The four most promising worlds for alien life in the solar system

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The four most promising worlds for alien life in the solar system
by Gareth Dorrian, The Conversation

NASA’s Curiosity Rover takes a selfie on Mars in June, 2018.
Credit: NASA/JPL-Caltech/MSSS, CC BY-SA

The Earth's biosphere contains all the known ingredients necessary for 
life as we know it. Broadly speaking these are: liquid water, at least 
one source of energy, and an inventory of biologically useful elements 
and molecules.


But the recent discovery of possibly biogenic phosphine in the clouds of 
Venus reminds us that at least some of these ingredients exist elsewhere 
in the solar system too. So where are the other most promising locations 
for extra-terrestrial life?

Mars

Mars is one of the most Earth-like worlds in the solar system. It has a 
24.5-hour day, polar ice caps that expand and contract with the seasons, 
and a large array of surface features that were sculpted by water during 
the planet's history.

The detection of a lake beneath the southern polar ice cap and methane 
in the Martian atmosphere (which varies with the seasons and even the 
time of day) make Mars a very interesting candidate for life. Methane is 
significant as it can be produced by biological processes. But the 
actual source for the methane on Mars is not yet known.

It is possible that life may have gained a foothold, given the evidence 
that the planet once had a much more benign environment. Today, Mars has 
a very thin, dry atmosphere comprised almost entirely of carbon dioxide. 
This offers scant protection from solar and cosmic radiation. If Mars 
has managed to retain some reserves of water beneath its surface, it is 
not impossible that life may still exist.

Mars has polar ice caps. Credit: ESA & MPS for OSIRIS Team 
MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA), CC BY-SA
Europa

Europa was discovered by Galileo Galilei in 1610, along with Jupiter's 
three other larger moons. It is slightly smaller than Earth's moon and 
orbits the gas giant at a distance of some 670,000km once every 3.5 
days. Europa is constantly squeezed and stretched by the competing 
gravitational fields of Jupiter and the other Galilean moons, a process 
known as tidal flexing.

The moon is believed to be a geologically active world, like the Earth, 
because the strong tidal flexing heats its rocky, metallic interior and 
keeps it partially molten.

The surface of Europa is a vast expanse of water ice. Many scientists 
think that beneath the frozen surface is a layer of liquid water—a 
global ocean—which is prevented from freezing by the heat from flexing 
and which maybe over 100km deep.

Evidence for this ocean includes geysers erupting through cracks in the 
surface ice, a weak magnetic field and chaotic terrain on the surface, 
which could have been deformed by ocean currents swirling beneath. This 
icy shield insulates the subsurface ocean from the extreme cold and 
vacuum of space, as well as Jupiter's ferocious radiation belts.

At the bottom of this ocean world it is conceivable that we might find 
hydrothermal vents and ocean floor volcanoes. On Earth, such features 
often support very rich and diverse ecosystems.

Europa’s icy surface is a good sign for alien hunters. Credit: 
NASA/JPL-Caltech/SETI Institute, CC BY-SA
Enceladus

Like Europa, Enceladus is an ice-covered moon with a subsurface ocean of 
liquid water. Enceladus orbits Saturn and first came to the attention of 
scientists as a potentially habitable world following the surprise 
discovery of enormous geysers near the moon's south pole.

These jets of water escape from large cracks on the surface and, given 
Enceladus' weak gravitational field, spray out 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 that 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 
90˚C. This is very strong evidence for the existence of hydrothermal 
vents on the ocean floor, providing the chemistry needed for life and 
localized sources of energy.

Titan’s atmosphere makes it look like a fuzzy orange ball. Credit: 
NASA/JPL-Caltech/Space Science Institute, CC BY-SA
Titan

Titan is the largest moon of Saturn and the only moon in the solar 
system with a substantial atmosphere. It contains a thick orange haze of 
complex organic molecules and a methane weather system in place of 
water—complete with seasonal rains, dry periods and surface sand dunes 
created by wind.

The atmosphere consists mostly of nitrogen, an important chemical 
element used in the construction of proteins in all known forms of life. 
Radar observations have detected the presence of rivers and lakes of 
liquid methane and ethane and possibly the presence of 
cryovolcanoes—volcano-like features that erupt liquid water rather than 
lava. This suggests that Titan, like Europa and Enceladus, has a 
sub-surface reserve of liquid water.

At such an enormous distance from the Sun, the surface temperatures on 
Titan are a frigid -180˚C—way too cold for liquid water. However, the 
bountiful chemicals available on Titan has raised speculation that 
lifeforms—potentially with fundamentally different chemistry to 
terrestrial organisms – could exist there.

Explore further

Saturn's moon Enceladus could be another location for life beyond Earth
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