Carol Stoker is the principal investigator for the Mars Analog Research and Technology Experiment (MARTE). MARTE has just begun its second field season drilling into the subsurface near the headwaters of the Río Tinto in Spain, searching for novel forms of microbial life. 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 first part, Stoker describes the field site and discusses some of the research team’s early results.

Astrobiology Magazine (AM): You’re heading up a project that is going to be drilling into the subsurface near the Río Tinto in Spain, looking for microbial life. Where exactly is the drill site?

Carol Stoker (CS): We’re drilling in a massive pyrite deposit in southwestern Spain, in a site called the Iberian Pyrite Belt, which is this massive sulfide deposit, probably the biggest in the world. We’re drilling at the headwaters of the Río Tinto. Río Tinto is “river of red wine.” “Tinto” is both the color and a brand of red wine in southern Spain. The place where we’re drilling is a site called the Peña del Hierro, which is Iron Mountain. It’s the site of a major mine that was mined from Phoenician times – actually Paleolithic times – there were miners there in the Stone Age, mining for copper. It was mined through Roman times and then in the early part of the 20th century it was pit-mined, so the Peña del Hierro, which was a mountain, is now a pit.

AM: What is it about this site that interested you?

CS: We wanted to go into the subsurface at the source of the Río Tinto, because the Río Tinto is coming out of the ground there in little seeps. The pit crater is filled with water at its base and that water is acidic. To the south of the pit crater there are little seeps coming out of the ground, at a pH of 2 or 1. And the process that makes the acidity is thought to be a microbial process. Microbes are eating sulfide minerals and excreting sulfuric acid and protons as a byproduct.

Our project was based on the hypothesis that that kind of chemistry was being created by an underground ecosystem. So our project is about proving that that ecosystem is there: drilling into where we think it is and sampling the rock to determine whether we can find subsurface life.

Our location is on the rim of this pit crater on the Piña del Hierro. We drilled there because we could actually see the stratigraphy. It’s exposed in the wall of the pit crater and we could see a place where there were massive sulfides remaining, even though most of it was dug out. Geology, until it gets folded or faulted, starts out as layer cake. Things get piled on top of each other. But massive sulfides don’t end up following that rule because they come up in plumes, chimneys really; they are little localized towers. So we chose this site because we could see that there was massive sulfide there, and also because it was geographically related to where these seeps come out of the ground.

AM: How far down do you plan to drill?

CS: Last year we drilled 160 meters (525 feet). That got us to where we could see the mass of sulfide started and once we were in it we just kept drilling until we went all the way through it and back out below it.

AM: How many meters deep is the sulfide layer?

CS: At the site where we drilled, it starts at about 50 meters (164 feet) below the surface.

AM: And it goes down how far?

CS: Almost another 100 meters (328 feet). We popped out of it at about 150 meters (492 feet).

AM: Did you extract material and analyze it?

CS: We brought core out. It was a coring-drill operation, and the cores came out in 3-meter (10-foot) lengths. They were then put into a glove box, designed to be an anaerobic [oxygen-free] chamber. the idea was to get those cores into an environment that was like the underground environment as quickly as possible, with the assumption being that there wasn’t free oxygen in the underground environment. Then we sampled the cores at 1-meter (3-foot) intervals. For every meter of core we put into the anaerobic chamber we took some subsample out of it and then subjected that subsample to a suite of biological analyses.

AM: So you did this 160 times?

CS: Right. More, actually, because in the interesting places we took more than one subsample.

AM: And you looked for organisms in these cores?

CS: Right.

AM: And…?

CS: We found organisms in these cores. Not in all of them; they were localized. We are still in the process of characterizing their life styles, figuring out exactly what kind of organisms we have. But we found a number of locations, or hot spots, as we called them – hot in the sense that they had a microbial signature. And there was also a mineralogical signal associated with those locations as well.

AM: The mineralogical signal was the result of their metabolism?

CS: Presumably.

AM: So you don’t know yet anything about the nature of these organisms, whether they’re novel organisms or similar to ones that have been found other places?

CS: We don’t know anything yet about the specific identities. At least, I don’t know anything yet. It’s possible that my colleagues do. Because a lot of the microbial analysis is being done in Spain. The first was sort of a quick look, yes-no, is-there-life-or-not? sort of analysis, using fluorescent in situ hybridization. Then on the basis of what we saw we did a number of other types of hybridizations to try to key down the organism.

In addition to that, cultures were grown up. Some of those cultures were started in the field last year. A lot of things that you would find in soil, you couldn’t grow in pure culture. The fact that you can grow pure cultures [from your samples] gives you a lot more confidence that what you’re seeing is real and really came from your location.