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When it comes to the question of whether Mars ever had life—or
just possibly still has it—Christopher McKay knows whereof he
speaks. A planetary scientist at NASA Ames Research Center, McKay
has traveled as far as the Siberian arctic and the Antarctic Dry
Valleys to study how life makes do in Mars-like environments, and he
is actively involved in planning for future Mars missions, including
those that would settle humans on the red planet. While McKay thinks
the chances that life still exists on Mars are vanishingly remote,
he is optimistic that the planet once hosted living things, and he
says that, if asked, he would willingly go help search for their
remains. Why such great expectations? Read on.
The necessary ingredients
NOVA: Why do we think Mars might have had life?
McKay: Well, it's not because of anything that's there today.
It's a dry, frozen desert. But there's evidence recorded on its
surface that Mars at one time had water, lots of it. There were
rivers, lakes, maybe even an ocean. Mars had water early in its
history, possibly at the same time Earth first had life. It's that
comparison—water on Earth, water on Mars, life on Earth, what
happened on Mars? That's the question.
NOVA: What else about Mars would support the notion that life
might once have been there or might even still be there?
McKay: Mars is not that far from Earth. It's one of the
terrestrial planets, along with Earth and Venus. It formed from
roughly the same materials as Earth. We know that it has carbon,
water, nitrogen. Right now Mars has everything needed for life
except one thing—liquid water. But we see evidence that it had
liquid water in the past.
In fact, there's a distinct parallel between early Mars and early
Earth. Every environment that would have been on the early Earth
could also have been on Mars. So wherever life made its initial
evolution on Earth, that same environment should have existed on
Mars as well.
NOVA: Also the atmosphere?
McKay: Right. The only way to understand that Mars had liquid
water in its past is to suppose that it had a much thicker
atmosphere, presumably one made of carbon dioxide. Long ago Mars
lost its atmosphere. Where did it go? We think that most of it is
tied up now in rocks. It's been turned into carbonate. It's been
mineralized.
That thicker atmosphere is needed to stabilize the water, but it
would also have made Mars warmer, and it would have provided the
material that life needs. Life could have taken up the carbon
dioxide from that atmosphere. The atmosphere would have protected
life from cosmic rays and other radiation sources, and it could have
provided weather and all the sorts of things we have here on Earth.
NOVA: All the conditions necessary for life.
McKay: Exactly. When we look at life on Earth, we see that
life needs a series of things. It clearly needs energy, it needs
carbon, it needs a few other elements. The most important
requirement for life is liquid water; we think that's the defining
requirement for life in our solar system [see
Life's Little Essential]. There's plenty of energy, there's plenty of carbon, there are
plenty of other elements on all the planets in our solar system.
What's rare and, as far as we know, only occurs now on Earth and
early in Mars' history, is liquid water on the surface.
NOVA: Why is carbon so important?
McKay: That's a good question, and we don't have a
fundamental answer to it except that we see that all life we observe
uses carbon. Carbon has some very good properties for life in terms
of its ability to make molecules that link together and make
polymers. Maybe other molecules could do this, too; people have
speculated on silicon, for instance, which is in many ways similar
to carbon. It's hard to know if carbon is really the essential
ingredient for life, or just the ingredient that we happen to use
here on Earth.
NOVA: Was there oxygen on the early Mars?
McKay: Well, when we look at the Earth, we see that through
most of its history life was very small—microscopic. It's only
with the rise of oxygen that we see the development of large animals
and ultimately intelligence. Based on that observation, we think
that early Mars probably was only microscopic as well in terms of
life. But it's possible that oxygen rose more rapidly there. It's a
smaller planet, and it lacks the sort of tectonic recycling that
early on prevented the buildup of oxygen on Earth. Mars could have
become oxygen-rich much faster than the Earth.
“I’m optimistic that Mars had life.”
And that could have led to large creatures on Mars much faster than
on our planet. This is speculation, of course, but it's possible
that evolution on Mars went faster than evolution did on Earth. So
we have to be careful when we use the Earth as the model for Mars,
because the planets are different.
Chances for life
NOVA: Are you saying Mars could have had complex life?
