To those who know rocks, this chondrite meteorite could not be mistaken for an Earthly stone. Photos by Andrew Alden

The other week I mentioned, in talking about concretions, that people can be fixated on the idea that they have found a dinosaur egg or meteorite. This last week meteorites featured in two news stories, one excitingly true and the other almost certainly bogus.

The exciting story was about a set of meteorites recovered in the desert of Morocco, a few months after their fall from space had been recorded. That doesn't happen very often—once meteors arrive in the atmosphere, their unguided trajectory means that a rather large area must be searched to find them. What was extraordinary was that these rocks were from Mars, certified as such this week by an expert scientific panel.

Meteorite hunting has become a cottage industry in the Sahara Desert, where conditions are ideal for space rocks to be preserved and for practiced observers to spot them. The locals who found the new Martian rocks sold them to dealers, who in turn marked up the price to almost a thousand dollars per gram even before the meteorites were formally certified as Martian.

Meteorites are most easily found in two places on Earth, the Sahara and Antarctica. In the case of Antarctica, they fall on the ice cap, where no other rocks exist at all. Movements of the ice can concentrate these meteorites, including the rarest stones from Mars and the Moon, in certain areas that are surveyed regularly and exclusively by scientists. That's good for science. For its part, the Sahara is good for the rest of us who can acquire these rarities for our own pleasure. And scientists can still study Saharan stones because meteorite hunters must donate pieces of their finds to a museum to qualify for authentication, without which the stones have no value. It's a tidy system with little impact on the environment.

The California deserts are also promising places for meteorite hunters. At least one Martian stone has already come from the Mojave. Meteorite hunting is simple in principle, yes, but far from a casual hobby. First you acquire a very intimate knowledge of the rocks that belong there, and then you examine approximately a million rocks to find one that doesn't belong there. And with that, you start to learn about meteorites. I love rocks inordinately, but I think I would still go mad. Dr. Randy Korotev is a genuine meteorite expert at Washington University who gets torrents of email from would-be meteorite finders. On his excellent "What to Do" page, he says that of over 2000 serious inquiries over the years, only eight people had real meteorites.

Easier to dream of fabulous wealth falling into your back yard. That dream, fed by an extremely rare handful of true stories, can blind people to the obvious. And that leads me to the Castro Valley man who got a reporter to feature his story in the San Jose Mercury News last week. His story did not even point to a meteorite, let alone prove it. He said he responded to his dog's barking and found a fresh pit in his back yard with a smoking, red-hot stone in it. That scenario is a old urban legend about meteorites that is never true. He said he talked to experts from Lawrence Livermore National Lab, who had him hold a magnet against the stone and try to cut off a piece of it. He claimed that after finding it both non-magnetic and hard enough to break a hacksaw blade, those experts told him that was positive evidence. None of that is what an expert would say. And the object he showed a photographer had a silvery color and finish (which could not be iron because it was non-magnetic), and a multiply-layered structure that is very common in Earth rocks. In short, it looked nothing like a fresh meteorite and everything like an ordinary metamorphic rock. But he was fervent enough in his belief to fool a reporter, and at least one editor, into running the story anyway.

This is the back side of the chondrite shown at the top. Note the dark fusion crust and the hollows, called regmaglypts, carved by erosion in passing through the Earth's atmosphere. And a magnet sticks to it because it has small grains of iron metal throughout it.

Easier to save up some of your birthday money and buy a nice little meteorite at a rock show from a large, well-run dealership. My advice is to wait until the afternoon of the last day for the best price; dealers hate to pack up their inventory. That's how I got my 1/3-pound chunk of meteoritic nickel-iron from the Sikhote-Alin fall. There's nothing like the feeling of this ancient space metal in your hand.

The Sikhote-Alin fall occurred in eastern Siberia in 1947. Specimens like this are readily available.

I've written more about Martian and lunar meteorites on my About.com site. I also have a photo gallery there. Dr. Tony Irving has a deep and erudite page on Martian meteorites.

