Frosty dunes

I love this photo, because it shows that Mars is a lively place with wind and water. These dunes near the north pole, occupying a region the size of Texas, have been sculpted by wind into long lines with crests 500 meters apart. Their hollows are covered with frost, which appears bluish-white in this infrared photograph. The big white spot near the bottom is a hill 100 meters high.

For more info, go here:

• THEMIS, North polar sand sea.

If you download the full-sized version of this photo, either by clicking on my picture or going to this webpage, you’ll see it’s astoundingly detailed!

THEMIS is the Thermal Emission Imaging System aboard the Mars Odyssey spacecraft, which has been orbiting Mars since 2002. It combines a 5-wavelength visual imaging system with a 9-wavelength infrared imaging system. It’s been taking great pictures—especially of regions that are too rugged for rovers like Opportunity, Spirit and Curiosity.

Because those rovers landed in places that were chosen to be safe, the pictures they take sometimes make Mars look… well, a bit dull. It’s not!

Let me show you what I mean.

Barchans

These are barchans on Mars, C-shaped sand dunes that slowly move through the desert like this:

C

C

C

And see the dark fuzzy stuff? More on that later!

Barchans are also found on Earth, and surely on many other planets across the Universe. They’re one of several basic dune patterns—an inevitable consequence of the laws of nature under fairly common conditions.

Sand gradually accumulates on the upwind side of a barchan. Then it falls down the other side, called the ‘slip face’. The upwind slope is gentle, while the slope of the slip face is the angle of repose for sand: the maximum angle it can tolerate before it starts slipping down. On Earth that’s between 32 and 34 degrees.

Puzzle: What is the angle of repose of sand on Mars? Does the weaker pull of gravity let sandpiles be steeper? Or are they just as steep as on Earth?

Barchans gradually migrate in the direction of the wind, with small barchans moving faster than big ones. And when barchans collide, the smaller ones pass right through the big ones! So, they’re a bit like what physicists call solitons: waves that maintain their identity like particles. However, they display more complicated behaviors.

This simulation shows what can happen when two collide:

Depending on the parameters, they can:

c: coalesce into one barchan,

b: breed to form more barchans,

bu: bud, with the smaller one splitting in two, or

s: act like solitons, with one going right through the other!

This picture is from here:

• Orencio Durán, Veit Schwámmle and Hans J. Herrmann, Simulations of binary collisions of barchan dunes: the collision dynamics and its influence on the dune size distribution.

In this picture there is no ‘offset’ between the colliding barchans: they hit head-on. With an offset, more complicated things can happen – check out this picture:

It may seem surprising that there’s enough wind on Mars to create dunes. After all, the air pressure there is about 1% what it is here on Earth! But in fact the wind speed on Mars often exceeds 200 kilometers per hour, with gusts up to 600 kilometers per hour. There are dust storms on Mars so big they were first seen from telescopes on Earth long ago. So, wind is a big factor in Martian geology:

• NASA, Mars exploration program: dust storms.

The Mars rover Spirit even got its solar panels cleaned by some dust devils, and it took some movies of them:

Geysers?

This picture shows a dune field less than 400 kilometers from the north pole, bordered on both sides by flat regions—but also a big cliff at one end.

Here’s a closeup of those dunes… with stands of trees on top?!?

No, that’s an optical illusion. But whatever it is, it’s something strange. Robert Krulwich put it nicely:

They were first seen in 1998; they don’t look like anything we have here on Earth. To this day, no one is sure what they are, but we now know this: They come, then they go. Every Martian spring, they appear out of nowhere, showing up—70 percent of the time—where they were the year before. They pop up suddenly, sometimes overnight. When winter comes, they vanish.

In 2010, astronomer Candy Hansen tried to explain what’s going on, writing:

There is a vast region of sand dunes at high northern latitudes on Mars. In the winter, a layer of carbon dioxide ice covers the dunes, and in the spring as the sun warms the ice it evaporates. This is a very active process, and sand dislodged from the crests of the dunes cascades down, forming dark streaks.

