Comparative Sedimentology and Stratigraphic Record on Earth,
Mars, Titan, and Venus
J. Grotzinger, A. Hayes, M. Lamb, S. McLennan
Chapter Outline
1. Introduction
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Introduce each planet, and note that they all have vastly different
conditions. Tabulate similarities and differences. Despite these
differences, we find surprisingly similar transport and depositional
processes. Therefore….
Each planet should have a rock cycle of weathering, erosion/transport,
and deposition.
Atmosphere critically affects all of these processes – even the fluid
medium of transport.
Introduce “source-to-sink” concept, standard now for integration of Earth
science data related to geomorphic evolution of source terrains where
weathering and erosion occur, and their transition into systems of
sediment transport and deposition at terminal sediment sinks (basins).
Note that terminal sediment sinks differ between planets.
2. Weathering
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Approach: Start with Earth and establish as a baseline. Then compare
and contrast the weathering processes of each planet
Chemical weathering and production of soils, regolith. Note potential
role of minute quantities of water in Mars soils driven by
diurnal/seasonal/longer-term (obliquity?) climate cycle.
Physical weathering, freeze-thaw cycles, diurnal temperature fluctuations
and fracturing.
Sediment production via impacts. This is likely to have been very
important during late heavy bombardment, especially on Mars.
Weathering/sediment production on Titan. Several points here that Alex
contributed: 1) A form of physical weathering may occur on Titan when
methane wets and subsequently weakens water-ice it through volumetric
expansion. 2) Sediment generation through evaporitic processes and
atmospheric deposition. 3) Water-ice is insoluble in liquid hydrocarbon,
but ammonia-ice (which can be generated through disequilibrium
processes) and solid hydrocarbons are soluble.
3. Transport/Erosion
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This section may work better by discussing processes in a conceptual way,
followed by parameterization for Earth, then comparison with other
planets.
Physics of first motion. Describe general mechanics then contrast each
planet.
Bedforms. Subaqueous versus subaerial. I think we could cover the
ripple bedform stability diagram for each planet as an example of how
these features might change. But then we just describe and illustrate
how subaqueous versus subaerial dunes might differ.
Fluvial systems, stream networks. Show Malin/Edgett map of channels
on Mars (equatorial to subequatorial distribution). One issue here will be
to move from discussing channel networks (Section 3, above) as
endproducts of erosion to channels networks as major conduits of
sediment.
How does transport induce erosion, and formation of bedrock channels?
Eolian systems, ergs….control by climate. Major point to make is that
what we see today is not necessarily the final place of deposition. The
fact that we see bedforms means that they cannot yet be part of the
stratigraphic record.
4. Deposition/Stratigraphic Record
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Major Point: Deposition occurs in multiple locations and varying time
scales. The terminal sink for Earth is the oceans (because of rivers), the
terminal sink for Mars may be the northern plains (perhaps because of
winds); the terminal sink for Titan maybe many distributed lake basins.
Furthermore, Titan lacks the dichotomous topography of Earth
(continents vs. ocean basins) and Mars (southern highlands vs. northern
lowlands).
Earth. Our emphasis here should be on climatically sensitive sediments,
and where they tend to accumulate. Earth is wet, so overwhelming
majority of sediments are of fluvial and marine origin. Eolian deposits
and evaporites are rare. Comment on role of plate tectonics in
controlling rock cycle.
Mars. In contrast to Earth it is possible that the stratigraphic record of
Mars is dominated by evaporites and eolian sediments, in addition to
possible volcaniclastic deposits. No evidence for marine sediments, and
lacustrine strata are probably uncommon. Comment on absence of plate
tectonics and how Mars may have had only a partial rock cycle. Note
initial importance of impact-produced sediment.
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Titan. Do the lakes on Titan get filled with fluid resulting from runoff or
emerging groundwater? Given this uncertainty are sediments ever
transported to the lakes by channel networks? Evidence for deltas?
Evidence for any strata of any type? If the lakes formed by emerging
groundfluid tables then it seems likely that no sediments would be
transported to the lakes, and therefore accumulate in the lakes. Given
dune fields on Titan, maybe lakes could fill with wind-transported
sediments that get trapped by fluid? Titan's seas seem to be sediment
filled with large channel networks flowing into them, but the smaller
lakes are typically seen without associated drainage networks. The empty
lake basins are steep-sided deep (150 m - 300 m) depressions, suggesting
that there are not being filled in. There is evidence for deltas (notably at
Ontario Lacus) and flooded river valleys (notably at Ligeia Mare).
6. Summary
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The stratigraphic record is the archive of ancient sedimentary processes,
which in turn document interactions between the atmosphere and
surface of planets.
These interactions occur during weathering - the chemical and physical
transformation of surface materials (can we call these minerals and rocks
for Titan?); Erosion - disaggregation and mobilization of loose materials
by gravity and moving fluids; Transportation - the processes that move
loose sediment particles longer distances and cause hydraulic segregation
of minerals and grain size; and Deposition - where sediments are stored
in intermediate or terminal repositories whose architecture reflects longterm climate change, tectonic modulation, and variations in sediment flux.
(emphasis here on modern or recent processes)
The stratigraphic record for Earth emphasizes the role of water as the
dominant process, a process that has sustained for 4 billions years. By
comparison, the stratigraphic record for Mars shows that it once had a
wetter climate, drying over time. Titan may or may not have a
stratigraphic record. (Emphasis here on notion that modern may not be
key to very ancient past)
On Earth, the oxygenation of the atmosphere and diversification of life is
the outstanding event preserved in the stratigraphic record; for Mars it
may be the transition from wet climate, with neutral pH weathering,
transitioning to acid pH weathering and very limited water/rock
interactions, followed by planetary desiccation. For Titan it is not
currently possible to read a record of long-term global environmental
change due to the absence of a stratigraphic record and the tools to
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decipher it even if it existed. (emphasis here on examples of first order
difference between modern and very ancient past)
In the absence of a stratigraphic record for Titan our understanding of
surface processes is based on the assumption of equilibrium between the
current atmosphere and the materials and morphology of the surface.
(emphasis here on asking if we really believe this, or if there is any reason
to believe that we have a “Pleistocene” history preserved by
disquilibrium relationships that may speak to a different climate state in
the recent past, perhaps due to orbitally-driven change?)