This solid vein of white mineral near Endeavour crater, up to 2 centimeters thick, has every appearance of the well-known mineral gypsum. NASA/JPL-Caltech/Cornell/ASU image.

We have been studying Mars with spacecraft for almost 40 years now, starting with fly-bys and progressing to landers and then rovers. Each of these scientific missions has been more ambitious than the one before, and each has found more and more Earth-like features on Mars. The latest example was announced last month in San Francisco at the annual AGU meeting: Mars has veins of gypsum. I've seen gypsum veins myself, right here in California. And you can too.

The currently operating Mars rover is named Opportunity, and it has managed to roll across the pristine Martian ground for more than 33 kilometers since it landed in 2004. Nosing around the rampart of the crater Endeavour in a monotonous area of basalt rock, Opportunity came upon the vein of solid bone-white mineral shown above. It approached closer and turned its alpha particle X-ray spectrometer on the stuff, along with its close-up camera. The spectrometer indicated sulfur and calcium in the right proportions, and the camera images reminded NASA scientists of nothing but gypsum, hydrated calcium sulfate or CaSO₄·2H₂O.

Close-up of the Martian gypsum vein. NASA/JPL-Caltech/Cornell/ASU image.

All right, so what? Well, gypsum points directly and unmistakably to liquid water, something that we've been looking for on Mars but never quite proving. There is gypsum-like dust blowing around parts of Mars, like it does at White Sands in New Mexico, but no sign of its origin. The mission's chief scientist, Steve Squyres, said at the AGU press conference, "This stuff was formed right here. There was a fracture in the rock, water flowed through it, gypsum was precipitated from the water. End of story. Okay, there is no ambiguity about this. This is what makes it so cool. . . . Here the chemistry and mineralogy of it just scream of water."

On Earth, gypsum is found in two main settings: wherever seawater begins to dry up, gypsum comes out of solution first and can accumulate in thick beds. And wherever hot-spring type fluids concentrate dissolved matter, gypsum is a likely mineral. The Martian vein appears to be of the latter, hydrothermal type. California's Central Valley, for much of recent geologic time, has been the former type of setting—a shallow sea basin or brackish lake.

Gypsum is especially abundant in the Kettleman Hills, an area of former Central Valley seabed that has been gently lifted by recent tectonic activity. If you have an extra hour next time you're on Interstate 5 going to Los Angeles, turn off at 25th Avenue, south of Kettleman City, and pull over once you enter the hills. Gypsum litters the ground there, and farther south near Lost Hills gypsum is being mined to make drywall, soil additives and plaster of Paris.

Gypsum of the Kettleman Hills. Photo by Andrew Alden.

If you're there in nice weather, enjoy the scene and think of Mars. Maybe you'll share the tantalizing feeling that the discovery of Martian gypsum arouses in geologists despite their best mental effort.

Kettleman Hills, 26 May 2010. Photo by Andrew Alden.

Endeavour crater, Mars, 6 August 2011. NASA/JPL-Caltech/Cornell/ASU image.

Like one of those 1950s-vintage Cuban taxis, Opportunity has hung on far longer than people originally planned. Planetary space missions have a fairly short "prime mission" of a few months or maybe a year, but everyone knows without admitting it that the rugged, overdesigned machinery will last a lot longer. NASA makes a big deal out of extending the mission, a year or so at a time, and we all feel good about getting our money's worth. In the case of Opportunity, its prime mission began eight years ago and the solar-powered contraption is still plugging along. Imagine what its successor, the rover Curiosity, might do once it lands in Gale crater in August of this year.

Read more about the discovery from NASA.

Gale Crater, the destination for NASA's new rover Curiosity. Credit: NASA, Google Earth

Ready for yet another great Martian adventure? Another prestigious interplanetary mission to that fabled world? The next technological ambassadorship of space-age robots and exotic landscapes? Or, how about an inglorious romp to go dumpster diving to sift through a pile of geological garbage…?

