Workshop on the Habitability of Icy Worlds (2014)
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TRANSPORT AND CONCENTRATION OF ORGANIC MOLECULES ON TITAN-LIKE WORLDS. Michael J. Malaska, Robert Hodyss, Morgan L. Cable,1 Mathieu Choukroun1, Tuan Vu1, Jason W. Barnes2, Shannon
MacKenzie2. 1Jet Propulsion Laboratory / California Institute of Technology, Pasadena, CA. 2University of Idaho,
Moscow, ID. ([email protected]).
Introduction: Titan represents a class of icy
worlds where organic chemistry reigns supreme [1].
Understanding how complex organic molecules are
moved and concentrated on planetary surfaces will
quantify the potential for feedstock deposition and perhaps even the chemical processes before the initiation
of life itself. In our Solar System, Titan represents a
frozen laboratory where prebiotic chemical processes
can be examined.
solution to build up a layer of organic material in the
bottom of the lakes and playa basins.[8] Complete drying creates a remnant evaporite deposit of concentrated
organic materials.[9] Several transport/drying cycles on
seasonal or epochal scales could build up a multi-layer
stack of deposited organics. A model assuming an average rate of 1 cm pure ethane precipitation over 1 year
(terrestrial) moving a non-limiting supply of aromatics
with a 10:1 drainage:lake area ratio, gave 2.2 microns
of accumulated aromatic deposit. A similar model including the theoretical solubility of HCN resulted in a
yearly deposit thickness of 1.6 mm.
Figure. 1. Hydrocarbon fluid cycle on aTitan-like world.
Titan Hydrocarbon Cycle: On Titan, high-altitude
photochemistry creates complex organic molecules that
eventually deposit on the surface.[2, 3] Many of these
molecules represent the earliest precursors and starting
materials for prebiotic synthesis. The deposits are subjected to hydrocarbon rains that occur as part of a volatile cycle similar to the terrestrial water cycle (Figure
1). On exposure to the cycling fluids, soluble organic
molecules are dissolved, transported, emplaced, and
concentrated. The goal of our laboratory experiments is
to understand these processes on Titan-like worlds.
Dissolution: We experimentally determined equilibrium solubilities and the kinetics of dissolution for
benzene, naphthalene, and biphenyl in ethane at 94
K.[4] We found that dissolution is relatively rapid,
even at cryogenic temperatures. The speed of dissolution is comparable to salts such as gypsum that are
commonly leached out of terrestrial deposits.[5]
Transport: Once in solution, the fluids are carried
by the numerous rivers and channels that have been
described on Titan’s surface[6] and possible subsurface
conduits.[7] The dissolved materials follow fluid paths
down local and regional topography to collect in local
basins and depressions.
Concentration and Emplacement: As the solvent
volume is reduced via evaporation, the dissolved solids
will reach their saturation points and precipitates out of
Figure 2. Model showing amount of yearly transport of benzene,
naphthalene, and biphenyl in ethane fluid at 94 K..
Conclusions: Our laboratory measurements of
equilibrium solubilities and evaporite structures, when
combined with atmospheric flux rates, precipitation
rates, equilibrium solubilities, drainage areas, and
evaporation rates, can help determine how organic materials are moved and concentrated on a Titan-like
planetary surface.
Acknowlegements: Support from the NASA Postdoctoral Program, NASA Astrobiology Institute (Titan), ASTID, and Outer Planet Research (OPR) programs is gratefully acknowledged.
References::[1] Lorenz et al. (2008) GRL, 35,
L2206. [2] Krasnopolsky, V.A. (2009) Icarus . 201,
226-256. [3] Lavvas et al. (2008) PSS, 56, 67-99. [4]
Malaska and Hodyss, (2013) LPSC 44, Abstract 2744.
[5] Jeschke et al. (2001) Geochimica et Cosmochim
Acta 65, 27-34. [6] Burr et al. (2013) GSA Bulletin
125, 299-321. [7] Mitchell et al. (2008) LPSC 39 Abstract 2170. [8] Cordier et al. (2009) Astrophysical J.
707 L128-L131. [9] Barnes et al. (201) Icarus, 216,
136-140.