She focused our attention on this piece of the image:

and she wrote:

In the subimage falling material has kicked up a small cloud of dust. The color of the ice surrounding adjacent streaks of material suggests that dust has settled on the ice at the bottom after similar events. Also discernible in this subimage are polygonal cracks in the ice on the dunes (the cracks disappear when the ice is gone).

More recently, though, scientists have suggested that geysers are involved in this process, which might make it very active indeed!

Geysers formed as frozen carbon dioxide turns to gas, shooting out clumps of dark, basaltic sand, which slide down the dunes… that’s the most popular explanation. But maybe they’re colonies of photosynthetic Martian microorganisms soaking up the sunlight! Or maybe geysers are shooting up dark stuff that’s organic matter formed by some biological process. A bunch form right around sunrise, so something is being rapidly triggered by the sun.

This has some nice prose and awesome pictures:

• Robert Krulwich, Are those spidery black things on Mars dangerous? (maybe), Krulwich Wonders, National Public Radio, 3 October 2012.

The big picture above, and Candy Hansen’s explanation, can be found here:

• HiRiSE, Falling material kicks up cloud of dust on dunes.

HiRiSE, which stands for High Resolution Imaging Science Experiments, is a project based in Arizona that’s created an amazing website full of great Mars photos. For more clues, try this:

• Martian geyser, Wikipedia.

Cryptic terrain

What’s going on in this region of Mars?

Candy Hansen writes:

There is an enigmatic region near the south pole of Mars known as the “cryptic” terrain. It stays cold in the spring, even as its albedo darkens and the sun rises in the sky. This region is covered by a layer of translucent seasonal carbon dioxide ice that warms and evaporates from below. As carbon dioxide gas escapes from below the slab of seasonal ice it scours dust from the surface. The gas vents to the surface, where the dust is carried downwind by the prevailing wind. The channels carved by the escaping gas are often radially organized and are known informally as “spiders.”

This is from:

• HiRISE, Cryptic terrain on Mars.

Vastitas Borealis

Here’s ice in a crater in the northern plains on Mars—the region with the wonderful name Vastitas Borealis:

Many scientists believe this huge plain was an ocean during the Hesperian Epoch, a period of Martian history that stretches from about 3.5 to about 1.8 billion years ago. Later, around the end of the Hesperian, they think about 30% of the water on Mars evaporated and left the atmosphere, drifting off into outer space… part of the danger of life on a planet without much gravity. The oceans then froze. Most of them slowly sublimated, disappearing into water vapor without ever melting. This water vapor was also lost to outer space.

• Linda M. V. Martel, Ancient floodwaters and seas on Mars.

But there’s still a lot of water left, especially in the polar ice caps. The north pole has an ice cap with 820,000 cubic kilometers of ice! That’s equal to 30% of the Earth’s Greenland ice sheet—enough to cover the whole surface of Mars to a depth of 5.6 meters if it melted, if we pretend Mars is flat.

And the south pole is covered by a slab of ice about 3 kilometers thick, a mixture of 85% carbon dioxide ice and 15% water ice, surrounded by steep slopes made almost entirely of water ice. This has enough water that if it melted it would cover the whole surface to a depth of 11 meters!

There’s also lots of permafrost underground, and frost on the surface, and bits of ice like this. The picture above was taken by the Mars Express satellite:

• ESA, Water ice in crater at Martian north pole.

The image is close to natural color, but the vertical relief is exaggerated by a factor of 3. The crater is 35 kilometers wide and 2 kilometers deep. It’s incredible how they can get this kind of picture from satellite photos and lots of clever image processing. I hope they didn’t do too much stuff just to make it look pretty.

Chasma Boreale

Here is the north pole of Mars:

As in Antarctica and Greenland, cold dense air flows downwards off the polar ice cap, creating intense winds called katabatic winds. These pick up and redeposit surface ice to make grooves in the ice. The swirly pattern comes from the Coriolis effect: while the winds are blowing more or less straight, Mars is turning around its pole, so they seem to swerve.

As you can see, the north polar ice cap has a huge canyon running through it, called Chasma Boreale:

Here’s an amazing picture of what it’d be like to stand near the head of this chasm:

Click to enlarge this—it deserves to be bigger! Here’s the story:

Climatic cycles of ice and dust built the Martian polar caps, season by season, year by year—and then whittled down their size when the climate changed. Here we are looking at the head of Chasma Boreale, a canyon that reaches 570 kilometers (350 miles) into the north polar cap. Canyon walls rise about 1,400 meters (4,600 feet) above the floor. Where the edge of the ice cap has retreated, sheets of sand are emerging that accumulated during earlier ice-free climatic cycles. Winds blowing off the ice have pushed loose sand into dunes, then driven them down-canyon in a westward direction, toward our viewpoint.

The above picture was cleverly created using photos from THEMIS. The vertical scale has been exaggerated by a factor of 2.5, I’m sad to say. You can download a 9-megabyte version from here:

• THEMIS, Chasma Boreale and the north polar ice cap.

and you can see an actual photo of this same canyon here:

• THEMIS, Dunes and ice in Chasma Boreale.

It’s beautifully detailed; here’s a miniature version:

and a sub-image that shows the layers of ice and sand:

Scientists are studying these layers in the ice cap to see if they match computer simulations of the climate of Mars. Just as the Earth’s orbit goes through changes called Milankovitch cycles, so does the orbit of Mars. These affect the climate: for example, when the tilt is big the tropics become colder, and polar ice migrates toward the equator. I don’t know much about this, despite my interest in Milankovitch cycles. What’s a good place to start learning more?

Here’s a closer view of icy dunes near the North pole:

Martian Sunset

As we’ve seen, Mars is a beautiful world, but a world in a minor key, a world whose glory days—the Hesperian Epoch—are long gone, whose once grand oceans are now reduced to windy canyons, icy dunes, and the massive ice caps of the poles. Let’s say goodbye to it for now… leaving off with this Martian sunset, photographed by the rover Spirit in Gusev Crater on May 19th, 2005.

• NASA Mars Exploration Rover Mission, A moment frozen in time.

This Panoramic Camera (Pancam) mosaic was taken around 6:07 in the evening of the rover’s 489th martian day, or sol. Spirit was commanded to stay awake briefly after sending that sol’s data to the Mars Odyssey orbiter just before sunset. This small panorama of the western sky was obtained using Pancam’s 750-nanometer, 530-nanometer and 430-nanometer color filters. This filter combination allows false color images to be generated that are similar to what a human would see, but with the colors slightly exaggerated. In this image, the bluish glow in the sky above the Sun would be visible to us if we were there, but an artifact of the Pancam’s infrared imaging capabilities is that with this filter combination the redness of the sky farther from the sunset is exaggerated compared to the daytime colors of the martian sky. Because Mars is farther from the Sun than the Earth is, the Sun appears only about two-thirds the size that it appears in a sunset seen from the Earth. The terrain in the foreground is the rock outcrop “Jibsheet”, a feature that Spirit has been investigating for several weeks (rover tracks are dimly visible leading up to Jibsheet). The floor of Gusev crater is visible in the distance, and the Sun is setting behind the wall of Gusev some 80 km (50 miles) in the distance. This mosaic is yet another example from MER of a beautiful, sublime martian scene that also captures some important scientific information. Specifically, sunset and twilight images are occasionally acquired by the science team to determine how high into the atmosphere the martian dust extends, and to look for dust or ice clouds. Other images have shown that the twilight glow remains visible, but increasingly fainter, for up to two hours before sunrise or after sunset. The long martian twilight (compared to Earth’s) is caused by sunlight scattered around to the night side of the planet by abundant high altitude dust. Similar long twilights or extra-colorful sunrises and sunsets sometimes occur on Earth when tiny dust grains that are erupted from powerful volcanoes scatter light high in the atmosphere.

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