In any case, get set; on November 25th the launch window opens for NASA’s next Mars rover, “Curiosity,” which will arrive over yonder next August.

Inglorious dumpster diving? What’s NASA got planned this time, anyway?

Answer: Gale Crater, a large impact blast about 1400 miles to the west of Gusev Crater, where the rover Spirit now sits motionless—and incommunicado—in its last rusting place.

When I heard that Curiosity was bound for Gale Crater, my first impulse was to start up Google Earth, switch to Mars mode, and zoom in on Gale Crater for a landing to see, at least superficially, what might be of interest there to an explorer. (You can do this too; download Google Earth at www.google.com/earth, select Mars, and search for Gale Crater.)

My first Google-vista of Gale, which included several overlaid strips of high-resolution imagery from the Mars Reconnaissance Orbiter (MRO), hinted that there may be a LOT of interest here.

Gale Crater, about 90 miles across, sports a really large mountain right in the middle—mostly filling the bowl, in fact. The mountain, whose peak rises around 2700 feet above average Martian surface level (can’t say sea level there…not in the present day, at least), towers 3 miles above the deepest part of the crater surrounding it.

I zoomed in on an MRO overlay of a canyon high on the mountain, finding what looks like layered deposits exposed by whatever cutting action had carved the canyon in the past. The landscape I find there is stunning, the details rich. I almost feel as though I’ve stood where Curiosity is soon to tread.

The rover will be set down by a sort of rocket-propelled winch, lowered gently to the base of the mountain where an alluvial fan promises potential riches. Not wind-worn pebbles of solid gold, not fist-sized chunks of diamond—well, probably not—but rather the riches of dirt that may have been deposited there by the action of water. A dumpster of chemical, geological, and potentially biological history.

One of the reasons Gale Crater was chosen as Curiosity’s destination is that it is a deep, low-altitude impact crater, situated at a “downhill” destination where water, if indeed it did flow on Mars long ago, is very likely to have converged, dumping all sorts of soil, rock, and whatever else might have come along for the ride.

Curiosity carries ten instruments designed to conduct a range of measurements, including chemical analysis of soil and rock samples in search of organic compounds that may have been preserved. Where Curiosity’s predecessors, Spirit and Opportunity, have looked for the chemical signs of past water to help tell us whether Mars was ever hospital to life, Curiosity will look for the leftovers of life itself. And in the bottom-lands of Gale Crater, at the foot of a huge mountain from which the rubble of layer upon layer of history has been scoured, it will be in a really good place to do it.

Endeavour Crater; Credit: NASA, Google Earth

Who would have imagined three years ago, when the already veteran Opportunity set forth from its two-year prospecting site at Victoria Crater on a long march to the much larger Endeavour Crater, that the Fates would actually NOT stop this Energizer-Bunny dead in its tracks?

Okay, enough jaw-dropping incredulity. Some things CAN be built to last….

About three weeks ago, Opportunity reached the rim of the 14-mile wide Endeavour Crater, after clocking nearly 21 miles since its landing seven and a half years ago. By Earth-rover standards, that’s about one round trip to work and home again for me, and about 25 minutes of my time—but Opportunity’s commute is a far greater feat, alone on another world, long minutes away even by radio waves, no service garages for maintenance, no fuel stations other than the daily dose of energy doled out by the Sun.

One of the first things Opportunity did upon reaching the rim of the giant crater, after taking some pictures to give us the lay of the land, was to examine some rocks. After all, more than anything else, the Mars Exploration Rovers are geologists, or rock hounds, sent to tell us about the Martian environment, today and in the past, through the chemical makeup and stratigraphy of the rocks and soil.

Data from orbital spacecraft have shown that the materials at the rim of Endeavour may date back to early in Martian history, making for fertile ground in Opportunity’s quest to uncover clues to the planet’s past. Evidence for the presence of clay minerals, possibly formed under wet conditions favorable to life, has been brought to light—which really adds some excitement to the rover’s rock hounding exploits to come.

