On Tuesday March 13, NASA's Curiosity science team announced that the Martian rover had found the first confirmed site other than Earth where conditions were right to have once hosted ancient life, if it ever evolved on the red planet.

John Grotzinger, project scientist for the Curiosity mission, offered his perspective on the rover's journey of exploration and the historic find at the John Klein drill site.

A lot has happened in the seven months since Curiosity has landed. How does it feel to have accomplished so much in that time?

It feels terrific. I think our team has a really strong feeling of accomplishment. Everybody's worked really hard. We had a year of preparation for surface operations. Before we landed, we were able to get the orbiter data and do advanced mapping of the landing ellipse, that no matter where we landed, we would have an initial guess about where we might want to go. All of that preparation paid off.

We always felt that the placement of the landing ellipse would put us in a good position to have potential for discovery. We never really felt that we had to race out of there right away. Between the particular place that we ended up landing and having done all that preparation, we were really in a good position to have a rapid path of discovery.

However, you don't know what you've got until you see your cards, and so until that, we've been nervously waiting for the drilling to happen to see what we would get into CheMin and SAM. It worked out just about as well as we could have hoped for.

You mentioned that on reaching the site John Klein, the discoveries that were made were not serendipitous. They were not accidental or luck, but they were very deliberate in terms of actually getting to that point.

Yes, finding the right geological place was something that we did very deliberately, working with the geological model that we began to develop before we landed, that we added to after we landed with the discovery of the ancient pebble bed. All the signs were directing us towards this area.

Now, what was then serendipitous was the discovery of the clays and the sulfates. We would have been happy with either one of them, let alone both of them occurring simultaneously.

Would you have come up with different conclusions if you had found only one rather than both?

It's possible and it depends. The clays point to a neutral pH environment. The calcium sulfate could be consistent with a variety of pH's, but I think together it really adds up to a strong story for the habitable environment.

This is the first definitive habitable environment outside of Earth. Would you like to speak to that?

It feels pretty great. That's always been the goal. Before Mars, we've been getting closer and closer all the time, and we've known that the very ancient [terrain] is the place to go to. We've done a decade of mapping from orbit and we've tried places on the ground with previous rovers. All of this has been adding up to an increasingly positive situation that we've now been finally able to demonstrate.

In principle, this is an ideal kind of habitable environment for microbes, so we feel really good about that. It's the kind of thing that you look at and you realize as a team that we really have been able to do something pretty profound. We benefited from all those that came before us. We had state-of-the-art equipment and we had an incredibly capable rover with what seemed to be a highly improbable landing configuration. So we took risks where we needed to take risks and they were always what we viewed to be relatively small risks, but being aggressive in that kind of exploration has paid off and we feel really, really good about it.

I think it's kind of obvious how the orbiters have helped you with the mapping and pegging sites. How did previous rovers and the work that they've done help to lay the foundation?

Opportunity showed us the vastness over which water can be active but it also showed us that chemically and mineralogically, just water alone isn't enough. The environment at Meridani, which turned out to be a subsurface groundwater environment, was probably very acidic. It was probably extremely salty and we don't see the chemical energy that we have at John Klein. So it provided a calibration point for the orbiters that were trying to map the sulfates. Because we had an instrument on Opportunity that was able to confirm the presence of sulfate minerals, we were then able to do a cross-correlation between the surface and the orbiter. Thus the orbiter was able to do a better job of mapping.

Then with Spirit, at Gusev Crater, it took years and years and years of exploration to finally find something that was really good. When it did, it was very encouraging exploring the much more ancient part of Mars. We saw what looked like a hot spring deposit there that we weren't able to do the full chemical characterization on, lacking instruments like CheMin and SAM, but we saw that water was able to exist in a different, more promising type of environment than it had existed in Meridiani. Now here at Gale, we're exploring something that looks like an ancient river and lake type environment.

Could you kind of touch on why John Klein has good preservation potential for organics?

