ESA Science & Technology30-Jun-2005 05:48:14

Close Inspection for Phobos

Last Update: 30 Aug 2004

Phobos, Mars's tiny innermost moon, is under unprecedented scrutiny now that Europe's mission to Mars is in orbit around the Red Planet. Mars Express is due to pass within 3000 km of the 22 km diameter moon a few hundred times during its two-year nominal mission lifetime.

"There will be many more opportunities for close fly-bys than during NASA's Viking mission," says Tom Duxbury from NASA's Jet Propulsion Laboratory, a veteran of previous Phobos observations who is helping to plan the Mars Express observation campaign.

At a distance of 3000 km, the High Resolution Stereo Camera (HRSC) on board Mars Express will be capable of taking images with a resolution at least as good as any taken by Viking. At 1000 km, the Infrared and Visible Mapping Spectrometer (OMEGA) will also be switched on to map the mineral composition of the satellite's surface; and when the distance is only a few hundred kilometres, the Planetary Fourier Spectrometer (PFS) and the Sub-surface Sounding Radar/ Altimeter (MARSIS) will record measurements. The Energetic Neutral Atoms Analyser (ASPERA) will monitor the plasma environment around the orbit of Phobos during all fly-bys.

"Mars Express will provide the first global map of Phobos. This is important because we know one side of the satellite well, but the other side is less well-known," says Agustin Chicarro, ESA's Mars Express project scientist. "We should get a complete view of the satellite in terms of its topography, sub-surface and composition. This should help us determine once and for all whether it is a captured asteroid or not," he adds.

Mars Express could generate the most comprehensive suite of observations ever recorded for Phobos. They will help to resolve some of the outstanding puzzles about the Martian moon, many of which came to light during previous observation missions. In addition to Viking, these included the ill-fated Russian mission Phobos 2, and NASA's Mars Global Surveyor, which is still on mission around the Red Planet. Phobos 2 was due to send a lander to the tiny satellite in 1989, but was lost just 100 km above the surface.

Phobos, a world of mysterious origin

One of the most striking features on the irregularly shaped Phobos is the giant Stickney crater (left), clearly visible in this Viking Orbiter image (NASA)

Phobos is a world of mysterious origin and destiny. It is light, with a density less than twice that of water, and orbits just 5989 km above the Martian surface. Recent observations by Mars Global Surveyor have revealed that the surface is covered in one metre thick dust, suggesting erosion due to meteor bombardment as the lack of atmosphere rules out other means. One idea is that Phobos and Deimos, Mars's other moon, are captured asteroids. Data returned by the infrared mapping spectrometer experiment on board the Phobos 2 mission supported this view.

"In general the composition matched that of the inner asteroids. But we don't know whether Phobos is primordial or whether it has undergone transformation," says Jean-Pierre Bibring, principal investigator for the infrared spectrometer on Phobos 2 and also for OMEGA on Mars Express. "We found some tantalising evidence for transformation in the form of interesting minerals at the bottom of a crater, which were absent elsewhere on Phobos, suggesting that the bulk composition is inhomogeneous. I'm very excited about observing Phobos again because OMEGA will have much higher resolution and will be able to determine the surface composition unambiguously and pick out minerals at the bottom of craters, thus determining the bulk composition."

However, there are other ideas about the origin of Mars's moons. One, favoured by Duxbury, is that they are lightly accumulated ejecta from asteroid impacts on the Martian surface, with Phobos composed of ejecta orbiting Mars faster than the planet rotates; and Deimos, whose orbit is further out and orbital motion slower, composed of ejecta orbiting more slowly than the planet rotates.

Another mystery about Phobos concerns the origin of deep grooves on its surface. Some of these radiate from Stickney, the large impact crater that distorts the shape of the moon, and are thought to indicate that the force of the impact nearly shattered the small body. However, others on the central portion of Phobos run almost parallel and have prompted another thought.

John Murray, from the Open University, United Kingdom and a co-investigator on the HRSC, thinks that the grooves are strings of impact craters made by ejecta thrown up by major impacts on Mars. He has calculated the velocity of such ejecta and the position and direction of the 'grooves' they might be expected to leave on Phobos. "The direction of the grooves corresponds to what the hypothesis predicts," he says. "There are no grooves on the trailing edge of Phobos, which is also what you'd expect."

