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.
For more about the mission, visit: http://www.nasa.gov/mro
Media contacts: Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
Dwayne Brown 202-358-1726
NASA Headquarters, Washington