With this mission, to take our exploration for past or present habitable environments and life on Mars to the next level, we will need to respond to discoveries made by the present-decade missions. Under the existing strategy, after MSL, potential science pathways diverge, contingent on what we find out.

I'm going to run through several different scenarios described in a report entitled "Mars Exploration Strategy 2009-2020." This report, published last year by NASA's Mars Science Program Synthesis Group, headed by Dan McCleese, identified a number of alternative exploration pathways. The pathway we follow will come out of what we're learning now.

Continue the Search for Evidence of Past Life. This is an approach we might follow if the present-decade missions going on now confirm that ancient Mars was habitable for extended periods of time.

By this time it is assumed we will have identified a whole bunch of places where we can go to look for fossil biosignatures in ancient rock sequences. I think we've already gotten indications that we might be, at least in part, on this pathway.

Explore for Ancient Hydrothermal Habitats. The discovery of modern or ancient hydrothermal environments in the present decade, might lead us to send missions to this very specific, and highly regarded environment.

Search for Present Life. If we discover modern habitat, say active hydrothermal systems capable of supporting life, we would have to start thinking seriously about in situ life detection and Mars sample return, which is perhaps the most reliable way of detecting life and understanding it. Of course, this discovery would also bring along important planetary protection issues to address: how to mitigate the risks of forward or back contamination.

Explore the Evolution of Mars. This is a pathway we might follow if in the present decade we failed to find evidence for past or present liquid water environments, at least long-lived environments that are capable of supporting life. I don't think we're on this pathway any more. I think we've obliterated this possibility with the results from recent missions.

The nice thing about this pathway analysis is that it was done considering cost constraints, to make it as realistic as possible. For example, for the "search for evidence of past life" pathway, there's a set of missions going out to 2020. This pathway has a Mars sample return in 2016. It's bounded on either side by Mars Scout missions, so that we can afford to do that, because this kind of initial sample return is going to be expensive. Under this scenario, deep drilling comes in 2020.

For the "explore for hydrothermal systems" pathway, there is an alternative set of missions. If we found hydrothermal systems, we'd have a mission in 2013, called the Astrobiology Field Laboratory, that would look for biosignatures in rocks. And out in 2018 deep drilling is identified as a possibility, particularly if we're trying to get into subsurface hydrothermal systems.

The discoveries that have been made just in the last year have somewhat outdated the pathways document. So the community is organizing to develop another set of scenarios that will take into account the discoveries that have occurred, so that the options that we have on the table are truly discovery-driven.

So, to summarize, what have we learned? Mars has had a prolonged aqueous history with widespread surface water during its early period, and hints of a subsurface hydrosphere through much of its younger history, possibly up to the present time. What do we need to know? We still have a lot to learn about the availability of basic nutrients and energy sources in both surface and subsurface environments, essential information for reliable assessment of habitability.

What are the steps we need to take to move toward extant life detection? Ultimately, that's what we're really shooting for. One vital step is to provide flight-ready instrumentation for definitive in-situ life detection at Mars. The astrobiology community is just starting to awaken to the need to develop new approaches to life detection and get them on the discussion table, and to mature those instruments very quickly for flight readiness.

One study has shown that it takes about 8 years from concept to flight, to get an instrument to Mars. So if we want to be out there in the middle of the next decade with a life-detection instrument, we have to be developing that instrumentation now!

We also need to obtain a more thorough understanding of the potential for forward contamination of Mars and how to mitigate against false positives. Carrying some level of "bio-load" with us to Mars is probably unavoidable. We have to know how to deal with that. The whole idea of planetary protection, whether forward to Mars, or backward to the Earth, is still in its infancy as far as actual implementation is concerned.

We need to conduct the first in-situ life detection surveys on the surface of Mars at locations that have proven to be past or present habitable environments. We're still identifying the places to go. Where we want to go may be very difficult to get to and missions will probably need to be specifically designed to go there.

That's technology that needs to be developed as well. If we're going to the subsurface, we have to develop sterile drilling methods to search for subsurface ground water, biochemistry and life.

Eventually, we want to undertake targeted sample returns from high-priority sites where we think we may have detected life, so we can characterize that life in Earth-based labs. But that introduces another whole set of problems, because of the potential for planetary back contamination.

So these are things that have to be dealt with. Now, we have a new presidential initiative. The idea is to get humans to the moon by 2020 and use that as a stepping-stone for humans to Mars by 2030. We'll see what happens. But perhaps one of the most important scientific justifications for sending humans to Mars may be the exploration of the deep subsurface.

It may simply prove impossible - it seems very difficult right now - to drill to multiple-kilometer depths with a robotic system. Maybe that will change, but there will need to be a lot of technology development. It may be much easier to send humans to Mars to run drilling rigs. But we have to be very careful about what we're doing, because the back-contamination issues are really a problem when you talk about human exploration. How do you decontaminate an astronaut?

So there are lots of things to consider for the second decade of exploration. But we're on a path, a phased program of exploration. We've had very interesting discoveries and I think we're going to have a lot more. MRO is going to open a lot of eyes with this new data that we're going to get, data complementary to Mars Express. So I'm very excited about the future of Mars exploration, and I hope you are too!

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