On Opportunity’s approach to the crater rim, it spotted in the distance unusual outcroppings, and a “shelf” of what looks like sedimentary rock with inclusions of material that may have been deposited by water action.

Water water water, the watch-words of Martian exploration for many years. Where there is, or was, water, perhaps there is, or was, some form of life. And while the Mars Exploration Rovers weren’t designed to look for signs of life directly, their larger, better equipped descendant, Curiosity, to be launched in November, is. Curiosity, do tell….

As a child I liked to imagine what it would be like to land on and walk about the surface of Mars. Mind you, back then we had no images from Mars’ surface—not until 1976 when Viking landed. We had low-res images taken from space, and plenty of science fiction sound stage backdrops and sets from various TV shows and films (and a Mars stand-in, Death Valley, in Robinson Crusoe on Mars).

Opportunity still appears to be in good shape, so the odyssey of its exploration seems to have a good chance of delivering yet another episode of the Life (?) and Times of Mars….

"Mars has become a kind of mythic arena onto which we have projected our Earthly hopes and fears." – Carl Sagan, Cosmos. (1980: New York, USA: Random House.) p. 106.

Science to the side, Mars has meant many things to us earthlings. The Romans named the Red Planet after their god of war, and it has long been associated with masculinity. The symbol for the planet is the same circle extruding an arrow we use for the male gender. (Remember the 1992 self-help best-seller, Men are from Mars, Women are from Venus?)

Images of Mars and Martians have also provided much grist for the mills of popular culture. When I interviewed the scientists we feature in Searching for Life on Mars, I asked them about their favorite appearances of Mars in popular culture. Their answers–Edgar Rice Burroughs’ pulp swashbuckler from 1917, A Princess of Mars, and the 1960’s American sit-com, My Favorite Martian–represent the antipodes of the ways in which the modern imagination has regarded Mars.

In 1917, Edgar Rice Burroughs (best-remembered as the man who brought us Tarzan) published the first in what came to be a series of novels about the adventures of John Carter, a civil war veteran mysteriously transported to Mars in the midst of an Indian attack. Mars, called “Barsoon” by its inhabitants, is strictly divided along racial lines. Princess Dejah Toris belongs to the humanoid and civilized Red Martians, who are beset upon by the barbaric Green Martians, twelve feet tall, with four arms and huge eyes on the sides of their heads. As with Superman on Earth, the lower gravity on Mars endows Jon Carter with extraordinary powers, while his white skin, belonging to a noble race now extinct on Barsoon, gives him mythic status.

At the same time, some authors imagined a more threatening Mars. War of the Worlds, H.G. Wells’ science fiction novel from 1898, was one of the first to imagine a conflict between humankind and a superior alien race. Orson Welles’ realistic 1938 radio adaptation triggered panic as listeners mistook the broadcast for live coverage of an actual invasion. Successful movie adaptations of War of the Worlds were produced in 1953 and 2005.

The period after World War II became the heyday for Mars in popular culture, as Red Scare anxieties about invasion or infiltration at the hands of godless communists were given form in the artifacts of the day, from movies to bubble gum trading cards. Some of the highlights:

-Ray Bradbury’s The Martian Chronicles (1950) which recounts the efforts of earthlings to colonize an ancient Martian civilization, inadvertently succeeding through the chicken pox virus they brought with them;

-Red Planet Mars (1952) where radio transmissions from Mars incite revolutions on Earth–climaxing in the installation of the head of the Russian Orthodox Church on the throne of the Tsar–and are eventually revealed to be the voice of God Himself;

-Invaders from Mars (1953) where a boy is awakened to see a flying saucer disappear into a sandpit behind his home, and later, noticing an unusual red cut beneath his father’s hairline, sees him turn cold and hostile;

-Devil Girl from Mars (1954) in which a statuesque and dominating Martian, dressed head-to-toe in black vinyl, leads a mission to Earth to restock the vanishing male population on Mars;

-The 1962 Topps trading card series Mars Attacks, which depicted an invasion by sadistic and hideous Martians with so much explicit gore and implied sexuality that a public outcry led the company to halt production.

