Beginners Astronomy: Mars 3 – The Moons of Mars

Mars: Credit NASA

By Duncan Lunan

The two moons of Mars were discovered in 1877 by Asaph Hall.   The largest crater on Phobos, the inner moon, is named ‘Stickney’, the maiden name of Asaph Hall’s wife, who encouraged him to keep looking when he was ready to  give up.   Both are so close to the planet that neither Phobos nor Deimos can be seen from the poles of Mars, and at first Asaph Hall thought that there were several inner moons of Mars, because Phobos is the only natural satellite to revolve in less than its planet’s day, and Deimos is almost in synchronous orbit.   Deimos circles the planet four times in five Martian days.

It led to a Russian suggestion that the moons of Mars might be artificial, by I.S. Shklovskii, a distinguished radioastronomer who collaborated with the late Carl Sagan on the book “Intelligent Life in the Universe”  (1966), in which the idea was given a whole chapter.  It was based on the observations of US astronomer B.P. Sharpless, who discovered in 1945 that the inner moon Phobos was accelerating in its orbit, indicating that it was spiralling towards Mars.  If Phobos was made of material with the same density and albedo  (percentage of light reflected)  as Mars itself, it would be a very small object and if the braking effect was produced by atmospheric drag, Phobos’s density would be low enough to imply that the moon had to be hollow.  Perhaps they’d only just been launched when Asaph Hall discovered them?  Actually, telescopes of the size required to see them had only recently become available.

In 1968 I suggested that Phobos might be the galactic library, and in “The Cosmic Connection”  (Doubleday, 1973), Carl Sagan recounts how the first close-up of Phobos by Mariner 9 showed what appeared to be a bright light on the surface, but turned out to be just a data error.  That didn’t stop the artist Ed Buckley, who was illustrating “New Worlds for Old” and “Man and the Planets” at the time:  he produced a painting, which hung in ASTRA’s lecture room for many years when we had our own premises, showing an astronaut filming what appears to be a door in the face of the moon.

In the mid-1970s the idea was thought to be so fantastic that the Sharpless observations had to be wrong, but they weren’t.  The Viking Orbiter flybys showed that both Phobos and Deimos are much darker and larger than anyone expected, and consequently much less dense than Mars is.  Their density is barely higher than water’s and although they appear to be captured asteroids, or fragments of one larger one, they’re darker and less dense overall than any known class of asteroids, even the dark, fragile carbonaceous chondrites, and have turned out to be fragments of the Martian crust, blown off by impacts.   Carbonaceous chondrites have a high water content, and if Phobos and Deimos do, that could be very important.   Dr. Fred Singer proposed a manned mission to Mars called the Ph-D mission, making use of propellants manufactured on the moons.  Dr. Iván Almár of Budapest counter-proposed that Phobos should be made a science park, kept pristine for future investigators with more advanced equipment, like the northwest quadrant of Stonehenge.      

Nevertheless, in the late 1980s Soviet plans for the exploration of Mars turned on the manufacture of propellant on Phobos, and the two ‘Phobus’ probes were intended to determine the surface constitution of the moon.  Unfortunately the first of them was accidentally switched off in flight, and the second one failed in Mars orbit on 25th March, 1989, in mysterious circumstances.  Some accounts say that it was ‘attacked’ by a rocket-shaped projectile from Phobos, and my friend Andy Nimmo saw a video of it which presumably was a simulation, since Phobus 2 did not carry movie cameras!   I have been sent a single frame, allegedly one of the last transmissions from the probe, extracted from a book by David Childress called “Extraterrestrial Archaeology”  (Adventures Unlimited Press, 1994/5), but it turns out to be an artefact of processing the picture with high contrast.    Unfortunately the only detailed account I’ve seen of Phobus 2’s last minutes came from Dr. Zecharia Sitchin, in his book “Genesis Revisited”, and I would need independent verification to accept it.  He reproduced a photo of a ‘long, thin ellipse’, which he alleges to be an object between the spacecraft and the planet.   But I’ve yet to see a convincing argument that it isn’t simply the elongated shadow of Phobos itself.

These color-enhanced views of Deimos, the smaller of the two moons of Mars, result from imaging on Feb. 21, 2009, by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter.

NASA Deimos

It’s sometimes alleged that the existence of Phobos and Deimos was known in classical times, hence Homer’s allusion to them as the horses drawing Mars’ chariot.  Immanuel Velikovsky suggested that the Solar System had been so disturbed in classical times that the moons were visible to the naked eye, in which case they’d have to be well within 20,000 miles of Earth.  In his Foreword to “Return to Mars”, Hodder & Stoughton, 1955, Captain W.E. Johns wrote, “Why horses?   Did they have tails?  Comets have tails.  Whatever they were they were visible to the naked eye, from which we are forced to the conclusion that Mars, the ancient God of War, was at one time nearer to us than it is today.”  To say the least, it’s very unlikely that was the case.  Phobos and Deimos were actually named by Henry Madan, a science master at Eton, the uncle of the 11-year old Venetia Burney who suggested calling the new planet of 1930 ‘Pluto’.  

