By Paul Rincon
BBC News Online science staff
Nasa's Spirit rover is gearing up for a challenging ascent on Columbia Hills, the high ground it will explore for clues to the history of water on Mars.
The rover has been examining unusual rocks at the base of the Columbia Hills
Spirit is undergoing a "tune-up", a kind of training regime to prepare it for the climb.
Its twin, Opportunity, is exploring inside the 130m-wide Endurance Crater, after which it will probably head south to the mysterious "etched terrain".
Both rovers will soon begin preparing for the harsh winter on the Red Planet.
Spirit is currently at the base of Columbia Hills and is exploring some unusual rocks which have been given names such as "breadbox" and "sourdough".
"What we think is that these started out as basalts, but they've been corroded by groundwater. For example, they show these little balls on stalks," Dr Ray Arvidson, deputy principal investigator for the science payload, told BBC News Online.
"The balls we think used to be holes or vesicles in the volcanic rock, where chemical precipitate formed from groundwater. The surrounding rock has been eroded out, so you wind up with this inverted topography."
Dr Arvidson said there were plenty of analogous geological formations on Earth.
"In the western desert of Egypt, we find these river valleys or wadis that are actually curved hills on the surface," he explained.
"The interiors of the valleys filled with spring deposits that were harder than the surrounding chalk. The wind blew away the chalk and left the curved mounds."
The rover's analysis of the Martian rocks also shows that they contain sulphur, chlorine and haematite - a chemical signature that suggests they were altered by water in the past.
Time for bed
Spirit's tune-up involves a series of diagnostic engineering tests.
Firstly, scientists are using the instrument arm as a guide to gain precise information on the orientation of the hazard avoidance cameras, which are thought to have moved with the changing temperatures on Mars.
Second, Spirit is doing visual odometry, which involves taking pictures of the rover's surroundings to update its knowledge of where it is.
Third, the rover will be put into deep sleep mode, which involves turning all systems off at night and relying on the Sun to wake Spirit up.
Opportunity continues to examine rocks in Endurance Crater
Fourth, scientists are trying to heat and rotate a "sticky" right front wheel to distribute lubrication. This wheel continues to draw roughly twice the current of the other wheels.
After this is complete, the rover will drive north to the top of the western spur before turning east. Spirit will tilt its solar panels toward the Sun to soak up as much energy as possible and continue to do remote sensing as it drives uphill to an outcrop.
By sol 240 (the number of Martian days the mission has been on the planet), ground controllers will be looking for a place near this outcrop for Spirit to bed down for the winter.
Viewed from above
Meanwhile, rover scientists will have to decide whether to keep Opportunity in its crater during the Martian winter, or drive to the etched terrain.
This site, to the south of Opportunity's present position, contains large exposures of the bright bedrock the rover has been examining in craters at its landing site on Meridiani Planum.
These rocks seem to have been deposited as sediments in a shallow sea, which evaporated to form chemical precipitates. They were then reworked into so-called cross-beds in the layers of sediments.
After this, ground-water seeped through the rocks, forming the characteristic concretions, or spherules, which drop out of the rock as it is eroded.
The etched terrain lies south of Opportunity's landing ellipse (in white) (Malin Space Science Systems)
"Presumably, the etched terrain carries more of these sulphates and other salts that formed during the evaporation period of that shallow sea," Dr Arvidson told BBC News Online.
"We've been looking at some of the spectral imagery from the Omega instrument on the orbiting Mars Express and the etched terrain is very busy spectrally. We can go downhill and see the strata that are deeper in the section.
"We don't know yet if the materials we've been seeing at Eagle and Endurance Crater are representative of what's in the vast etched terrain. That's why we'd like to drive south to investigate."