Rinds differ chemically from subjacent outcrop, notably showing enrichment of Na and Cl and depletion of S. They are particularly well developed where they have formed at the interface between outcrop surfaces and thin coverings of soils. This observation suggests that the rind formation process may be ubiquitous and ongoing on exposed or thinly covered outcrop surfaces, but that its rate is substantially slower than the eolian sandblasting of soft outcrop rock by saltating grains that is pervasive across the plains. In this model, only where rock surfaces have been protected from sandblasting, for example, when buried by a thin veneer of soil and only recently uncovered, is thick rind formation observed.
Fracture fills are erosionally resistant, often vertically oriented features associated with linear fractures of possible impact origin. These features are spectrally distinct from adjacent outcrop but differ chemically only in detail. APXS data indicate that fracture fills contain siliciclastic materials in amounts similar to or slightly greater than nearby outcrop lithologies; the fill is typically slightly enriched in Al and Si and depleted in Mg and S. Unlike rinds, fracture fills show no substantial Na or Cl enrichment. The high abundance of silicates means that the fills are not primarily precipitated, but the absence of basaltic minerals indicates that fractures are not filled by present-day soils. Instead, the close similarity of fracture fill and country rock lithologies suggests that fractures were filled primarily by intraclastic material derived from adjacent outcrops. The limited total volume of alteration rinds and fracture fill indicates very low aggregate rates of fluid flow and chemical weathering during the time since the outcrop rocks were deposited.
Pretty standard geological mumbo-jumbo, eh? Could be from anywhere on earth.
But it's for Mars.
Admittedly, I deleted a couple of words that would have given it away. But I was reading Science this afternoon, specifically Two Years at Meridiani Planum: Results from the Opportunity Rover by Squyres et al. (Science 8 September 2006: 1403-1407) and halfway through it occurred to me that this is the same sort of stuff that is done here all the time, but now they're doing it on a different.freakin'.planet.
I'm old enough (barely) to remember when it was still thought that there were 'canals' on Mars, because telescopes couldn't get enough detail to see any sort of land features other than dark smudges on the surface. I still remember as a kid one day watching TV as pictures from one of the Viking landers came in (live). I'm too young to remember the moon landing, but this really struck me: looking at pictures from the surface of another planet.
So now here we are 30 years later talking about the micro-stratification and grain characteristics of Martian rocks. Wow.