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Phil Christensen

Planetary Geologist/Principal Investigator
Arizona State University
Arizona, USA

When you were a young grad student during the Viking missions of 1976, did you ever dream that, twenty-six years later, you would have two instruments at Mars and another two on the way?

No.  I got started doing Mars research.  I loved space and I loved following the Space Program but it never occurred to me that I would actually get to do it.  That's what I try to stress with the kids, "Don't think that being a NASA scientist is this weird unattainable thing, because that is what I thought of it, even in college.  As a Viking intern, I had a wonderful opportunity.  I took it.  I got to work on the Viking Project.  All through that, and even as a post-doc after I graduated with my PhD, every step of the way, this has been an incredibly fortunate thing.  As long as people are willing to keep paying me to do this, I'll keep doing it.

I certainly, as a Viking intern, had no idea that 26 years later I would be doing this.  At that point in time, people weren't sure what we would be doing at Mars in 26 years.  So most people involved said, "This is great.  I am going to do it, and then when it ends, I'll go back to my real job of being a geologist or whatever that is."  So, yeah, I would definitely have had no idea I would get here.  It has been a wonderful period of time, in that many of us who were working on Viking, have had the opportunities.  There are far more opportunities now than there were 20 years ago.  There are many missions and many opportunities for people to do this type of work.

Were there very many Viking Interns?  Did they continue in the field?

There weren't that many interns.  There were only ten of us, or something.  Bruce Jakosky was one.  Alan Peterfreund was one.  He ended up doing a PhD and went into business and educational outreach type things.  Most of the people involved have gone on to be scientists.  Not that many of them have gone on to be Mars scientists.

How did sitting in the back of the room, listening to Viking scientists, influence or inspire you?

The whole process was incredibly inspiring.  It was the first time we landed on Mars.  Everybody was totally geared up.  They were excited about this.  Fifty year olds, who had waited a decade for this, and they were excited.  It was impossible not to get caught up in that.  It was just unbelievable excitement.  Who knows what the next picture is going to bring?  Who knows what we are going to find tomorrow?  It was fun to watch a group of very bright individuals interact.  It was a remarkably professional group of people that had worked together for a while.  The leadership was great.  They knew exactly where people's strengths were and how they interacted.

Viking Landing Site Selection
Credit: Hans-Peter Biemann, 1977
It was fun to watch real professionals at work.  I was, literally, this kid sitting on the floor in these standing room-only conferences, just happy to be there.  There was actually a picture taken.  They were voting on whether or not they should land at one of the Viking landing sites.  So, they were going around the table of all of the Princpal Investigators voting and three or four of us interns were there, literally sitting up against the back wall, watching.  It really felt like you were part of history.  Like something very momentous was going on and you were there.

That experienced hooked me.  I realized right there and then that I didn't want to be a researcher sitting in a lab, chugging along writing a paper now and then.  I wanted to be involved with missions.  That's where the excitement was.  That's where the fun was.  So, it was an addicting experience and I haven't changed.

Hugh and Phil
Imagiverse Educational Consortium
Some people, like my advisor Hugh Kieffer, said, "This is great but I'm not going to do it again.  It's too much stress."  Me, I can't get enough of it.  I'll have to send missions to Mars until I die, because it's REALLY fun.  A lot of that fun is just the interaction with people, and the talent that comes together and the enthusiasm and dedication.  They're just fun people to be around.

Hugh was my mentor.  His attitude was, "I couldn't care less how you look, how you dress, when you come to work.  I work 20 hours a day.  If you like this job as much as I do, you'll want to work as hard as I do."  And people did.  I've tried to do the same thing here.  It's a wonderful environment.  You get the most talented people you can find, you put them together and you just stand back and see what happens and let people do their thing.  It's amazing, and that, to me, is how you run a group like this.  No meetings, no schedules... just: "Hey!  If you love this as much as we do, work hard and we'll have a good time."

How did you get the chance to continue in Mars exploration?  Why didn't more of the Viking interns follow that path?

It's part chain of events; it's partly how badly you want to do something.  I mean, people have to make choices.  An opportunity might mean that you have to move or uproot your family.  A lot of people aren't going to do that.  In planetary science, it has always been something like: "There a job opening in... Phoenix, Arizona... if you want it, you move to Phoenix, Arizona."  It's not, "Gee, I really like living in... San Diego... maybe I'll see if I can do planetary science from here."  So, for most people it was just other priorities that went other directions.  I really wanted to do this.  It was clear that I was going to do it, no matter what.  That is another thing I tell students, "Don't listen if someone says, 'oh, there aren't very many jobs.'  That's true, there aren't very many jobs, but if you want it bad enough, you'll be one of those who can get one of those jobs."

