[FPSPACE] Periodicity in Mars' Climate

[E.P. Grondine]

While the following is interesting, I suspect that the Earth is not the only planetgettting hit every 26 million years or so. Comet impacts with Mars should be releasingvolatiles from its crust on a periodic basis. 

NASA Orbiter Finds Martian Rock Record With 10 Beats to the Bar 
                    PASADENA, Calif. -- Climate cycles persisting for millions of years on ancient Mars left a record of rhythmic patterns in thick stacks of sedimentary rock layers, revealed in three-dimensional detail by a telescopic camera on NASA's Mars Reconnaissance Orbiter.
                    Researchers using the High Resolution Imaging Science Experiment
camera report the first measurement of a periodic signal in the rocks
of Mars. This pushes climate-cycle fingerprints much earlier in Mars'
history than more recent rhythms seen in Martian ice layers. It also
may rekindle debates about some patterns of rock layering on Earth.
                    Layers of similar thickness repeat dozens to hundreds of times in rocks exposed inside four craters in the Arabia Terra
region of Mars. In one of the craters, Becquerel, bundles of a 10-layer
pattern repeat at least 10 times, which could correspond to a known
10-to-one pattern of changes in the tilt of the planet's rotation axis.
                    "Each
layer has weathered into a stair step in the topography where material
that's more resistant to erosion lies on top of material that's less
resistant to erosion," said Kevin Lewis of the California Institute of Technology,
Pasadena , who is the lead author of a report on the periodic layering
published in the Dec. 5 edition of the journal Science. 
                    Some
periodic change in the environment appears to have affected how
resistant the rock-forming sediments became, perhaps from changes in
what size of sand or silt particles were deposited by the wind, or from
how the particles were cemented together after deposition.
                    Some of the individual layers are less than three feet thick. 
                    The
camera, called HiRISE for short, took pairs of images of each site from
slightly different angles in orbit, providing the stereo information
necessary for determining each layer's thickness.
                    "It's
easy to be fooled without knowing the topography and measuring the
layers in three dimensions," said Alfred McEwen of the University of
Arizona, Tucson, principal investigator
for the camera and a co-author of the new report. "With the stereo
information, it is clear there's a repeating pattern to these layers."
                    Geologists
commonly find "rhythms," or repeating patterns, in sedimentary layers
on Earth. Determining the source of the rhythms can be difficult. Some
result from annual or tidal cycles, or from episodic flooding that may
not be periodic at all, but the role of longer-term astronomical cycles
has been debated. One step in showing that astronomical cycles can
leave their mark in sediments came from finding repeating five-layer
sets in some terrestrial bedrock, matching a known five-to-one ratio of
two cyclical variations in Earth's orbit. 
                    Lewis and colleagues found something similar on Mars: "Our findings suggest that cycles of climate change
led to the patterns we see recorded in the Mars rock layers today,
possibly as a result of similar variations in Mars' orbit," he said.
"Mars has a 10-to-one ratio in cycles of how its tilt changes --
smaller wobbles within larger packages. Sure enough, we see a 10-to-one
ratio in one of these layered deposits. It's like trying to identify a
song -- it's easier if there are multiple instruments playing different
parts, rather than just a single rhythm."
                    In
addition to having rhythm of 10 beats to the bar instead of Earth's
five-beat pattern, Mars has characteristics that make it a good
laboratory for studying how astronomical cycles affect climate. The
tilt of Mars' axis varies much more than the axis of Earth, because
Earth's relatively large moon provides a stabilizing effect. And, at
least for most of its history, Mars has lacked the oceans and thick
atmosphere that, on Earth, modulate the effects of orbital variations
and add their own cyclical patterns. 
                    The
10-beat pattern of Mars' wobble lasts about 1.2 million years. If the
10-layer bundles in Becquerel crater are indeed signatures of that
cycle, the 10 or more bundles stacked on each other record about 12
million years when environmental conditions affecting sedimentation
were generally steady except for effects of the changing tilt. 
                    NASA's Jet Propulsion Laboratory, a division of Caltech, manages the Mars Reconnaissance Orbiter for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems,
Denver , is the prime contractor for the project and built the
spacecraft. The HiRISE camera was built by Ball Aerospace and
Technologies Corp., Boulder , and is operated by the University of Arizona . 
                    
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