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?
Another envisioned habitable moon: an artist’s impression of a view from above a hypothetical moon (with an atmosphere and liquid surface water) of the exoplanet Upsilon Andromedae d. Image available at https://commons.wikimedia.org/wiki/File:UpsilonAndromedae_D_moons.jpg, User: LucianoMendez, Author: Luciano S. Méndez
This artist’s impression shows a sunset (speculated and hypothetical) as seen from an exoplanet in another solar system – super-Earth Gliese 667 Cc, in a triple star system. Image Credit: ESO/L. Calçada. Source: http://www.eso.org/public/images/eso1214a/.
While candidates beckon even within our Solar System (particularly moons of other planets), so do the stars at night … and their potential planets and moons.
In this last regard, evidence continues to mount that some such star systems do indeed include planets (known as exoplanets), and that some of those planets host natural satellites, or moons (known as exomoons).
While there are many factors speculated to potentially influence planet and moon habitability, some key ones boil down to an assessment of the presence, or likely presence, of stable liquid water, the availability of energy accessible for life, and likely presence or potential formation of organic compounds.
An Artist’s hypothetical impression of suspected exoplanet Gliese 667 Cb with the the stars Gliese 667 A and Gliese 667 B in the background. Source: http://www.eso.org/public/images/eso0939a/, Image Credit: Credit: ESO/L. Calçada.
Titan is a large, in many ways planet-like, moon of Saturn.
Many aspects of Titan bespeak its astrobiological possibilities, although there are also drawbacks in this regard.
Titan is composed largely of rock and water ice. It receives very little sunlight. It has a dense, nitrogen-rich atmosphere that also include organic compounds. Furthermore, it is evidenced as likely having surface lakes of hydrocarbons (it’s surface is essentially too cold for liquid water, but hydrocarbons are liquid at much lower temperatures than water), and well as subsurface liquids that may be either liquid ammonia or a water-ammonia solution. Titan is also evidenced as having active weather, such as may include hydrocarbon rain and cryovolcanism. Many of these aspects of Titan suggest astrobiological possibilities, particularly the organics-rich surface liquids that interface with a dense, organics-rich atmosphere, as well as the energy and mixing/reacting effects of it’s chemically dynamic atmosphere and weather patterns.
On the other hand, Titan, being very cold, has essentially no surface or near-surface liquid water, although liquid water may exist deep within.
In addition to having subsurface liquids that might possibly contain liquid water (which raises interesting possibilities in itself), Titan is the only known astronomical body, besides Earth, that is believed to have surface liquids (though not water). Moreover, it has a dense atmosphere with which such liquids, or lakes, can interact, which combination could hypothetically be very conducive to microbial life. However, since these surface lakes lack water, any life in them would have to utilize hydrocarbons as a solvent, instead of water. All known life lives “in” water and uses water as the critical solvent; as such, any hypothetical life in the surface lakes of Titan would have to be a very different and exotic form of life as compared with known life. However, the possibility of life that uses hydrocarbons instead of water as a solvent, while highly speculative, is not beyond the realm of reasonable hypothetical speculation.
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).
An actual color image of the upper atmosphere of Titan (not it’s surface). The bluish upper portion is chemically dynamic, and the orange portion is a “smog”; the dense atmosphere of Titan, which may contact and interact with surface hydrocarbon lakes, is rich in both organic compounds and astrobiological intrigue. Public domain image. Source: http://photojournal.jpl.nasa.gov/jpeg/PIA06236.jpg
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
An artists conception of the Cassini-Huygens spacecraft in orbit around Saturn. Data from this spacecraft provides images of cryovolcanism on Enceladus, and strong evidence of a subsurface liquid water ocean. Image: public domain. Source: http://photojournal.jpl.nasa.gov/catalog/PIA03883.
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)
Huygens Probe replica, by David Monniaux, available at http://commons.wikimedia.org/wiki/File:Huygens_probe_dsc03686.jpg
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
Methane polar clouds of Titan (left) compared with water/ice polar clouds on Earth (right), http://www.nasa.gov/sites/default/files/thumbnails/image/titan-earth-polar-clouds-lg.jpg
like.
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
Mosaic of images of the surface of Europa taken by the Galileo spacecraft, including the Conamara region, and showing surface features including lineae. Public domain image, http://photojournal.jpl.nasa.gov/catalog/PIA01092
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.
