Steve Squyres, principal science investigator for the US Mars rovers, Opportunity and Spirit, tells BBC News Online about the ongoing mission to explore the Red Planet.
Prof Squyres says the mission has been immensely gratifying
Why are you convinced the rocks at Opportunity's landing site were once drenched in water?
SS: There are several lines of evidence. The spherical objects found in the rock outcrop. In all of their characteristics: their shapes, their sizes, their distribution in the rock and their composition - which we now know is dominated by haematite - they appear to be concretions which are things that form in rock when liquid water has stuff in it that precipitates out, finds a nucleation site and begins to precipitate in concentric layers.
Another piece of evidence is that some parts of the outcrop are shot through with these little holes - tabular-shaped holes within the rock. They are 1cm long, 1mm or two wide and at all kinds of crazy angles through the rock.
And those are the kind of things that commonly form in rocks on Earth when you have growth of tabular-shaped crystals - particularly some kinds of sulphur-type minerals like gypsum will do this; where they will grow in one place and then later they either get eroded away or dissolve away and they leave behind these little holes.
The next observation which I think is pretty compelling is the huge amount of sulphur in this rock. And that's very hard to explain with any process not linked with water.
Finally, with our Moessbauer spectrometer, which looks at iron-bearing minerals - we have been able to look at the rock and confirm there is a sulphate salt there called jarosite. And jarosite is an iron sulphate hydroxide. It's got water in it and requires water for its formation. You take those pieces of evidence together and you are led pretty quickly to the conclusion that water has interacted in a pretty fundamental way with this rock.
What does this tell us about possibilities of life?
SS: These rocks clearly indicate there was liquid water here and an acid groundwater environment because jarosite is a mineral that requires fairly acidic conditions - low pH - for its formation.
And an acid groundwater environment is one that is suitable for some kinds of organisms. The other thing is that with minerals precipitating out of a solution, those minerals as they precipitate can trap chemicals and organic materials and whatever else might be in the water at the time that the precipitation takes place, providing a very good long-term preservation mechanism.
The spherules seen by Opportunity are an important water clue
So if we can get our hands on some of these rocks and get them back to Earth and a laboratory, I think they would have a very interesting story to tell.
So are you hopeful life might be found there?
SS: I wouldn't use a word like hopeful. One of the worst things you can do in science is to hope for a certain outcome because if you start doing that you can start to skew your judgement and threaten your objectivity.
So I will simply point out that if there was biology going on in this environment, these rocks might have been pretty good at preserving it. It would be a great location for sample return.
What is this mystery in the Mini-Thermal Emission Spectrometer (Mini-Tes) data from Adirondack, the rock that has been investigated by the Spirit rover?
SS: In the Mini-Tes spectra of some of the rocks at Gusev, we are seeing the spectral signature of some mineral or minerals that we simply don't recognise - something that we don't have in our spectral library.
So we have to give it some more thought, do some hunting to see if there are some less well known minerals on Earth that have a spectral signature like that. But right now, we don't know what it is.
So it could be a new mineral?
SS: It's possible I suppose; it is a different planet.
The rovers had a specified drive capability of 600m. How confident are you that they will exceed this and even do a couple of km?
SS: I think the chances are pretty good. The 600m number was a design target for pretty difficult terrain. Of course, we had no idea before we landed how navigable the terrain was going to be.
We tried to come up with a design that over the course of a 90-sol mission had a reasonable chance of going 600m in really tough terrain.
Spirit may have discovered a new mineral on the Red Planet
As it turned out, the terrain was much more navigable at both sites, especially at Meridiani; but at both the terrain is easier going than we designed for, which is good news.
And then the vehicles are looking like they are going to last a lot longer than 90 sols so you can pretty much forget about the 600m number.
You've done some projections which show the rovers could work for at least 240 sols. But you have raised the spectre of cost - both to Nasa's budget and to the scientists. Would Nasa consider euthanasia? Would it kill off healthy rovers?
SS: I have no idea - that's a Nasa decision. There's plenty of science for us to continue to do and I think the rovers will continue to hold out for a long time.
It's been a gruelling schedule for you. Would you keep going?
SS: Yes it is. I don't think any of us anticipate continuing to work on Mars time for periods longer than 90 sols. We are already trying to transition to an easier schedule, something that's more sustainable in the long term and that includes trying to shift most of the team to Earth time. It's not done yet, but we're in the process of beginning it.
Don't you just want to get your hands in the dirt?
SS: Well we are on Mars and the fact that we are actually doing real field geology on the surface of Mars goes a very long way towards making up for any frustration I may feel.
If they were launching a manned mission tomorrow, would you sign up?
SS: Sure absolutely. I think ultimately we should be sending humans to Mars and the sooner we do it the better but I think there are some pretty tough technical obstacles to get over first.
Would you lead another robotic mission like this again?
SS: I would not do it again - absolutely. Doing a mission like this requires an enormous amount of effort and commitment and more than that it requires a tremendous amount of sacrifice. It has required a tremendous amount of sacrifice on the part of my family and my goal for future missions is to find roles where I can have a lot of fun but not so much responsibility.