Science results at Home Plate

Last week, we published the results of last fall's science campaign at Home Plate, including the evidence for and against a couple of our hypotheses for its formation - see the Science article here (most people need a subscription to download the full article online, but you can also visit your library or bookstore to see it in print). I love alphabetical order sometimes - I get to be 4th author on this Athena team publication :)

If you've been following along, you'll already know we think it is a volcanic feature formed when lava met wet terrain, like a maar volcano (hey! Maars on Mars!). If lava interacts with groundwater, the water can flash into steam and make shallow eruptions. These eruptions don't make normal volcanic cones, but looke like low rings or craters. Zuni Salt Lake in New Mexico is a well-preserved example of a maar, which is why it was a stop on our MER field trip last summer. Check out team intern Megan Ennis' terrific poster comparing Zuni Salt Lake to Home Plate to see some of the interesting and diagnostic features of each.

It's good to keep an open mind among the team, though, and we're doing all a lot of new work with the rover to look for more evidence that would help us rule scenarios in or out. For instance, one of the things many people find compelling is this photo, which shows a dark rock and some apparently bending layers. To many, this is a bomb sag, formed when the lighter rock was still wet or deformable, and the dark rock got plunked down onto the layers, bending them under it like when you sit on a soft chair. I'm still a little skeptical though - to me the dark rock looks like the other dark rocks in this frame, which might be weathering out of the light rock or might just be rolled onto the light rock later. We can't tell which from this photo and we didn't have time last fall to investigate more thoroughly. It would be great to find another example like this one!

We are planning a throrough characterization of the rocks as we guide Spirit up the side of Home Plate in the next couple of weeks. And then we've got an exciting area ahead of us - the Home Plate surface itself! We just scooted on by as we headed for Winter Haven and we're all anxious to get on up there and check it out.

IDD bonanza at Home Plate

This week, I've been working with Spirit as it investigates the eastern side of Home Plate. Home Plate is turning out to be super-cool in a science kind of way. Every outcrop we get close to turns out to be a gem. In these last few weeks, we've deployed the instrument arm (IDD) onto several rocks called Elizabeth_Mahon, Madeleine_English, and Examine_This, plus a light-colored soil near Madeleine_English. The instrument arm holds the RAT, the Microscopic Imager, the APXS, and the Mossbauer spectrometer, and we've put all of them to good use.

I've already explained that Mossbauer spectroscopy determines the kinds of iron-bearing minerals in the rock. The APXS is the Alpha Particle X-Ray Spectrometer. It works by exposing the rock target to a source of alpha particles and X-rays and then measuring the alpha particles and X-rays that come back. Each element interacts with X-rays differently, the X-ray interaction depends on the electron shells of an atom. When an atom is hit with an X-ray, the atom can absorb the energy by having one of its electrons go to an excited state. The excited state is unstable, so the electron will want to come back down and when it does, the atom emits energy. The exact frequency of the emitted energy is specific to the flavor of the atom - that is, the exact element. Therefore, you can look at a specific frequency and count the number of returned X-rays and know how much of that element is in the target. In practice, the MER APXS looks at a whole spectrum of frequencies returned and we pick out the peaks at specific energies to say how much of each element is in the rock. Unlike Mossbauer, which is tuned to iron, APXS detects the amount of every element in the rock. But APXS doesn't tell us anything about how those elements are combined into minerals. So, we like to use the APXS and Mossbauer on the same rock to get both the elemental makeup and the mineralogy.

APXS and Mossbauer spectra aren't much to look at unless you're a geek who likes squiggles on graphs (ok, I admit I like them). But this week we also found a nice flat rock exposure that we could brush with the RAT. The Rock Abrasion Tool grinding bit on Spirit wore out long ago, but it still has fantastic wire bristles that can brush soil away. Check out these before and after hazcam shots of the rock, the RAT brush, and placing the APXS on the brushed spot. The RAT brush spot is the spectacular shining beacon in the middle left. (I made this using gifninja.com.)

Then we looked at the shining beacon of brushed rock close-up. I mean REALLY close up. Here's the Microscopic Imager mosaic of the brushed spot, which is about 3cm across. Though it's a mostly flat-ish rock, it's got a knobby texture that we're still discussing. Compare this texture with what we saw at Madeleine_English, described below.

Spinning wheels

Spirit is on the east side of Home Plate, which is this raised platform-like structure that we're trying to understand. It could be some kind of volcanic structure, possibly formed by hot lava inteacting with groundwater, and it would be the first volcanic feature either rovers or landers have gotten to investigate. Because the surface of Mars is largely volcanic, it's important for us to understand the processes of volcanism and so we're trying to do a really full-up investigation of Home Plate.

Our next target is called Madeleine_English (the team is informally naming targets after deceased members of the All-American Girls Professional Baseball League, complementing the names from the Negro Leagues on HP's west side last fall) and is a really interesting target for understanding Home Plate. It is pretty obviously a layer of rock, near the bottom of the stack of layers that make up Home Plate. We'd really like to understand what the different layers are made of and how they are put together. We got to take a look at Madeleine_English a couple of days ago in all 13 glorious Pancam filters (you can see the pictures filter by filter, like here, or combined to make the true-color filter combination at left) and the results are intriguing, showing a very interesting texture in the rock.

So all this week, Spirit and her handlers tried valiantly to approach Madeleine_English so we could deploy the arm instruments. Unfortunately, we're really feeling the effects of having a gimpy wheel. During these short drives over rough terrain angling for a precise approach, the rover has to strain against the wheel, because the wheel doesn't rotate freely. But as I've said here before, the rover drivers are amazing at coaxing the rover into places to wring more science out of this planet. Here's their summary of the driving maneuver to get to this target: "The route involves driving backward, turning around, backing up, parking in parallel between two sizable rocks flanking the target, pivoting clockwise on the stuck right front wheel, and finally "crabbing" forward to the target. Spirit performs crabbing by steering the two rear wheels toward the stuck right front wheel, thus opposing resistance from the right front wheel and keeping yawing (swinging from side to side) to a minimum." (You can see weekly summaries of rover activities in the JPL Mission Manager reports)

Whew! Check out where Spirit ended up (through her front hazcam). Sweet! That's the target rock right in front of us! Can't wait to have a closer look!