Our planet is so far the only world known to contain all the ingredients required for life as we know it. These are liquid water, energy sources, and an array of elements and molecules. However, the recent discovery of bio-signatures in the atmosphere of Venus is a timely reminder of how a selection of these ingredients exists elsewhere in the solar system too. Space agency NASA has now listed the most promising planets for hosting alien life.
Earth’s nearest neighbour Mars is the most obvious candidate for hosting life as we know it, as it is the one most similar to ours.
Mars has a 24.5-hour day, ice caps at its poles, and numerous surface features sculpted by water during the planet’s early history.
But arguably most significant of all was both the discovery of a lake beneath the southern polar ice cap and bio-signature of methane in the Martian atmosphere.
However, the specific source for the methane on Mars has yet to be confirmed and may simply be abiotic (not derived from living organisms) in origin.
Experts suspect this is prevented from freezing by the heat from flexing and which might potentially be more 62 miles (100km) deep.
Evidence for this ocean includes geysers erupting through cracks in the surface ice, a surprisingly weak magnetic field and desolate surface terrain, which may have been made chaotic by ocean currents stirring underground.
This icy shield insulates the sub-surface ocean from the extreme cold and vacuum of space, as well as Jupiter’s deadly radiation.
In this oceanic world, NASA believes they may one day discover hydrothermal vents and ocean floor volcanoes.
This hypothesis is very exciting because such features often support very rich and diverse ecosystems on Earth.
Enceladus is another ice-covered moon with a subsurface ocean of liquid water.
The moon orbits ringed planet Saturn and initially came to the attention of scientists as a potentially habitable world following the surprise discovery of giant geysers close to its south pole.
These geysers spew from huge cracks on the surface and, given Enceladus’ weak gravitational field, fire into space.
They are clear evidence of an underground store of liquid water.
Not only was water detected in these geysers but also an array of organic molecules and, crucially, tiny grains of rocky silicate particles.
These can only be present if the sub-surface ocean water was in physical contact with the rocky ocean floor at a temperature of at least 90C.
This is consequently strong evidence for the existence of hydrothermal vents on the ocean floor, providing the requisite chemistry for alien life.
Titan is Saturn’s largest moon and is the only one in the solar system with a significant atmosphere.
Titan contains a thick orange haze of complex organic molecules and a methane weather system in place of water.
This results in seasonal rains, dry periods and surface sand dunes formed by wind.
The atmosphere is mostly made up of nitrogen, a chemical used in the construction of proteins in all known forms of life.
In addition, NASA has detected the presence of rivers and lakes of liquid methane and ethane and possibly the presence of cryovolcanoes.
These are volcanic features that erupt liquid water rather than lava, which indicates Titan also has a sub-surface reserve of liquid water.