Did the discovery of hematite on Mars, using your TES instrument, change your position in the scientific community in any way?

Imagiverse Educational Consortium
I think from the day TES was first picked to go on Mars Observer and then later on Mars Global Surveyor, being the Principal Investigator on an investigation like this clearly raises your visibility in the science community.  When you make discoveries like hematite (or snow deposits), people get to know who you are.  The hematite was particularly fun because it really validated what the whole TES experiment was about.  Namely, let's go to Mars, look for different minerals and let's use that information to figure out where we should go land, instead of just pictures and morphology and "gee, it looked like a lake or it looked like a river bed."  It was very, very gratifying to me to say, "hey, we actually picked the landing site based on minerals," which is what I had said we could do, a decade ago.  It was a lot of validation in terms of "hey, this really did work," and I think that it paid off.

The flip side of the fun of getting an instrument is the pressure of getting an instrument.  You say, "I'm going to go spend a bunch of NASA's money on an experiment because I think it will work and here's why."  But there's on awful lot of sleepless nights wondering: "Will it really turn out?" or that I'd hear "We flew your experiment and it didn't tell us anything."  So, to me it was personally satisfying when it worked and it discovered the hematite.  Now that we're spectrally going to land there, I can breathe a sigh of relief.  I didn't waste everybody's money after all!

How important was that discovery to the Mars exploration program?

I think the discovery of hematite by TES was important for a couple of reasons.  First of all, it validated the idea of using complex spectral measurements to study Mars, learn enough and then go to those places.  Secondly, it gave us a wonderful landing site.  It's unbelievably lucky sometimes.  It is an interesting place; very likely had water; it's easy to get to.  It's safe and the engineers liked it.  We couldn't have come up with a better place.  We had something that the engineers liked and something that is extremely interesting scientifically.  It's a perfect landing site for the MER rovers.

A number of things are funny about the site (Meridiani Planum).  It's located right at zero latitude, zero longitude.  It's like this beacon sitting there.  It's very easy to remember where it is at: it's just 0, 0.  Mars could have turned out to not have these weird, interesting mineral deposits.  Or, we could just as easily found the hematite at 70N latitude and there's no way to get there.

To me, that discovery single-handedly justified the entire TES investigation.  We've discovered lots and lots of really neat things but that one thing said, "TES was worth it."

What do you expect to find when Mini-TES arrives at Mars aboard the two MER spacecraft in early 2004?

The beauty of Mini-TES is that it is really a laboratory instrument that we're taking to Mars.  TES, which is also an infrared spectrometer, looks down from orbit through an atmosphere of dust, carbon dioxide and water.  It looks at an area of several miles across.  That's the smallest thing we can see.  So, actually, to be able to discover minerals from that far away is pretty remarkable.

Groundtruthing in the field
Now, with Mini-TES, we'll be on the ground.  We can see rocks the size of my fist.  Basically, we'll be doing what we do in our lab: take a rock, look at it with an infrared spectrometer, and then map the minerals that are in that rock in detail.  I'm sure we're going to find the basalts, the volcanic rocks that make up most of the hematite area in Meridiani.  We'll undoubtedly find the 10 or 15% hematite that's there.  What is more important, I think, is up close we can see the other minerals that hopefully are there as well.  Hematite itself isn't all that interesting.  What it says though is water was at this site.  If there was water there, other minerals (that tell us more about the water, its temperature and how long it was there) might be found.  In particular, minerals that might tell us about the possibility of life also having once existed there.  So, I am hoping that we will land and find some other minerals that give us more insight into the environment... what this place was like when it was warmer.

How do the MER missions differ from past landings on Mars?