By the 1960’s, the sci-fi movie cycle had generated into cheapies such as Santa Claus Conquers the Martians (1964) and Mars Needs Women (1966). In mainstream American culture, extraterrestrials from the Red Planet were sufficiently domesticated for Ray Walston to play Uncle Martin in My Favorite Martian (1963-1966). Martin’s ability to turn invisible when he raised his retractable antennae was played strictly for laughs.

Besides nostalgia for the Martians of their youths, the scientists we interviewed, Chris McKay, David Blake, Nathalie Cabrol and Tori Hoehler, all offered speculation about a more serious consequence if life were confirmed on Mars. Christianity earlier embraced the geocentric views of Ptolemy and Aristotle where the Earth was the center of the universe, around which all else revolved, just as mankind was the center of God’s creation. The ideas that the Earth moved around the Sun, or that men evolved from apes were–and in some quarters still are–strenuously resisted as undermining the belief that life was created on Earth as part of a divine plan. If there were another, perhaps earlier creation, a second genesis–as proof of life on Mars would imply–what becomes of our starring role in history? What would be the effect on our pride, could our religions survive? For now, the answers to such questions are the province of the writers of science fiction, but if the scientists find what they’re looking for, that might not remain the case in the near future.

Watch Searching for Life on Mars.

QUEST on KQED Public Media.

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Robinson Crusoe on Mars

The idea of a one-way, one-astronaut mission to Mars isn't brand new, even in the non-sci-fi world of real space exploration chatter, but it has recently resurfaced in the news. In a nutshell, the reason for the idea of a hop to, but not back from, Mars is a mixture of cost and technical feasibility—that the biggest hurdle in setting humans down on Mars is not the act of doing so, but the conventional returning of the astronauts to Earth.

Technical feasibility and cost and long lists of volunteers potentially interested in being the first person on Mars aside, my thoughts focused in on one main aspect of such a venture: the solitary soul to make the trip. Even if they knew what they were getting into, went through months, or years, of psychological training and conditioning, and said all their goodbyes—can you imagine the life they would be committing to?

Forget about the "normal" risks of space travel; I'm thinking about the fact of absolute planetary solitude, possibly for the rest of one's life. There would be no guarantees that other humans would follow in later missions—even cheap space programs get cancelled. The only guarantee is that you'd never leave Mars.

And no Star-Trekish two-way conversations with folks back on Earth. In the real world, radio signals travel at 186,300 miles per second, making a one-way signal time of three or four minutes at best (at closest approach between Mars and Earth). So you say "hello Earth!" into your microphone, and six to eight minutes later you get a "hello back at you Mars!" (And most of the time, the round-trip hello/hello-back time is much longer.)

Okay, so you have your never-ending power source (nuclear), lifetime supply of food and medicine, entertainment (books, videos, games), and, of course, you're living your dream exploring another planet where no one has gone before—and making huge strides in humankind's exploration of space and understanding of the natural history of a completely different world.

I just can't help think about one of my favorite 60's sci-fi films, Robinson Crusoe on Mars, specifically the piece where the lone astronaut (monkey doesn't count, much) thinks he might go mad in such isolation, with the potential of never being rescued. Or the film version of Journey to the Center of the Earth, when Professor Lindenbrook stared incredulously at the three scratches on a plumb-bob made by his predecessor, and whispered, "Alone…alone!"

Even though our would-be Mars lifer would be going into this voluntarily and deliberately—what if, when they got there, the true reality settled in and they got an unexpected and extreme case of homesickness….

I'm not poo-pooing the idea of sending a person to Mars–even though I am an advocate of the much more achievable robotic exploration of the solar system, and all the knowledge it brings at a relatively affordable price. A scientist on a multi-decade prospecting trip to our neighboring world would reap huge rewards for us all. But, the solitude…the solitude…. (The blogger shakes off a shiver.)

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Close-up of a nickel-iron meteorite discovered on Mars by
the rover Opportunity. Image Courtesy of NASA.

Trundling along the wind-swept plains of Meridiani Planum, NASA's Mars rover Opportunity stumbles upon yet another rock that looks like it could be an iron meteorite. Does the image of a Gold Rush era prospector leading his burro across desert sands and pecking at a rock with a hammer come to mind? Ah, but more precious than gold is common sand and stone when it's on Mars!

Sounds a bit lonely, like the lives of real gold prospectors; right now, Opportunity is the only active explorer on the surface of the entire planet, surrounded by the now eternally sleeping derelicts of the Vikings, Pathfinder, Spirit, Phoenix, and a few lost souls like Beagle and some Soviet landers….

But, it really is an amazing time to be alive. Each new report from our exploration of space—Mars in particular—reminds me of the state of our knowledge of the solar system when I was a starry-eyed child, back in the 1960s. I recall having to imagine what the surface of that planet might look like; I remember poring over pictures in books and posters of artists' concepts of what the Martian desert might be like, or the even more mysterious shrouded surface of Venus, or how a scene of Saturn and it rings from the surface of one of its moons—Titan, Enceladus, Iapetus–might look.

(Speaking of Venus, I remember a page from the old Time-Life book series–I think it was "The Planets" volume–that showed different possibilities of what might lie under the perpetual shroud of cloud: a rocky desert, a vast ocean, a steamy swamps. Isn't imagination great for filling in the gaps in our knowledge? Remember, prior to 1964, we had sent no spacecraft to any planets, so all we had to go on was what we saw through ground-based telescopes.)

But back to Opportunity, lone prospector of Mars now traversing its 15th mile as it marches steadily on to Endurance Crater. NASA sent the rover to the meteorite suspect it spotted from a distance to take a closer look. Why? We sent the rovers to Mars to examine Martian geology, not interplanetary debris that happened to fall from the sky. Sounds a bit like going to Paris to sample French cuisine and running into the McDonalds.

Actually, even a meteorite can tell us something about Mars. Depending on when the meteorite fell, an examination might reveal clues as to the thickness of the atmosphere at the time. And a reading of the content of certain radioactive isotopes (though Opportunity does not possess this capability) might reveal how long ago the meteorite stopped being exposed to interplanetary radiation—see my earlier blog, Mars Rock Talks, Opportunity Listens, for more on that.

Though the highly imaginative ideas about Mars and other places in our solar system are being summarily swept away by the extremely revealing close-up images taken by our robotic explorers, the reality of those places is at least as enthralling. No, no steamy, dinosaur-filled swamps on Venus; no Martian-constructed water-bearing canals on the Red Planet; no strange black obelisks on a Moon of Saturn (that we know of). But, being privileged to stand in the wheels of Opportunity and gaze across real Martian sands at a lump of extra-Martian iron through the rover's eyes is, just, awesome….

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NASA's Mars Science Laboratory–artist concept

In 1997 there was the grandmother of all Mars rovers, Sojourner—a high-tech breadbox on wheels that crawled about the immediate area of its landing site in Ares Vallis, at the downstream end of an outflow structure. Sojourner (the rover portion of the overall Pathfinder lander) took 3-D images, performed chemical analyses on rocks, and successfully tested the "bounce around the landscape in an airbag" concept of landing on another planet.

Next up were Sojourner's successors—dare I say Prodigal Sons?—Spirit and Opportunity. A step up in stature, these self-sufficient science labs on wheels are about the size of small dinner tables with camera masts stretching 5 feet into the sky. Equipped with the almost prerequisite stereoscopic cameras, rock grinders, microscopic cameras, and a plethora of other instruments, these Mars Exploration Rovers far outlasted their original mission timelines—by years. Landing in 2004, Spirit only recently ceased transmission (temporarily or permanently is yet to be seen), and Opportunity is still roving, on its way to Endeavor Crater to see what it may see.

And now? It's bigger! It's better! It's the new Mars Science Laboratory, Curiosity! The next rover mission is currently being assembled and tested, in preparation for a launch in the Fall of 2011 and arrival at Mars early in 2012. Weighing in at 1,875 pounds (Earth-weight) and with a camera-mast height of nearly 7 feet, Curiosity will carry the most extensive and advanced set of instrumentation and analysis capability of any lander yet sent to Mars.

It's mission: to determine if the environment on Mars is, or ever was, capable of supporting microbial life.

Past missions have been focused incrementally on making new discoveries about the once mysterious, and currently astonishing (and still mysterious), Mars. The first robots to venture forth to the Red Planet—Mariner 4 and Mariner 9, particularly—gave us our first close-up look at Mars' cratered desert surface, its thin atmosphere, and the global dust storms that it is famous for. Missions to follow include the twin Viking landers in the late 1970's that gave us our first views from the surface of another planet, and also performed chemical experiments to search for the telltale signs of life—though results on that score are inconclusive.

Continuing exploration missions—Mars Global Surveyor, Mars Odyssey, Mars Express, and now the powerful Mars Reconnaissance Orbiter—have filled in many details of Mars' exquisite geography, water/ice processes, meteorology, and chemical composition.

The principal mission of the twins Spirit and Opportunity was to find evidence of past—or present—liquid water on Mars…the conclusion of which is that we're nearly certain that there was.

And it is the knowledge of past Martian seas, and possibly present submartian liquid water, that seems to make some almost taste the presence of microbial Martians, be they live or fossilized. Hence, the mission of the Mars Science Laboratory takes us on the next step of our journey to answer that age old question: is there life beyond Earth? Do tell….

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Martian dunes, captured by NASA's Mars Reconnaissance Orbiter

The motor in question powers a fan in an exhibit built to demonstrate the physical processes of duning—the fluid transport and deposition of solid particulates into collections and patterns. The fan blows up a constant micro-gale within the exhibit enclosure, and visitors get to play Mother Nature by turning a handle and redirecting the wind. Meanwhile, a mass of tiny white glass beads is constantly whipped up into a fair recreation of a sand storm on planet Arakis….

After the chore of installing the new motor, I rewarded myself by enjoying the exhibit a bit. I piled up all of the sand on one side of the tank to see how the fan would redistribute it; I sent the wind from different directions, watching how the freshly blown grains were scattered across the pristine black undersurface; I placed all of the pyrite rocks, which serve as wind obstacles, in one pile. It was a lot of fun.

One thing I noticed that I hadn't paid much attention to in the past was how the dune actually moved, or migrated. Maybe I hadn't watched long enough before, or maybe it was easier to witness because I had stacked the deck by mounding the sand all in one corner, but it was fascinating to see the process.

On the windward side of the giant dune, the scouring wind picked up the sand and carried it racing to the top—slowly peeling away the front face of the dune. As soon as the sand-laden wind reached the crest and took a sudden turn downward, it was slowed a bit, becoming less able to support the sand grains, which then fell out onto the leeward side of the dune in a sandy-wind version of precipitation. The buildup of sand on the lee side eventually formed small avalanches that slid down the face in little dry floods.

In this fashion, the dune moved along, slowly being erased on its windward side and formed on the lee.

Almost coincidentally, a few days later I read a report from NASA about sand dunes on Mars. In some areas, dunes have been observed to migrate over time, while on others the patterns have remained stock-still—some of them for perhaps thousands of years, or longer.

So I had successfully created the right conditions for a migrating sand dune. What about static dunes? Well—I had noticed already that some of the pyrite rock obstacles that I placed in the sand stream formed small dunes in the wind-shadows of their leeward sides. The rocks weren't moving, and so the dunes they were nurturing and protecting remained in place.

