Could Phobos Be Hollow?

by Nancy Atkinson on March 5, 2010

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A mosaic image of Phobos composed by 53 pictures. Credits: ESA/ DLR (S. Semm, M. Wählisch, K.Willner)/ FU Berlin (G. Neukum)

Back in the 1950s and 1960s, there was some speculation that Mars’ moon Phobos could possibly be hollow due to the its unusual orbital characteristics. While scientists now agree that the moon is very likely not hollow, vast caverns may exist within the moon, and it might be a porous body instead of solid. The Mars Express spacecraft made a close flyby of Phobos on Wednesday to help provide more data on the interior of Phobos, and all indications are the event was a big success. The spacecraft skimmed smoothly over the odd-shaped moon at just 67 km, the closest any manmade object has ever been. No images were taken from this flyby. Instead all the instruments were turned off so that ground stations could listen for a pure radio signal of how Phobos “tugged” on the spacecraft. Scientists say the data collected could help unlock the origin of Phobos and other ‘second generation’ moons.

“Phobos is probably a second-generation Solar System object,” said Martin Pätzold, Universitat Koln, Cologne, Germany, and Principal Investigator of the Mars Radio Science (MaRS) experiment. Second generation means that it coalesced in orbit after Mars formed, rather than forming at the same time out of the same birth cloud as the Red Planet. There are other moons around other planets where this is thought to have been the case too, such as Amalthea around Jupiter.

Previous flybys of Phobos have shown that it is not dense enough to be solid all the way through. Instead, it must be 25-35% porous. This has led planetary scientists to believe that it is little more than a ‘rubble pile’ circling Mars. Such a rubble pile would be composed of blocks both large and small resting together, with possibly large spaces between them where they do not fit easily together.

The March 3rd flyby was close enough to give scientists the best data yet about the gravitational field of Phobos.

The radio waves travel at the speed of light and took 6 minutes 34 seconds to travel from Earth to the spacecraft on Wednesday night, and by analyzing the data on Phobos’ gravity field, scientists should be able to estimate of the density variation across the moon and detect just how much of Phobos’ interior is likely to be composed of voids.

This flyby was just one of a campaign of 12 Mars Express flybys taking place in February and March 2010. For the previous two, the radar was working, attempting to probe beneath the surface of the moon, looking for reflections from structures inside. In the coming flybys, the Mars Express camera will take over, providing high resolution pictures of the moon’s surface.

Source: ESA

About 

Nancy Atkinson is Universe Today's Senior Editor. She also is the host of the NASA Lunar Science Institute podcast and works with Astronomy Cast. Nancy is also a NASA/JPL Solar System Ambassador.

Starhunter March 6, 2010 at 5:49 AM

Will be intersting to see if the moon has vast cavers, can’t wait for the pictures, wonder if it has moon quakes be tugged on by Mars .

Jon Hanford March 6, 2010 at 2:24 AM

Hasn’t anyone heard of this hollow Phobos theory? ( http://en.wikipedia.org/wiki/Phobos_%28moon%29#.22Hollow_Phobos.22_suggestions )?

According to Wikipedia:
“In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow.

Around 1958, Russian astrophysicist Iosif Samuilovich Shklovsky, studying the secular acceleration of Phobos’s orbital motion, suggested a “thin sheet metal” structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin.[34] Shklovsky based his analysis on estimates of the upper Martian atmosphere’s density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light — one calculation yielded a hollow iron sphere 16 km across but less than 6 cm thick.”.

Mars Express did leave room for doubt for at least a few hollow spots within Phobos interior. Again, from Wiki: “…mapping by the Mars Express probe and subsequent volume calculations do suggest the possible presence of vast caverns within the moon and indicate that it is not a solid chunk of rock but a porous body instead”.

Reminds me of “For the World is Hollow and I Have Touched the Sky” from the original ST.

It’s just not totally hollow :)

Torbjorn Larsson OM March 6, 2010 at 2:56 AM

If a Mars size last impactor on Earth threw up enough material to coalesce into the Moon, the putative Moon sized last impactor that scuffed the northern part of Mars in a glancing impact (and perhaps destroyed Mars chances for a bipolar magnetic field) may have resulted in a moon as well, right?

But even if that would be so, the question is then why Phobos is so puny. (Deimos doesn’t help here, it is but a 1/10th the mass. And looks like a captured asteroid anyway.)

Either impact energies scale really funny, or it is a neat test of the glancing impact prediction.

And of course at this stage any misguided guesses are likely to be much less exciting than the actual facts when they are uncovered! The upcoming Phobos-Grunt mission 2011 suddenly looks a lot more interesting than I initially thought.

Jon Hanford March 6, 2010 at 3:12 AM

@Torbjorn Larsson OM , let’s all wish the Russians (and Chinese) researchers have better luck here than the original Russian Phobos probes (near 0% success rate, to date). I really want to see this mission work. However, given the long track record of interplanetary probes built and launched by China and Russia., my hopes for such an ambitious and technical mission cast doubts on even a partial success. :(

Sirius_Alpha_1 March 6, 2010 at 8:37 AM

Nancy, if I may ask, where did you find that cylindrical map of Phobos? And more importantly, is there a larger one available? :D

RUF March 6, 2010 at 9:53 AM

Sirius Alpha 1:

Click on the ESA link at the bottom of the article for the original source. There are pics there.

Aqua March 6, 2010 at 6:59 PM

Does look kind of like a layered cake in some views. So you are saying its a ”fluffy’ layered cake?

What’s that mean? That it’s a piece of a highly processed cometary crust? loaded with gases?

Nancy Atkinson March 6, 2010 at 7:01 PM

RUF- Click on source link to find the ESA source for the image or here: http://www.esa.int/SPECIALS/Mars_Express/SEMIPX6K56G_1.html#subhead2

Vedic March 7, 2010 at 9:15 AM

Oooh, vast caverns is interesting, sounds as though it could be a sort of prêt-à-porter space station. Just need to line the walls and then fill it with air…

I’m looking forward to finding out the facts a lot more now.

Hauerg March 7, 2010 at 10:46 AM

IF Phopos is a result of an impact it should be REALLY dry. I hope it is something else, more useful as a target for exploration missions.

Dark Gnat March 8, 2010 at 9:55 AM

That’s cool! Maybe there are giant worms living inside of it, ready to eat the Millenium Falcon!

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