The beauty of both of our landing sites, the hematite site at Meridiani Planum and the Gusev site is that we have scientific hypotheses.  We have a prediction, if you will, of what we are going to find.  We are predicting at Meridiani Planum that it was formed by water.  Once we land, once you can look at a rock, you can find out does it have layers in it?  Is the hematite cemented between the grains or is it a coating that covers the rock?  We basically have several hypotheses that water could have formed hematite in a variety of ways.  I think, once we have that first rock, we may very well have the answer: It was this way as opposed to these others.  So, there's nothing like being on the ground looking at the landscape and basically looking at a rock and saying, where is the hematite?  The same at Gusev: people have hypotheses that it's a lakebed, that water flowed in.  Within minutes of turning on the instruments after landing, we should be able to look around and see.  Will we see rocks with layers in them, which is what we would expect to see if it was a lakebed?  I think we can actually test both of these hypotheses.  For the Viking missions and even to much extent, Pathfinder, we were just, "hey, let's go land on Mars and see what's there."  The beauty of the current MER is now we have very detailed predictions and hypotheses and that's a much better place to be... to be testing something rather than just, "Hey, let's go land and see what we see."

You start with, "Let's just land on Mars."  That's an incredible accomplishment.  You take a picture and look around.  Over time, you get more sophisticated and can ask more complicated questions.  That's really where we are.

The beauty of science and exploration is, even if our hypotheses of how water might have formed the hematite (or that Gusev was a lake) are wrong, we are still going to learn a lot.  Because we can go back and say, "okay, why were we wrong?  What did happen here?"  That will improve our knowledge.  We'll know more about Mars, even if we are wrong.

Do you have any major concerns about the MER missions?  Does any part of the mission make you a little nervous?

Not really.  Sending anything to Mars is unbelievably complicated.  There are a thousand things that can go wrong, not for want of trying.  I'm worried about, "will they work?"  I think they will.  The team that has worked on these missions is unbelievable.  They are dedicated people willing to do anything to make this work.  So, I think it's got a really good chance.  I think we're going to some very exciting places and Mars will cooperate.  Once we get on the surface, it will be a very exciting mission.  Having two rovers is great.  It's the way NASA used to do things.  There was a period of time when due to budget reasons, they couldn't.  Having two makes a huge difference.

Do you support or reject the theory that Mars was once an Earthlike planet with a large ocean?

Artist's impression of an early wet Mars
Michael Carroll
I don't believe Mars was ever Earthlike.  You can see surfaces on Mars that are unbelievably old and yet there is no good, conclusive evidence that it ever rained.  There are a lot of channels, but it doesn't look like it rained on the surface.  If you have an ocean and it's Earthlike and it's warm, you are going to make clouds and they are going to rain.  So my view of Mars is much more glacial.  It has a lot of water, but it's always been cold.  If it did have large lakes, they were probably frozen.

Some people talk about an ocean on Mars.  I'd rather think of a continental glacier on Mars.  There was lots of water, but don't get the idea of tropical beaches and inclement warm days.  It was cold.  So, that ocean, if it was there, I believe was frozen.  That doesn't mean Mars wasn't a really interesting place.  It's just being that much farther from the sun, it's really hard to get Mars warm enough to have an ocean.

Was Mars similar to Europa?

I think Europa is going too far.  That is really cold.  Mars may be more like Earth in the middle of a big Ice Age, where the continental glaciers on Earth stretch all the way down to Iowa.  If you flew over Earth at that time, then you'd say, "it's this iceball planet."  I think Mars is more like that.  There are places where there was running water and frozen lakes but more like Minnesota in the wintertime than Hawaii in the summertime.

What have been some of the most significant findings from the orbiting Mars Global Surveyor or Odyssey spacecraft?

Gullies carved by melting snow
I think that the pictures of the gullies that the camera on Mars Global Surveyor took rank as the most, or near the top of the list, the most important pictures we have ever taken of Mars.  I think until that moment, Mars had an interesting past.  It was a fascinating place three billion years ago.  Ancient volcanoes...  Everything was extinct.  Everything was "then".  Then pictures said, "Wait a minute.  It's now."  These gullies are forming more or less today.  Whatever is going on, Mars isn't a dead place anymore.  There's water trickling down the surface of Mars today.  I think those are some of the most important pictures we ever took.

Does your current theory about how the Martian gullies formed, contradict previous theories?

The idea I put forward for Martian gullies is quite different from most of the previous ideas.  The previous theory was that the water came out of the ground.  On Earth we have ground water.  We have springs.  If the ground water comes up to a cliff, it will come out as a spring, trickle down and form little gullies.

