ESA / NASA / JPL / UNIVERSITY OF ARIZONASaturn’s largest moon has a remarkably
complex atmosphere, with all the chemical
ingredients for life as we know it.WXHUYGENS’ DESCENT These images were taken by the Huygens
probe during its descent to Titan’s surface on January 14, 2005.
Each row across the page spread shows the four cardinal directions,
and each column down the page has views taken at five different
altitudes. Altitudes from top row and moving down: 150 kilometres,
30 km, 8 km, 1.5 km and 300 metres.T
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 )
atmosphere and an active hydrologic cycle — though Titan’s
cycle doesn’t involve water. The atmospheric surface
pressure on Titan is about 1½ times that at sea level on
Earth, and its surface temperature is 94K (−179°C),
conditions in which water is rock-hard ice.
Instead, Titan’s environment is very near the triple
pointof methane (CH 4 ), which means that methane
can exist as a solid, liquid or gas depending on local
conditions. (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 fields
— all made of methane and related compounds.
Much of our understanding of this landscape comes
from 13 years of intensive study with the Cassini-
Huygens mission, undertaken jointly by NASA and ESA.
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
and an important place to study the chemistry that might
have occurred in our planet’s atmosphere before lifeTitan’s
Veil
26 AUSTRALIAN SKY & TELESCOPE April 2019
ALIEN ‘EARTH’ by Sarah Hörst