Yesterday I was checking out the latest photos from Curiosity on Mars and came across one Mastcam image that was particularly rock-layer-rich:
It was part of a larger panorama exploring a finely layered hill in the foreground and the flanks of Mount Sharp in the background:
Later yesterday, I was working my way through some rover mission archives and came across another rock-layer-rich outcrop:
Except that isn't from Curiosity. It's from Spirit. It's a view of the western edge of Home Plate, taken on the very sol that Spirit became trapped in the sand at Troy. I was struck by the fact that, superficially, the thinly layered rocks are similar, but geologically, they're very different.
Curiosity's layered rocks are virtually all sedimentary, formed when either water or wind transported and deposited broken-up bits of other rocks into the Gale crater basin. Rocks around Home Plate are mostly related to volcanism: they are deposits of tuff (volcanic ash that fell out of the air) or accretionary lapilli (little beads of ash that formed in the sky before falling to ground, like volcanic hailstones). They are sort of like sedimentary rocks, but the "sediments" that they're made of did not start out as other rock deposits; they are brand new baby rock material, freshly formed from a melt. In rare places at Home Plate, there are windblown sandstones, but most of the story is volcanic. The rocks at Home Plate have been changed by the action of hot mineral-rich groundwater, so water was involved, but only after the fact, and even that was related to heating from underground volcanism.
It just goes to show that as much as we all love images from Mars, images are not usually enough to understand the geology of a place. There are lots of different kinds of ways to make layered rocks. Images provide crucial context for other kinds of observations, which are usually much coarser and sparser: spectral measurements from orbit, elemental compositions from robotic arm instruments, that sort of thing. But you can't understand Martian (or Earth!) geologic history from photographs alone.
It was fun to play with the archival Spirit data. In case you're interested in playing with it, too, here are some resources:
- Three papers describe the Spirit mission in detail, explaining what Spirit did, when, and why, and giving high-level overviews of the scientific results. Because American Geophysical Union journal articles are now open-access beginning 24 months after their publication, you can download them all:
- Arvidson et al., 2005: "Overview of the Spirit Mars Exploration Rover Mission to Gusev Crater: Landing site to Backstay Rock in the Columbia Hills" covers landing to sol 512, when the rover was at Larry's Lookout, before peaking Husband Hill.
- Arvidson et al., 2008: "Spirit Mars Rover Mission to the Columbia Hills, Gusev Crater: Mission overview and selected results from the Cumberland Ridge to Home Plate" covers sols 513 to 1476: summiting Husband Hill, descent to the Inner Basin, circumnavigation of Home Plate, and reaching the third winter parking spot off the north edge of Home Plate.
- Arvidson et al., 2010: "Spirit Mars Rover Mission: Overview and selected results from the northern Home Plate Winter Haven to the side of Scamander crater" covers sols 1410 to 2169: the third winter campaign, drive around the northwest corner of Home Plate, and entrapment at Troy.
- There are three ways to browse and download Spirit data, and I worked with all of them to identify the images I was interested in:
- Midnight Mars Browser works as well for Spirit and Opportunity as it does for Curiosity. You can browse by sol or by site or even locate named targets. However, it only gives you access to raw JPEGs; you need to go to the Planetary Data System (PDS) for archival quality science data.
- The Analyst's Notebook is the main Planetary Data System location for rover data. There you can also browse by sol, though because the Notebook also includes all the other kinds of data as well as mission planning documents, it's a little klunky if all you want is images. But the maps, annotations, and contextual information can be really helpful. You can download images through the Notebook, but if you want to download a great many images, it becomes a bit tedious. So once I've used Midnight Mars Browser and/or the Analyst's Notebook to identify the images I want, I usually go to...
- The Imaging Node of the Planetary Data System, where you can access a folder tree of rover images. Each type of image (Hazcam, Navcam, Pancam, Microscopic Imager, Descent Camera) has its own folder, and there are a couple of folders for mosaics. Within each folder there can be very many different versions of images, processed in slightly different ways. It's all explained in the MER Camera Software Interface Specification (pdf). I prefer radiometrically processed and linearized versions of images, meaning that they've been calibrated and corrected for camera distortion. These will have an "R" and an "L" in their filenames right after the 9-digit string of numbers that come from the spacecraft's clock. Once you download these, you'll need to convert them from PDS format to something you can work with, like PNG; there are many ways to do this, but I like IMG2PNG.
- When you make color combinations from these data, the colors usually don't look quite right, because most Spirit and Opportunity color data are not taken through red, green, and blue filters; they're taken through infrared, green, and slightly-beyond-blue filters. I downloaded some nice-looking color images from the Pancam websiteas a guide and set about correcting the color combinations to look a little more real, a mostly subjective process. In case you're interested (and also for my own records!), here's what I ended up doing to the images:
- Brightened the red channel by 224%, the green channel by 196%, and the blue channel by 210% (in Photoshop, this is equivalent to setting the white level slider to 113, 130, and 121 for the red, green, and blue channels, respectively)
- Subtracted 36, 25, and 25 DN from the red, green, and blue channels, respectively
- This looked much better but it was still much more saturated (colorful) than seemed real to me, so I added a Saturation layer in Photoshop; after trial and error I ended up with a Saturation value of -44
- Also, the sky contained various artifacts (hot pixels and whatnot). I used the Healing Brush on the worst of these, then I selected the sky and applied a 2-pixel-radius Gaussian blur to it to smooth it out without losing the gradient in color.
- If I wanted to take more time on it, I would also use a mix of Clone Stamp and Healing Brush on the hot pixels on the outcrop -- but I figured it was time for me to move on to something else!