Figure 1. Phobos as seen by Mars Reconnaisance Orbiter (MRO), in color. Click for full image.
All images in this article are courtesy NASA and JPL, and are in the public domain.



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Phobos as seen from Mars and Orbit

By F. E. Harris

October 15, 2011


Phobos is one of the most interesting and accessible objects in the solar system. The Russian space agency RosCosmos, is planning an unmanned expedition to Phobos, that may return a sample to Earth. There is talk that, instead of landing people on the surface for the first manned expedition to Mars, the astronauts should land on Phobos. Why?


Section 1: Ease of Access

For one thing, after the Moon, Phobos is probably the easiest body in the solar system to return samples. When arriving at Mars, a spacecraft can brake to orbit using Mars's atmosphere. Rendezvous with Phobos is fairly simple, after that.

Phobos Eclipse
Figure 2 : Phobos Passes in Front of Sun's Face, Nov. 9, 2010, as seen by Mars rover Spirit.

Phobos, only about 22 kilometers (13.5 miles) in diameter, has less than one-thousandth the gravity of Earth. That's not enough gravity to pull the moon into a sphere, so it's oblong.1,2 Phobos has such low gravity, that landing on Phobos is more of a docking maneuver. Getting a sample could be as easy as having coring tubes or grasping claws mounted on the landing legs, to capture dust and small pebbles. Roving on Phobos could be done with springs that make the rover hop, much like a grasshopper on Earth.

Phobos eclised by Mars
Figure 3. Phobos eclipsed by Mars, as seen by MER (Mars Expedition Rover) Spirit on Sol 675. Time lapse photo.

Taking off requires only slight rocket thrust. Returning to Earth from Mars orbit requires far less fuel, than taking off from the surface.

Phobos and Diemos
Figure 4. Phobos and Diemos, as seen from the surface of Mars. Animated GIF, made from time lapse photos


Section 2: Sceintific Considerations

Some space scientists say Phobos is a captured asteroid. Others say it is a moon, much like our own. The best way to settle the question is probably to visit, and get samples.

Gaspra, Phobos and Diemos
Figure 5. Phobos and Diemos, with asteroid Gaspra, to scale.

Phobos's surface has clearly been extensively reworked by meteor impacts. Materials from deep within have probably been pulverized and brought to the surface. So not only would a sample return mission from Phobos provide us with samples from the surface of a second moon, or a second asteroid, but also possibly samples from deep within, as well.

Phobos has a unique topography. There are groove covering the surface of Phobos, which at first glance appear similar to the grooves on Vesta,3 but there are important differences. The grooves on Vesta are concentric circles, centered on the two largest impact craters on that asteroid. Vesta's grooves are like lines of lattitude. The grooves on Phobos run from the "leading pole," or West Pole, to the the "trailing pole," East Pole. What I mean by that, is that Phobos always presents one face to Mars, like the Earth's Moon does to the Earth. But Phobos orbits so low over Mars that it gives the impression of being like an aircraft (well, a giant blimp) flying over Mars, with a nose and a tail. The nose is the West, or leading pole, and the tail is the East, or trailing pole.

With this orientation, the grooves on Phobos appear as if they were lines of longitude, or the stitching on a football, with the "points" of the football at the East and West poles. See figure 1 at the top of this page. I strongly reccoment you click on the image, to see it at full size.

Phobos Grooves
Figure 6. Grooves on Phobos, from another angle.

How did the grooves on Phobos form? It's anyone's guess at this point, to be settled by future lander or rover missions. But my opinion is that Phobos orbits low enough over Mars, that it experiences a very slight atmospheric friction, or wind. The wind is very thin, but it always blows the same direction, at thousands of miles per hour. Over millions of years, it has blown boulders across the surface, from one pole to the other, leaving the grooves we see.

Phobos crater streaks
Figure 7. Phobos. Stickney crater, showing dark streaks.

Another process on Phobos is very similar to what we see on Vesta. That is dark streaks from landslides on the walls of craters.3

Vesta crater streaks
Figure 8. Vesta. A fresh crater, showing dark and light streaks.


Section 3: Heat and Resources

There is talk of using Phobos as a refueling station for future manned Mars expeditions. The plan is to send a robot lander first, with tanks and solar cells, that breaks down water, methane, or Carbon dioxide, into useful gasses for fuel like Hydrogen and Oxygen. If methane can be collected, it could be used directly for fuel.

Temperatures on Phobos
Figure 9. Phobos with temperature measurements overlaid at several points.

This photo puts the refueling idea for Phobos into some doubt. While the colder temperatures are low enough to preserve frozen gasses, the warmest temperatures on the moon are enough to cause evaporation, and all parts of the surface see sunlight, sooner or later. A probe might have to dig deep, to get to layers of frozen gasses.

On the other hand, 25 degrees C is like a balmy summer day on Earth, so human explorers might find Phobos a pleasant place to visit. Correction: A reader has pointed out that "25 degrees C" is an error, and that the temperatures on the photo are in Farenheit. 25 degrees F is certainly not balmy weather for astronauts. It is below the freezing point of water, but above the melting points of ammonia, methane, and carbon dioxide. Therefore the chances of discovering usable ice deposits are much greater than indicated above. (Thanks to Redditor Douces for this correction.)


Section 4: Conclusions

There is much more to be learned at Phobos. It is a good target for future probes and human expeditions. Here are some more pictures.4

Phobos, Diemos from Mars
Figure 10. Phobos, Diemos from Mars at night, with stars. Right picture is overexposed to better show stars. Photo by MER rover Spirit.

2 moons
Figure 11. Phobos, Diemos.
Time lapse photo.
2 moons
Figure 12. Phobos, Diemos in color.
This was the original top photo.


1. "NASA Spacecraft Images Mars Moon in Color and in 3D," Jet Propulsion Laboratory News, 04.09.2008
2. "Phobos (moon)," Wikipedia artcle, 11 October, 2011.
3. "Fresh crater with dark and bright material," Dawn- Image of the Day, October 9, 2011, JPL-Dawn-Vesta web site.
4. The source for most of the pictures in this article, and many more pictures of Phobos is the JPL web site's photojournal search page.
Off TopicFunny Pictures from Orbit

© 2011 F. E. Harris for Hypertek Publications. All Rights Reserved.