The nature
of strange ripples of sand on the Martian surface is clearer now thanks to pole-to-pole
images returned by a NASA spacecraft. But even with this new information,
scientists still are unsure just how the features, which are unlike anything on
Earth, came to be.
The ripples
are found over large swaths of Mars. They are smaller than the red planet's
gigantic dunes but larger than sand
ripple fields found on Earth.
Called
Transverse Aeolian Ridges (TARs), these unusual features are formed by
wind-driven particles (phenomena involving air movement are known as
"Aeolian processes"). The winds blow the ridges into many shapes:
simple ripples, forked ripples, snake-like sinuous waves, crescent shapes and
complex, over-lapping networks.
Scientists
study TARs because they hold clues to the past and present climate processes on
Mars, and because they can trip up NASA's rovers currently rolling around the
Martian surface, as has already happened to the Mars Exploration
Rover Opportunity.
Pole-to-pole
Matt Balme
of the Planetary Science Institute in Tucson, Ariz., and his colleagues
conducted a comprehensive pole-to-pole survey of more than 10,000 images of
TARs taken by the Mars Orbiter Camera, part of the Mars Global Surveyor
spacecraft (which launched in 1996 and orbited Mars at least until 2006 when it
stopped responding to signals), to gain more insight into the strange features.
The images
showed that the TARS:
- Are more
common in the southern hemisphere of mars than in the northern hemisphere.
- Are found
in an equatorial belt between 30 degrees north latitude and 30 degrees
south latitude.
- Exist in
two distinct environments: near layered terrain or adjacent to Large Dark
Dunes (LLDs).
- Are
abundant in the Meridiani Planum (a plain 2 degrees south of Mars'
equator) region and in southern-latitude craters.
Additional
information on TARs came from Opportunity's
2005 encounter with one of the ridges. The rover was bogged down for six
weeks with its wheels firmly stuck in the sand.
Opportunity's time stuck in the ridge showed
that at least this TAR was composed of an outer layer of granule-sized material
ranging from about 0.08 inches (2 millimeters) to 0.2 inches (5 millimeters).
Below that outer layer was a mixed mass of fine and coarse particles.
How they
form
TARs need
two things to form, Balme explained: a supply of sediment
and strong winds. The sediment requirement helps explain why the features are
found near dunes and layered terrain, he added.
Both dunes
and layered terrain (which could have been formed by ancient sand dunes, ocean
or lake deposits, or layers of volcanic ash) provide the raw material for the
TARs.
Steep
slopes can also provide additional particles for ripple formation as they
erode. This factor could explain why TARs are confined to a central belt around
the planet, Balme said, because steep slopes are not generally found in the
middle to far north and south latitudes of Mars.
TARs also
come in two age ranges: Those near layered terrain are generally several
million years old and inactive, while the ones near LLDs are young and may
still be actively forming and moving.
"My
theory is that the very young TARs are found near the Large Dark Dunes, which
are also very young, because the sand blowing off the dunes provides the energy
needed to form TARs," Balme said. "Meanwhile you have areas near
layered landforms that used to have active sediment transport, but no longer
[do]. This shows a dynamic environment that has changed, and we might be able
to use TARs as paleo-markers to help decipher ancient climates."
Past
climate, future work
Current
models that examine circulation patterns in Mars' atmosphere don't provide much
evidence that wind
patterns and atmospheric densities on Mars were significantly different in
the past than from what they are now.
"But I
think the geology we are seeing suggests that there might have been different
patterns and densities," Balme said. "The observations we're getting
now from Mars Global Surveyor [data analysis] and the HiRISE camera (flying
aboard the Mars Reconnaissance Orbiter [currently orbiting Mars]) are giving us
really good data to drive the models."
In addition
to figuring out what processes formed the TARs, scientists also still don't
know exactly what materials compose the various TAR fields or why they see
these large features on Mars, but not on Earth.
"Over
the next couple of years we should be seeing many more images from HiRISE that
can gives us more information, for example, about the heights versus spacing
and whether TARs have more in common with dunes or the ripple fields found on
Earth," Balme said.
HiRISE
images of the fields taken over long time intervals could show small movements
within some of the TAR fields, indicating which ones are still active and
possibly demonstrating how they form, Balme said.
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