“We're probably talking about
something in the 2020s, if we do have a sample return mission
Now, you might ask why do we want to
bring a sample back from Phobos? Why not bring one back from
Mars. The answer to that is simply a matter of propulsion
It's very, very difficult to run a
mission in which we land a spacecraft on the surface of Mars
that's big enough to get off the Martian surface and back to
On the other hand, Phobos is already in orbit around Mars. It's
much easier to plan a mission with chemical propulsion where we
rendezvous with Phobos, grab a sample and then come back to
THE EUROPEAN SPACE AGENCY IN TERMS OF IT WORKING ON A PLAN TO
GO TO PHOBOS TO TRY TO GATHER MATERIAL, DOES IT ALSO FIT INTO
INFORMATION THAT HAS BEEN CIRCULATING ON THE WEB SAYING THAT THE
EUROPEAN SPACE AGENCY IS DESCRIBING PHOBOS AS HOLLOW. AND IN
THESE WEB REPORTS, IT IS RAISING THE QUESTION: COULD PHOBOS BE A
CONSTRUCTED OBJECT AND WAS NOT NATURAL?
I have not seen those web reports. But there is absolutely
nothing that I know of that indicates that Phobos is not a
Mysteries of Phobos
But there are some
mysteries with Phobos.
We don't know how it got into orbit
around Mars. We don't know - it's composition seems to be like a
primitive meteorite (carbonaceous chondrite), which would mean
that Phobos was captured. We don't know any way that could
happen in the absence of quite a bit of gas being around, which
would put its capture very early in the solar system.
So, how it got into the orbit it's got is kind of a mystery.
But looking at the surface of Phobos,
looking at its mass, which we know - we've had many close
spacecraft encounters. Its density is relatively low, but then
it's full of fractures. What we see is not out of line with it
being a kind of ordinary chondrite that has been fractured.
The lines across the surface of
Phobos are just regolith draining into gaping fractures. So, its
(low) density is not terribly much of a surprise. I don't know
what those web reports are, but I would not pay too much
attention to them.
Wikipedia - Regolith is a layer of loose, heterogeneous material
covering solid rock. It includes dust, soil, broken rock, and
other related materials and is present on Earth, the Moon, Mars,
some asteroids, and other terrestrial planets and moons. The
term regolith combines two Greek words: rhegos, “blanket,” and
Is Phobos “Hollow”?
THE SOURCE IS SUPPOSED TO
BE SOMEBODY AT ESA AND I'M WONDERING, WHEN THEY USE THE WORD
‘HOLLOW,’ WHAT DOES THAT MEAN TO YOU AS A PLANETARY GEOLOGIST?
A hollow configuration is something that does NOT occur
naturally. I suppose that's where the speculation comes from. I
would say that Phobos is thoroughly fractured and has a lot of
void space in it.
That doesn't imply anything
Phobos is a very peculiar situation. It's closer to its planet
Mars than what is called the Roche Limit. Tidal forces are
actually tending right now to tear Phobos apart. It doesn't have
very much life left on a geological timescale. We expect that as
tides cause Phobos to spiral in towards Mars, within 100 million
years or so, Phobos will be ripped apart and Mars will have a
ring sort of like Saturn's rings for awhile.
But that doesn't imply anything
artificial. Phobos has probably been spiraling towards Mars for
most of its life time.
Wikipedia - The Roche limit, sometimes referred to as the Roche
radius, is the distance within which a celestial body, held
together only by its own gravity, will disintegrate due to a
second celestial body's tidal forces exceeding the first body's
gravitational self-attraction. Inside the Roche limit, orbiting
material will tend to disperse and form rings, while outside the
limit, material will tend to coalesce. The term is named after
Edourd Roche, the French astronomer, who first calculated this
theoretical limit in 1848. ]
BEYOND THE DESCRIPTION OF LIGHT DENSITY AND MIGHT BE HOLLOW,
WHAT ARE OTHER PECULIARITIES?
Well, from what we can tell from its detailed spectrum, colors
it reflects, it appears to be most closely related to a
primitive meteorite - doesn't look like the surface of Mars at
all. It appears to have been captured, perhaps from the asteroid
But nevertheless, it's in a circular
orbit and that does not happen very naturally. So, there are a
number of curious things about Phobos, but again, nothing
suggesting it's artificial.
HOW DOES PHOBOS COMPARE TO DEIMOS?
Well, Phobos is bigger than Deimos and it's closer to Mars than
Deimos. The reflectance spectrum between Phobos and Deimos is
quite similar. Phobos is a little more difficult to get to. It's
further down in Mars's gravity well. But Deimos is not inside
the Roche Limit and it doesn't have the striations across the
surface that Phobos does. So in appearance, it looks quite a bit
Phobos Source Is Unknown
IF PHOBOS IS A CAPTURED OBJECT, THAT MEANS THAT NO PLANETARY
GEOLOGIST IS EVER GOING TO BE ABLE TO KNOW PRECISELY WHERE
PHOBOS CAME FROM?
That's probably true of just about any captured object.
It's certainly related to things in
the asteroid belt, but that's one of the reasons it's so
interesting to bring back a sample to see what it's relations to
other objects in the asteroid belt actually are. Deimos would
also have been captured. It's also in a circular orbit and has
the same problems of where is its origin as Phobos.
