As Spirit closed in, the rounded nature of these hills becomes more apparent, as shown in the above images. From these images, one might get the impression that they are rather high. In actuality, the highest point is just 90 meters above the edge of the float block apron. This MOC image shows that these Hills are just a "blip" on the surface inside Gusev Crater. Nevertheless, they may reveal the nature of materials in the pre-impact surface.
This next image is a perspective of the Columbia Hills made from the same MOC data. It shows the path being followed at West Spur by Spirit during its explorations.
West Spur was examined in early June of 2005. Spirit then moved to Lookout Point, and then ran over a low pass into the Columbia Hills Inner Basin,
As these hills were neared, Spirit stopped to trench the outer slope sediments and found suggestive proof of water action. The trench showed varying amounts of Mg and SO4 (expressed as sulphur), with signatures suggesting possible Kieserite, a magnesium sulphate mineral present in abundance at the Opportunity site. Note this plot:
Spirit spent considerable time on the Columbia Hills, visiting a number of outcrops. This view shows inconspicuous, non-definitive evidence of layers at West Spur.
However, the loose float around this spot was indeed bedrock of a blocky nature. Some rocks were more slablike, possibly of a sedimentaary nature. Instrumental measurements will zero in on the question of their origin. Here are two scenes containing rocks.
Pot-of-Gold, the flat rock in the image above, may also be layered. Inspection with the Microscope Imager has revealed rather peculiar nodules, some at the end of tiny columns.
Mössbauer (spelled in label as Moessbauer to obviate use of 'o umlaut' in German) spectra for Pot-of-Gold shows the presence of Fe+2 peaks attributable to olivine and pyroxene, and four extra peaks representing Fe+3 which is best explained as hematite. The rock seems to be a basalt that has been altered to some extent to hematitic iron oxide.
While on West Spur, Spirit got its first good look at thin dark layers outcropping midst the regolith, rock fragments and small boulders. This is likely to be older basalt flow rock brought up from the crater floor during the Gusev cratering event.
A lighter-toned outcrop, dubbed "Clovis" rock, was found on West Spur. It is currently being analyzed.
APXS plots for a basaltic rock (Humphrey) and the Clovis rock show one major difference: At least some of the Clovis rock contains Magnesium, Sulpher, and Bromine - indicators of either a sedimentary unit or alteration that includes Magnesium Sulphate (Epsom salts).
Results from a processing of Mossbauer data from Clovis has brought to light one of the most rewarding discoveries yet at the Meridiani site. The mineral Goethite, hydrogen iron oxide, HFeO2, containing 10% water by weight has been identified, as shown here:
On Earth, Goethite - a very common mineral associate with Limonite - is found as an alteration product or as a direct precipitate in the so-called "bog iron" deposits, which result from a reducing, water-rich swampy environment. That form of Goethite is usually produced with the aid of bacteria but can also form inorganically. The mode of origin of the Clovis Goethite is still "up for grabs" but the presence of this mineral suggests a significant role for water in Mars' past.
Of renewed interest in the possibilities of significant layered rock units is this view looking further from the West Spur position.
The first close-up view of layered rocks is the detached block named Tetl, shown here:
Tetl comes from an outcrop area that consists of distinct layered units, seen in this composite image.
As Spirit drove off from the Columbia Hills it encountered in the soil and rock fragments a pronounced white encrustation that looked much like a crystalline coating. Chemical analysis show this to be mostly magnesium sulphate. Whether formed by surface water evaporation or by groundwater seepage containing dissolved salts, the discovery is the best evidence yet for the onetime activity of water at the Gusev crater site.
Despite "limping" along with two of its six wheels having problems, Spirit has visited more individual layering features - outcrops and loose rock. Uchben is an example where there is some evidence that the layering is caused by volcanic ash deposition. Microscope Imager views of surfaces close-up show angular flaky particles similar to those occurring in terrestrial ash deposits. Water has been found in its composition: the water may have been in the volcanic material derived from the interior, or may have been present in later altering hydrothermal solutions, or could have been incorporated from a standing body of water once at the site of the Columbia Hills (probably before their present uplift). So far, deposition by direct ashfall, by wind action, or in water all remain possibilities:
Because of wheel problems Spirit has been more cautiously guided. By disabling its front right wheel, and proceeding slowly, Spirit has covered much of the first ridge in the Columbia Hills, has moved over West Spur, and has moved into the flats:
As Spirit traversed the West Spur, it encountered a rock nicknamed "Wishstone" whose appearance enticed the JPL team to decide to investigate it in some detail. The RAT was used to cut into its surface. The next two images show Wishstone (near center) after the abrasion hole was produced and a close-up of what was exposed:
The exposed interior shows that Wishstone has a granular texture. A few dark fragments are noted. The texture is consistent with either a volcanic tephra deposit or water-deposited fine particles; the interpretation remains ambiguous. The APXS instrument produced this plot of elements detected. The most significant feature is a higher than average (using West Spur results as reference) amount of phosphorus. This points to some mode of aqueous activity affecting the rock before it left its initial location (by impact ejection or by rolling over the West Spur slope).
Spirit has continued to move through the Columbia Hills visiting Rover team-named landmarks as seen in this perspective from orbit:
These hills seem to be underlain by layered rocks. Most evidence so far points to volcanic deposits, perhaps of several types such as flows and ash falls. At an outcrop named Methuselah, a rock called Keystone shows thin layers:
With one exception (shown on next page), trenches in and beyond the Columbia Hills did not disclose a light-colored bedrock unit comparable to that we shall see at the Opporutnity site. The layers seen in the Columbia Hills appear to be made up of grains of basalt loosened and transported into thin units as local deposits. The interpretation now favored has subsurface water leaching out chemicals from bedrock and soil, rising towards the surface, and depositing the sulphate salt near the surface as a cement for the granular units. This is the best evidence to date for the action of water at the Gusev Crater site; this water probably came from beneath the surface rather than from any standing surface water.