Sky & Telescope - February 2019 - 28

Alien "Earth" Next Door

The atmosphere is dense enough in the mesosphere that a spacecraft can't
orbit there - but tenuous enough that it's diﬃcult to study with common
techniques. This region is therefore often called the "ignorosphere."

POLAR BEAUTY MARK
Far right: This large vortex,
shown here in a natural-color
composite, formed at Titan's
south pole as the hemisphere
approached winter. The cloud
system sticks above the
surrounding cloudtops. Near
right: A close-up reveals what
might be open-cell convection, during which air sinks in
the center of the cell and rises
at the edge, forming clouds at
cell edges.

FE B RUA RY 2 019 * SK Y & TELESCOPE

N ASA / JPL- CA LTECH / SPACE SCIENCE INSTIT U TE (2)

techniques. Likewise, Earth's mesosphere is at too high an
As the water molecules drift along, solar photons break them
altitude to study with balloons and too low to study directly
up, producing ionized oxygen atoms (O+). Calculations indiwith spacecraft. This region is therefore often called the
cate that those ions are deposited in the same region of the
"ignorosphere."
atmosphere where the heavy ions exist. That
means that oxygen may also be participatNevertheless, Cassini's Ultraviolet Imaging Spectrograph
was particularly well suited to studying Titan's mesosphere.
ing in the ionospheric chemistry, resulting
It ﬁnally provided us with our ﬁrst detailed measurements of
in the formation of very heavy ions that
#7
Hydrogen
this layer's composition and density, furnishing important
contain CHON.
cyanide
inputs and tests for models of Titan's atmosphere.
Indeed, laboratory experiments that try
At the bottom of the mesosphere is something called the
to simulate the processes at work in the upper atmosphere
detached haze layer, ﬁrst discovered by the Voyager 1 spaceshow that some amino acids and nucleobases may be prescraft. When Cassini arrived in the Saturn system,
ent. Therefore, there's every indication that complex
the detached haze layer was at a much different
molecules of prebiotic interest form in Titan's upper
altitude than where we'd left it during Voyager's 1979
atmosphere.
ﬂyby, which was very confusing to Titan scientists.
The incredibly heavy ions that CAPS detects are
#8
Carbon
Luckily, the length of Cassini's mission allowed us to
at the very beginning of their journey on Titan, not
dioxide
ﬁnd out that the haze changes altitude with season (a
the end. They will still undergo a number of proyear in the Saturn system lasts nearly 30 Earth years). When
cesses that may alter their composition and physical properwe returned to the same part of Titan's year that Voyager had
ties as they descend through the atmosphere and precipitate
explored, the layer returned to the same location where it had
onto the surface.
been observed before.
We still don't know exactly why the detached haze layer
Mesosphere: Changing Haze
changes altitude or very much about its composition, but we
Deeper down in the atmosphere, Cassini revealed other
do know that both chemistry and motion in the atmosphere,
secrets. The region below the ionosphere, the mesosphere,
as well as the feedback between them, play important roles.
tends to be difﬁcult to measure: The atmosphere is dense
For example, haze particles affect how energy moves through
enough that a spacecraft can't orbit there - but tenuous
the atmosphere. Energy's movement in turn affects atmoenough that it's difﬁcult to study the region with common

Sky & Telescope - February 2019

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