A friend sent me the link to this image (without annotations), yet another spectacular and provocative high-resolution image of Mars. The crest of each dune runs along the boundary between the light and dark blue-gray flanks, and the surface between the dunes is covered in boulders. However, I have a slight problem with this and would appreciate other views. Here's the caption that goes along with the image:
Dunes of sand-sized materials have been trapped on the floors of many Martian craters. This is one example, from a crater in Noachis Terra, west of the giant Hellas impact basin. The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter captured this view on Dec. 28, 2009.
The dunes here are linear, thought to be due to shifting wind directions. In places, each dune is remarkably similar to adjacent dunes, including a reddish (or dust colored) band on northeast-facing slopes. Large angular boulders litter the floor between dunes.
(Image Credit: NASA/JPL-Caltech/University of Arizona; http://www.nasa.gov/images/content/418311main_mro20100113-full_full.jpg)
Now, the fact that "large angular boulders litter the floor between dunes" is not the problem - on our own planet, one of the extraordinary things about dunes is the way in which they seem to self-accumulate and hoover up all the sand from the clear, rubble-strewn "streets" between them. It was Ralph Bagnold who set us firmly on the path to understanding why and how this happens - bouncing versus splashing. I saw this phenomenon very clearly in Bagnold's old stomping grounds of Egypt's Western Desert, as shown in the photo below - vast areas of pebbles and larger rock fragments, essentially swept clear of sand and encroached on by the flanking dunes.
If you cross the dunes, the only rocks to be found on them are either meteorites or surplus tossed out by a passing geologist. And therein lies my problem. Look closely at the Mars image - for example, the two areas that I've magnified - and very large boulders appear to be lying well up the flanks of the dunes, on the surface of the sand and clearly influencing the pattern of ripples around them. What's going on here - or am I misinterpreting something? (As is common, NASA frustratingly disdains providing a scale).
This is a good time to be celebrating the wonders of the Red Planet, because right now it's particularly close to us. As reported in Wired Science:
Shame about the weather and the light pollution here in London! [Friday night update - the London skies miraculously cleared - extremely cold and clear, a spectacular full moon and, yes, the red planet. Well worth a look.]
On Jan. 27, Mars will be closer to Earth than any other time between 2008 and 2014. A mere 60 million miles away, the red planet will be a great target for backyard telescopes, and will appear bright to the naked eye as well.
Every 26 months, the two planets’ orbits bring them closer together, sometimes closer than others. In 2003, Mars came within 35 million miles of Earth, a 60,000-year record.
Observers with a telescope will be able to see changes over the north pole of Mars as the carbon dioxide ice cap is nearing summer and evaporating into gas that affects the polar clouds. (If any of our reader-astronomers catch a nice image, send it our way!)
From the ground, Mars will look like an orange star almost as bright as Sirius, the brightest star in the sky. The display may actually be best on Friday, Jan. 29, when Mars will rise alongside the first full moon of the year, directly opposite the sun.
From the surface of Mars, we have an update on the stuck rover, Spirit. In spite of some recent success in the attempts to extricate it (progress of a few inches in the direction it was originally travelling), NASA has now designated the Rover as a "stationary science platform" - it will likely stay where it is for the rest of its working life, the duration of which will very much depend on its ability to combat and survive this Martian winter.
"There's a class of science we can do only with a stationary vehicle that we had put off during the years of driving," said Steve Squyres, a researcher at Cornell University and principal investigator for Spirit and Opportunity. "Degraded mobility does not mean the mission ends abruptly. Instead, it lets us transition to stationary science."
Meanwhile, Spirit's twin, Opportunity, has fallen very much out of the limelight during the stuck-in-the-sand saga, but it continues to trundle effectively about, conducting geological fieldwork. Most recently, it's been drilling away at a rock that would seem to represent the interior of the planet and something different from anything we've seen before.
And, on a completely different note, I have just cottoned on to the fact that this seems to be "Sand Week" on the geoblogosphere. Ron Schott pointed this out as part of his great series of "Deskcrop" posts of different sands - "We'll find out who knows their sands." See also his recent Outcrop post from Death Valley with great images of a sand storm and beautiful nascent dunes blowing across the road.