New images
of rock layers at Mars' surface have given scientists evidence of climate
swings on ancient Mars that were driven by the wobbling of the red planet's
axis — the same mechanism that causes Earth's ice age cycles.
The stereo
topographic maps of rock outcrops within four craters of Mars' Arabia
Terra region were obtained by the high-resolution camera onboard NASA's Mars Reconnaissance
Orbiter, which is still circling the planet.
The team
looked at the layering of deposits of sedimentary rocks in the outcrops to see
if they could discern any patterns. The layers were a few meters (yards) to
tens of meters thick. At Becquerel crater, the researchers found an alternating
pattern of layers within layers that suggests that each one formed over a
period of about 100,000 years as a result of cyclical climate changes.
These cycles
are a result of the changing degree of tilt of Mars' orbital axis, the
imaginary line running between a planet's two poles around which it rotates, said
study leader Kevin Lewis, a graduate student at Caltech.
"Due
to the scale of the layers, small variations in Mars's orbit are the best
candidate for the implied climate changes," Lewis said. "These are
the very same changes that have been shown to set the pacing of ice ages on the
Earth and can also lead to cyclic layering of sediments."
The study
is detailed in the Dec. 5 issue of the journal Science.
Changing
tilts
The tilt of
Earth on its axis varies between 22.1 and 24.5 degrees over a 41,000-year
period. That seemingly small variation leads to large changes in the amount of
sunlight reaching the polar regions of Earth — when less sunlight reaches the
poles, more ice can accumulate there, leading to ice ages that can last thousands
of years.
Mars' tilt
has more variation than Earth's, wobbling by tens of degrees over a
100,000-year cycle, which can produce even more dramatic changes in climate.
When Mars' axial tilt, or obliquity, has been low, the poles have been the
coldest places on the planet, which has resulted in atmospheric changes that
can impact how material is deposited.
And when
the poles are colder, water and carbon dioxide in the atmosphere migrate pole-ward
where they are locked
up as ice. When the obliquity is higher though, the poles get more sunlight
and the water and carbon dioxide migrate way.
"If
you move carbon dioxide away from the poles, the atmospheric pressure would
increase, which may cause a difference in the ability of winds to transport and
deposit sand," said study team member Oded Aharonson, also of Caltech.
This could change the rate at which material is deposited on the Martian
surface.
The
changing tilt would also affect the stability of any surface water, which would
alter the potential for sand grains to stick together and cement into rock
layers.
Million-year
cycles
In addition
to the apparent 100,000-year cycle of layering, every 10 layers in the craters
were bundled together into larger units that correspond to a longer climate
cycle of about one million years, the scientists found. In Becquerel crater,
the 10-layer pattern is repeated at least 10 times.
The
one-million-year cycle corresponds to a known pattern of change in Mars's
obliquity caused by the dynamics of the solar system.
Many
geologic processes, in addition to changes in atmospheric density and surface
water stability, could change with Mars' wobbling, so researchers can't tie the
sediment layering patterns to any particular geologic processes. But, "a
strength of the paper is that we can draw conclusions without having to specify
the precise depositional process," Aharonson said.
"This
study gives us a hint of how the ancient climate of Mars operated, and shows a
much more predictable and regular environment than you would guess from other
geologic features that indicate catastrophic floods, volcanic eruptions and
impact events," Lewis said. "More work will be required to understand
the full extent of the information contained within these natural geologic
archives."
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