Some of the static dune ripples observed in Meridiani Planum—where the rover Opportunity is exploring—have been explained as possibly being protected by the presence of "blueberries": tiny nodules of gray hematite that have eroded out of Martian rocks, but which themselves are erosion-resistant, and too large (1-3 millimeters) to be carried by the wind. The blueberries, as the explanation goes, embed in the sand and form a protective "armor" layer for the dune ripples, which remain safe and still in their lee.

Where else do we find dunes, other than Earth? Well, you need wind of sufficient strength and sand of sufficiently small size, for starters. We don't know about dunes on Venus; Venus has a thick enough atmosphere, but the winds may be too sluggish to whip up much of a sand storm. The only other object with a thick enough atmosphere and a solid surface is Saturn's moon Titan—and in fact we have pictures of Titanian dunes taken by Cassini.

Now I'm feeling that old itch to make another trip to my favorite place in the Solar System, Death Valley, to explore the macroscopic dunes of Stovepipe Wells . I'll send a postcard….

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Gullies in wall of Hale Crater. Credit: NASA/MRO

Here's a quick status on the active ones:

Mars Reconnaissance Orbiter: The most recent arrival at Mars (2006), MRO's 5-year mission (hmmm—sound vaguely familiar?) is to study the history of water and climate on Mars, as well as to serve as a telecommunications relay for other spacecraft. Armed with a suite of powerful instruments to study the atmosphere, surface, and subsurface of Mars—including a camera, HiRISE, that can almost read the license plates on Martian automobiles—MRO has to date sent back more data than all other Mars missions combined. It's not been a glitchless flight, however: in 2009, MRO's computer reset itself four times for unknown reasons; the last reset, in August, was followed by a 4-month operational hiatus as Earth-side controllers performed some careful programming updates to help guard against effects of any future resets. MRO resumed operation in December.

Mars Odyssey 2001: NASA's 2001 Odyssey is still going fine after nine years—although the computer glitchiness out around Mars seems to be catching: Odyssey's computer put itself into a safe mode last November 2009 in response to a memory error. This was corrected and Odyssey has resumed doing science. Among Odyssey's major discoveries was the detection of huge expanses of water ice just under the surface of polar lowlands, and the surveying of deposits of water-related minerals in various locations around the planet.

Mars Express: Arriving at Mars in 2004, Mars Express became the European Space Agency's first mission to another planet, which was recently extended to 2012. Though the Beagle 2 lander component of the mission fell to Mars and was never heard from again, the Mars Express orbiter has sent back years of captivating images and important data, including the confirmation of methane in Mars' atmosphere (whose source is in all likelihood subsurface, and the origin of which—organic or inorganic processes—is being debated).

Mars Exploration Rover Opportunity: After six years of crawling around Meridiani Planum discovering chemical and geological evidence for past water on Mars, Opportunity is now on a long march to a large impact crater, which it will reach (if it can keep on running) in about two years. Currently, the rover has stopped to RAT out chemical and geological information from a rock called Marquette Island—the RAT is its rock abrasion tool, or rock grinder. Still going….

Mars Exploration Rover Spirit: Also still alive after six years—almost 25 times longer than it was planned to run—Spirit has been stuck in a sand bog for the last six months. With a couple of wheels on the fritz, Earth-side operators have been confounded in trying to free the robot—but Spirit has continued to make scientific measurements anyway…and in fact made a significant discovery in the course of trying to get unstuck.

Phoenix lander: Although it's been in the deep dark freeze of a Martian winter since November 2008, the Phoenix lander has an outside chance of survival. Now that light is returning to Phoenix's landing site, NASA is listening for the robot's radio signal, in case the return of solar power means that Phoenix will rise from the frost and live again! So far, no such signal….

Next up: The Mars Science Laboratory rover, "Curiosity."

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