When you see the reducing compounds and the green color, it's an indication that, all other things being equal, you've got a better environment for preservation of organics than one in which all the minerals are red, which means they're more oxidized. If you introduce oxygen, at least in a chemical way, it can break down organics. That's why I said that really the important thing, the learning point for us going forward as a community, including the media, is that there's three parts to this preservation problem. It's not just one.

Initially, the issue is that of concentration of organic matter in the primary environment. The second thing is what's going on chemically during the conversion of sediment to rock -- what we call diagenesis -- where lithification occurs. That's where the color is relevant. If you have less oxygen available, then you have a better chance to preserve organics, but that presumes that there is something there to preserve to begin with. Then the third part is that, even if all that goes right and organics had accumulated and you also have the right colors, chemicals, and minerals, if you then expose the surface to radiation for a couple of billion years it can break those organics down.

All three of these things are important for preservation. The good thing for us is that, looking at that grayer color and finding those clay minerals and seeing iron in a not-so-oxidized state helps, but it's not the only thing.

One of the comments made during the press briefing was that, since there were four potential landing sites, there was a 75% chance that the wrong one was selected.

To be clear, the others could have also paid off. There were four final landing spots and we picked one that we thought was best for our payload. But with that we took a little bit of a risk, because within the landing site there was no evidence for sulfates or clays from orbit, which we considered to be potential leading indicators of not just water but also potentially the kinds of habitable environments that we would like to find. If we wouldn't have gone to Gale, that's not to say we wouldn't have found those things elsewhere. All four of the landing sites were known to contain clays at least in one place that would have been accessible to the rover.

Gale just seemed to offer the greatest diversity weighed against the risk that there was no signal in the landing ellipse that there were clays or sulfates there. We were willing to accept that risk. Gale Crater is full of rock, but the reason you don't see a signal from orbit is because it's got a thin coating of dust. That turns out to be a real problem for the spectrometers that look from orbit. Even a few microns-thick layer of dust is enough to prevent the signal from being seen.

So there could be other sites that have clays then that would be hidden from orbit.

Right, yep.

In preparation for solar conjunction, when the sun stands between Earth and Mars and you can't communicate with Curiosity, what kind of things will the team be doing?

We as a team will try to focus on getting more SAM and CheMin results. But mostly it will be the engineers working with Curiosity to make absolutely sure that she'll be safe during conjunction, while we have no ability to communicate.

There was a big hoopla over your NPR interview back in November. How did you feel going into that?

It was just a simple misunderstanding. My enthusiasm was about the proven capability of the payload.

Once you see an instrument as complex as SAM have everything work on it perfectly for the second time, that's when you feel really good about the mission. I
believe that, even without the results that we announced that, between the landing and the ability of the rover that was doing as well as it did and that all this sophisticated instrument payload technology was working as well as it has, that this would be historic.

It means that we as explorers can continue to do this. Even if we didn't find the stuff that we had set out to discover, you can at least turn around and say we have the capability to do this at one of the other landing sites.

Gale was a site that the team was just really happy with. It was one that we all embraced with very strong consensus as a place that harbored a lot of potential, though we didn't think that we would know about it this soon.

I think you look at something as complicated as this mission, and when you see it all working, that's what makes you feel like it succeeded.

What would you be most excited to see or discover on Mars with Curiosity?

Well, this is it. I feel at this point the rover is not going to ride off into the sunset. We're going to continue to be as aggressive and as focused and determined as we've been in the past to keep exploring.

At this point it would be an issue of what additional things we would like to see. Geologically speaking, we as a science team see that the base of Mt. Sharp has different ancient environments. We have geologic evidence that suggests there are things there that are different than they are here, and I would like for Curiosity to discover as many potentially different habitable environments as possible. So we have more to go.

Then of course, this is one that you can always hope for but you have to temper it with realistic expectation, and that is to find more complex organics.