22 July 2011

MSL to Land at Gale Crater

Posted by Ryan Anderson

Curiosity is going to Gale Crater! Mountains of layered rocks and spectacular canyons, here we come!

HRSC topography of Gale Crater with the MSL ellipse in the northwestern floor. (From Anderson & Bell, 2010)

Gale is a 150 kilometer diameter impact crater just south of the equator on Mars, and it is interesting because in its center there is a mountain of sedimentary rocks more than five kilometers tall. Gale is a very deep and very old crater, so it’s possible that the layers at the base of the mound formed in a lake, back when Mars was more habitable. The presence of hydrated clay and sulfate minerals in the lower mound means that the rocks have been altered by water, and that there have been multiple different environments (sulfates form in acidic waters, while clays tend to form in more neutral water).


A CTX overview of the Gale Crater landing site, showing some key features and a potential traverse. Credit: NASA/JPL/MSSS/Ryan Anderson


The landing ellipse is located on a fan of material eroded from the northwestern wall by flowing water, and the rover would head south from this fan onto a fractured surface. We’re not sure what this unit is, but it has a high thermal inertia, which means it heats up and cools down slowly. Dust has a low thermal inertia, solid rock has a high thermal inertia. The odd thing is that unlike a lava flow, this high thermal inertia stuff is not dark-looking, meaning it is probably sedimentary material cemented together.
Continuing south, Curiosity will encounter a line of dark, fresh-looking sand dunes, followed by the lowest layers of the mound. As MSL climbs the mound, it will be able to study the layered rocks like pages in the history book of Mars. The layer containing clay minerals is a little over 300 meters above the crater floor and should be pretty easy to access. Just above that, it looks like the mound transitions to layers with sulfate minerals, although the layers with clays and sulfates may be interbedded in some places.

Close-up of the potential MSL traverse with a CRISM mineral parameter map overlaid. Green indicates clay minerals and blue/magenta indicates sulfate minerals. The basemap is a mosaic of HiRISE and CTX data. Credit: NASA/JPL/University of Arizona/MSSS/JHUAPL/ASU/CRISM map by Ralph Milliken/traverse and composite map by Ryan Anderson.

From the easy-to –reach clay layer, MSL will pick its way upward along one of several potential paths. The rover drivers at JPL, along with some of us scientists, took a very careful look at the high-resolution topography of the Gale mound and found that the drive up will be challenging, but do-able.
On the way up, MSL will come across a water-carved channel filled with debris from higher on the mound, and some spectacular fractures that have been mineralized by the action of groundwater at that level. These mineralized fractures are more erosion-resistant than the surrounding rock, so when erosion occurs, they end up forming ridges. These sorts of things often mark the upper level of the groundwater table here on earth.

Figure from Anderson and Bell (2010) showing some examples of cemented fractures on the mound. Similar fractures are accessible on the proposed traverse.

Above those fractures, MSL will encounter a much lighter unit that was deposited on top of the mound after the mound had already eroded to its present shape. This younger unit is not traversable, and is the typical end point for the proposed traverse. But, assuming that MSL will last longer than that, there is plenty more to do, whether we decide to detouraround the younger material and continue up the mound, or head to the west to explore the lower mound layers more thoroughly.

Throughout all of this, the views will be mind-blowing. Right now, we’re getting excited about the little hills on the horizon that Opportunity is seeing as it drives toward Endeavor crater, but those hills are only a couple hundred meters high. In Gale, the landing site will be surrounded on all sides by spectacular topography. To the north, the crater rim will dominate the skyline, and looking east and west it will fade into the distance, driving home just how huge Gale Crater is. To the south, of course, the mound will be waiting, layers beckoning.

Once we reach the mound and start climbing, the view back down the hill will get even better, and the view uphill will be dominated by towering stacks of layers, eroded by the wind into canyons. I promise, the scenery alone will change the world’s perception of Mars.

HiRISE color image of a portion of the proposed traverse that passes through a spectacular canyon. Credit: NASA/JPL/University of Arizona/traverse by Ryan Anderson

On a personal note, it’s simultaneously wonderful and terrifying that MSL is going to the landing site I have been studying for the last few years. Suddenly my work is going to come under very close scrutiny, and I fully expect that MSL will discover things that completely invalidate some of my interpretations. But that’s ok, that is what science is about and I’m thrilled that we will get the answers to some of the big questions about Gale that I couldn’t answer using data from orbit.
Still, the landing site selection is a little bittersweet for me. For years, we have been living in a world with four or more potential landing sites, and even though I study Gale, I liked them all. I have pictured Curiosity exploring the delta at Eberswalde, or the spectacular minerals at Mawrth, or the wide range of features at Holden, and in a way it has been like having four rovers. Now, suddenly, we have one site. To abuse a concept from quantum mechanics, we had been living with a superposition of four possible missions, and now the wave function has collapsed to Gale Crater.
One of the most amazing things about the whole selection process is that, for the first time, site selection has been driven by science, not engineering. Yes, some of the earlier sites were excluded for safety reasons, but we ended up with 4 great sites and the ability to choose between them based on science! This is unprecedented. It made the decision really difficult, but investigating all four sites has been really rewarding for the Mars science community. We have ended up with a site with a spectacular story to tell, both in terms of the history of Mars, and the drama of striving to reach the mound and then climb ever-higher. I think people will fall in love with this site, and Curiosity.

The shuttle program just ended, and there are a lot of concerns about the future of the space program, but I think Curiosity’s mission at Gale crater will show that NASA is still very much in the business of exploring.


Note: For more information about Gale Crater, check out my paper about it, and the presentations from the last landing site workshop.