While conversing with Ken Edgett about the smiley face on Mars he remarked to me how different Mars looks at different pixel scales, and in particular that there is a transition somewhere in the neighborhood of six to seven meters per pixel. At lower resolutions, Mars looks "Viking-like," and at higher resolutions it looks different, what he calls "MOC scale." I thought it would be interesting to take a look at one feature and see how our view of it has changed with each mission. That has evolved into a gargantuan project as I dig through all these data sets. There is just so much data from Mars; researchers have hardly scratched the surface.
Mariner 8 or 9
Not every spacecraft has photographed the entire planet, though, so you have to select a spot carefully if you want to be sure there will be images from every mission. One of the most distinctive spots noticed early on in Mars exploration was this spot (below) dubbed "White Rock" based on its appearance to Mariner 9. For this one spot alone there's a handful of images from every mission, and for some, Viking and Mars Global Surveyor in particular, there are dozens. This was originally intended to be one blog entry but I realized I'm going to have to post it on the installment plan or it'll never get done. So we'll start our journey through time with Mariner 9.
Mariner 9 was the first Mars orbiter, arriving there on November 13, 1971. After waiting out a planet-encircling dust storm, it proceeded to photo-map the entire planet, returning our first complete look at Mars' diverse geology. It had two television cameras, one wide-angle and one narrow-angle. The black dots you see on the image below are reseaux, which were permanent marks on the faceplate of the camera. These markings were intended to allow the Mariner 9 camera team to correct for geometric distortion intrinsic to the camera; many other '60s and '70s-era spacecraft cameras, notably the Voyagers, also had reseaux markings.
NASA / JPL-Caltech
White Rock from Mariner 9
Mariner 9 captured the first view of light-toned rocks within Pollack crater on February 13, 1972. The image covers an area of about 34 by 43 kilometers and is centered at 8.23°S, 334.94°E.
A more distant view reveals that "White Rock" is on the floor of a crater which was later given the name Pollack.
NASA / JPL-Caltech
White Rock from Mariner 9
A distant view of Pollack crater from Mariner 9 shows its odd interior deposits. The image was taken on January 25, 1972 and has a resolution of approximately 500 meters per pixel.
It was interesting to browse through the Mariner 9 data set to locate these images, because evidently the Mariner 9 team wasn't sure to make of these strange bright deposits. The first one I posted was described as "white rock" in a comment on the image, and that name has stuck. But another image's descriptive comment read "curious ice patches." Another totally misidentified not only the nature but the location of the photo, describing it as "polar cap." Comments on images of other places in the Mariner 9 catalog generally reflect the fact that Mars had not yet been systematically surveyed by an orbiter, so the scientists often weren't quite sure what they were looking at, and even when it was clear there were craters, those craters had mostly not yet been named. I came across comments like "peculiar filametary structure" and "possible craters" and "streaky detail" and "cloud?" and "multitude of surface detail" and "odd fork-shaped bright pattern." It's fun to browse through that table and imagine surveying Mars, with a spacecraft stationed at the planet for the first time, made all the more dramatic by its initial obscuration by a dust storm that slowly cleared.
Mariner 9 is one of the more challenging data sets to work with because it's just so old. However, everything you need to access it, find images, view them, and convert them to more familiar formats is readily available online. First of all, the data itself can be found by browsing the data volumes at the PDS Imaging Node, and you can learn a little bit about the data at the National Space Science Data Center. To figure out what's what and to try to track down images of specific areas, you can download this spreadsheet (XLS format, zipped, 7.5 MB) containing an index to all the images. The images are all in a format that won't be familiar to most of you, but like most spacecraft data you can convert a folder full of images to PNG format using my favorite amateur-produced software, Björn Jónsson's IMG2PNG. However, if you're only working with a couple of images, I'd recommend a different amateur-produced piece of software for converting the images, Piotr Masek's MarinerView, because MarinerView can be used to correct the Mariner 9 images (one at a time) for the little white specks of noise that are spattered across every one.