In a four-part interview with Astrobiology Magazine Managing Editor Henry Bortman, conducted just before Stoker left for Spain, she explained what MARTE hopes to accomplish. In this third part, Stoker described how the MARTE team avoided contaminating their drill-core samples.

Astrobiology Magazine (AM): When you drilled last year at Rio Tinto, how did you know for sure that your samples weren't contaminated with organisms that you brought down from the surface on the drill or in the drilling fluid?

Carol Stoker (CS): We had a lot of procedures to control contamination. There's always a suspicion that what you're seeing is something you brought with you, contamination from the surface. That's especially important in doing a subsurface search, because you're drilling with fluid and the fluid is filthy.

It's literally mud - it's called mud. It's water with solutes placed in it to increase its density so that its density is more matched to the rock that it's in. When you're drilling at any significant depth, you need to match the density of the fluid you're using to the density of the rock that you're in.

Otherwise you can get pressure differentials that cause the hole to collapse, or things to start squirting out the top, or the drill to get blown out of the hole. Bad things happen.

There's no way you can sterilize this water. We're literally using a drilling-fluid truck full of water every couple of days. Fifty thousand gallons (nearly 200,000 liters), an Olympic-size swimming pool full of water, was getting dumped down this hole every couple of days.

The problem with this is that you're dumping huge microbial loads down the hole to do the drilling.

And the number densities of in situ bacteria in the subsurface are nine orders of magnitude or so (about a billion times) lower than what you get on the surface, so in order for you to see anything down the hole that you didn't bring with you, you have to be very, very clean.

The way that you do that is you put contamination tracers in the drilling fluid, and then you look for those contamination tracers in the rock that you extract.

You come out with a 4-inch-diameter (10-centimeter-diameter) core and then you subsample out of the center of that to get a sterile sample.

You assume that the edge of the core has been exposed to drilling fluid and some amount in from the edge, probably, and you hope that it isn't saturated with fluid all the way to the center. So we put tracers in the fluid and then searched for those tracers in our cores. Then we discarded everything that contained the tracers.

AM: Where do you find water for drilling fluid on Mars?

CS: You don't. On Mars you either have to use a different drilling fluid or you have to do dry drilling. Our robotic project can do dry drilling. And dry drilling on Mars is not impossible in principle. The difficulty comes in transport of cuttings. The fluid is just a way of lubricating the cuttings to get them out of the way.

If you don't get the cuttings out of the way, they end up becoming a source of friction that burns your bit up. Another problem is that, on Earth, you pick your drill it for the kind of rock you're in, and if you get into a different kind of rock you may have to change drill bits.

Or you wear out drill bits. And dry drilling is going to wear out drill bits faster than wet drilling. So these are problems that really need to be solved technically.

The nice thing about these dry drilling systems is that they are effectively sterile drilling systems. They're not really sterile in the sense that you aren't sterilizing your drill bits, but you can easily run them in such a way that they would self-sterilize just by getting hot enough.

But the point is that you're not pumping drilling fluid down the hole, and so you're not bringing a whole huge amount of contamination with you.

So on Mars you're not going to use drilling fluid, at least you're not going to use water. But for deep drilling you might conceivably want to use fluid. And an interesting candidate fluid would be liquid CO2, which you could make by compressing Mars air.

AM: I think we'll leave that for another day. You're not going to be doing that in Rio Tinto, I presume.

CS: Not yet.

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