Mars - Spirit came across an interesting spot of pure silica this past May. It literally ran right over it. Since then, scientists have been trying to figure out what might have caused it. Two theories include a previous water source which dissolved the silica and then deposited it, along with acidic steam springs emanating from deep within Mars. In preparation for the coming long, cold Martian winter, Spirit is being parked at a safe place near the previous silica discovery with the hopes of sending back more images for additional analysis.
The concern now is over the accumulated dust on Spirit's solar panels. The levels are worse than last winter when the vehicle had to be parked idle for 7 months. It was parked so that all of the energy it accumulated could be used to power the heaters. While the worst of the Martian winter will not arrive until June, 2008, Spirit is already needing all of the energy it is receiving from the distant sun to keep its heaters active. This heat protects its vulnerable components from the coldest Martian nights, which can sink well below -140C at the poles, and below -100C at the equator.
After moving into final position, Spirit will likely remain parked for longer than the previous 7 month period this time. This is due to the limited capacity of Spirit's solar cells. No word was given on Opportunity's condition, however the last report indicated both rovers were in approximately the same condition, though Opportunity is closer to the equator, and therefore will not suffer the same extreme cold temperatures. On both crafts, some minor systems were failing, or were limited in operation in some way, yet they were both mobile, active, responding to commands, and apparently still willing to serve mankind.
Scientists are very eager to determine the cause of the silica deposits, the base material for window glass. According to early reports, they were 90% pure, indicating an active process was involved in their deposition. According to Steve Squyres of Cornell University in Ithaca, NY, the principal investigator for their science payload, "There aren't many ways to explain a concentration so high". In short, it's looking more and more like Mars has had an active enough past that the planet should be teeming with microbial life.