I don't believe that happened.  I don't believe there was ground water.  The problem with that is, if I have ground water and it manages to survive for billions of years and manages to come out of a little cliff, today the atmosphere of Mars is so cold and dry, water is gone instantly.  It will either freeze or evaporate before it has time to trickle down and carve a gully.  So that is what has always bothered me about the initial theory of it coming from the ground.  The idea of it coming from snow gets around all that.  The water is in the snow.  It's cold, for sure, but the snow acts as a blanket.  It insulates to protect the melting water that is inside the snow.  The surface is bitter cold but the interior actually warms up and melts and trickles down through the snow eroding the gullies.  You've brought along your own protective blanket to keep the water from just evaporating and freezing.  So, I think it provides a good explanation for how you can have surface water in today's environment.  It's not really surface water, it's underneath this snow blanket.

Taken at the ASU Educator Workshop on March 1, 2003
That's the other problem on Mars, the ground water.  Why is it still sitting there after millions and billions of years and then suddenly decides to come out and trickle down the slope and last maybe tens of thousands of years?  Again, it gets back to the problem of rain.  If I have ground water, I've got to either keep it there for a very long time and then let it out or I have to have some way to recharge that ground water getting water back in again.

On the Earth, we get water back into our ground water system by rain.  You are not going to be able to recharge that ground water system on Mars, because there is no evidence that it rains.  Once that water ever comes out, you will never recharge it.  So, it just doesn't seem plausible to have a lot of underground water.  Why not let the water come in the form of snow?  Snow comes and goes: snows on the surface, it melts, snow goes away, comes and goes.  It just seems to work better.  But it's only a model.

How did you get the idea that the gullies were carved by melting snow?

I had seen the pictures of the gullies for two or three years.  I had been looking at images of this pasted-on material and trying to explain it.  One of the first pictures we took with THEMIS shows some of this pasted-on stuff.  I was well aware of the climate models that predict that the climate of Mars changes.  I had read Gary Clow's paper in the eighties predicting that if snow was present on Mars it would melt.  It's the classic case of learn, read, put as many ideas as you can in your head.  Someday they are all going to come together.  Literally, I looked at this one THEMIS image of pasted-on stuff on the cold part of the crater.  If you went around to the west, where it was warmer, suddenly there were gullies.  And literally (I've never experienced this before) a light bulb went off.  Something clicked.  It's like your brain took all those pieces and put them together and said, "I know how gullies are formed."

Taken at the ASU Educator Workshop on March 1, 2003
Imagiverse Educational Consortium
It's very unusual.  Most science, you plod along.  You make maps.  You try things.  Eventually, you struggle to get something out, pull together an idea.  This literally was, "click!"  All the pieces fell into place.  And it was fun.  It was probably the most fun I've ever had.  Without being conceited or anything else, it literally was, "I know how gullies formed."  It was very satisfying.  It was: "this is neat."  The definition of an idea, I think, is taking disconnected things and putting them together.  And it happened instantly!  I looked at a picture and it was like: "Wow!  I know how gullies formed."

How did you go about convincing others that this is how gullies formed?

When I try to explain this idea, I lead people through exactly.  "These are the pieces: gullies, pasted-on material, climate change model, a melting snow model."  I put all that on the table and then show them this image, and then it's like: "Yeah.  Of course.  It's obvious."

One of the people I respect most in this business said to me (when I explained this gully idea), "Gee, I'm embarrassed I didn't think of it myself."  Because once you see it, it's obvious.  But it's one of those things that is obvious after the fact.  There are a lot of things like that, things that suddenly become clear, once you had the idea.  It's like: "Oh, okay.  Gee, it's obvious."

What if you are wrong?

A really good friend of mine (Bruce Jakosky, we were Viking interns together) said just the other day, "I've written two kinds of papers.  Those that have been proven wrong and those that will be proven wrong."  Which is not true, but it points out how a good scientist thinks.  You don't cling to an idea, like: "I'll cling to this idea until I die.  You can't prove me wrong."  Every idea evolves.  Every idea is a step forward.  There are pieces of this gully story that I think are right.  There are pieces that will be proven wrong.  But the process is what is important.  We are moving in the direction of understanding Mars data.  That's what is important.  There are clearly gullies that didn't form by this process.  There are probably five different ways that gullies might form.  And they're probably all right at some place on Mars.  So it really isn't about whether it's right or not.  The thing I enjoy the most about the whole idea is that it's caused arguments.  It's caused people to debate and discuss.  It's caused people to go off and do experiments.  It's sent people into the Arctic to look for snowbanks and other places.  It's stimulated people's thinking.  It really doesn't matter whether it is right or not.  People are thinking harder about Mars and they're thinking in a different way about Mars.  The idea of oceans that came out fifteen years ago, I don't think it's right but it was one of the most important papers written.  It forced people to think in a new way about Mars.  That's what science is about.  It's not being right; it's being curious.

Taken at the ASU Educator Workshop on March 1, 2003
Imagiverse Educational Consortium
A number of things are coming together right now that are causing people to think much more about ice and snow and frozen ground on Mars.  In people's minds, Mars is a much wetter place than it was a decade or five years ago.  It's a cold wet place but the more you look, it's like: "Wow, that feature could be a glacier.  These features could be caused by freezing and thawing of ground."  It's helping to create this atmosphere of, "Oh, let's look at Mars and think of it as an icy place."

How would you test this hypothesis?

The simplest test will be a shovel.  I think proving that it is snow doesn't take much.  What I want to argue for is to land on some of this stuff and look for life.  There are complications for sure, but if I'm a bug on Mars, I've got sunlight within a few inches from the surface.  The sunlight gets down into the snow and it melts.  The temperature is above freezing so I've got liquid water.  That's a bug's dream!  For lots of organisms on this Earth that's all they need  When things die, they leave behind organic residues.  So, I think what you'd send to one of these sites would be experiments to look for current microbial life, microscopic life: chemistry experiments that could say, was life ever here?  The other thing I would want to do is a core.  It is very likely that just like on the Earth, if you go to places where it snows, you can look at a snowbank and you'll see a layer of snow, then a layer of dirt, a layer of snow, then a layer of dirt, a layer of snow and a layer of dirt.  Each one of those layers is a snowstorm, and after the storm it gets dirty and the snow gets a layer of dirt on it.  The same probably happens on Mars.  Each layer may be a year or an Ice Age.  You can probably see layers of dust on top and layers of ice and dust and snow and dust.  That's a climate history, a climate record that may go back tens of thousands of years.  In each one of those layers, you might look for evidence that there was life growing in the snowbanks at some time.  So you clearly want a way to drill down as far as you can: a few feet, tens of feet, as far as you can.

Could life on Mars be dormant, just waiting for an opportunity to wake up and flourish?

Life on the Earth certainly goes dormant for tens of thousands of years.  So it might be that, when the snow is first formed and it is melting, life can invade and grow and flourish.  Then as the climate changes, the snow stops melting.  The life might go dormant.  Who knows?  There are certainly organisms we can take from the Earth and put in these snowbanks and they would thrive.  Whether they got started on Mars or not, of course, is the question.  But these snowbanks are perfect environments for simple life to flourish.

Are there organisms living in similar environments here on Earth?

Red algae.  That is what it is doing.  It's living in snow, absorbing sunlight and flourishing.  An alga is a very complex organism.  Photosynthesis is one of the most complicated chemical reactions we've ever studied.  That's not primitive life.  Could life on Mars get to where it could do photosynthesis?  That's a big question and the answer might be "no".  If there was life on Mars, I think these are the perfect places for it to have migrated to.  If you ask me, "Where should we go look?"  The snow deposits are the number one place.

What fascinates you the most about geology?

The thing I love about geology is it's all about history.  It's telling a story.  It's so much fun to go to a rock outcrop and see a bunch of layers, and put your hand on a layer of rock and say, "When this rock formed where I'm standing was a muddy, smelly mosquito-infested swamp.  And the layer above it looked like the Bahamas and here was a deep ocean, and up here were sand dunes, baking desert."  I love telling stories and listening to good stories.  That's what geology is.  It's "let's tell a story about what happened here.  Close your eyes and imagine what the Earth looked like 200 million years ago, when dinosaurs were around."  That's what I love about it.  It's trying to figure things out.  It's like detective work.  "What happened here?"

What fascinates you the most about planetary geology, specifically Martian geology?

Earth, Europa and Mars
Earth, Europa and Mars.  Credit: NASA/JPL/MSSS
Exactly the same thing, it's a story.  What happened on Mars?  If I could make a time-lapse movie that showed Mars, when it first began, to when impact craters formed and volcanoes erupted, ice came and went and winds howled... to me that would be the ultimate goal.  Doing the detective work of looking at the rocks, the pictures and the minerals unravels this time-lapse movie.  We can't go back in time but we can learn an unbelievable amount from a single rock.  One rock on Mars might tell us, "Gee, where I'm standing used to be underwater.  So, to me that's a goal.  You see, by the time I was born the Earth's history had been figured out: continental drift, plate tectonics, and dinosaurs.  Scientists had been working for hundreds of years and they had it pretty well figured out.  Well, Mars is this whole new, untouched place.  Our generation can kind of unravel its history.  We might get part of it right.  The next generation will unravel some more.