McKay: It's possible that Mars, being smaller than the Earth,
evolved more rapidly than the Earth in terms of oxygen, and that if
life started on Mars, it could have reached the level of complex
life faster than Earth. I've done calculations that suggest it could
have reached a level of complex life a thousand times faster than
our planet. Instead of taking two billion years for the increase of
oxygen and complexity of life, on Mars this could have happened in a
few million years. So on Mars we'll look for microscopic life, but
we should keep our eye open for something more interesting, more
complex. It might be there. It's worth looking for.
NOVA: Some scientists believe life never developed on Mars.
Why do they say that?
McKay: Well, right now we have no scientific data that tells
us that life did or did not develop on Mars. My intuition tells me
that what life needs is water, and we see a planet that had water,
so I'm optimistic that Mars had life. Other people may think that
it's more difficult to start life and think that just because Mars
had water, it's not necessarily probable that it had life. We really
don't know. We don't know how life started on Earth. We don't know
if it would have started on another planet. We don't understand the
details of planetary evolution well enough. The only way to advance
our knowledge is to go look on Mars.
NOVA: I've heard "Yes, Mars had water, but not long enough
for life to have existed."
McKay: We don't know how long Mars had water, but we also
don't know how long it takes for life to evolve. Some people argue
that life can start very quickly, a million years or less; some
people argue that it takes billions of years. We don't know.
The one bit of evidence that we might bring to bear on this is the
record of life on Earth. It appears that life started quickly here.
Life seems to be present soon after the formation of the Earth
billions of years ago. If that's true, then you might argue that
life starts quickly. But it's hard to reach that conclusion based on
only one example. If you move to California and win the lottery the
first day, that doesn't mean it's easy to win the lottery.
Life may have started on Earth very quickly but purely by accident.
It may be a very rare, difficult event. On the other hand, life may
be easy to start under Earth-like conditions on any planet. These
are questions that we'll never answer staying here on Earth. We've
got to go look at another example. We've got to go see if it
happened on Mars.
If we go to Mars and we find evidence for life there, a separate
origin of life, I think it's clearly telling us that life starts
readily on any Earth-like planet. If we go to Mars and find that it
had water, it had a thicker atmosphere, it had everything needed for
life, and it never developed life, then I think that would make us a
little pessimistic in our predictions about life on other planets.
Planet postmortem
NOVA: Why did Mars die?
McKay: Well, suppose you were on Mars three to four billion
years ago, and you were walking around on this very nice world with
a thick atmosphere and water, and everything was just fine. Well,
gradually things would start getting worse and worse and worse.
What's happening, you'll notice, is that the atmosphere is getting
thinner.
Meanwhile, your friends on Earth would be finding that their planet
was just fine, that there was recycling due to plate tectonics, that
the Earth was maintaining its atmosphere. So the two planets start
off the same, one goes down, and the other maintains itself. That's
the fundamental difference between the history of Mars and of Earth.
NOVA: What happens when you lose your atmosphere?
McKay: Well, the main problem of living without an atmosphere
is that there's no greenhouse effect. It's very cold. Everything
freezes. And the pressure is so low that water goes directly from
solid to vapor without forming a liquid. So this is a double whammy
from the point of view of life—temperature and pressure too
low for liquid water.
NOVA: Plus lots of nasty ultraviolet radiation.
McKay: Yes. There is both UV and cosmic radiation coming
through the thin atmosphere and hitting the surface. But those are
not really powerful detriments to life. Ultraviolet light and cosmic
radiation are bad for humans, of course; we would have higher
incidents of cancer and so on. But many organisms on Earth have
learned to cope with UV radiation—microorganisms in
particular. And organisms that live in the subsurface have no
worries about that sort of radiation.
“It’s possible that there are still places on Mars today
where life is a going thing.”
I don't think that the radiation itself would prevent life on Mars
if the atmosphere was thicker and if liquid water could be present.
Life can figure out a solution for everything else, but liquid water
seems to be the one thing that life can't work around.
Weighing the evidence
NOVA: Are there any recent findings to support the
possibility of past life?
McKay: The most interesting recent results from Mars all
focus on water. From the Odyssey spacecraft we now have direct
evidence that Mars has massive ice in the polar regions, in the
permafrost there. Also, there is clear evidence of ancient rivers
and channels being carved by water. The more we learn about Mars,
the more we're convinced that it was a water planet.