However, the leading edge of Phobos has never been imaged in close up before and the appearance of grooves here will be critical to the theory. The HRSC on Mars Express should provide the required images for the first time. "If I'm right, we should see wider chains of grooves crossing at all angles here," says Murray.

Facts and historical anecdotes about Phobos and Deimos

Scientists speculate that Deimos (depicted here in a computer mosaic constructed from Viking Orbiter images) and its companion moon Phobos were once passing asteroids that were pulled in by the gravity of Mars (NASA)

Unlike the Earth, which has one oversized satellite (the Moon), Mars has two very small satellites. Asaph Hall at the Naval Observatory in Washington D.C. discovered them in August 1877 and they were named after the nasty twin children of Mars, who are mentioned in a few scattered lines of the Iliad. Deimos means 'terror' or 'panic' and Phobos means 'fear' (or, possibly, 'rout' in the mind of Homer).

Both satellites have equatorial, almost circular orbits, with Phobos orbiting once every 7 h 39 min just 5989 km from the surface of Mars and Deimos orbiting once every 30 h 18 min at 20 062 km from the surface of the planet. The orbital period of Phobos is three times faster than the rotation period of Mars, with the unusual result among natural satellites, that Phobos rises in the west and sets in the east as seen from Mars. Also, it orbits so close to the surface of Mars that the curvature of the planet would obscure its view from an observer standing in Mars' polar regions.

The two bodies are too light for gravity to make them spherical. Both are somewhat potato-shaped, with major diameters of 22 km for Phobos and 12 km for Deimos. Phobos, at least, has a very lumpy appearance. It is heavily cratered and is covered with dust one metre thick, suggesting that it has suffered heavy meteor bombardment. Both satellites are probably composed of carbon-rich rocks. In 1988, Phobos 2, one of the last great scientific missions of the Soviet Union, detected outgassing from the satellite, but could not ascertain its nature because the spacecraft malfunctioned before fully accomplishing its objectives.

A remarkable feature on Phobos is a (relatively) large impact crater, named Stickney after the maiden name of Mrs Hall. Streaks on the surface of Phobos indicate that the satellite was almost shattered by the impact. Deimos and Phobos are thought to be captured asteroids, but an accepted (and complete) theory of their origin is still missing. Whatever its origin, Phobos will have an end: its orbit is shrinking by 1.8 cm per year. Thus, Phobos could either crash into Mars or break up to leave a ring of fragments in less than 100 million years time.

Deimos and Phobos are linked to an enigma in the history of astronomy. Although the two moons were not discovered until 1877, Jonathan Swift had written in 1720 (in Gulliver's Travels, chapter 3) that the inhabitants of Laputa had made important astronomical observations of 10 000 fixed stars and of the two satellites of Mars, one orbiting with a period 10 hours and the other with a period of 21 hours.

All sorts of theories have been invented to explain this remarkable coincidence. That Mars should have two satellites was an old idea, which Kepler had expressed in 1610 and others had elaborated further. Thus, Earth had one satellite, Mars should have been endowed with two, Jupiter with four, Saturn with eight et cetera. But guessing the periods and distances of Deimos and Phobos to within 30% is a less likely coincidence, which we will probably have to accept without explanation.

The two satellites were eventually discovered only after many years of searching. Why did Hall find them after others, no less well equipped, had failed? Some astronomers at the time advanced the hypothesis that the satellites were artificial, launched by the Martians just before Hall made his discovery.

Vital statistics for Phobos and Deimos

Parameter Phobos Deimos
Distance from centre of Mars (km) 9378 23 459
Distance from surface of Mars (km) 5989 20 010
Radius (km) 13.4 x 10.8 x 9.4 7.5 x 6.1 x 5.5
Mass (kg) 10.6 x 1015 2.4 x 1015
Rotational Period Synchronous Synchronous
Orbital Period (days) 0.32 1.26
Orbital Eccentricity 0.015 0.0005
Orbital Inclination 1.1 1.8

For further information please contact: