The pictures are in!  The flyby maneuvers of Phobos performed by Mars Express were successful, returning lots of data and some beautiful, clear pictures of the moon.  While details of the radiometric study of Phobos’ density will be coming out later, ESA has already released the pictures showing the proposed landing site for Russia’s Phobos Grunt lander in 2012.  It’s a nice, clear area with a relatively level surface, perfect for a lander.  With this final detail, the Russian mission is set.  Originally scheduled for launch in 2009, the Phobos Grunt mission went through delays and schedule changes, as many space projects do, but now appears to be set for launch next year.  The probe will travel for 11 months, arriving at Phobos in 2012.  When it completes its mission, it will be the longest sample-return mission ever undertaken.

Phobos Grunt is a comprehensive mission to study both Phobos and Mars itself.  It will be conducting studies of Mars, Phobos, and their spatial environment (radiation, plasma, space dust, etc.).  While the return of Phobos surface material will be the tour-de-force, and undoubtedly will be the thing for which the mission is remembered, this is really a larger project regarding Mars and its entire area of space.

The probe will also be carrying a culture of Terrestrial bacteria as a biology experiment.  When the mission successfully returns its samples to Earth, the bacteria will be studied to determine the effects of the long space voyage.

The power for the operation of the probe will be supplied by two rectangular solar panels.  These will be folded down like the eaves of a roof during the voyage to Mars, then deployed in a standard “paddle-wheel” configuration during use.  Between these two panels will be a doughnut-shaped propellant tank, and in the hole of the doughnut will be the rocket that will be used for the return voyage.  This assembly will be connected by eight narrow struts to the ring-shaped landing gear underneath.  Before it is deployed, this entire unit will sit atop a completely separate propulsion system, which will be used for the pre-deployment maneuvers.

Phobos Grunt will go up in the same launcher with Yinghuo-1, China’s first mission to Mars, which will investigate one of the great mysteries of Mars: where did all the water go?  It is now abundantly clear that Mars had much more surface water in its youth than it does today.  The process that deprived Mars of its surface water is still only poorly understood.  Recent findings indicate that some of this water is now locked up in frozen subsurface deposits (see our articles on the Phoenix and Odyssey spacecraft) but exactly how it ended up there, leaving the planet’s seas and river systems dry, is something that will bear much further study.

The origin of Phobos is open to question.  It seems to share surface characteristics with some types of asteroids, which would indicate that it was captured from the nearby asteroid belt.  That scenario is perfectly believable, except for one detail: Phobos orbits Mars in a nearly circular path, exactly on Mars’ equatorial plane.  Now, if this were a random asteroid that had been captured, you would expect its orbit to be random; it probably would not be a perfect circle, and it probably would not be exactly on Mars’ equator.  That kind of symmetry is what we would expect from a body that had been formed along with Mars, in the original planet-forming period of the solar system.  In that case, Mars and its moons could have formed out of one big, spinning glob of dust and gas, and therefore would spin in the same plane.

So with Phobos (and its sister moon, Deimos, too) we see a body that looks like an asteroid, but orbits like something that formed along with Mars.  If it’s an asteroid- or a rubble pile composed of several chunks of asteroidal rock- then it’s hard to explain the orbit.  If it formed along with Mars in the distant birth of the solar system, then it’s hard to explain its surface characteristics.  This is the great enigma that is emerging about the two moons of Mars, and all research regarding them will be aimed at clearing up the question.  We haven’t even looked at Deimos in-depth yet, but when we do, all of the questions that are now being asked about Phobos will also be asked about it.  Where did these moons come from?  Exactly what are they?  In the years to come, we will be trying to find out, and Phobos Grunt will be an attempt to get closer to an answer.

There is also another mystery about Phobos: it just looks funny.  There are long, straight grooves running for many kilometers across its surface, as if it had been sandblasted.  That may be literally what happened: asteroid impacts on Mars in the distant past may have thrown up huge amounts of ejecta, reaching so high that it scored the moon’s surface.  Such asteroid collisions may have happened repeatedly throughout Mars’ early history, and provide us with still another possible origin for the Martian moons: they could be formed out of material thrown up from Mars by early collisions, then scored again and again by the ejecta from later collisions.

Alternatively, it is theorized that these grooves may be long, straight fissures in the underlying rock, into which surface dust has settled.  Phobos Grunt will be taking a closer look at these formations to determine which theory is correct.