The on the above image, actress Jodie Foster’s character shows
From the 1997 movie, “Contact”
the flash of excitement and amazement at the recognition of what seems to be non-“naturally”, non-randomly occurring radio signals from a distant star system – not merely signals, but, in fact a communication – a repeating pattern, a structure, bespeaking an intelligent formulation, and thus an intelligent source.
One area of astrobiology includes
The Very Large Array (VLA) Radio Astronomy Observatory in New Mexico, USA. Source: http://en.wikipedia.org/wiki/Radio_astronomy#mediaviewer/File:USA.NM.VeryLargeArray.02.jpg
the search for intelligent life beyond earth (and generally beyond the solar system), often including the use of radio astronomy in attempting to receive, identify and interpret hypothetical radio frequency communications from such intelligent life (a large part of the activities collectively known as SETI – the Search for ExtraTerrestrial Intelligence).
The ultimate homerun of astrobiology, success in this effort would not only affirmatively answer the massive question of whetherlifeexists beyond Earth (and indeed beyond the Solar System), but also the exponentially more massive question of whether intelligent life exists beyond Earth and in another star system.
Radio communications (that is, radio frequency electromagnetic communications) would seem the most promising form of communication for potential interstellar communications.
Just as visible electomagnetic signals (i.e., light) can be received from sources very close by (look up and see light with the source being a ceiling light fixture a few feet away) or incredibly far away (look into the night sky and see light from a star a hundred light years away, or even a thousand), so can radio electromagnetic signals.
Like light, radio signals can be received from sources very close by (such as by a receiver in radio controlled car, the source being the radio controller held by a child across the room), or from not as close by (such as by a person’s car stereo receiver from an FM broadcasting tower), or, in fact, from incredibly far away, such as by a radio telescope, from a distant star or another galaxy.
Radio astronomy generally, and radio SETI, can use telescopes with large “dish”-like receivers, or arrays of many such telescopes essentially working together (called radio interferometry), to receive, in the strongest and clearest fashion practical, radio signals from distant star systems and celestial bodies.
Image: public domain, http://mars.nasa.gov/mer/gallery/press/spirit/20050420a.html
The above strikingly detailed image shows a region of Mars called
Artist’s concept of a Mars Rover, public domain image, http://photojournal.jpl.nasa.gov/catalog/PIA04413
Gusev Crater. It was taken by the Mars Spirit rover in 2005.
Mars, today, can be described as appearing similar to a harsh desert on Earth. It is indeed mostly void of liquid water – essential to all life as known on Earth.
Yet, abundant evidence, including present-day Martian landforms, strongly suggests that, during some times in its extremely ancient history, Mars flowed with water over large portions of its surface – a wet world truly different than the Mars of today.
Did life perhaps thrive in the waters of ancient Mars? Conditions may have been too salty or acidic, at least for life as known on Earth, but perhaps not.
Mars echos, but moons in the Solar System – Europa, Titan, Enceladus – ring out, now, with the real possibility of harboring life.
Archaea are a type of microorganism – a relatively simple, ancient form of life, and yet intriguing.
Extant since early in the history of life on Earth, and still widely
Archaea, in high magnification, public domain image, available at http://en.wikipedia.org/wiki/Archaea
distributed on Earth today, archaea include extremophiles: microorganisms that thrive in extreme environmental conditions. For example, certain extremophiles live in highly saline water environments (halophiles), extremely hot water environments such as the hot spring depicted above (thermophiles) and extremely cold water environments (cryophiles or psychrophiles).
Extremophiles are particularly interesting to astrobiologists, who attempt to identify environments beyond earth that may harbor life – environments that are often extreme by Earth standards.
As an important example, microorganisms, including archaea, have been found to thrive around hydrothermal vents, which are essentially underwater volcanoes. These microbes use chemicals in the heated, mineral rich water for energy (leading to entire ecosystems there). Yet hydrothermal vents are evidently not unique to Earth, even in the Solar System; they are thought to likely exist, for example, on Europa, moon of Jupiter, as well as Titan and Enceladus, moons of Saturn. If archaea can thrive around earth’s hydrothermal vents, could microorganisms thrive around hydrothermal vents on Europa, Titan, Enceladus and elsewhere beyond Earth?
