Though both Mars and Earth possess wind-blown sand dunes with very similar characteristics, it seems Martian dunes have a little something extra.
Mars is a planet shaped by aeolian -- or "wind-driven" -- processes. So it probably doesn't come as a surprise to know the Red Planet also sports some pretty big sand dunes.
From afar, these dunes strongly resemble the dunes we have on our planet. But in a new study carried out by NASA's Mars rover Curiosity, an active dune field on Mars has revealed that, though many of the processes that shape Martian dunes are the same processes that shape terrestrial dunes, there's an extra ripple that can only form in Mars' atmosphere.
"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 graduate student Mathieu Lapotre, of Caltech in Pasadena, Calif., in a NASA statement.
On both Earth and Mars dunes can be as large as a football field and consist of a gently-sloping upwind face and a steep downwind face that is shaped by continuous sand avalanches as the prevailing wind keeps pushing material over the apex of the dune. Classical arc-shaped barchan dunes can often result on both planets and Mars satellites have captured some stunning observations of these types of dunes from orbit. Just look at them, they're amazing.
On Earth, the surfaces of these dunes are often rippled with peaks and troughs spaced around 30 centimeters (12 inches) apart. These rows of ripples are created by wind-carried grains of sand colliding with stationary grains, eventually creating a corrugated texture on dunes covering sandy deserts and beaches.
Until Curiosity started its approach to the active dark Bagnold Dunes six months ago on the northwestern slopes of Mount Sharp, scientists didn't know whether these small-scale "impact ripples" existed. From orbit, larger ripples measuring around three meters (10 feet) from peak to peak could be seen and it was generally assumed that these larger-scale ripples were equivalent to Earth's impact ripples, only much larger owing to the thin Martian atmosphere and lower gravity.
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.
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."
Read more at Discovery News