Mars lake was discovered using the MARSIS radar on board the "Mars Express" orbiter, and the profiles were collected between May 2012 and December 2015. The lake is centered at 193°E, 81°S, a flat area that does not exhibit any peculiar topographic characteristics. It is mostly surrounded by higher ground except on its eastern side, where there is a depression. Mars has two relatively small (compared to Earth's) natural moons, Phobos (about in diameter) and Deimos (about in diameter), which orbit close to the planet. Asteroid capture is a long-favored theory, but their origin remains uncertain. Both satellites were discovered in
Transit of Phobos from Mars the autumnal equinox and the vernal equinox, while farther from the equator they happen closer to the winter solstice. Observers at high latitudes (but less than 70.4°) will see a noticeably smaller angular diameter for Phobos because they are considerably farther away from it than observers at Mars's equator. As a result, transits of Phobos for such observers will cover less of the Sun's disk. Because it orbits so close to Mars, Phobos cannot be seen north of 70.4°N or south of 70.4°S; observers at such latitudes will obviously not see transits, either. Mars Rover "Opportunity" rover photographed transits of