My first Magazine Sky & Telescope - 02.2019 | Page 24

ALIEN “EARTH” NEXT DOOR
by Sarah Hörst
Titan’s
Veil
Saturn’s largest moon has a remarkably com-
plex atmosphere, with all the chemical ingredi-
ents for life as we know it.
itan is often described as Earth-like. Besides our
home planet, it’s the only world in the solar system
to possess a dense molecular nitrogen (N 2 ) atmo-
sphere and an active hydrologic cycle — though Titan’s
cycle doesn’t involve water. The atmospheric surface pres-
sure on Titan is about 1½ times that at sea level on Earth,
and its surface temperature is 94K (−179°C, −290°F),
conditions in which water is rock-hard ice.
Instead, Titan’s environment is very near the triple
point of methane (CH 4 ), which means that methane can
exist as a solid, liquid, or gas depending on local condi-
tions. (Earth’s surface is similarly near the triple point
of water.) The result is a marvelous world of lakes and
seas, rivers carving channels into water-ice bedrock, and
intense rainstorms of large droplets that occasionally
punctuate the relative calm of extensive sand-dune fi elds
— all made of methane and related compounds.
Much of our understanding of this landscape comes
from 13 years of intensive study with the Cassini-Huy-
gens mission, undertaken jointly by NASA and ESA (S&T:
Sept. 2017, p. 16). Before that, Titan’s thick atmosphere
and characteristic orange haze largely obscured our view
of the surface.
Although we often look at Venus and Mars to teach
us about our world, Titan’s atmosphere is the best solar
system analog we have for the early Earth environment
tu HUYGENS’ DESCENT These images were taken by the Huygens
probe during its descent to Titan’s surface on January 14, 2005. Each
row shows the four cardinal directions, and each column has views
taken at fi ve different altitudes. Altitudes from top row and moving
down: 150 km (93 mi), 30 km (19 mi), 8 km (5 mi), 1.5 km (0.9 mi),
300 m (1,000 ft).
22
FE B RUA RY 2 019 • SK Y & TELESCOPE
T