Do you compare everything you see on Mars to things you can find on Earth?

Ubehebe Crater
If you've never been outside on the Earth, you can't study the geology of Mars.  You have to have some basic understanding of the Earth and its processes to study Mars.  I tell my students, I insist, if you are going to study volcanoes on Mars, you've got to spend six months walking around on volcanoes on the Earth.  You have to understand the processes.  On the other hand, you can get into a mindset where you think too much like Earth.  It has never rained on Mars.  There is no place on the Earth that you can go, that comes closest to Mars.  The landscape of the driest desert on this planet, is formed mostly by running water.  If it only rains once every hundred years, that's still enough.  Everywhere you look, you still find gullies and channels, the effects of rain.  So putting that out of your thinking, looking at Mars and trying to imagine a place that has never rained, that is the biggest challenge.

Well... maybe you won't find cacti on Mars.  -Sunset Crater
Imagiverse Educational Consortium
You have to understand the Earth, but you have to have an open mind and not get locked into Earth kind of thinking.  A simple example: If you're standing around in the desert long enough, you'll experience a little dirt devil.  It will blow over and it will stir up the dust and blow newspapers around and it will blow by.  You'll think, "Ok.  Big deal."  You won't think of that as a major process that's shaped the landscape.  On Mars, if you stand there, it's possible that a dust devil will blow over you once a week per year, per ten years, per a hundred years, per a thousand years, per a million years, per a billion years.  Imagine 50 billion dust devils blowing over you.  Well, after a while, that might actually do something!  One does nothing, but a billion years of wind blowing around on Mars could have an incredible effect on the landscape.  So, you have to be careful when you look at a picture of Mars.  You've got to keep this mental image of "it's never rained but wind may have blown dust through here, every day for a billion years."

Have you discovered anything very unlike Earth?

Grand Canyon - Water as a dominant process vs. wind
Imagiverse Educational Consortium
Early on, there were huge impacts, volcanic eruptions, lava flows, but over the last few billion years, I think wind is the dominant process.  There is nowhere on the Earth that we can go and make that same statement (where wind has been the most important factor).  You can imagine a thousand years of little dust devils blowing over you and all it takes is one good rainfall and all of that effect of the dust is gone.  So, there's nothing like it on the Earth.  That again, is the hardest part about not trying to get caught up in Earth-centric, water-centric view of geology.

Cholla - Almost nothing on Earth you can't find on Mars!
The thing I find is the most fun about Mars is there is almost nothing on Earth that you can't find on Mars.  Mars has volcanoes; it has canyons; it has riverbeds; it has sand dunes; it has glaciers; it has polar caps; it has weather; it has dust storms; it has clouds; it snows on Mars...  So, there is almost nothing that you can't find on both planets.  The one key thing that we haven't found yet is life.  But almost everything else is there.  So, if you're a glacial expert on the Earth, Mars is a fascinating place.  If you love volcanoes, Mars is a fascinating place.  I love sand dunes.  Mars is covered with sand dunes!  So, there are tremendous similarities.  The difference is just the balance.  If you went there, it would feel like home.  Except for the lack of green plants, you'd walk around and go, "Hey, I've been here before."  The main difference between Earth and Mars is this idea that Mars was shaped through thousands and tens of thousands of years of wind howling across the surface.  But again, I grew up in Kansas.  I can imagine the wind howling across the surface, so there is really nothing that we've found on Mars that you can't find something similar to it on Earth.

Hugh and Phil at Meteor Crater
Imagiverse Educational Consortium
To me, that's what makes it so attractive.  It's very familiar but its different enough that it makes it fun to explore.  There's a lot of dust, but even on the Earth you can find places where there's a lot of dust.  I think living on Mars, the dust could get old really fast.  Everywhere you go kicking up piles of dust.  You'll have dust in your teeth; you'll have dust in your eyes; you'll have dust in your ears.  You'd be constantly covered in dust.  I suspect the first astronauts will not have kind things to say about Martian dust.  You'll want to take a shower every ten minutes and yet, there's not a lot of water around.  So it's going to be a challenge.  It's going to be a real challenge.  It's like those classic 1950s westerns of cowboys riding in, covered head to toe in dust.  That's what Mars will be like.  These dust storms that you see in the Middle East and here in the Southwest (United States)... get used to it.  On Mars, you're going to be covered with these huge dust storms.

