This is the second installment in my look at one enigmatic feature on Mars as seen by all its orbiters through the more than thirty years of spacecraft observations. The feature called "White Rock" was first spotted in Mariner 9 images in 1972, a strange, relatively light-toned splotch of something sitting within a crater named Pollack near Mars' equator. Mariner 9 seems to me to belong to the "prehistory" of Mars exploration, and I don't think it's just because Mariner 9 happened before I was born. Mariner 9 performed a useful survey of Mars, but its image data was surpassed and supplanted by the two Viking orbiters, which operated at Mars from 1976 to 1980.
Even though the Viking data is now three decades old, it is still relevant and widely used by Mars scientists. The lasting accomplishment of the Viking orbiter missions was to produce globally complete data sets that were carefully assembled into digital maps of the entire globe of Mars. The Viking mosaics provided base maps and context images for just about every subsequent study of the planet; it's only very recently that better-quality global image maps have come available, from the wide-angle camera on Mars Global Surveyor MOC and Mars Odyssey THEMIS. (You can download that data here, or wander around it using Google Mars. To wander around the Viking global maps, you can visit the PDS Map-a-Planet website.) Even with the advent of the MOC and THEMIS maps, Viking is still the only approximately true color global data set that is readily accessible.
There were two main Viking mosaics, a black-and-white map produced at 256 pixels per degree (which translates to a resolution of 231 meters per pixel at the equator) and a color map produced at 64 pixels per degree (which translates to 925 meters per pixel at the equator). Here's what Pollack crater, and White Rock, look like in the Viking global mosaics:
Valuable as those global data sets are, they short-change the capability of the Viking orbiters. There are many areas of Mars that Viking imaged with resolutions of 25 meters per pixel or even higher, nearly ten times better than the highest resolution offered by the global mosaic. Using the wonderful map interface hosted by Arizona State University, I dug up the four images required to make this mosaic of White Rock, captured through a red filter with a resolution of roughly 30 meters per pixel.
This high-resolution Viking image reveals much more subtle detail in the tonal variations in the crater floor surrounding White Rock than is visible in the Mariner 9 image. However, I can't say that the Viking images really improve much on Mariner 9 to illuminate what the heck White Rock is. One detail you can discern from the Viking images is that windblown deposits probably play some role in the appearance of the feature, because there appear to be windswept tails trailing off to the southeast of the little bits of bright material, especially near the top of White Rock. These remind me a lot of the similarly vague windswept features trailing from bright spots visible in Cassini images of the boundary between bright and dark areas on the surface of Titan (an example of which is at right).
There is one valuable lesson to be learned from the Viking images of White Rock which is best illuminated by the tiny little low-resolution color view in the mosaic above. The lesson: White Rock really isn't white, which is to say, it's not exactly that bright. It looks really bright because it is totally surrounded by unusually dark material that seems to fill the southern half of the floor of Pollack crater. But its apparent brightness is actually an optical illusion; if you compare the pixel values in the "White Rock" area with the pixel values in the crater floor in the northern part of Pollack, you'll see that they're pretty much the same; White Rock is no more white than the rest of Mars. This is important, because some early attempts to explain White Rock involved unusual deposits of very light-colored minerals like carbonates or salts. You don't need to invoke such unusual stuff if "White Rock" is not different in color from other nearby areas of Mars.
Here's a parting view of White Rock from Viking. Most of the Viking Orbiter views were captured pointing straight down at Mars, giving the bird's-eye view that is best for making global maps. However, there were some shots where Viking pointed significantly off-nadir. These give pretty perspective views across the surface. Here, Viking was looking toward the dawn terminator (off the image at left), so we see White Rock not long after sunrise. This image is pretty but there's also another valuable lesson contained within it. Notice how most of the craters in the image are pretty dramatically lit; the low-angle early morning sunlight beams down on their western walls, while their eastern walls are darkly shadowed. Now look at the White Rock feature itself. There is a tiny hint of shadow, but not much. That tells you that whatever topography White Rock has, it is much less dramatic than the topography found in the walls of the crater; it's a pretty flat deposit, or outcropping, or whatever it is.
The low angle of the sunlight also brings much subtler topographic features into relief. All over the place, you can see evidence of low valleys carved into the landscape. They're almost invisible under high Sun, but with low Sun you can see that this landscape, although very heavily cratered (and thus very ancient), still bears the marks of a historical period marked by the action of liquid water flowing across the surface.
In the next installment, we'll fast forward through almost twenty years without a successful Mars mission, to Mars Global Surveyor.
Finally, I'd like to point to a similar (and, characteristically, much more poetical) effort to look at one spot on Mars through all the orbiters over at Cumbrian Sky.