Curious about Life: Interview with Jeff Moersch
The Mars Science Laboratory Curiosity rover has 10 science instruments, and each will be used in the coming weeks and months to help characterize the environment of Mars and determine if the planet ever had the potential for life.
Donated by the Russian Federal Space Agency, the Dynamic Albedo of Neutrons, or DAN, instrument will search for water underground without any need for digging. Jeff Moersch, at the University of Tennessee-Knoxville, is one of the American scientists working on DAN. Astrobiology Magazine spoke with Moersch at the start of his Mars day, at 2 pm local time, when he was just waking up.
How has working on Mars time affected you?
Right now Mars is about 180 degrees out of phase with Pasadena. It’s good if you’re a night owl, if you’re a person that tends to stay up later and later, which I am, but morning people have a really hard time with it.
Mars rotates about 40 minutes per day slower than the Earth, so we stay up 40 minutes later each night than the previous night, and get up 40 minutes later than the previous morning. I think it takes about five weeks to walk around the clock on the Earth completely, one full cycle, so that we’re back where we started.
In a few weeks, all of the morning people will have the advantage. When we’re getting up at four or five in the morning and we’re working until noon or so, that’s pretty rough on me.
What kind of science or research do you usually do?
I’m a Mars researcher. I specialize in remote sensing. This is my sixth Mars mission; a lot of my work is with missions. Other than that, I do field work on Earth. I do a lot of terrestrial analog work, places like the Atacama desert and the Andes in Chile, the Mojave desert, and Death Valley.. I also have an interest in instrumentation, so I’ve had some grants to build prototype instruments for missions.
So what’s your favorite terrestrial analog to travel to?
Well, it really depends on what you’re trying to study. There’s no perfect analog, so you’ve got to pick which aspect of it you’re trying to replicate and then select from there. I really like working in the Atacama a lot. It’s a very unspoiled desert in a lot of the places we go. Death Valley is fantastic, too, of course.
You said this is your sixth Mars mission, so what is your favorite Mars mission?
Favorite Mars mission? I don’t want to play favorites. Right now, this one is full of potential, I would say, but the one I worked on before this was demonstrated potential. That one was just a blast. That was the Mars Exploration Rovers, Spirit and Opportunity. This one, I think, will get there. It’s kind of slow at the beginning, because we take a while to do all the instrument check-outs. Right now, they’re checking out the arm. The site we’re at is spectacular. I think once we really get going, this will be just as exciting as Spirit and Opportunity were.
That’s one thing that I think the general public have a hard time appreciating is how slow this process can be. We land and they expect to see instant results on the news that night, and it just does not work that way. You have to be very patient.
What do you do, specifically, with MSL?
I’m working on an instrument called DAN, which stands for Dynamic Albedo of Neutrons. It’s an instrument that was contributed by the Russian space agency, IKI. You can think of it as a hydrogen detector for the subsurface. As we drive along, the instrument measures the energy of neutrons leaking out of the ground. This tells you how much hydrogen is underneath the rover. You can think of the detector footprint as being a hemisphere, about two or three feet in radius, right underneath the rover. So as we drive, we stop, make a measurement, stop, make a measurement, etc., and in doing so, we build up a profile of subsurface hydrogen abundances along the traverse route.
The general idea is that this instrument is sort of a “sniffer.” If we find something truly anomalous—if we find a significant excess abundance of hydrogen—then maybe that’s a good place to stop and use the rest of the instruments on the rover. We don’t call DAN a “water detector” because it can’t tell the difference between hydrogen that’s bound up in H2O versus hydrogen in minerals such as hydrated sulfates or clays. But all of those minerals have something to do with water. That’s a useful piece of information that contributes to the overall science of the mission.
There are lots of hydrogen-bearing minerals. The more common ones that we might expect to find in this location on Mars would be clays, which have a lot of H2O in their structure, and also OH. There’s also hydrated sulfates, like gypsum, that have H2O in them. Zeolites have water in them. There’s a whole range of minerals that form in liquid water that end up incorporating H into their structure.