If you could design a mission to land a spacecraft anywhere on Mars, where would you choose?

The MER landings are going to be great.  They'll tell us a lot.  If I had my complete choice, I'd go to the middle latitudes.  Not the Poles, it's too cold.  The ice never melts.  The equator is pretty dry.  I'd go somewhere in the middle.  We have what we think are these snow and ice deposits.  They melt occasionally.  You could land, drill a core, look for layers.  See the climate change.  These are going to be fascinating places.  I think we'll get there.  I'm certainly going to argue long and hard for a rover going to places somewhere there's ice on the ground today.  The biggest challenge with going places where there's ice and snow, is you are going places where it's cold.  The mid-latitudes are colder than the equator of Mars.  Your spacecraft have to bring alternative heat sources and they probably can't survive just on solar panels alone.  There's not enough sunlight.  It's too cold at those latitudes.  Other than that, I think driving around on some of these snow packs might be easier than driving across a lava flow, for example.  It's mostly the temperature.  You have to bring enough power, enough energy to keep your spacecraft warm at night.  The Poles are even harder to survive at.  They're harsher conditions, and because they are colder, the water doesn't melt.  So I think they are even less interesting.  I think at the middle latitudes we have this nice middle ground.

Yeah, it's going to be tough for the engineers, but they can do it.  This is not impossible.  If you give an engineer enough power and mass, let them build a big enough thing with enough power, he or she will give you anything you want.  They can do it.  No problem.

Is the THEMIS instrument on Mars Odyssey performing to your expectations?  Has it surprised you in any way?

THEMIS is one of those experiments where you have an idea, you write a proposal, and you convince people that it's going to work.  They believe you and then you lay awake nights for two or three years in a cold sweat hoping that it really does work the way you thought it would.  The first picture from THEMIS was this incredible weight off my shoulders, because it really did work!  The THEMIS camera is unbelievably better than I hoped it would be.  It's performing exactly as the engineers said it would but there is a difference between, "It should be able to do this" and "Oh my God, look at that incredible picture!"  It's working beautifully and Mars is cooperating.  The things we are seeing on Mars with the infrared pictures are much more interesting than I thought they would be.  It's exceeded my wildest expectations.

Have you made any extraordinary discoveries with THEMIS?

Not specifically.  The THEMIS data are kind of different.  We may not make that kind of discovery although finding this explanation for snow related to gullies might fall in that category.  It's still really early in the THEMIS mission, believe it or not.  We're looking for hot spots and hot springs.  We haven't found them yet.  They are hard to find.  We're looking for interesting mineral deposits.  We are just starting to look.  But, even if we don't make the "Aha!" discovery with THEMIS, the quality and volume of data (complete imaging of the entire planet) will be significant.  Twenty years from now, people will still be pouring over THEMIS data looking for answers.  The deeper you get into a science, the harder it is to go, "Oh my gosh, Mars has this."  That is just because the questions are more complicated, more sophisticated.  So THEMIS is worth every nickel and more.  In many ways, I think it will be more revolutionary in our thinking than TES was.  I can still look at most THEMIS images and say, "I have no idea what formed that surface and why it looks like that."  That tells me we have an awful lot to learn from these data.

If you look at TES, THEMIS and Mini-TES, it almost seems like we had this grand design and planned it that way from the beginning, but that was really not the way it worked out.  I proposed TES and it got accepted.  I proposed THEMIS and it got accepted.  I proposed Mini-TES.  There was no grand plan, but taken together, you couldn't have had a better set of experiments.  TES gives us this incredible detail about what minerals are there but it's kind of a low resolution so you can't see the details.  THEMIS can't distinguish minerals as well, but you can see things the size of a football field.  Mini-TES only gets to see a small piece of Mars down on the surface, but all of that together...


If we find something interesting in the THEMIS image, we can go back to the TES data and find out exactly what minerals are there.  We'll land on the ground with Mini-TES and we'll look and we'll see this stuff and we'll say, "Wow, up close these rocks look like this and they have these minerals in them.  So that's what we were seeing in other places on Mars with TES from orbit."  The big picture, the little picture, the detailed picture...  You put all that together and the sum really will be much, much better than the individual parts.

