civilizations](https://www.britannica.com/science/extraterrestrial-life/Searching-for-technical-civilizations)
    

[References & Edit History](https://www.britannica.com/science/extraterrestrial-life/additional-info) [Related Topics](https://www.britannica.com/facts/extraterrestrial-life)

[Images & Videos](https://www.britannica.com/science/extraterrestrial-life/images-videos)

 [![Image 3: Planet TOI 700e](https://cdn.britannica.com/60/239460-004-4452E0D8/Newly-discovered-earth-sized-planet-TOI-700-e-orbits-within-the-habitable-zone-of-its-star.jpg)](https://cdn.britannica.com/60/239460-050-21C85CE4/Newly-discovered-earth-sized-planet-TOI-700-e-orbits-within-the-habitable-zone-of-its-star.jpg)[![Image 4: Photosynthesis](https://cdn.britannica.com/22/94122-004-5EC15559/trees-plants-carbon-dioxide-sunlight-sugars-water.jpg)](https://cdn.britannica.com/22/94122-050-93F6BAE0/trees-plants-carbon-dioxide-sunlight-sugars-water.jpg)[![Image 5: Mars](https://cdn.britannica.com/66/73766-004-D58CECCE/neutrons-map-data-Mars-surface-spacecraft-levels-2001.jpg)](https://cdn.britannica.com/66/73766-050-C3198E26/neutrons-map-data-Mars-surface-spacecraft-levels-2001.jpg)[![Image 6: Earth at night](https://cdn.britannica.com/69/118369-004-EB5FEA86/Earth-satellites-composite-images-US-Air-Force.jpg)](https://cdn.britannica.com/69/118369-050-EB6A7AA3/Earth-satellites-composite-images-US-Air-Force.jpg)[![Image 7: telecommunication antennas](https://cdn.britannica.com/61/96161-004-9601FFD6/Shortwave-microwave-telephone-telecommunication-antennas-types-buildings.jpg)](https://cdn.britannica.com/61/96161-050-4563DDA3/Shortwave-microwave-telephone-telecommunication-antennas-types-buildings.jpg)[![Image 8: Saturn: surface of Titan](https://cdn.britannica.com/12/96612-004-20AD7080/Image-surface-probe-Titan-Huygens-High-Resolution.jpg)](https://cdn.britannica.com/12/96612-004-20AD7080/Image-surface-probe-Titan-Huygens-High-Resolution.jpg)[![Image 9: Moon](https://cdn.britannica.com/29/23329-004-2091AFDE/Earth-impact-basins-side-Moon-way-spacecraft-December-7-1992.jpg)](https://cdn.britannica.com/29/23329-050-E4D2ABF7/Earth-impact-basins-side-Moon-way-spacecraft-December-7-1992.jpg)[![Image 10: Mercury](https://cdn.britannica.com/16/75516-004-7ABFB8F8/Photo-mosaic-Mercury-Mariner-10-spacecraft-1974.jpg)](https://cdn.britannica.com/16/75516-050-C40070F2/Photo-mosaic-Mercury-Mariner-10-spacecraft-1974.jpg)[![Image 11: seasons of Mars](https://cdn.britannica.com/24/73524-004-3BC32C7E/result-seasons-Mars-planet-inclination-plane-cap.jpg)](https://cdn.britannica.com/24/73524-050-3C2F3432/result-seasons-Mars-planet-inclination-plane-cap.jpg)[![Image 12: Mars as seen by Hubble Space Telescope compared with an 1894 map of Mars](https://cdn.britannica.com/68/118368-004-EB16804D/Eugene-Antoniadi-Mars-map-observations-Hubble-Space-1894.jpg)](https://cdn.britannica.com/68/118368-050-47C28A3F/Eugene-Antoniadi-Mars-map-observations-Hubble-Space-1894.jpg)

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Contents

[Science](https://www.britannica.com/browse/Science) [Astronomy](https://www.britannica.com/browse/Astronomy)

Life in the [solar system](https://www.britannica.com/science/solar-system)
===========================================================================

in[extraterrestrial life](https://www.britannica.com/science/extraterrestrial-life)

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Written by

[Lynn Margulis Distinguished University Professor, University of Massachusetts, Amherst. Coauthor of _Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth; What Is Life?;_ and others.](https://www.britannica.com/contributor/Lynn-Margulis/5466)

