John Hossain
Planetary Evolution
Professor Gilmore
11/17/15
Notes from November 10th, 2015
Crustal Processes in Titan’s Methane Cycle
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Titan is Saturn’s largest moon and the second biggest in our solar system (about the
diameter of Mars).
From Earth, methane was observed spectroscopically and so indicates an atmosphere.
Voyager I then launched, finding that there is an atmosphere on Titan and nitrogen is the
principle gas in it (about 90% of the atmosphere). Methane and ethane make up the
remainder.
The atmosphere is relatively young, which indicates a possible external methane source.
o Source possibilities include:
 Space (in the form of impactors)
 A surface reservoir (possibly an ocean of methane)
 The moon’s interior (best theory at the time because nothing indicates
otherwise)
Clouds on Titan were found in the Southern Hemisphere in 2001 by Keck Observatory.
Cassini put to the rest the idea of a global methane ocean.
o Found convecting clouds and, via radar mapping, the first evidence of fluvial
processes.
The Huygens Probe then landed and found hydrologic geomorphology such as stream
channels and rounded rocks. There are also dendritic features which indicate rainfall.
Cassini radar found both light and dark channels; the lighter colors are not as deep and
have a higher water-ice content.
Cassini radar also found both darker and lighter lakes, with the darker ones being deeper.
Rainfall rates and erosion: low temperature water-ice can act like soft rock. Also, there
may have been a wetter climate in the past.
Dune formation: dunes weren’t expected because Titan’s atmosphere is so thick, and so
winds strong enough to form dunes weren’t expected.
o But we did find dunes! Long, linear features.
o The dunes tell us that the dominant wind direction is west to east.
What are the possible sources of the sand in the dunes?
o Breakdown of impact ejecta
o Fluvial sediments
o Agglomeration of atmospheric haze
All of this suggests that the methane-ethane cycle on Titan is similar to Earth’s
hydrologic cycle.
Recent hydrologic phenomena may be renewed periodically as climate cycles between
wet and dry conditions.
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Remaining Questions:
What are the densities of the fluvial features?
What are the methane sources and sinks?
What’s up with “alkanofers”? Like aquifers, but for alkanes.
Water and D/H Ratio
 Comets and other ice-rich bodies are one way to bring water onto a planetary body after
dry accretion.
 Deuterium/Hydrogen (D/H) fractionates according to mass-dependence. When water
leaves a system, hydrogen differentially leaves and deuterium differentially stays because
deuterium is the heavier isotope. Many of these processes happen through abiotic
processes that are dependent on temperature.
 Another way mass-dependent fractionation can occur is via the consumption of elements
by microbes. Life is lazy, and so likes to preferentially take in lighter isotopes.
 When it comes to galactic bodies, “wherever you are born affects your basic D/H ratio.”
 Water on Earth likely had a lower D/H ratio in the past, but was then subject to impacts
from carbonaceous meteorites. The current Terran D/H ratio is therefore higher, at about
200*10^-6.