Upon arrival in Mars orbit, Phobos Grunt will first study Mars’ magnetosphere and atmosphere, and release the Chinese Yinghuo-1 into Mars orbit.  When these operations are completed, the landing on Phobos will be attempted.  This operation is a bit challenging, simply because of the small size of the target.  Phobos is a rugged little rock about 20 or 30 km. wide (depends on which way you measure it; Phobos isn’t even close to a sphere) and simply hitting it will require some sharpshooting.  Landings on small objects are always a time of uncertainty and anxiety for the crew back on Earth; there are so many things that could go wrong.  If you’re a little off-target, you’ve missed the moon altogether, and if you hit it a bit too hard, you’ve smashed your multi-million-dollar probe.  Assuming that Phobos Grunt can get past this nail-biter, it will arrive on the surface of Phobos and collect its samples.  While the object of the mission is to return the samples to Earth for further study, the probe will be able to do some preliminary work on the spot.

Phobos Grunt will be carrying three instruments contributed by France’s Centre Nationale d’Etudes Spatiale.  One of these is a microscope that can see in visible and infrared wavelengths, which will be used to spot interesting places to collect soil samples.  An identical instrument was used with great success on the Rosetta mission (see article at this site).  The other French instruments are a gas-phase chromatograph and a laser spectrometer, which will be used to determine soil composition.   While the samples returned by Phobos Grunt will undoubtedly be studied for years to come, the preliminary examination by these instruments will give us some idea of what we have, without having to wait for the samples to arrive.

Simply getting the samples is only part of the job; they will then have to be returned to Earth.  The long voyage back will be the easy part.  The real anxiety starts when the ground crew starts to think about reentry.  Getting delicate samples to the ground intact has proven a problem in the past.  For instance, in the return of the samples from the Stardust comet mission (see our article), the sample capsule was damaged during the impact, nearly compromising the samples.  Disaster was narrowly avoided that time, but it’s a safe bet that the Phobos Grunt crew will be thinking about it when they try to bring their probe down.

Hopefully, everything will come out all right, and the Russians will have the world’s first samples of Phobos.  As part of their agreement with France, Roscosmos is sharing the samples with CNES.  Within days or weeks at the most, scientists all over the world will finally be able to learn something about this strange little body.

Even if the sample return is unsuccessful, this mission will give us some data about Mars and its largest satellite that will prove valuable for future researchers.  It’s a neat mission, and the spacecraft is a classy device that will probably be copied for other sample-return missions in the future.

As developments happen, you can read about them here.

In conducting these flybys, the ESA is working in cooperation with ROSCOSMOS, the Russian Federal Space Agency.  Russia is planning to place an unmanned lander on Phobos by 2012 to collect soil samples, and images taken on these flybys will be used to select the landing site.

It is hoped that all of this scrutiny will unlock the secret of this moon’s origin, which is a point of much speculation now.  There are three conflicting theories about this, which we will examine in more detail in a moment, and while the final verdict won’t come out until the actual return of soil samples, these flybys are already filling in some of the blanks in our understanding of this body.

We talked about the Mars Express probe in a previous article (still posted at this site).  This spacecraft was launched by the ESA on June 2, 2003, and arrived at Mars in December of that year.  Since then, it has returned huge amounts of data on the red planet, including many beautiful and striking images of Martian landforms.  One of the accomplishments of this mission has been the detection of methane in some areas of Mars, which may be an indication of some sort of biological activity.  Mars Express has also contributed to the growing body of evidence for frozen water just beneath the Martian surface.

This is not the first time Mars Express has encountered Phobos.  In fact, it’s a regular event for this probe, whose orbit periodically brings it close to the moon.  The folks at ESA call it “Phobos flyby season,” and typically use this time to study the body.  But this time is the closest approach to the moon yet, and will present a chance for more exact measurements than ever before.  The observations will include very precise radiometric readings to determine exactly what the gravity of Phobos is, and how mass is distributed within the body.  This information will be useful to the Russians in planning their lander expedition, and will also address a key fact which has come to light about Phobos: it seems to be hollow.

Well, maybe not completely hollow.  Scientists are estimating that between 25 and 35 percent of Phobos’ interior is empty space.  They have arrived at this conclusion because Phobos simply doesn’t have as much gravity as it should.  The dimensions of this moon are quite well established, so it’s possible to get a rough estimate of how much gravity there should be if the whole body is solid rock.  While the flybys that are happening now will give us our most precise measurement of the moon’s real gravity to date, less precise measurements have already revealed that the gravity of Phobos is much less than it should be.  The conclusion is inescapable: this body doesn’t have as much matter as it seems to have.

