How does the wind blow on Mars? It sounds like a simple question, but scientists are still learning a lot about how that happens. With few meteorological stations on the surface, one of the bestindicators is observing sand dunes from orbit
-- vast formations that march across the Martian surface, sometimes as much as several feet per year. In recent months,Curiosity has been maneuvering
for an unprecedented close-up look at these dunes to see, in great detail, how the grains of sand shift. Scientists are trying to figure out mysteries such as why ripples appear larger on some Martian dunes than Earth ones, and why they're of a different texture. Read on to explore some of the most recent findings and past observations of Mars' mysterious dunes.PHOTOS: Curiosity Plays in Sandy Martian Dunes
Image: This observation was imaged by Curiosity's front Hazard Avoidance Camera (Hazcam) on Sol 1184 (Dec. 5) as it carried out a closeup investigation of dunes on the slopes of Mount Sharp.
This is one of the first images ever of a rover next to a large dune on Mars. Previously, NASA Mars Exploration Rovers Spirit and Opportunity were by much smaller ripples. NASA is trying to learn more about how dunes, which are distinguished by steep flanks like the one pictured here, move about on Mars. The challenge is modelling Martian winds and sands, which are obviously not as well-known as what is found on Earth. Curiosity is on the lookout for evidence of sand slides,which happened relatively recently in this picture
-- a season ago in Martian terms.MORE: The Weird Wind-Rippled Surface of Mars
Image: Panoramic image of the Curiosity rover next to "Namib Dune" on Dec. 18, 2015.
In 2009, the Spirit rover was exploring Gusev Crater -- filled with fluffy regolith -- when it became mired in a sand trap nicknamed "Troy." While not technically a dune, scientific investigations of the region before Spirit got stuck and eventually died showed what sand can hide underneath. In this case, the rover was sitting atop a sulfate deposit, associated with a hydrothermal (steam) vent. The rover's demise showed how difficult it is to predict the composition of sand in even flat regions on Mars.MORE: Mapping Mars' Ripples Unveils Dune Mysteries
Image: The Spirit rover captured the region where it got stuck, nicknamed "Troy", in April 2009.
For the last few years, Opportunity has been exploring Endeavour crater on Mars.This picture was taken while the rover was en route
, in April 2010. The dunes pictured here are relatively modest -- just 20 centimeters (8 inches) tall. The brighter spot in the foreground is a rock outcrop. Opportunity's controllers changed its route to Endeavour after discovering sand ripples (larger than the ones pictured here) that could have posed a threat to the rover.MORE: Mind-Blowing Beauty of Mars' Dunes
Image: Ripples in Endeavour crater as seen by the Opportunity rover in April 2010.
NASA/JPL/University of Arizona
In 2014, the Mars Reconnaissance Orbiter's powerful camera, HiRISE, captured dunes in the extremely active area of Nili Patera. The dunes have been tracked moving an average of 1.7 meters (5.5 feet) in less than four Earth months, according toa Geophysical Research Letters journal article
. What's even more astonishing is just how big these dunes are: up to 50 meters (164 feet) high. You can see moreincredible HiRISE pictures of Martian dunes in a previous Discovery News article.
MORE: The Psychedelic Landscape of Mars
Image: Nili Patera, a very active dune field on Mars, caught by the HiRISE camera on the Mars Reconnaissance Orbiter.
Once again, this image shows the amazing extent of dunes on Mars. This image is of the formations in Bunge Crater, taken by NASA Mars Odyssey's thermal emission imaging system in 2006, covering a region about 14 kilometers (9 miles) wide. The dunes are moving towards the left of the picture in this perspective. Scientists are doing their best to understand these processes by using computer models and comparing these dunes with ones on Earth.PHOTOS: The Weirdest Craters on Mars
Image: Bunge Crater dunes -- described as "fans and ribbons" of dunes by NASA -- take on the appearance of a rough skin on the surface of Mars. Picture taken by Mars Odyssey in January 2006.
This ESA Mars Express picture shows dunes towering in Rabe Crater as high as 200 meters (656 feet) high, nearly twice the height of the Empire State Building. They're mostly made up of basalt (volcanic rock) and have complex geology, according to the European Space Agency: "Its flat floor has a number of smaller craters and large sunken pits within it. The bulk of the dune material sits atop the flat remnant of the original crater floor, but then some of it spills dramatically down into the pits below."MORE: Sand Dunes Could Reveal Weather on Alien Worlds
Image: Ripples of dunes inside a crater are visible in this 3-D view based on Mars Express data on Rabe Crater. Images to construct this view were taken in 2005 and 2014.