From time to time, both Spirit and Opportunity's cameras have captured the quick passage of atmospheric micro-cyclones or "dust devils". One was shown on page 19-11; here is another example:
In a remarkably serendipitous event, Spirit has been given a new rebirth that makes its already exceeded design lifetime even longer. Over the year plus of operation both Spirit and Opportunity have been gradually coated with martian dust. Their solar panels were becoming sufficiently plastered by dust to cut the efficiency that the panels should have for generating electricity from solar photon input. This cuts down operational time and threatens to fall to minima in which instruments cannot work. Then, sometime between March 5 and 15, 2005, the power output of Spirit dramatically spiked upwards. By looking at its surroundings, the JPL team concluded that a dust devil, the atmospheric whirlwind that occurs in large numbers of mini-tornados, must have swept over Spirit, sucking up (vacuuming) the dust coating, and exposing the solar plates to near optimum efficiency. TV images of a part of the spacecraft taken before and after this lucky encounter show conclusively that Spirit has been cleaned:
(The Opportunity team is hoping intensely for a similar quirk of fate to rejuvenate its power supply.)
By late August, 2005 Spirit had climbed towards Husband Hill, a peak in the Columbia Hills (see map above), and spent several months gathering data. Here is a panoramic view made as Spirit (put in the scene by trick photography) in November started its descent from the top; in the scene are some layered rocks which will likely be visited in the future:
Spirit came to an area in which a low embankment contains outcroppings of slablike rocks. These are nicknamed the "Home Plate" site. Here is a Mars Reconnaissance Orbiter image of Home Plate (named from its shape) - the site consists of volcanic rocks which may get this platy structure from jointing or the individual plates are parts of a succession of thin deposits:
This view shows how the Home Plate rocks are interspersed with darker rocks:
From the ground, the Home Plate layered rocks looks like this:
Here are two close-ups of several of the thin layers at Home Plate. These have been interpreted as possible volcanic ash deposits (pyroclastics).
In May, 2006 Spirit, with one wheel broken, was placed in a "hibernation" mode, with few activities that require power and with its solar panels oriented towards the Sun while Mars passes through an 8-months winter. The vehicle was parked facing northward on a gentle slope named Low Ridge. Spirit did resume exploration in 2007. While spending the martian winter at one place, when conditions are favorable Spirit has gathered data during this period. For example: Here is a close-up of the immediate surface on which Spirit sat, showing several light yellowish-brown exposed patches of altered bedrock determined to be rich in CaSO4 with perhaps some FeSO4.
In early 2007, during renewed exploration Spirit's wheels exposed a thin continuous layer of a yellow deposit rich in sulphates:
In May of 2007 an intriguing discovery was made when a light-colored layer was uncovered during Spirit's traverse; it came to light serendipitously because a stuck wheel on the Rover dug deeper than normal as it spinned in place:
A mini-TES analysis of the layer revealed it to be nearly 90% silica. This was not quartz sand but a fine crystalline mass or perhaps amorphous. Its specific nature has not yet been determined but speculation centers on alteration of the material by acidic vapors or fluids, leaving the silica as a residue. The silica deposit is similar to that found in thermal hot springs associated with volcanism on Earth, and as such is an indicator of aqueous activity. This next image, with a different color shading, shows the white material to better advantage:
As martian spring began in 2009, Spirit has been reactivated (its was parked in 2008 during the martian winter), and has backed away from Home Plate to explore elsewhere. It has started towards the Columbia Hills but as of August 2009 has been stuck for a while in loose sand. Efforts to get it unstuck have so far failed. Although efforts continue to pry it free, the consensus in 2010 is that Spirit is now permanently trapped in its present location.
Thus, the history of exploring the Gusev crater site and beyond was at first suggestive of the absence of any strong evidence for water activity but once the Columbia Hills were reached, possible evidence was acquired in several settings. However, the water, if verified, seems associated with volcanic activity rather than any marine or lacustrine deposition. The results from the MER Opportunity rover, which discovered minerals almost certainly of hydrous origin and has found indicators of deposition from discrete water bodies, are covered on the next page (19-13b).