Where to Hunt for New Life-forms in the Solar System
The best place in the solar system to find new life-forms is not Mars, Europa, or even Earth, argues one astrobiologist. Instead, we should focus our efforts on Titan.
If we want to
find life elsewhere in the solar system that has arisen independently of life
on Earth, where should we look? According to Jonathan Lunine, at the University
of Arizona in Tucson, the somewhat surprising answer is Saturn’s giant moon,
Titan.
That’s not quite
as crazy as it sounds. Lunine’s argument is based on the idea that the
important question for humanity is not whether there is life elsewhere in the solar
system but whether there is life elsewhere in the universe. And that is an
important difference.
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We know that life
has arisen once in the solar system, but unfortunately this tells us nothing
about its ubiquity elsewhere. Life on Earth may be a one-off.
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But were we to
find evidence that life had independently arisen twice in the solar system, it would
provide strong evidence that the universe is teeming with ET creepy crawlies.
The key word here is “independently.”
For many
astrobiologist, the most obvious place to look is wherever there is liquid
water. Top of their list is Mars, which once had a warmer and wetter climate
and may still be damp in some places beneath its crust.
The trouble with
Mars, says Lunine, is cross-contamination. Any life on Mars may well have been
sent there from Earth by large meteor impacts (and vice versa).
Then there is
Europa, Jupiter’s moon, which hides a huge salty ocean some 100 kilometers beneath
its icy crust. Europa’s oceans hold about twice as much water as Earth’s. Cross-contamination
is less of a concern with Europa (although it cannot be ruled out entirely) but
the real problem is penetrating its thick icy surface. If Europa’s oceans
contain life, it’s going to be mighty difficult to reach it, although NASA has
recently grasped the nettle by announcing a major new mission to the moon.
Which leaves
Titan, an object with a nitrogen-based atmosphere about four times as dense as
Earth’s. Titan’s allure is based on its extraordinary climate. At -176 degrees
centigrade, the water on Titan is safely locked up as ice. But on the surface,
ethane and methane play the role that liquid water plays on Earth. There are
ethane oceans and methane rain and cloud.
Could life have
arisen in these conditions? If it has, Lunine suggests, it would look very different
to our own, perhaps based on hydrogen bonds rather than the covalent bonds that
hold life on Earth together. And if so, then there can be no question of
contamination.
What’s more,
Lunine argues that Titan-like climates may be much more common in the Milky Way
than Earth-like ones, because the most common stars are smaller and cooler than
the Sun.
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So Titan-like
life would be much more widespread than Earth-like life.
With its dense
atmosphere and low gravity, Titan ought to be much easier to explore than
Europa.
That’s an
interesting argument but it raises some questions, not the least of which is
how to make an unambiguous detection of such a life-form. That would be hard,
given our ignorance of every aspect of its existence.
One idea that Lunine
ignores is panspermia–the idea that life on Earth was seeded from space. We
know that some interstellar clouds are riddled with ice and organic molecules.
We also know from lab experiments that, when zapped with UV light, these
organic molecules can form cell-like vesicles, even at the freezing temperatures
of interstellar space.
Could the solar system
have passed through one of these clouds soon after it formed? Possibly, and if
so, the problem of contamination may be much bigger than Lunine suspects.