Is life common in the universe? The successful detection of a second independent origin of life within our own solar system would be absolute proof that life is common in the universe. Our solar system is just one out of billions of other planetary systems in the galaxy and if life can arise twice in a single planetary system, then the galaxy is expected to be teeming with billions of living worlds. Mars, Europa and Titan are undoubtedly the best places in the solar system to search for the existence of life. Of these three worlds, Titan may prove to be the best place to look for a second independent origin of life in the solar system and to test life’s cosmic ubiquity.
Throughout Earth’s history, hypervelocity
impacts caused by asteroids and comets crashing into the surface of the Earth
are known to be able to throw up rock material into space. These rocks can
carry terrestrial life within them through the vacuum of space and a tiny
fraction of these rocks do eventually find their way to the surfaces of other
planets and moons in the solar system. Mars and Europa have environments just
beneath their surface where the conditions are suitable to support terrestrial
life and Earthly microbes that have hitched a ride on these rocks can indeed
survive there. As a result, if any life is detected on Mars or Europa, it would
be difficult to definitively proof if it has an origin that is independent from
that of life on Earth unless the type of life turns out to be biochemically
distinct from life on Earth.
Titan is the largest moon of Saturn and it
has a thick nitrogen-rich atmosphere laden with a wealth of organic molecules.
The mean surface temperature on Titan is a frigid -179 degrees Centigrade and
this is so cold that any water on Titan is literally rock solid. In fact,
frozen water makes up the crust of Titan, creating a geological landscape where
features such as mountains, sand dunes and boulders are actually made of frozen
water. Instead of liquid water, liquid methane and ethane are the working fluids
in Titan’s “hydrological cycle”. The surface of Titan contains widespread
fluvial features such as rivers and deltas that were created through the action
of liquid methane and ethane. In the high latitudes of Titan, there are lakes
and seas of liquid methane and ethane. This makes Titan the only other place in
the solar system besides the Earth that has stable bodies of surface liquid.
Since all life on Earth is born of liquid water
and sustained by liquid water, the absence of liquid water on Titan creates an
environment that is completely inhospitable to terrestrial life. Any
terrestrial life that may have hitched a ride to Titan will not survive.
However, it would be naive to assume that liquid water is the only solvent
suitable for biology just because all life on Earth is water-based due to water
being the most common solvent on Earth. Like the Earth, Titan also has the
basic requirements for life such as the presence of a fluid environment, a
source of energy and abundant organic molecules. If any form of life exists on
Titan, it is expected to use the abundant liquid methane and/or ethane as a
biosolvent due to the absence of liquid water in such a cryogenic environment. This
means that any form of life detected on Titan is expected to be so
biochemically distinct from life on Earth that it would be proof for a second
independent origin of life in the solar system.
The most common type of star in the
universe are not G-dwarf stars like our Sun but the much less massive M-dwarf
stars, also known as red dwarfs. M-dwarf stars vastly outnumber G-dwarf stars.
A typical M-dwarf star is so much less luminous than the Sun that an Earth-sized
habitable planet with surface liquid water can only be sustained at one-tenth
the Earth-Sun distance from the star. However, such a close proximity causes
the planet to be tidally locked where one hemisphere perpetually faces the
M-dwarf star; creating permanent day and night sides. As a result, an
Earth-sized habitable planet around an M-dwarf star is unlikely to resemble the
Earth. On the contrary, a Titan-like habitable planet could exist at a distance
equivalent to the Earth-Sun distance from a typical M-dwarf star. The planet would
not be tidally locked and is expected to rotate freely about its axis. If life
is indeed found on Titan, it means that Titan-like habitable planets are likely
to be very common and may even be more common than Earth-like habitable planets,
given that M-dwarf stars vastly outnumber G-dwarf stars.
When considering the exploration of Titan,
the only disadvantage is the large distance between Titan and Earth. Apart from
that, everything else about Titan such as its low gravity, dense atmosphere,
low radiation environment and calm low altitude winds are advantages compared
to the exploration of other places in the solar system. For future missions to
Titan, the challenges associated with finding an exotic form of life that is
biochemically distinct from life on Earth cannot be underestimated. If life is
found on Titan, such life would have an origin that is independent from life on
Earth and will be proof of life’s cosmic ubiquity.
1. J.I. Lunine (2009), “Saturn’s Titan: A
Strict Test for Life’s Cosmic Ubiquity”, arXiv:0908.0762 [astro-ph.EP]