NOVA: Is there evidence that liquid water might still exist
there?
McKay: There is some evidence that suggests there is still
some activity that could be related to the presence of liquid water,
or the melting of snow, or the melting of ice in recent times. That
evidence is very interesting, but it's still controversial.
NOVA: Might the Mars Exploration Rover scientists find signs
of ancient life, say, in Gusev Crater?
McKay: We're pretty sure Gusev Crater was full of water. It
was really a crater lake. The idea is that maybe if there was life
in that lake and it died and settled to the bottom, it's preserved
in the sediments as fossils. We might find fossils right there on
Mars, and that would be interesting.
What I'd like to do next is then go down into the ancient terrain
near the south polar region in the permafrost there and drill and
try to find not a fossil but an actual dead martian organism frozen
in the ground, a corpse, something we could do an autopsy on.
NOVA: Any possibility there could be life still extant?
McKay: My guess would be that if Mars had life in its early
history that it has all died out, but we're not sure. It's possible
that there are still places on Mars today where life is a going
thing, say, near the polar regions, where there are possibilities
for water from the melting of ice or more likely deep underground,
where geothermal heat from the interior of the planet may be enough
to keep the water liquid. Those are the possibilities for life
today. I'm not optimistic.
NOVA: You mentioned geothermal heat, but earlier you said
Mars has lost its heat.
McKay: The geothermal heat on Mars is much lower than that on
the Earth, but it's still there. If you were to drill down a
kilometer or two below the surface, it would become warm enough that
the ice would melt.
NOVA: Could microbes from Mars' early history that are frozen
into the subsurface potentially still be viable?
McKay: Well, imagine in the permafrosts on Mars a bug frozen
into the ground, waiting for things to warm up. How long might it
survive? We think the answer might be hundreds of millions of years.
Unfortunately, on Mars they may have been waiting for several
billion. So even for these guys it may have been too long a wait.
But we're not sure of that. We should be prepared for the
possibility that we'll go to Mars, we'll dig up bugs, and they'll
still be viable.
Costs of exploration
NOVA: Given your hopes for the polar regions, it must have
been extremely disappointing when the Mars Polar Lander vanished in
1999.
McKay: Indeed. It was going to land down near the south polar
cap, down in that ice-rich material that may hold the organic or
even biological record of life on Mars. So needless to say, we were
very disappointed when it crashed, not just for the loss of the
mission and the loss of the time and effort that went into it, but
for the loss of the opportunity to advance our scientific
understanding of Mars in that way.
“I think sending humans to Mars is a possible task.”
But that's just the way it is when you explore planets. They're far
away, and it's hard to make sure things are working without someone
there to fix them. On average, only one out of three of the missions
that we Earthlings have sent to Mars have succeeded. The odds are
not good, but that's just the cost of doing this kind of
exploration. It's like the major leagues. If you're batting 300,
that's pretty good. We're batting 300.
NOVA: Why is it so difficult to get a mission, go to the
planet, and dig up this stuff?
McKay: It's difficult to go to Mars. It's a long way away. If
you send a robot and something goes wrong, there's no one there to
fix it. If you send humans, you've got to make sure they have enough
food and water and air to make it there and back. It's a challenging
prospect.
NOVA: We've sent people to the moon. What's so hard about
sending them to Mars?
McKay: Well, imagine you were going to send a well-trained
scientific team to search for life on Mars. It would take them at
least six months to get there, and on the way you'd have to make
sure that their bones and muscles didn't get weaker in the
microgravity of space. Once they got there, of course, anytime they
went outside they'd have to wear a spacesuit. Pressure, oxygen,
food, water—everything would have to be provided for their
entire trip.
Nonetheless, I think sending humans to Mars is a possible task. We
know how to do it. We have the technology. If we wanted, we could
set up a research station on Mars and do the scientific exploration
that would answer these questions. But I think it's still some time
in the future before we do that.
NOVA: If there was a mission tomorrow, would you want to be
on it?
McKay: If there was a mission going tomorrow to Mars and they
were looking for somebody to go out in the field and dig for
fossils, I would volunteer. Why not? As long as they promised to
bring me back after a few years!
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