[meteorite-list] MRO Finds Martian Rock Record With 10 Beats to the Bar

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 4 Dec 2008 16:26:40 -0800 (PST)
Message-ID: <200812050026.QAA12669_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2008-227

NASA Orbiter Finds Martian Rock Record With 10 Beats to the Bar
Jet Propulsion Laboratory
December 04, 2008

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.

For more about the mission, visit: http://www.nasa.gov/mro .

Media contacts: Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
guy.webster at jpl.nasa.gov

Dwayne Brown 202-358-1726
NASA Headquarters, Washington
dwayne.c.brown at nasa.gov

2008-227
Received on Thu 04 Dec 2008 07:26:40 PM PST