Transit of Phobos


phobos_transitMars has a couple of tiny moons named Phobos and Deimos. From the point of view of the Curiosity rover near Mars’ equator, the moons occasionally pass in front of, or “transit,” the Sun. These transits are as near as a Martian observer comes to seeing a partial eclipse of the Sun because the outlines of the moons do not completely cover the Sun. Earth’s Moon, of course, does blocks the entire Sun during a total solar eclipse. These eclipses, like those on Earth, occur in predictable “seasons” a few times each Mars year.

As part of a multi-mission campaign, the Curiosity rover is observing these transits, the first of which involved the moon Phobos grazing the Sun’s disk. The event was observed on Martian day, or sol, 37 (13 September, 2012) using Curiosity‘s Mast Camera, or Mastcam, equipped with special filters for directly observing the sun. This animation shows the transit as viewed by the Mastcam 100-mm camera (M-100).

Mission scientists use these events to very accurately determine the orbital parameters of the Martian moons. Phobos’ orbit is very close to Mars, and it is slowly spiraling in because of tidal forces. These forces change the orbital position of Phobos over time, and accurate measurements of those changes can provide information about the internal structure of that moon and how it dissipates energy. Deimos orbits much farther away and is slowly spiraling out.

NASA’s Mars Exploration Rover Opportunity will also attempt to observe a different set of Phobos and Deimos transits, seen from the other side of the planet, in Meridiani Planum.

Image Credit: NASA

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