The MER missions are short-lived.  The orbiter missions will keep going.  From TES and THEMIS we have a global view.  With MER, we are going to land in two places.  That will suddenly give us the: "Oh, aha!  I know what these places look like."  Then you go back to the global data and extrapolate and say, "Okay, Mini-TES sees this on the ground here.  This spot, over here on the other side of the planet, to TES and THEMIS looks the same.  So, therefore, this is probably the same.

An analogy I use with students a lot is: Imagine you are a Martian and you could pick one place on the Earth to land, where would it be?  Would it be as desert?  Would it be a glacier?  Would it be the bottom of the ocean?  Would it be New York City?  Imagine you are seeing from space all these funny things that are kind of different places and they look like amoebas.  You decide to land in one of those places and it's New York City.

Suddenly you go, "Ok.  Los Angeles and Chicago and New Orleans and Orlando.  They're probably similar."  You have an orbital view, a few places on the ground can give you tremendous insight into everywhere else.  You don't have to go to Los Angeles to know it's probably similar to New York City.  It looks more like New York than a desert.  So, the landers and the orbiters link together.  We'll learn so much from the MER landing sites that we can then extrapolate to similar places on the planet.  We can then say, "Aha!  You know, I think these kind of look like what we saw over at Gusev Crater!"

What is the benefit of studying Mars?

We didn't fully realize the importance of impact craters until we went to the moon.  You walk around the Earth and you don't see impact craters anywhere.  They've eroded away.  A group of scientists looked at a layer of rock that occurred right above where the fossils of dinosaurs disappeared.  They came up with the idea that an impact hit the Earth and threw up a huge cloud of dust and smoke that lasted for years.  Vegetation died.  The dinosaurs died and went extinct.  I don't think we would have had that idea fifty years ago, because fifty years ago, people weren't even thinking about impact craters as an important process.  So going to the moon showed us how important impact processes were.

On Mars we see incredible climate change with a surface shaped by wind.  I think Mars has changed our thinking about how important climate change can be.  We've realized that planets don't have to be warm and wet and covered with oceans.  They can be cold and frozen and shaped by the wind.  That insight, just expanding your horizons, opens you up to more ideas.  Now, when I look at rocks and layers on the Earth, I may think that maybe these were formed by the wind, because we saw that happen on Mars.  So it's really just sort of an education, broadening your thinking.  It clearly has changed how people think about the Earth.

What's next for you?

We had an incredible string of luck.  To have TES, THEMIS and two Mini-TES, possibly all operating at once... that's never been done before.  Most people consider themselves extremely lucky if they get one mission to Mars and I had five.  But I have no intention of stopping.  This is way too much fun.  It's too exciting and I've assembled too talented of a group of people to let them go away.  There's going to be a big lander that goes back to Mars in 2009.  We're going to propose experiments for that.  NASA has a program they call Scout Missions that give a group of scientists the opportunity to dream up anything they want, not just an instrument but the whole mission.  I have some ideas for that and they have to do with these snowbanks and looking at them and looking for life.  So, I'm hooked.  It's too much fun to send things to Mars to stop now.

Read Phil's Previous Interview (3 March 2001)

- 25 May 2003


MER - Two Mars Exploration Rovers were launched towards Mars in the summer of 2003, shortly after Phil Christensen did this interview.  The rovers, named Spirit and Opportunity, are robotic geologists on wheels.  They will be landing on Mars in January of 2004 to begin their mobile missions which are scheduled to last approximately 90 sols (90 Martian days) each.

MGS - Mars Global Surveyor was launched in 1996 and has been sending back photographs and data from its orbit around Mars since it arrived at the planet in September 1997.

Mini-TES - small thermal emission spectrometer, which is essentially a laboratory instrument, sent out "into the field".

Odyssey - Mars Odyssey spacecraft was launched in April of 2001 and arrived at Mars in October 2001.  It has been sending back images and data from orbit since the science instruments were turned on in February 2002.

TES - Thermal Emission Spectrometer instrument aboard the Mars Global Surveyor spacecraft.

THEMIS - Thermal Emission Imaging System aboard the Mars Odyssey spacecraft.


Mars Orbital Camera takes pictures of Gullies

Science@NASA's article on Snow Gullies

Latest Results from THEMIS

Latest Results from TES

Mars Global Surveyor

Mars Odyssey

Mars Exploration Rovers

ASU Mars Education Program

Grand Canyon

Meteor Crater


Last Updated:
8 January 2015

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