But when Curiosity arrived at Bagnold, the rover didn't only see the 10 feet-wide ripples, but it also saw the small-scale ripples just like Earth's impact ripples.
"As Curiosity was approaching the Bagnold Dunes, we started seeing that the crest lines of the meter-scale ripples are sinuous," said Lapotre, who's also science team collaborator for the Curiosity mission. "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."
So it turns out that Mars dunes have an added complexity that could only be proven by rolling up close and taking photos. Mars dunes have the small impact ripples, plus medium-sized "sinuous ripples" that can be resolved from space.
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Interestingly, though Earth's dunes don't possess sinuous ripples, they can form underwater -- on a riverbed, for example. Rather than particles colliding, these sinuous ripples are created as flowing water drags particles, causing them to settle in a rippled pattern.
Lapotre, who is lead author of a study that was published on July 1 in the journal Science, thinks that the Martian sinuous ripples are being driven in a similar way, but it's the Red Planet's thin atmosphere that's dragging the particles to form the medium-sized ripples on the sand dunes. Lapotre's team have nicknamed 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."
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But after studying observations (carried out by Curiosity and NASA's veteran rover Opportunity) of Mars' sandstone dating back to 3 billion years ago, the researchers found evidence of these wind-drag ripples preserved in the material of the approximate same size as the ripples that exist in today's Martian dunes. This means the planet lost most of its atmosphere early in its geological history and for the past 3 billion years the atmospheric pressure has remained fairly constant -- a finding that fits with other Mars atmosphere evolution models.
"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.
It's pretty amazing to think that a fairly simple observation of an active sand dune on Mars can reveal so much about Mars' current and ancient atmospheric conditions. But as the sophisticated wheeled robot continues its quest to seek out past and present habitable environments, and this is all in a day's work.
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