When Galileo Galilei first published his discoveries he cast them as anagrams, to keep them secret while ensuring his priority.  Johannes Kepler, who was not given the key, tried to decode them and came up with, “There is a red spot in Jupiter which rotates mathematically” and “Mars has two satellites”.  He wasn’t so far wrong with the second reading, because Galileo was actually announcing his glimpse of the rings of Saturn, which he misinterpreted as two large moons.   However, students struggling with Kepler’s Laws have been known to suggest that he should have stuck to solving anagrams instead of studying planetary motions!

Much has also been made of Jonathon Swift’s statement, in “Gulliver’s Travels”  (1726), that the astronomers on the flying island of Laputa have discovered two Martian satellites, for which he gives orbital periods and distances of three and five Mars diameters, superficially resembling those of Phobos and Deimos, and states that they conform to Kepler’s Laws.   There was even a suggestion in 1964 that Swift knew about the moons of Mars because he had come from there by flying saucer!  However, the fit isn’t close enough.  The figures were analysed by Robert S. Richardson  (“Mars”, 1965), who pointed out that from them the supposed mass of Mars can be calculated, and it’s six times too large.  Richardson toys with the idea that Swift might have meant ‘radius of Mars’ when he wrote ‘diameter’, and that gives a much better fit;  but the plain fact is that no telescope large enough to show the Martian moons had been built by the end of Swift’s lifetime.  

The most likely explanation is that if Earth had one moon, and Jupiter was then known to have four, Mars would have two by simple interpolation.  In fact the idea was widely circulated in the 18th century, and one well-known citation of it is in Voltaire’s “Micromégas”.  Swift presumably made up the figures to make it look convincing, as he did with the dimensions of Laputa itself.  But in “A Space Traveller’s Guide to Mars”  (Gollancz, 1957), Dr. I.M. Levitt suggests that Swift based the orbits on those of the moons of Jupiter, using figures in “Astronomy” by David Gregory, whose second edition was also published in 1726 and which gives their distances as ratios to Jupiter’s diameter.  That was all that could be done before astronomers determined the value of the Astronomical Unit, the Earth’s distance from the Sun, after which all the other distances could be calculated by Kepler’s Laws.  Gregory gave ratios of 2 5/6, and 4 ½, and Levitt suggests Swift rounded those up to 3 and 5 for his imagined moons of Mars.  (David Gregory was one of a distinguished family of 18th century Scottish scientists, including the first translator of Euclid and the inventor of the Gregorian telescope, still in use today.)

Phobos and Deimos have their real-life mysteries, however.  Both moons have thick coatings of regolith, broken rock, and that came as a surprise.  Although it was expected that they’d have undergone many collisions, since Mars is nearer to the Asteroid Belt than we are, it was thought that the gravity was too low to hold the debris.  Deimos’s mass is so low that a human being could jump off it, and Phobos’s orbital velocity is high enough for it to be sandblasted clean.   But many of the same arguments apply to the asteroid Eros, which turned out to be deeply covered with regolith when the NEAR-Shoemaker probe got there.

Even stranger, however, are the grooves on Phobos which were discovered by the Viking Orbiters.  At first sight they looked for all the world like strata, as if Phobos was a crustal fragment of some much larger, differentiated body.   More detailed examination revealed they are parallel to the short axis of Phobos, concentric with the long axis which permanently points towards Mars.  On one hemisphere, the grooves break up into long chains of craters.   Those could have been formed by impact debris blasted off into orbit, coming back like machine-gun fire, but that doesn’t explain why they should be parallel or why they should merge into grooves on the other hemisphere.  One suggestion was that tidal stresses might have released the internal water from the rock to explode as steam at the surface, but the forces acting on tiny Phobos aren’t sufficiently powerful to make that convincing.  Another suggestion is that the grooves formed due to resonance when Phobos’s crust was stripped off by the impact which formed the crater Stickney.  That too is possible, but some of the grooves cross the floor of Stickney and that makes it seem less likely.  Deimos is covered in thick regolith, a rock dust layer like the Moon’s, but doesn’t have grooves like the ones on Phobos.

In 2010 a close flyby of Phobos, by Europe’s Mars Express probe, detected hydrated rocks of types which had already been located on the Martian surface.  It was already known that there were void spaces within Phobos, accounting for the low density measured by the Viking Orbiters in the 1970s.  The startling conclusion was that Phobos and presumably Deimos were formed from material blasted off the surface of Mars in big impacts – like the formation of Earth’s Moon, on a smaller scale.   It ruled out the idea that the moons of Mars are captured asteroids, which until then had been widely believed.

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