Sunday, February 27, 2011

Fog on Mars

One of the things about Martian Surface Missions that appeals so much to me is how no other place in the solar system is so distant, yet so familiar and comprehensible on a human scale. Other places make your imagination run wild. Underneath the ice of Europa? Skimming the rings of Saturn? Standing on distant Pluto? They make for great art, and we can picture the locations, but their appeal is mostly intellectual, not guttural. But what about Mars? Exotic? Yes. But still it remains within the realm of places that feel familiar, where we could picture ourselves sitting and watching a sunset in person:

The sun setting over the columbia hills in Gusev Crater, Mars. If the sunset weren't blue, we could easily mistake this scene for one shot on Earth, say in Morocco or the Gobi Desert, or perhaps even the badlands of Alberta.



My work on the Phoenix Mission also emphasized the familiar. I set out to make observations of frost, of falling and swirling dust and of clouds scudding across the sky (described in my 3rd 1st author paper last January) and I wasn't disappointed:


Click on the image above to watch the animation which compresses 10 minutes into a few seconds! One of my favourite images of scudding clouds on Mars as seen by Phoenix. Note how the upper streaky clouds move in an opposite direction to the lower fluffy clouds, suggesting wind shear aloft.

I've kept up this work, and I'm proud to now be able to report some new results to you! On Friday, my fifth 1st author paper (11th overall) was published in Geophysical Research Letters. While this isn't the highest impact paper I've participated in, it is the most significant one that I have personally helmed. As such, I feel a bit of pride to be able to report to you on the first direct observation of ice-water fog on Mars.

Now wait, you tell me, hasn't it been known that ice fogs form on Mars for awhile? Well, it's a bit of a complicated subject. As far back as 1984, results were published of frosts on the ground at the Viking Lander 2 site. Based on what was seen, falling snow from an ice-fog was a reasonable assumption at the time. But the fog itself was not seen, and the frosting up of the ground could not be entirely ruled out.

During the hiatus of martian surface exploration between 1980 and 2004, modellers started to attack the problem of martian weather, and in many of these models fog was predicted. But seeing a result in a model isn't the same as seeing on the ground.

Lastly, as late as 2008, the CRISM instrument on MRO reported a distinct "blueing" of the ground overnight at high latitudes. This "blueing" was interpreted as water ice, but was it fog or was it frost? It was impossible to say and again both made sense.

But myself and my fellow authors have been able to finally put this mystery to bed. And like much of planetary science, the way in which we did so is straightforward. On Earth, you could find a fog by shining a flashlight into the sky and looking for the scattered light from the droplets. In the same way, we used the LIDAR laser to illuminate the particles up to 200m above the lander and exposed the CCD on the Surface Stereo Imager at the same time. The result speaks for itself:

Ice-Water fog on Mars as seen by the Surface Stereo Imager on sol 113 of the Phoenix Mission. We report our findings and the first observation of fog on Mars in this past week's Geophysical Research Letters.

When you do the math, there's not a whole lot of ice there, only a couple of milligrams per cubic meter, comparable to cirrus clouds on the Earth, or perhaps a thick diamond dust. But it is there.

Aside from the esoteric, why care about this discovery? Well, it has to do with understanding the martian water cycle. Unlike ice that forms high in the atmosphere, ice that forms as fogs can snow out onto the ground and interact directly with the ground ice Phoenix found at its northern latitude. In the paper, we used computer models to look at how much ice falls out every night onto the ground, based upon how much we saw using the LIDAR as a flashlight. It comes out to 6% of all the water in the atmosphere ends up on the ground overnight.

That's not a lot of water, in fact it would form a layer only about 2.5 thousandths of a millimetre thick. But over time, this water can transform the landscape as it diffuses down into the subsurface. It gives us the polygons you see around the phoenix lander and perhaps combines with powerful deliquescent salts, like the magnesium perchlorate we found. Together these two compounds offer any life a power source and an aqueous medium which can remain liquid down to a staggering -68 Celsius in thin films!

So perhaps the timeless face of Mars is not so unchanging after all, all it takes is water, and time. And time on Mars is the one thing suggested by the landscape that boggles the human mind!
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In searching for the human scale, I am reminded of another mission I participated in - the Cassini-Huygens mission - and the only other place in the solar system besides Mars where I feel this human connection: Titan. I was fortunate enough to be brought onto this project by my advisor, Peter Smith, and grandadvisor, Marty Tomasko. As such, when the probe touched down on January 15, 2005, I was sitting with the rest of the DISR team at Mission Control in Darmstadt, Germany watching the images come back. Now, I grew up in Newfoundland, Canada where it is often foggy. I can remember seeing rivers cutting the terrain and emptying into the sea, glimpsed dimly through the haze from aircraft on approach. As we watched the images from Huygens, I can tell you that I felt right at home:

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