Lynn Margulis,

[Carl Sagan Astronomer and science writer Carl Sagan (1934-96) was a highly popular and influential figure in the United States and controversial in scientific, political, and religious circles for his views on extraterrestrial...](https://www.britannica.com/contributor/Carl-Sagan/2564)

Carl Sagan•[All](https://www.britannica.com/science/extraterrestrial-life/additional-info#contributors)

Fact-checked by

[The Editors of Encyclopaedia Britannica Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.](https://www.britannica.com/editor/The-Editors-of-Encyclopaedia-Britannica/4419)

The Editors of Encyclopaedia Britannica

Last Updated: Sep 5, 2024 • [Article History](https://www.britannica.com/science/extraterrestrial-life/additional-info#history)

Table of Contents

[![Image 25: Saturn: surface of Titan](https://cdn.britannica.com/12/96612-004-20AD7080/Image-surface-probe-Titan-Huygens-High-Resolution.jpg?w=300)](https://cdn.britannica.com/12/96612-004-20AD7080/Image-surface-probe-Titan-Huygens-High-Resolution.jpg)

[Saturn: surface of Titan](https://cdn.britannica.com/12/96612-004-20AD7080/Image-surface-probe-Titan-Huygens-High-Resolution.jpg)Image of the surface of Titan from the Huygens probe's High Resolution Imager.(more)

A brief survey of [life](https://www.britannica.com/science/life)’s prospects on the moons and [planets](https://www.britannica.com/science/planet) of the [solar system](https://www.britannica.com/science/solar-system) follows. In the solar system there are many different [environments](https://www.merriam-webster.com/dictionary/environments) that could contain significant clues to the origin of life and perhaps even life itself. However, there is not yet definitive evidence for or against extraterrestrial life on these planets.

The [Moon](https://www.britannica.com/place/Moon) and [Mercury](https://www.britannica.com/place/Mercury-planet)
----------------------------------------------------------------------------------------------------------------

[![Image 26: Moon](https://cdn.britannica.com/29/23329-050-E4D2ABF7/Earth-impact-basins-side-Moon-way-spacecraft-December-7-1992.jpg?w=300)](https://cdn.britannica.com/29/23329-050-E4D2ABF7/Earth-impact-basins-side-Moon-way-spacecraft-December-7-1992.jpg)

[Moon](https://cdn.britannica.com/29/23329-050-E4D2ABF7/Earth-impact-basins-side-Moon-way-spacecraft-December-7-1992.jpg) The familiar near side of Earth's Moon, photographed on December 7, 1992, by the Galileo spacecraft on its way to Jupiter. Two primary kinds of terrain are visible—the lighter areas, which constitute the heavily cratered and very old highlands, and the darker, roughly circular plains, traditionally called maria, which are relatively young lava-filled impact basins. Among the maria are (from left) the crescent-shaped Oceanus Procellarum near the left limb, the large, almost perfectly circular Mare Imbrium, or Imbrium Basin (with the crater Copernicus a bright dot at its lower margin), Mare Serenitatis immediately to the right of Imbrium, Mare Tranquillitatis to the lower right of Serenitatis, and Mare Crisium, isolated near the right limb. Another bright crater, Tycho, stands out at the bottom left of the image. (more)

Recent News
-----------

Sep. 4, 2024, 6:16 AM ET (Newsweek)

[Alien Warning: Growing Belief in UFOs Is Dangerous 'Political Tsunami'](https://www.newsweek.com/alien-warning-growing-ufo-belief-political-tsunami-1948675)

Aug. 29, 2024, 12:40 AM ET (EarthSky)

[New SETI search goes beyond our Milky Way](https://earthsky.org/space/seti-galaxies-technosignatures/)

The [Moon](https://www.britannica.com/science/moon-natural-satellite)’s surface is inhospitable to life of any sort. Diurnal temperatures range from about 100 K (−173 °C, or −279 °F) to about 400 K (127 °C, or 261 °F). In the absence of either an atmosphere or a [magnetic field](https://www.britannica.com/science/magnetic-field), [ultraviolet light](https://www.britannica.com/science/ultraviolet-radiation) and charged particles from the [Sun](https://www.britannica.com/place/Sun) penetrate unimpeded to the lunar surface. In less than an hour, they deliver a dose lethal to the most radiation-resistant [bacteria](https://www.britannica.com/science/bacteria) known. The subsurface [environment](https://www.merriam-webster.com/dictionary/environment) of the Moon is not nearly so inclement. Ultraviolet light and solar [protons](https://www.britannica.com/science/proton-subatomic-particle) do not penetrate more than 1 meter (3.3 feet) below the surface, and the temperature is maintained at a relatively constant value of about 230 K (−43 °C, or −45 °F). Nevertheless, the absence of any surface fluid, atmosphere, or liquid to cycle matter and [energy](https://www.britannica.com/science/energy) makes prospects for life dim.

[![Image 27: Mercury](https://cdn.britannica.com/16/75516-050-C40070F2/Photo-mosaic-Mercury-Mariner-10-spacecraft-1974.jpg?w=300)](https://cdn.britannica.com/16/75516-050-C40070F2/Photo-mosaic-Mercury-Mariner-10-spacecraft-1974.jpg)

[Mercury](https://cdn.britannica.com/16/75516-050-C40070F2/Photo-mosaic-Mercury-Mariner-10-spacecraft-1974.jpg)Photo mosaic of Mercury, taken by the Mariner 10 spacecraft, 1974.(more)

The environment of [Mercury](https://www.britannica.com/place/Mercury-planet) is rather like that of the Moon. Its surface temperatures range from about 100 K to about 620 K (347 °C, or 657 °F), but, about 1 meter below the surface, the temperature is constant at roughly room temperature. However, the absence of any significant atmosphere, the unlikelihood of bodies of liquid, and the intense [solar radiation](https://www.britannica.com/science/solar-radiation) make the prospect for life on Mercury remote.

Martian “vegetation” and “canals”
---------------------------------

[![Image 28: seasons of Mars](https://cdn.britannica.com/24/73524-050-3C2F3432/result-seasons-Mars-planet-inclination-plane-cap.jpg?w=300)](https://cdn.britannica.com/24/73524-050-3C2F3432/result-seasons-Mars-planet-inclination-plane-cap.jpg)

[seasons of Mars](https://cdn.britannica.com/24/73524-050-3C2F3432/result-seasons-Mars-planet-inclination-plane-cap.jpg)The seasons of Mars, a result of the planet's inclination of 24.9° to its orbital plane. At present, southern summer occurs when Mars's elongated orbit brings it nearest the Sun. As the seasons change, the polar caps alternately grow and shrink. At its maximum size the southern cap extends about 5° more equatorward than the northern cap.(more)

Evidence for life on [Mars](https://www.britannica.com/place/Mars-planet) has been claimed for more than a century. The first such argument was posed by a French astronomer, Étienne L. Trouvelot, in 1884:

> Judging from the changes that I have seen to occur from year to year in these spots, one could believe that these changing grayish areas are due to Martian vegetation undergoing seasonal changes.
> 
> [](https://yahoo.com/)

The seasonal changes on Mars have been reliably observed, not only visually but also photometrically. There is a [conspicuous](https://www.merriam-webster.com/dictionary/conspicuous) springtime increase in the contrast between the bright and dark areas of Mars. Color changes with season have also been reported. Space probes have found no vegetation on Mars, but seasonally variable dust storms provide a convincing explanation of the color changes.

[![Image 29: Mars as seen by Hubble Space Telescope compared with an 1894 map of Mars](https://cdn.britannica.com/68/118368-050-47C28A3F/Eugene-Antoniadi-Mars-map-observations-Hubble-Space-1894.jpg?w=300)](https://cdn.britannica.com/68/118368-050-47C28A3F/Eugene-Antoniadi-Mars-map-observations-Hubble-Space-1894.jpg)

[Mars as seen by Hubble Space Telescope compared with an 1894 map of Mars](https://cdn.britannica.com/68/118368-050-47C28A3F/Eugene-Antoniadi-Mars-map-observations-Hubble-Space-1894.jpg)Mars as seen by the Hubble Space Telescope (left) compared with a map of Mars based on French astronomer Eugène Antoniadi's observations in 1894. Antoniadi later concluded that what he thought were Martian canals are an optical illusion.(more)

Historically, life on Mars was argued for on the basis of the “canals.” This apparent set of thin straight lines across the Martian bright areas extends for hundreds, even thousands, of kilometers and changes seasonally. First systematically observed in 1887 by Italian astronomer [Giovanni V. Schiaparelli](https://www.britannica.com/biography/Giovanni-Virginio-Schiaparelli), the lines were further catalogued and popularized about the turn of the 20th century by American astronomer [Percival Lowell](https://www.britannica.com/biography/Percival-Lowell). From the unerring straightness of the lines, Lowell argued they could not be natural in origin. Instead he interpreted them as artificial constructs built by intelligent Martians. Lowell suggested they might be channels that carry [water](https://www.britannica.com/science/water) from the melting polar caps to the parched equatorial cities. However, many other astronomers were not able to see the canals, and the canals are now believed to be an optical [illusion](https://www.merriam-webster.com/dictionary/illusion). Approximately rectilinear features do exist on the Martian surface, but these are natural features such as crater chains, terrain [contour](https://www.merriam-webster.com/dictionary/contour) boundaries, faults, mountain chains, and ridges [analogous](https://www.merriam-webster.com/dictionary/analogous) to the suboceanic ridge features of [Earth](https://www.britannica.com/place/Earth).

[![Image 30: surface of Mars from Viking 1 lander](https://cdn.britannica.com/95/74295-050-04FFA0B4/surface-rocks-material-spacecraft-Viking-1-1976.jpg?w=300)](https://cdn.britannica.com/95/74295-050-04FFA0B4/surface-rocks-material-spacecraft-Viking-1-1976.jpg)

[surface of Mars from Viking 1 lander](https://cdn.britannica.com/95/74295-050-04FFA0B4/surface-rocks-material-spacecraft-Viking-1-1976.jpg)Martian surface of rocks and fine-grained material, photographed in 1976 by the Viking 1 spacecraft.(more)

In July and August 1976 two U.S. probes, [Viking](https://www.britannica.com/topic/Viking-space-probe) 1 and 2, successfully landed on Mars with equipment designed to detect the presence or remains of organic material. Analyses of atmospheric and soil samples yielded conclusive results; the data were interpreted as negative. At least in the vicinity of these probes, no evidence for life exists. In 1996 analysis of the Allan Hills Martian [meteorite](https://www.britannica.com/science/meteorite) ([ALH84001](https://www.britannica.com/topic/ALH84001)) yielded structures and sedimentary [magnetite](https://www.britannica.com/science/magnetite) that some have interpreted as direct evidence for extremely small microbial life on [Mars](https://www.britannica.com/dictionary/Mars). However, most scientists are very skeptical that the Allan Hills meteorite actually contains traces of past Martian life. The culprits are more likely to be tiny carbonate crystals and abiogenic magnetite. The search for past and current life on Mars continues.

  

[Venus](https://www.britannica.com/place/Venus-planet)
======================================================

[![Image 31: Venus](https://cdn.britannica.com/09/78009-050-49FFFBA9/ultraviolet-light-Venus-spacecraft-Pioneer-Orbiter-bands-Feb-26-1979.jpg?w=300) 1 of 2](https://cdn.britannica.com/09/78009-050-49FFFBA9/ultraviolet-light-Venus-spacecraft-Pioneer-Orbiter-bands-Feb-26-1979.jpg)

[Venus](https://cdn.britannica.com/09/78009-050-49FFFBA9/ultraviolet-light-Venus-spacecraft-Pioneer-Orbiter-bands-Feb-26-1979.jpg)Venus photographed in ultraviolet light by the Pioneer Venus Orbiter (Pioneer 12) spacecraft, February 26, 1979. Although Venus's cloud cover is nearly featureless in visible light, ultraviolet imaging reveals distinctive structure and pattern, including global-scale V-shaped bands that open toward the west (left). Added color in the image emulates Venus's yellow-white appearance to the eye.(more)

[![Image 32: profile of Venus's atmosphere](https://cdn.britannica.com/94/54994-004-4ACA970E/atmospheres-middle-Profile-measurements-Venus-probes-mission.jpg?w=300) 2 of 2](https://cdn.britannica.com/94/54994-004-4ACA970E/atmospheres-middle-Profile-measurements-Venus-probes-mission.jpg)

[profile of Venus's atmosphere](https://cdn.britannica.com/94/54994-004-4ACA970E/atmospheres-middle-Profile-measurements-Venus-probes-mission.jpg)Profile of Venus's middle and lower atmospheres as derived from measurements made by the Pioneer Venus mission's atmospheric probes and other spacecraft. Below 100 km (60 miles) the temperature rises slowly at first and then more rapidly with decreasing altitude, well surpassing the melting point of lead at the surface. By contrast, the wind, which near the top of the middle atmosphere is comparable in speed to the more powerful tropical cyclones on Earth, slows dramatically to a light breeze at the surface.(more)

The average surface temperature of [Venus](https://www.britannica.com/place/Venus-planet) is approximately 750 K (477 °C, or 891 °F). Even at the poles or on the tops of the highest Venusian mountains, surface temperatures do not fall below 400 K (127 °C, or 261 °F). The temperatures on Venus are too hot for [Earth](https://www.britannica.com/place/Earth)\-style life. However, [carbon dioxide](https://www.britannica.com/science/carbon-dioxide), sunlight, and [water](https://www.britannica.com/science/water) (according to the results of the [Venera](https://www.britannica.com/technology/Venera) space vehicles) are found in the clouds of Venus. These three are the prerequisites for [photosynthesis](https://www.britannica.com/science/photosynthesis). Molecular [nitrogen](https://www.britannica.com/science/nitrogen) also is expected at the cloud level, and some minerals are likely convectively raised to the cloud level from surface dust. The cloud pressures are about the same as those on the surface of Earth, and the temperatures in the lower clouds also are quite Earth-like. Although highly acidic by virtue of their [sulfur](https://www.britannica.com/science/sulfur), the lower clouds of Venus are the most Earth-like extraterrestrial [environment](https://www.merriam-webster.com/dictionary/environment) known. No organisms on Earth lead a completely airborne existence, so most scientists dispute the possibility that organisms exist buoyed in the clouds of Venus.

[Jovian planets](https://www.britannica.com/science/giant-planet)
-----------------------------------------------------------------

[![Image 33: Jupiter's Great Red Spot](https://cdn.britannica.com/58/21158-050-238ED4DD/Jupiter-Great-Red-Spot-surroundings-Voyager-1-February-25-1979.jpg?w=300)](https://cdn.britannica.com/58/21158-050-238ED4DD/Jupiter-Great-Red-Spot-surroundings-Voyager-1-February-25-1979.jpg)

[Jupiter's Great Red Spot](https://cdn.britannica.com/58/21158-050-238ED4DD/Jupiter-Great-Red-Spot-surroundings-Voyager-1-February-25-1979.jpg)Jupiter's Great Red Spot and its surroundings, photographed by Voyager 1, February 25, 1979. Included are the white ovals, observed since the 1930s, and immense areas of turbulence to the left of the Great Red Spot.(more)

The atmosphere of [Jupiter](https://www.britannica.com/place/Jupiter-planet) is composed of [hydrogen](https://www.britannica.com/science/hydrogen), [helium](https://www.britannica.com/science/helium-chemical-element), [methane](https://www.britannica.com/science/methane), [ammonia](https://www.britannica.com/science/ammonia), some [neon](https://www.britannica.com/science/neon-chemical-element), and water vapor. These are exactly the gases used in experiments that simulate the early Earth. Laboratory and computer experiments have been performed on the application of [energy](https://www.britannica.com/science/energy) to simulated Jovian atmospheres. Immediate gas-phase products include significant quantities of [hydrogen cyanide](https://www.britannica.com/science/hydrogen-cyanide) and [acetylene](https://www.britannica.com/science/acetylene). More-complex organic molecules, including aromatic [hydrocarbons](https://www.britannica.com/science/hydrocarbon), are formed in lower yields. The clouds of Jupiter are vividly colored, and their hue may be attributable to organic [compounds](https://www.merriam-webster.com/dictionary/compounds). An apparent absorption feature near 260 nanometers in Jupiter’s ultraviolet spectrum may be due to aromatic hydrocarbons or even due to [nucleotide](https://www.britannica.com/science/nucleotide) bases. Jupiter may be a vast planetary brew that has operated for 4.5 billion years as a laboratory of [organic chemistry](https://www.britannica.com/science/organic-chemistry).

[![Image 34: Saturn](https://cdn.britannica.com/00/81400-050-8BBE1CDA/Saturn-rings-composite-images-spacecraft-Cassini-Shadows-October-6-2004.jpg?w=300)](https://cdn.britannica.com/00/81400-050-8BBE1CDA/Saturn-rings-composite-images-spacecraft-Cassini-Shadows-October-6-2004.jpg)

[Saturn](https://cdn.britannica.com/00/81400-050-8BBE1CDA/Saturn-rings-composite-images-spacecraft-Cassini-Shadows-October-6-2004.jpg)Saturn and its spectacular rings, in a natural-color composite of 126 images taken by the Cassini spacecraft on October 6, 2004. The view is directed toward Saturn's southern hemisphere, which is tipped toward the Sun. Shadows cast by the rings are visible against the bluish northern hemisphere, while the planet's shadow is projected on the rings to the left.(more)

The other Jovian planets, [Saturn](https://www.britannica.com/place/Saturn-planet), [Uranus](https://www.britannica.com/place/Uranus-planet), and [Neptune](https://www.britannica.com/place/Neptune-planet), resemble Jupiter, although less is known about them. Their cloud-top temperatures progressively decrease with distance from the Sun. [Microwave](https://www.britannica.com/science/microwave-radiation) studies of Saturn indicate that the atmospheric temperature increases with depth below the clouds. A similar situation is expected to exist on Jupiter, Uranus, and Neptune. These planets of the solar system are associated with many natural [satellites](https://www.britannica.com/science/satellite). Some, such as [Titan](https://www.britannica.com/place/Titan-astronomy), a satellite of Saturn, and [Io](https://www.britannica.com/place/Io-satellite-of-Jupiter), a satellite of Jupiter, have atmospheres. Despite the relative suitability for life’s preconditions, no evidence is known for life on the outer planets or their satellites.

[Europa](https://www.britannica.com/place/Europa-satellite-of-Jupiter), other Jovian moons, comets, and asteroids
-----------------------------------------------------------------------------------------------------------------

[![Image 35: Europa](https://cdn.britannica.com/85/75385-050-E8279E3D/cracks-moons-one-Crescent-view-Europa-Galilean-1998.jpg?w=300)](https://cdn.britannica.com/85/75385-050-E8279E3D/cracks-moons-one-Crescent-view-Europa-Galilean-1998.jpg)

[Europa](https://cdn.britannica.com/85/75385-050-E8279E3D/cracks-moons-one-Crescent-view-Europa-Galilean-1998.jpg)Crescent view of Europa, one of Jupiter's four large, Galilean moons, in a composite of images made by the Galileo spacecraft in 1995 and 1998. Colors have been exaggerated in processing to reveal subtle differences in surface materials. The reddish lines in the moon's icy crust are cracks and ridges, some of them thousands of kilometers long, while the reddish mottling indicates areas of disrupted ice, where large ice blocks have shifted. The red material may be salt minerals deposited by liquid water that emerged from below the surface. The relatively few craters indicate that the icy crust has been relatively warm and mobile for at least a good part of Europa's early history.(more)

[Europa](https://www.britannica.com/topic/Europa-Greek-mythology), the fourth largest satellite of [Jupiter](https://www.britannica.com/place/Jupiter-planet), may be the best candidate for extraterrestrial life in the [solar system](https://www.britannica.com/science/solar-system). The [Galileo](https://www.britannica.com/topic/Galileo-spacecraft) orbiter revealed a crust of water ice and a complex surface on this moon. Optical imaging, thermographic temperature probes, and [magnetic field](https://www.britannica.com/science/magnetic-field) measurements support the strong [inference](https://www.merriam-webster.com/dictionary/inference) that a liquid saltwater ocean surges beneath the frozen crust. A wisp of an [oxygen](https://www.britannica.com/science/oxygen) atmosphere has also been detected by spectrographic techniques. Furthermore, since organic molecules including [methane](https://www.britannica.com/science/methane) and [nitrogen](https://www.britannica.com/science/nitrogen)\-rich gases such as [ammonia](https://www.britannica.com/science/ammonia) abound on Jupiter and some of its other moons, such “prebiotic chemicals” are highly likely to be present on Europa. The Galileo flyby also detected abundant [sulfuric acid](https://www.britannica.com/science/sulfuric-acid), a potential chemical power source, on the surface of Europa. (Such discoveries in the Jovian planets inspire further investigation of the limits to [diversity](https://www.merriam-webster.com/dictionary/diversity) of life on Earth. Lakes such as [Vostok](https://www.britannica.com/place/Lake-Vostok) in [Antarctica](https://www.britannica.com/place/Antarctica) reside under more than 3 km \[2 miles\] of [ice](https://www.britannica.com/science/ice). Studies of [bacteria](https://www.britannica.com/science/bacteria) in these lakes and of water seeps within cavities in granitic and carbonate rocks provide models for the viability of possible Earth-like life-forms on Europa and other Jovian moons.)

[![Image 36: Ufo, alien, space over trees.](https://cdn.britannica.com/57/200357-118-AFCBF4FD/Ufo-alien-space-trees.jpg) More From Britannica Unidentified Flying Objects: What We Know](https://www.britannica.com/story/unidentified-flying-objects-what-we-know)[Io](https://www.britannica.com/place/Io-satellite-of-Jupiter) is the most volcanically active place in the solar system, and [Ganymede](https://www.britannica.com/place/Ganymede-satellite-of-Jupiter) and [Callisto](https://www.britannica.com/place/Callisto-satellite-of-Jupiter) may also have water ice under their surfaces. The immense tidal influence of Jupiter regularly pumps energy into these planetary systems. Now that it has become clear that chemoautotrophic life-forms do not require sunlight as sources of energy, some scientists argue that a shift of focus from [Mars](https://www.britannica.com/place/Mars-planet) and the other inner planets is in order. The outer planets’ satellites, especially Europa and Saturn’s [Titan](https://www.britannica.com/place/Titan-astronomy), promise new insights into the search for extraterrestrial life in the solar system. In 2008, for example, the [Cassini](https://www.britannica.com/topic/Cassini) spacecraft reported several hundred lakes and seas of organic materials on [Titan](https://www.britannica.com/dictionary/Titan), dozens of which contain more liquid [hydrocarbon](https://www.britannica.com/science/hydrocarbon) (such as methane and [ethane](https://www.britannica.com/science/ethane)) than all of Earth’s [oil](https://www.britannica.com/science/petroleum) and gas reserves combined.

Tens of thousands of [comets](https://www.britannica.com/science/comet-astronomy), as well as some thousands of [asteroids](https://www.britannica.com/science/asteroid) and asteroidal fragments revolving about the [Sun](https://www.britannica.com/place/Sun) between the orbits of Mars and Jupiter, contain organic molecules. The asteroids are the presumed sources of the carbonaceous [chondrites](https://www.britannica.com/science/chondrite)’ organic matter. [Pluto](https://www.britannica.com/place/Pluto-dwarf-planet) has a predominantly nitrogen atmosphere covering a surface of frozen nitrogen, [carbon dioxide](https://www.britannica.com/science/carbon-dioxide), and methane. The intense cold and paucity of [solar radiation](https://www.britannica.com/science/solar-radiation) on Pluto and the lack of atmosphere and liquid waters on the asteroids argue against the likelihood of finding life on these bodies.

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Margulis, Lynn , Sagan, Carl and Sagan, Dorion. "extraterrestrial life". _Encyclopedia Britannica_, 5 Sep. 2024, https://www.britannica.com/science/extraterrestrial-life. Accessed 26 September 2024.

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*   [LiveScience - How Do Scientists Search for Extraterrestrial Life?](https://www.livescience.com/59153-how-to-search-for-extraterrestrial-life.html)
*   [Physics LibreTexts - Life Beyond Earth](https://phys.libretexts.org/Courses/HACC_Central_Pennsylvania%27s_Community_College/Astronomy_103%3A_Introduction_to_Planetary_Astronomy/15%3A_Astrobiology_and_the_Search_for_Extraterrestrial_Intelligence/15.03%3A_Life_Beyond_Earth)
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*   [LiveScience - How Do Scientists Search for Extraterrestrial Life?](https://www.livescience.com/59153-how-to-search-for-extraterrestrial-life.html)
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*   [European Space Agency - Science & Exploration - Extraterrestrial life](https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/Exploration/Extraterrestrial_life)
*   [Space - SETI & the Search for Extraterrestrial Life](https://www.space.com/33626-search-for-extraterrestrial-intelligence.html)
*   [Frontiers - How Will We React to the Discovery of Extraterrestrial Life?](https://www.frontiersin.org/articles/10.3389/fpsyg.2017.02308/full)

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

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