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NASA Rover's Sand-Dune Studies Yield Surprise
Jet Propulsion Laboratory
June 30, 2016

Some of the wind-sculpted sand ripples on Mars are a type not seen on
Earth, and their relationship to the thin Martian atmosphere today provides new clues about the atmosphere's history.

The determination that these mid-size ripples are a distinct type resulted from observations by NASA's Curiosity Mars rover. Six months ago, Curiosity made the first up-close study of active sand dunes anywhere other than
Earth, at the "Bagnold Dunes" on the northwestern flank of Mars' Mount
Sharp.

"Earth and Mars both have big sand dunes and small sand ripples, but on
Mars, there's something in between that we don't have on Earth," said
Mathieu Lapotre, a graduate student at Caltech in Pasadena, California,
and science team collaborator for the Curiosity mission. He is the lead
author of a report about these mid-size ripples published in the July
1 issue of the journal Science.

Both planets have true dunes -- typically larger than a football field
-- with downwind faces shaped by sand avalanches, making them steeper
than the upwind faces.

Earth also has smaller ripples -- appearing in rows typically less than
a foot (less than 30 centimeters) apart -- that are formed by wind-carried sand grains colliding with other sand grains along the ground. Some of
these "impact ripples" corrugate the surfaces of sand dunes and beaches.

Images of Martian sand dunes taken from orbit have, for years, shown ripples about 10 feet (3 meters) apart on dunes' surfaces. Until Curiosity studied
the Bagnold Dunes, the interpretation was that impact ripples on Mars
could be several times larger than impact ripples on Earth. Features the
scale of Earth's impact ripples would go unseen at the resolution of images taken from orbit imaging and would not be expected to be present if the meter-scale ripples were impact ripples.

"As Curiosity was approaching the Bagnold Dunes, we started seeing that
the crest lines of the meter-scale ripples are sinuous," Lapotre said.
"That is not like impact ripples, but it is just like sand ripples that
form under moving water on Earth. And we saw that superimposed on the
surfaces of these larger ripples were ripples the same size and shape
as impact ripples on Earth."

Besides the sinuous crests, another similarity between the mid-size ripples
on Mars and underwater ripples on Earth is that, in each case, one face
of each ripple is steeper than the face on the other side and has sand
flows, as in a dune. Researchers conclude that the meter-scale ripples
are built by Martian wind dragging sand particles the way flowing water
drags sand particles on Earth -- a different mechanism than how either
dunes or impact ripples form. Lapotre and co-authors call them "wind-drag ripples."

"The size of these ripples is related to the density of the fluid moving
the grains, and that fluid is the Martian atmosphere," he said. "We think
Mars had a thicker atmosphere in the past that might have formed smaller wind-drag ripples or even have prevented their formation altogether. Thus,
the size of preserved wind-drag ripples, where found in Martian sandstones, may have recorded the thinning of the atmosphere."

The researchers checked ripple textures preserved in sandstone more than
3 billion years old at sites investigated by Curiosity and by NASA's Opportunity
Mars rover. They found wind-drag ripples about the same size as modern
ones on active dunes. That fits with other lines of evidence that Mars
lost most of its original atmosphere early in the planet's history.

Other findings from Curiosity's work at the Bagnold Dunes point to similarities

between how dunes behave on Mars and Earth.

"During our visit to the active Bagnold Dunes, you might almost forget
you're on Mars, given how similar the sand behaves in spite of the different gravity and atmosphere. But these mid-sized ripples are a reminder that
those differences can surprise us," said Curiosity Project Scientist Ashwin Vasavada, of NASA's Jet Propulsion Laboratory in Pasadena.

After examining the dune field, Curiosity resumed climbing the lower portion of Mount Sharp. The mission is investigating evidence about how and when ancient environmental conditions in the area evolved from freshwater settings favorable for microbial life, if Mars has ever hosted life, into conditions drier and less habitable. For more information about Curiosity, visit:

http://mars.jpl.nasa.gov/msl

News Media Contact
Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.webster@jpl.nasa.gov

Robert Perkins
Caltech, Pasadena, Calif.
626-395-1862 / 626-658-1053
rperkins@caltech.edu

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