Sky and Telescope - October 2017 - 27

EUROPA: N ASA / JPL- CA LTECH / SE TI INSTIT U TE; PLU TO: N ASA / JHUA PL / SWRI

p NURSERY MYSTERY Will the inevitable warming of Jupiter's satellite
Europa (seen here in close-up, fractures webbing its ice-covered surface)
spur the evolution of life on the moon - if it hasn't already evolved there?

But the possibility that life could develop and life actually developing are two different things. We only have one
example of life arising, and we just don't know how things
will transpire elsewhere. "I think the idea that Europa will
warm up and turn into this clement, habitable water world
- that's wrong," Laughlin says. "It will be even more interesting than that, in the sense that there are all sorts of weird
chemical pathways." The same holds for Titan. Laughlin feels
that it's naïve of us to speculate on what would happen when
Saturn's largest moon warms up (except to say that, as he told
me, "it's gonna smell really bad"). We just don't know.
Eventually, we may get an idea by studying exoplanets.
Laughlin considers Titan probably the best proxy in our solar
system for certain short-period "super-Earths" that have
similar compositions to Titan's but are balmy. Yet he stresses
that any biosignatures would not necessarily look like ours. "I
think the prospects for giving hints of weird, crazy chemical
disequilibria that have nothing to do with our own biology
but are hinting toward truly alien biologies - that might pan
out," he says. "It's important that we keep our expectations
completely open. Seeing blank-slate what's there is going to be
a really exciting adventure."

sites of biochemical, if not biological, evolution.
The window of time for complex life to potentially develop
in that distant realm is comparatively short, on the order of
300,000 to 100 million years, versus the roughly 3 billion
years Earth had for such life to arise, Stern says. But at their
enormous distance from the Sun, these bodies will be safe
from wholesale evaporation, they enjoy less harmful stellar radiation, and they suffer fewer collisions from errant
asteroids and other smaller objects. So maybe they'll have a
chance at life, even the sophisticated variety.
For our solar system, this is way off in the future, of
course. But the Milky Way has an estimated 1 billion red
giant stars. If icy organics are common 20 to 50 a.u. out from
these stars, as they are from ours, then such zones, Stern
says, "may form a niche type of habitable zone that is likely
to be numerically common in the galaxy." Thus they might
enable us to test the hypothesis that simple life can evolve on
short timescales, or that the slow evolution of complex life is
not necessarily the norm. They might even allow the potential for intelligent, space-traveling civilizations to escape the
destruction of their home habitable zones during red giant
stretches. Perhaps the locations of advanced civilizations are
even biased toward such late-term habitable zones, Stern says.

Other Scenarios
Earth, meanwhile, might face other destinies as lethal as
burning to a crisp. Simulations by Jacques Laskar and Mickaël
Gastineau (both at Paris Observatory) have shown that
collisions or ejections from the solar system are possible for
the inner planets long before the Sun becomes a red giant.

Pluto and Other Kuiper Belt Objects
The drifting habitable zone will not stop at Saturn, of course.
During the hottest stages of the Sun's red giant phases, the
zone will push right out into the Kuiper Belt, as much as 50
astronomical units (a.u.) from the Sun. Alan Stern (Southwest
Research Institute) calls these "delayed gratiﬁcation habitable
zones." Triton, Neptune's largest moon, Pluto and its largest
moon Charon, and large Kuiper Belt objects are all rich in both
water and organic compounds, and thus may become possible

p LAKES ON PLUTO? As the Sun matures, all that nitrogen, carbon
monoxide, and methane ice in Sputnik Planum may become liquid.
s k y a n d t e l e s c o p e . c o m * O C T O B E R 2 0 17

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Sky and Telescope - October 2017

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