Solar Eclipses on Mars

Phobos passes in front of the sun as seen from the Curiosity rover on March 26, 2019.

Earth isn't the only place in the solar system where solar eclipses occur. They are quite common on Jupiter, and Mars has them as well. During March 2019, NASA’s Curiosity rover on Mars witnessed two solar eclipses and took photos of both. The first eclipse, really a transit, took place on March 17th, when the smaller of Mars’ two moons, Deimos, passed in front of the sun. Deimos is only 7.5 miles (about 12km) across, but since it is much closer to Mars than the Moon is to Earth, it still is quite evident when passing in front of the sun.

Phobos, the larger of the two moons (about 14 miles across or 22km), whips around Mars once every 8 hours and does so almost directly over the Martian equator resulting in eclipses occurring virtually every day. Phobos is also closer to Mars than Deimos, only 3,700 miles above the surface (for comparison, the Moon is 239,000 miles from Earth). Phobos's high rate of speed and close proximity to the Martian surface means than solar eclipses, when they occur, are very brief though. The animation above was sped up 10x, but even in real-time the eclipse only lasted about 35 seconds. Deimos, being farther away from the surface and moving slower, takes longer to transit the sun as seen from the rover. The clip below shows the transit of Deimos on March 17, 2019, again sped up 10 times.

Both moons are spiraling closer to Mars, and so their relative size as observed from the ground will become larger with time. That could allow Phobos to eventually appear large enough to fully cover the disk of the sun during an eclipse, but gravitational stresses may tear Phobos apart before that happens.

Neither Deimos nor Phobos are large enough to be perfectly spherical. There small size and low mass keeps their gravitational pull low enough to prevent them from becoming round, and so they look rather potato-like.

Image of Phobos taken by NASA’s Mars Reconnaissance Orbiter on March 23, 2008. Image Credit: NASA/JPL/University of Arizona .

Besides being interesting to look at, the solar eclipses observed by the rovers on Mars allow scientists to better refine the orbits of both moons, which were rather uncertain prior to data gathered by the rovers. Nearly 50 eclipses have been observed from either Spirit, Opportunity, or Curiosity over the years, each observation helping to better refine the precise orbit of the moons.

Deimos as seen from NASA's Mars Reconnaissance Orbiter. Image Credit: NASA/JPL-Caltech/University of Arizona

So, while eclipses on Mars may not be quite as spectacular as those on Earth, they are still quite fascinating and help to provide valuable data.

Watch a YouTube video about the eclipse below: