Public Domain Image, created by NASA. Original source: http://planetquest.jpl.nasa.gov/images/NEWextrasolar-medium.jpg
The above artist’s impression shows an imagined habitable moon in orbit around a gas giant planet in another star system: the bluish habitable moon (with liquid surface water and an atmosphere) in the foreground, the gas giant planet in its background, another moon visible to the left, and the system’s host star in the upper left.
Beyond our Solar System, many stars have been identified that are believed to have orbiting planets, and many planets have been identified that probably have one or more moons. Increasingly, moons in the solar system are considered attractive candidates for potentially harboring life, such as in subsurface liquid water. Could moons orbiting planets in other star systems also harbor life?
Image: Public Domain, http://photojournal.jpl.nasa.gov/catalog/PIA08409
The above is an actual mosaic image showing the heavily cratered surface of the northern hemisphere of Enceladus, moon of Saturn.
At just over 300 miles across, Enceladus is a small moon, yet large in astrobiological promise. Its ancient, crater-marked northern surface belies the energetic, dynamic nature of it’s southern polar surface and interior; there, evidence strongly suggests a subsurface ocean of liquid water, under a thick layer of surface ice, and possibly heated (and thus prevented from freezing) by tidal flexing related to the force of gravity from Saturn.
And the lie told by it’s northern surface even stands visibly
betrayed by the dramatic story of its southern polar region: imaging performed by the Cassini-Huygens spacecraft during a 2005 fly-by of Enceladus confirms active surface cryovolcanoes (“ice volcanos”) spewing geyser-like jets of massive amounts of icy particles and water vapor out at enormous speeds from its interior, beyond its surface, and into space.
Recently mounting evidence suggests the presence of many of the “ingredients” required by known life in the likely subsurface ocean of Enceladus. In addition to the presence of stable, liquid water as a solvent for life, cryovolcanism and possible geologic activity increase available energy that could help drive life-friendly chemical reactions. Furthermore, simple organic molecules have been detected in the cryovolcano plumes, of types that, on Earth, support chemosynthetic microbes, such as methanogens (methane-producing microbes) that live near hydrothermal vents in Earth’s oceans.
Image: public domain, available at http://en.wikipedia.org/wiki/Titan_(moon)
The above image, taken from the surface of Titan, moon of Saturn, is the only image from a moon other than the moon (of Earth). It was taken by the Huygens probe, which landed on Titan in 2005.
While the portion of the surface of Titan shown in the surface image appears bare, Titan is in fact strongly evidenced as elsewhere having surface bodies of liquid hydrocarbons (as well as an atmosphere with which such liquids may chemically interact). These surface liquids represent the first stable bodies of liquid found beyond Earth. In addition, Titan is evidenced to have a subsurface liquid water/ammonia ocean.
It has been (very speculatively) theorized (including by astrobiologist Chris McKay) that methanogenic (methane-producing) microbes may live in the surface lakes of Titan, using, as a solvent, not water as all Earth-based life does, but liquid hydrocarbons, such as methane, in the lakes of Titan (for some more overview information on this, see the Fosdick’s Astrobiology Series article on this subject).
Image: public domain, http://photojournal.jpl.nasa.gov/catalog/PIA06230
The above image shows roughly what Titan would actually look
Titan is the only moon known to have a substantial atmosphere – and, like Earth’s, it is rich in nitrogen.
The orange color of Titan is attributable to hydrocarbons in its atmospheric haze. Organic compounds in Titan’s atmosphere, as well as, it is thought, in its surface and subsurface liquids, give rise to intriguing speculations regarding hypothetically possible life there.
Image: public domain, http://photojournal.jpl.nasa.gov/catalog/PIA00502
In the above image, the dark orange lines crisscrossing Europa’s
surface are called lineae, and may result from fracturing of the surface crust of Europa from tidal flexing, a force related to the gravity exerted on Europa by Jupiter.
Europa is strongly evidenced as having subsurface oceans of liquid water, with tidal flexing believed to provide the heat that keeps its oceans from freezing.
Possible hydrothermal vents in the subsurface oceans of Europa (which could be caused by tidal flexing) could hypothetically provide a habitat for life, just as life clusters around hydrothermal vents in the oceans of Earth.