Our own moon, the thinking goes, formed when a huge rock slammed into the Earth billions of years ago. Is the same true of one of Mars’ dual moons?
The Martian moon Phobos hides an unknown history. One idea has been that the 12-mile by 17-mile rock came from the nearby asteroid belt, and Mars’ gravity captured it. However, new evidence from the European Space Agency’s explorer Mars Express suggests that the stuff of Phobos is more Mars-like than asteroid-like, and therefore its origin goes back to a violent collision that knocked material from Mars into its own orbit. That material would have eventually coalesced into Phobos.
Speaking in Rome, the scientists led by Marco Giuranna said that the data sent back to Earth by a spectrometer aboard the Mars Express show a poor match between the rock of Phobos and material from chondritic meteorites that have landed here on Earth, and which scientists believe reflect the makeup of an asteroid that could have become Phobos under the capture scenario.
Also, the density doesn’t match up. Phobos appears to be a loose assemblage of material, not dense like many asteroids.
“This number is significantly lower than the density of meteoritic material associated with asteroids. It implies a sponge-like structure with voids making up 25%-45% in Phobos’s interior,” said Dr Rosenblatt. A highly porous asteroid would have probably not survived if captured by Mars. Alternatively, such a highly porous structure on Phobos could have resulted from the re-accretion of rocky blocks in Mars’ orbit. [BBC News]
However, because of the potential for amalgamation in orbit, some scientists maintain that these findings don’t rule out a capture origin for Phobos:
Pascal Lee of the Mars Institute in Moffett Field, California, says it may be too soon to rule out a capture scenario, since some of the silicate minerals observed by Mars Express are also seen on carbon-rich asteroids. “The data seem exciting, but from what I have seen, they may still be consistent with Phobos being a captured asteroid,” he [said]. He adds that a captured asteroid could also explain Phobos’s low density, if it was broken apart by a large impact and later reassembled from the resulting debris. [New Scientist]
Humans may soon get a closer look at Phobos. As DISCOVER noted when Mars Express beamed back stunning images of the moon in March, Russia is planning a mission to send an unmanned probe to land there.
DISCOVER: Russia’s Dark Horse Plan To Get To Mars
Bad Astronomy: More Incredible Phobos Imagery
Bad Astronomy: Phobos Ahoy!
80beats: Released: Stunning Close-Up Photos of the Weird Martian Moon, Phobos