DAN has two operational modes: an active mode and a passive mode. Passive mode works the same as neutron detectors that we have had in orbit around Mars before. It relies on the fact that cosmic rays are zipping around in space. Cosmic rays are very high-energy particles, mostly protons, traveling at some appreciable fraction of the speed of light. They come from the Sun and from the galaxy. On a planet that has a very thin atmosphere like Mars, they rain down on the surface. On Earth they don’t make it to the surface because the atmosphere is thick enough to stop them. On Mars, they actually make it through the thin atmosphere and bombard the surface.
When they get to the surface, these high-energy protons and cosmic rays will strike the nuclei of any atoms that are in the surface, and this really gives them a wallop. When that collision occurs, it knocks neutrons out of the nucleus in a process called spallation. These neutrons are moving very fast, and they rattle around, bouncing off the different nuclei in the surface. If the nuclei that they bounce against are heavier than neutrons, then every time a neutron bounces against a heavy nucleus, it just bounces back off at about the same speed that it came in at. But if the nuclei the neutrons bounce against are of comparable mass to the mass of a neutron, then it’s kind of like a pool ball hitting another pool ball: every time there’s a collision, the neutron gives up about half of its speed. If this happens enough, the population of neutrons that was initially going very fast can be significantly slowed down, or “thermalized” by the time the neutrons leak out of the surface, where they can be measured by DAN.
Chemically speaking, the only nucleus that has a mass that’s comparable to the mass of a neutron is a hydrogen nucleus. It has one proton in its nucleus and the electron weighs almost nothing. The next element after hydrogen is helium, and that’s four times the mass of a neutron. Really, when you look at the energy that a neutron is leaking out at the surface, you’re getting a measure of how much hydrogen was there in the subsurface to slow it down. You can actually detect the energy of these neutrons and say that the more slow neutrons you see leaking out of the surface, the more hydrogen is there.
We also have something that’s unique in this instrument that we don’t have in orbit: we brought with us a tiny little particle accelerator. It’s about the size of a shoebox. It’s pretty neat. I actually bought one to do experiments. They’re made in Moscow. There are two normal uses for these things. One is as triggers for nuclear warheads, because neutrons are needed to set off nuclear bombs. The other thing they use them for is in oil wells. They put these little particle accelerators down the oil well to generate neutrons, and then you look at the leaked neutrons coming back out to tell if there are hydrocarbons down there, because of course oil has hydrogen just like water does. So these are used a lot by the oil companies.
So this accelerator on DAN sends out a very short— one microsecond— and intense pulse of neutrons. Those high-energy neutrons get shot into the ground, and they leak back out just like the ones that are made by the cosmic rays through spallation. Except in this case, you can look at the timing of the slowed neutrons that leak back out relative to the time of the pulse. What that gets you, in addition to the bulk hydrogen abundance, is the vertical distribution of hydrogen. The neutrons that come back first are from the shallow layers, because they’re closer and they don’t have to travel as far. The ones that come back later are the ones that are coming from deeper.
So it’s the same idea. You’re still using the slowdown of the neutrons to figure out how much hydrogen is there, but we get a crude layering idea of how deep the hydrogen is buried in the top meter of the soil. That’s something you can only do if you are close to the ground. You can’t do that from an orbiter because the generator would be too far away and you wouldn’t get an appreciable number of neutrons making it to the ground. With the rover sitting right over the surface, we can pulse the accelerator and we can watch the neutrons as they come back, and get a decent idea of how the hydrogen is layered below us.
It’s a neat experiment. It’s pretty clever. I take no credit for the design. That was all the Russians.
In November, NASA selected a bunch of additional scientists for the mission, including myself. There’s three or four of us American scientists, who are working with this Russian-contributed instrument. The Russians have their own team, and we’ve been brought on to work with them. We help them interface with the rest of the spacecraft being run out of JPL.
How could your work help to answer astrobiology questions?
Well, these minerals that I was talking about, like clays or hydrated sulfates, they all form in specific environments, environments that have an aqueous history. One of the main goals of the mission is to search for potential habitats for past life.
It’s not a life detection mission. It’s not likely to identify actual life or fossil life. What it’s designed to do is look for evidence of places that would have been hospitable to life. When we find these minerals that form in the presence of liquid water, that’s one of the key ingredients you would need in a habitat. Finding these habitats addresses the overall mission goals, which are very astrobiologically oriented.