So they are both peculiar in that
We know that Jupiter and Saturn have a large number of so-called
irregular satellites that look like they have been captured, but
none are in circular orbits and none is close to their primary
(planet) as Deimos and Phobos are.
FROM YOUR POINT OF VIEW AS A PLANETARY GEOLOGIST, HAVE YOU
HAD ANY DISCUSSIONS EVEN IN CASUAL CONVERSATION WITH ANY
PLANETARY GEOLOGIST AT ESA ABOUT THE POSSIBILITY THAT ALL THESE
PECULIARITIES OF PHOBOS MIGHT INDICATE THAT IT'S SOME LEFT OVER
CONSTRUCTION FROM SOMETHING FROM OUTER SPACE?
I've not had any such conversation at all. I've had many
conversations with people in ESA and with people involved in
spacecraft exploration of Mars and there is no serious
discussion that Phobos is any thing other than a natural object.
Odds for Life Evidence in Phobos
WHAT IS YOUR BEST SPECULATION TODAY ABOUT WHETHER OR NOT YOU
WOULD FIND SOME FORM OF LIFE IN MATERIAL COLLECTED FROM PHOBOS?
We get meteorites from Mars all the time. About one ton of
Martian surface material falls into the Earth's atmosphere every
year kicked off by impacts on the surface of Mars.
In fact, one can make a pretty
cogent argument that life might have been started initially on
Mars and then perhaps 4 billion years ago came to Earth as a
result of these natural impacts.
IS THERE AN ARGUMENT FOR WHY CURIOSITY (ROVER) WOULD NOT FIND
MICROBIAL LIFE ON MARS?
Well, you could say that life never got started on Mars,
although the evidence is it was once as hospitable - or more so
- than the Earth through conditions in which life might form.
But we don't know the circumstances
under which life forms. It might be a rare and happy accident
that just happened to have occurred on Earth. Or it might be it
occurs whenever conditions are right.
Now, the surface of Mars is an extremely dry, cold, hostile
desert that's bathed in ultraviolet light that is lethal to any
terrestrial organism. It's not a place where at least Earth-type
life could thrive now.
But it might be that life got
started in the past (on Mars) and that's the point of the
Curiosity rover investigation to find out if there is evidence
of past life; and even better, if there is something still
surviving deep below the surface.
I THINK YOU SUGGESTED EARLIER THAT THERE IS A POSSIBILITY
THERE MIGHT HAVE BEEN METEORITES THAT MADE IT FROM MARS TO EARTH
IN EARTH'S DISTANT PAST THAT COULD HAVE STARTED LIFE HERE?
That's right. That is a possibility.
It's a speculation, but it's based
on the fact that we have Martian meteorites that came to us from
Mars without any alteration and under conditions that life could
have survived. It appears that Mars had a stable crust and
flowing water perhaps earlier than Earth's moon existed and
perhaps earlier than when Earth was hospitable for life.
That would have been about 4 billion
SO THAT MIGHT HAVE HAPPENED WHILE THERE WERE LAKES AND RIVERS
That's exactly right.
IF THAT'S THE CASE AND THERE MIGHT HAVE BEEN LIFE ON MARS
DEVELOPING, WHAT DO YOU THINK COULD HAVE HAPPENED THAT WOULD
HAVE CHANGED THAT WATERY LIFE CONDUCIVE PLANET TO THE COLD
DESERT THAT IT IS NOW?
Well, Mars evidently lost most of its atmosphere.
Meteorite impacts themselves eject a
certain amount of the atmospheric gas. The solar wind impinges
directly on the atmosphere of Mars unlike the Earth because Mars
presently lacks a magnetic field. It once had a magnetic field
that would withstand the solar wind.
But Mars cooled off faster. Its core
froze and its dynamo died and the magnetic field died at the
same time. That allowed the solar wind to directly hit the
atmosphere, which would have stripped off quite a bit of it as
well as the ultraviolet hitting directly on the surface.
Earth has a pretty good shield of water loss by ultraviolet
light in the form of ozone. The ozone shield for the Earth comes
So life developed on Earth to the
point that it started producing oxygen that has since about 2.5
billion years ago been a shield against the evaporation of
water, whereas that is not the case with Mars.
Did Earth Life Originally Come from
Hubble Space Telescope.
WHAT IS THE MOST EXCITING PART OF YOUR RESEARCH IN THIS SOLAR
The excitement is:
What did the large number of
impacts in Earth's early history do to Earth?
Where did life come from?
Did life on Earth come from
Mars and how did life start?
What's the relation of life
to the large impacts that we know were occurring?
WHAT IS YOUR PERSONAL THINKING NOW IN FEBRUARY 2013 ABOUT
WHAT YOU MOST STRONGLY THINK MIGHT BE THE ORIGIN FOR LIFE ON
I think the ultimate origin is chemical; that is, we had an
environment in which carbon in particular had an opportunity of
interacting with other elements and an environment that changed
constantly would have gone through a chemical evolution where
the chemicals that grow faster get to be more abundant than the
chemicals that don't grow so fast.
Eventually they evolved into what we
recognize as life.
FINALLY, DO YOU HAVE A BET WITH ANY OF YOUR PLANETARY
GEOLOGIST COLLEAGUES ABOUT CURIOSITY FINDING LIFE ON MARS?
I have no such bet. (laughs) I have a fond hope that Curiosity
will find life, but I think it's possible it will not.
AND IF IT DOESN'T, WHAT WOULD YOUR REACTION BE?