Also Available in:
This article is from
Creation 40(3):50–51, July 2018

Browse our latest digital issue Subscribe

Mars moons mystery

How did Phobos and Deimos get to where they are?



It wasn’t until 1877 that the two moons of Mars were discovered.

American astronomer Asaph Hall, acting on suspicions that Mars had at least one moon, had conducted a night-by-night methodical search using the telescope facility at the U.S. Naval Observatory in Washington, D.C. Frustrated at not finding anything, he was about to give up but his wife Angelina urged him to continue. The following night, 12 August, he discovered Deimos, closer in to the red planet than anyone had thought of looking before, orbiting just 23,000 km away.

 (By way of comparison, our Moon is about 384,000 km from Earth.1) Six nights later, Hall discovered Phobos, even closer in—only about 9,000 km from Mars.2

Their close proximity to Mars and relatively small size had kept the moons hidden in the glare from the red planet until Asaph Hall’s dogged persistence finally found them. It helped that he had a much larger, and thus light-gathering, telescope than that available to astronomers like Galileo—26 inches (66cm) in diameter. He named them from Homer’s ancient poem, The Iliad, in which Phobos (Gk. = fear) and Deimos (Gk. = flight, as in fleeing after defeat) were the twin sons of Ares (Mars in the Roman pantheon), accompanying him into battle.

Over the next century, as more information came to light about the two Martian moons, there was much to fascinate astronomers. Phobos is about 22 km in diameter, while potato-shaped Deimos is even smaller, being about 12 km across at most. While our Moon takes a leisurely month to orbit the Earth, Deimos takes about 30 hours to go around Mars, in a nearly perfectly circular orbit around its equator. Meanwhile the remarkable Phobos goes around Mars in only eight hours, i.e. about three times per Martian day. Being faster than the red planet’s rotation, Phobos would appear to an observer on Mars to rise in the west and set in the east, with that moon passing through all its phases in a few hours.

The biggest puzzle in the minds of many cosmologists, however, concerns theories about the formation of the moons.

Deimos bears more resemblance to an asteroid than to any of the other moons in the solar system. This and other factors led many to suggest that both Deimos and Phobos might hail from the asteroid belt, pushed from there by Jupiter then captured by the gravity of Mars. But the near-circular orbit of both moons does not sit well with that idea. Also, the atmosphere on Mars is so thin that it “would have a hard time providing the necessary braking to settle the pair into their present-day orbits.”2 And the moons are less dense than objects in the asteroid belt.

Other ideas—that the moons formed around Mars through dust and rock being drawn together there by gravity, or that they originated as a result of something colliding with Mars—also have serious problems. So the ‘captured asteroid’ theory remains the most popular, despite the awkward facts already mentioned and that “the capture mechanism is unknown and the scenarios unlikely”.3

A major challenge for the various formation theories about Phobos and Deimos concerns the constraints in timing. The difficulty in a nutshell: Deimos is inexorably spiralling away from Mars (just as our Moon is moving away from Earth), and Phobos is ‘doomed’ in the other direction—it is progressively being drawn closer to the red planet. Spiralling inward at a rate of about 1.8 metres per century, Phobos is on track to crash into Mars within 50 million years2—“an eyeblink in [evolutionary] astronomical terms”.4

That is why formation theories for the Martian moons have to try to come up with an origins scenario that is substantially younger than the billions-of-years evolutionary age ascribed to Mars.

The problem is so stark, and the unusual characteristics and decaying orbit of Phobos in particular so distinctive, that in the 1950s and 1960s “reputable scientists”, including the science advisor to USA President Eisenhower, suggested that Phobos had been put there artificially.2 I.e. that it was a “hollow artificial satellite” deliberately lofted into position from the Martian surface by intelligent beings living there.

However, when subsequent data and imaging definitively showed the rocky characteristics of both moons, the idea of them being artificial satellites was abandoned.

Note however that the original primary reason for invoking an artificial origin to explain the mystery hasn’t gone away, i.e. the distinctive motion of the moons. The conclusion of the scientists that it pointed to intelligent placement was perfectly reasonable—but they were wrong in thinking that it was ‘Martian Intelligence’ that had thwarted naturalistic origins theories. Rather, the Intelligence was (is!) eternal, and revelatory:

The heavens declare the glory of God, and the sky above proclaims His handiwork.” (Psalm 19:1)
Posted on homepage: 4 November 2019

References and notes

  1. Redd, N.T., Deimos: Facts about the smaller Martian moon, space.com, 21 June 2016. Return to text.
  2. Redd, N.T., Phobos: Facts about the doomed Martian moon, space.com, 21 June 2016. Return to text.
  3. Dick, S.J., Under the moons of mars, nasa.gov, 19 November 2007. Return to text
  4. The rate of our Moon’s recession from Earth raises huge difficulties for evolutionary timelines there, too.
    See: Sarfati, J., The moon—the light that rules the night, Creation 20(4):36–39, creation.com/moonReturn to text.