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SYFY WIRE Bad Astronomy

10 Years Over the Red Planet

By Phil Plait

On March 10, 2006,* the Mars Reconnaissance Orbiter arrived at the fourth planet from the Sun and took up orbit there.

Forty-five thousand orbits and 10 years later, it has provided us humans back on Earth with a revolution in the way we think about Mars.

It’s almost impossible to list all the accomplishments of MRO over the past decade. It’s shown us how wet Mars was in the past, and how that’s affected the surface today. Images have shown how diverse the environment there is, helped map out landing sites for future missions, tracked seasonal changes (over five Martian years), and supported the other missions that have set their wheels upon the ground there.

Here’s a video put together by the people at the Jet Propulsion Laboratory to give you a taste of what MRO has accomplished:

For me personally, it changed the way I think about Mars. To be clear, I’ve always supported scientific exploration of it. At the very least we need to understand how planets work; we live on one. By studying the other terrestrial planets we can understand our own better. Given how we humans are changing the Earth, having an outside example to study is critical; our future depends on it.

That’s true and a worthy motivation for exploring any planet. But as to Mars itself, years ago when I thought of it I pictured a dry, dead planet, cold and barren—a lifeless rock.

I was wrong. MRO showed us that Mars was a world, with all that implies. Before MRO I thought of Mars as an interesting scientific object of study. After MRO it became a place.

And it was not dead. Far from it! It has an atmosphere, clouds, weather of a sort. It changes, it’s dynamic.

It’s wondrous, and it’s beautiful.

Let me show you. Here are just a handful of some of my favorite images sent back to Earth from MRO using HiRISE, the High Resolution Imaging Science Experiment, which is capable of capturing fantastic details on the surface of Mars. See how gorgeous and amazing Mars really is.

Of all the images returned from MRO, I think this is the one that made me literally gasp out loud the hardest. That is an avalanche. On Mars.

This was from 2008, showing a region near the Martian north pole. It’s the edge of a scarp, a cliff, and as the Sun warmed the pole, carbon dioxide frozen underground turned into a gas and leaked out, weakening the cliff edge. Eventually, something had to give. Quite a few of these avalanches are seen every spring as the season changes. Here’s one from 2010:

Of everything we’ve seen, these drove home to me just how dynamic the surface is.

Mars has a thin atmosphere, with a surface pressure less than 1 percent that of Earth. It’s so thin that much smaller comets and asteroids can penetrate it to hit the ground, so the surface is covered in craters. They come in all kinds of weird shapes and sizes.

I love this one! It was a bit of mystery when it was first seen, but now we understand that under the rockier surface is an ice layer, and as the impactor releases its huge energy as an explosion, the shock wave penetrates the ground, reacting differently to the different strengths of the layers. In the end, you get these odd concentric rings. In the wide-angle shot of this crater a smaller, similar crater can be seen, too. Quite a few terraced craters have been seen across Mars.

Most craters on Mars are very old, billions of years. There’s not a lot of erosion on the surface like there is here on Earth. Still, some craters are much younger than that. Like, say, this one:

The crater was seen in an image in 2012, but that same area of Mars showed no crater in 2010. So it’s only a few years old! The blue color isn’t real; the image is displayed in enhanced color to show how material has been moved around. The fresh features show its youth; those radial rays tend to erode away after a few centuries. Maybe less.

Here’s another young crater:

This one is about 10 meters across, but has an interesting streak many hundreds of meters long, possibly left by the shock wave as the object came in at a low angle. I love the debris splash pattern!

This new crater is very interesting indeed:

Note the white material around it: That’s ice! Yes, water ice, excavated by the impact from a layer just below the surface. This crater is at midnorthern latitude, a bit farther south than you might expect ice to be able to hang out for long periods of time. Clearly, the surface is a good insulator. That’s a promising fact for potential future colonists looking for easy access to water.

The surface of Mars is rocky. Over millions of years and more, erosion has turned a lot of that into dark sand. The air is pretty thin, and has a hard time moving that sand … but Mars has nothing but time. Over the eons, huge dune fields have built up, and from orbit, they are works of artistic sculpture.

That’s a field of barchan (BAR-kan) dunes, horseshoe-shaped piles that form when the wind blows primarily from one direction (in this case, from the left).

Here’s another one in slightly enhanced color:

In that link I describe how these form; in a nutshell wind flows around an obstacle, splitting, and you get a steep hill upwind with a long wave-shape downwind.

Besides the sand, there’s also dust: Much smaller particles heavy in iron oxide … rust! These particles are small enough that the wind can blow them around. In some cases, the reddish sand is over darker, grayer basaltic rock, so when the wind blows away the dust, it’s like a Martian Etch a Sketch. In one case, this creates what may be my favorite photo of Mars of all time:

How amazing is that? All those curlicues are caused by dust devils, vortices of air like mini-tornadoes (though they form in a different manner).

While that may be indirect evidence of dust devils on Mars, we have direct evidence too:

That monster was a devil 800 meters high, and blowing back even farther. I love the shadow of it on the ground.

Some of the most amazing images of Mars aren’t of the planet itself, but of missions that we sent there after MRO. One of my favorites shows the descent of the Curiosity rover, hanging underneath its parachute:

That’s incredible. Remember, MRO is in orbit, moving rapidly, and doesn’t see much of the planet at any one time, so the timing of this photo had to be perfect for it to happen. The engineers at JPL are magicians. HiRISE was also used to get shots of the various components of the hardware from Curiosity’s landing scattered over the ground, too.

HiRISE has also spotted Beagle 2, Phoenix, and Opportunity as well.

To date, no unequivocal evidence of extant liquid water on Mars has ever been found. We have tons of evidence of water having flowed there eons ago, and even of standing lakes and oceans back in the day. There have been lots of images showing hints of water today, but nothing we can point at and for sure say, here be water.

Crater rims and canyon walls are under intense scrutiny in the search for water. A lot of the tops of these rims are lined with gullies, and sometimes we see before and after shots where the gullies have darkened or changed shape, as if at some point water has recently flowed down them.

You can see the gullies, called lineae, fanning out into dark streaks. That’s what you’d expect for flowing water; it would follow the channels, then fan out, darkening the material underneath.

The problem is, you’d expect this with carbon dioxide, too, which can suddenly evaporate and cause very similar features. It’s not possible to tell them apart with current technology, but with higher resolution cameras, or even geologists on the ground there, this mystery may eventually be solved.

I could go on and on; MRO returns as much data back to Earth as every other orbiter around Mars combined, and it’s been doing so for a decade. I urge you to go to the MRO website and poke around, see how this interplanetary invasion has been orchestrated.

I can’t help but think of Ray Bradbury’s The Martian Chronicles, especially the last page. Spoilers: A father takes his family on a long trek across Mars, promising them they will see Martians. Finally, he shows them the Martians, pointing down at their reflections in the water-filled canal.

“The Martians stared back up at them for a long, long silent time from the rippling water ...”

There may not be standing water on Mars, and certainly no canals. But one day, there will be Martians. We’ve already started that journey.

*Correction, March 14, 2016: I originally misstated that the MRO entered Mars orbit in 1996 due to a momentary desire not to be 10 years older than I think I am inside my brain.

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