16 May 2011

Utah Mars Analogs

Posted by Ryan Anderson

Greetings from Los Angeles! I’m in California this week for the 5th and final MSL Landing Site Workshop. Since that is sure to provide some tasty blog-fodder, I thought I should sit down and write about my trip to Utah two weeks ago.

Why did I go to Utah? Because the latest MSL camera team meeting was held in Moab, and I was hoping to give a brief presentation about some work I’ve been doing on the side (in all my copious free time) with the calibration data for the Mastcams. Unfortunately, I can’t write about what happened at the business part of the meeting because then I would have to kill you. Or more likely Mike Malin would kill me. It turned out there wasn’t time for me or my adviser to give our presentations, but it was still a great trip because after the “sit in a room all day and watch powerpoint presentations” part of the meeting, came the field trip!

Our field trip started with a stop at the Green River overlook in Canyonlands National Park. This was a great place to discuss erosion on earth and Mars, and particularly how incredible it is that so much erosion can happen. The Colorado Plateau contains so many spectacular geologic features, especially canyons, because it was uplifted. When that occurs on a wet, rainy planet like the earth (and yes, the desert southwest is wet and rainy compared to everywhere else in the solar system except maybe Titan) the flowing water immediately begins to carve into the uplifting rocks. I am notoriously bad at remembering numbers, but suffice it to say that A LOT of stuff has been removed from the Colorado plateau in the last few million years. Something like a kilometer of rock, across the entire Colorado Plateau has been removed. We think similar amounts of erosion have occurred on Mars (although probably more because of the ravages of billions of years than sudden tectonic uplift), and the maddening thing about it is that the erosion can uncover craters that looks fresh and pristine, like they were never buried at all!

An aerial photo of upheaval dome, which is likely the eroded remnants of an impact crater.

Our next stop was Upheaval Dome, also in Canyonlands. Upheaval Dome is a circular feature that has been the topic of considerable debate. Its name is quite appropriate: unlike most places in the Colorado Plateau, where the layers of rock are nice and flat, at Upheaveal Dome there has been some sort of … upheaval. The strata are crumpled and twisted and tilted and folded. There are two hypotheses for its formation. First, it is possible that Upheaval Dome is caused by something called a salt diapir. These occur when you bury a large amount of salt, which is less dense and quite a bit softer than most rocks. Like the wax in a lava lamp, the salt is buoyant and bulges upward, eventually making its way to the surface. In the case of Upheaval Dome, there is no trace of the salt left, but remember, the current surface is not necessarily the surface when the diapir was passing through. So, the theory is that the passage of a blob of salt into higher strata which are now missing, left behind the disruption seen at Upheaval Dome.

A photo of the center of upheaval dome, showing tilted bedding and dikes of material from deeper below the present surface.

The alternative theory is that Upheaval Dome is an impact crater. Or more accurately, it’s the exposed subsurface structure of an impact crater. The pattern of concentric and radial faults, and the injection of broken-up material from lower units all are what you expect in an impact. As a group of planetary geologists, we of course favored the impact crater hypothesis, especially since some recent papers claim to have found evidence of shocked quartz, which only forms in a hypervelocity impact. Although to be fair, someone pointed out that we can’t be sure that the impactor was not a giant blob of salt.

Could these distorted layers be caused by the same impact that caused upheaval dome? Maybe...

After Upheaval Dome, we drove over to Arches National Park through a landscape that would make Wile E. Coyote feel right at home and stopped at The Windows. Unlike 99% of people stopping at The Windows, we weren’t there to see the spectacular arches framing the distant snow-capped mountains. We were interested in the rumpled layers of sandstone, and the possibility that their disruption might have something to do with the Upheaval Dome impact 30 miles away. The theory is that seismic shaking from the impact may have caused the layers of sandstone to become deformed and injected breccia of rocks from lower units into the sandstone. The question was raised: why just some layers, why not all of the layers of rocks. One possibility is that the rocks that were most affected were an aquifer full of water at the time of the impact. Since water is incompressible, the beds had to deform, while drier layers could withstand the seismic wave without deforming. After much deliberation, I think the consensus here was that we couldn’t say for sure whether the deformation was caused by the impact.

Cross-beds with coarse poorly-sorted grains indicate that this sediment was deposited by flowing water rather than wind.

The second day of our field trip, we headed out to the countryside near Green River to look at some inverted channels. These are long chains of outcrop that used to be river beds. Since rivers tend to concentrate the coarsest grains in their beds while carrying fine grains in suspension, when the river dries up, the riverbed can actually be more resistant to erosion than the surrounding plains. So you end up with a river bed standing higher than its surroundings. These features are very common on Mars, and it was great to see one up close. One interesting thing was that the inverted channel was actually lots of channels in a braided or meandering stream. But then that raised the question: if this was braided or meandering, why is the erosion-resistant stuff concentrated in one sinuous chain of outcrops? Normally in braided or meandering systems, the channels move back and forth across the available surface to form an expansive sandy floodplain, not a localized riverbed. This had the real geologists among us scratching their heads, but one possibility was that the channel system that formed the inverted feature actually came after the surrounding sediments had been deposited and cut down into them. By cutting down, the system was localized and eventually formed the erosion-resistant material.

That concluded the formal field trip, but a few of us made an extra stop to look at some interdune carbonate deposits. These formed when the dune field that would eventually form the navajo sandstone was temporarily flooded with water in the low areas and carbonate minerals were deposited. The result is interbedded layers of sandstone ad carbonates at various levels in the stratigraphic sequence. It was interesting that the carbonates were full of lovely red chert, which I learned was formed because high pH (which is typically associated with carbonates) makes silica more soluble, so the silica from the sand dissolved and then re-precipitated in locations that were locally more acidic. Often, the acid is provided by living things.

So, that is a quick summary of the Utah field trip. It’s always great fun to get outside and walk around on the rocks with real geologists and try to soak up their knowledge. I still have a lot to learn, but I’m gradually getting better at reading the rocks.

Plus, the scenery is pretty nice.