This is not as big a mystery as it seems.  In fact, Phobos appears to be a type of body that was predicted before any were actually found, and which now seems to be quite common in the solar system.  Scientists call them “rubble piles,” and that’s exactly what they are.  You see, there are an awful lot of rocks flying around out there, especially in the asteroid belt, which is between the orbits of Mars and Jupiter.  In the early solar system, there were even more of them than there are today.  (Nowadays, the majority of these rocks have already crashed into some larger body, and the asteroids that are left are just the small percentage that have managed to avoid this fate.)

Since all matter exerts some gravitational attraction, every one of those rocks has its own gravity, and when rocks come close to each other, they tend to attract.  If two of these space rocks drift together and stick to each other because of their mutual gravitation, then of course, they’re exerting a stronger gravitational attraction than either one did separately, so they tend to attract still more rocks.  Eventually, if more and more of them come together and stick to each other, you end up with a big mass of rocks loosely held together by gravity.  Of course, they don’t fit together very well, and while they are touching each other in some places, there will also be a lot of gaps.  What you’ve got is a classic rubble pile.

Over millions or billions of years, more rocks will keep hitting this pile, smashing up the outer surface and spraying a lot of asteroid dust around.  Eventually, our rubble pile acquires a coating of this dust which fills in the cracks on the outside of the body, giving the illusion of a solid, unbroken surface.  To an outside observer, there is no obvious sign that this body is not solid- but deep inside, all those gaps are still there.  You wouldn’t suspect a thing unless you measured the body’s gravity, at which point it would become evident that there was an awful lot of empty space inside it.

So, that’s the giveaway: if you find a body that has a much lower gravity pull than it logically should have, you know you’ve got a rubble pile.  We have already found a few of these bodies out there.  For example, it is strongly suspected that Jupiter’s moon, Amalthea, is a rubble pile.

This brings us back to a point mentioned earlier: the fact that there are conflicting theories about the origins of the Martian moons.  One of them is the scenario that we’re looking at here: Mars, being so close to the asteroid belt, has attracted some far-roaming rocks into orbit around itself.  The fact that Phobos- and possibly the other moon, Deimos- is a rubble pile instead of a solid chunk doesn’t really effect this scenario; the rocks may have formed into a pile in the asteroid belt, or after they were captured by Mars.

Another theory about the formation of the Martian moons is that some large body slammed into Mars in the remote past, and Phobos and Deimos are fragments from the collision, thrown off the planet with enough speed to achieve orbit.  If this is the case, it is probable that we won’t know for certain until soil samples can be collected from Mars and both moons, so they can be compared.  If Mars and its moons were originally part of the same body, they should have similar compositions.

However, even if this is the case, it still will not be conclusive, and further research will have to be done to get a definitive answer.  If Mars and its moons are made of similar stuff, there is another possible explanation for it, and this gets to the third theory about the formation of these moons.  This theory holds that the planet and both moons were all formed at the same time, from the same primordial accretion disc that gave birth to all the other planets.  In other words, they may be first generation objects (formed in the birth of the solar system) rather than second generation (formed later from smaller fragments coming together).

So far, it’s all a big mystery, but we are fast closing in on the answer.   The information gained from this series of flybys has already taught us a few things about Phobos, and further analysis of it will reveal more.  Radiometric data from these flybys are being studied right now, and should be precise enough to tell us where the gaps are within the moon.  As said earlier, the gravity analysis from Mars Express will be used to help ROSCOSMOS select the spot to set down its lander in 2012, and the samples from that encounter will answer more questions.  In time, we will pry all of the secrets out of Phobos.

One final note about the possible future of Phobos: it may be a ready-made home for settlers.  What you’ve got is a big rock with a lot of holes in it, and some of those holes can probably be smoothed out and modified to make living spaces.  Since it now appears that at least a quarter of Phobos’ area is empty, that amounts to a lot of space.  If we should ever want a convenient space station orbiting Mars (and we will, eventually) Phobos might be it.  Using a body that’s already there would be a lot easier and cheaper than building something from scratch.  In some far-future time, this little rock might be riddled with underground colonies.

As you can see, the work is just starting on Phobos- and we haven’t even looked at Deimos yet.  The data from the recent encounters will be studied for years to come, and new findings will undoubtedly come to light.  Stick with us; we’ll keep you posted.