On May 12, 2016, the Hubble Space Telescope captured a series of images of Mars and in them the planet’s moon Phobos can be seen appearing from behind the western limb. This was just 10 days before opposition which, in 2016, was the closest Mars had been to Earth since 2005, lending particularly good opportunity for picking out its largest—yet still quite small—moon.
If you’ve ever visited the Grand Canyon in the southwestern U.S. you know why it has the name it does—the vistas from the rim of this geological wonder are simply breathtaking, and it’s even more amazing to realize that it was all carved over the course of millions of years through the erosive action of the Colorado River. But there’s an even grander canyon system in our Solar System, and it’s not on Earth: it’s Mars’ Valles Marineris, which spans over 2,500 miles and is four times deeper than the Grand Canyon in Arizona—in fact, if Valles Marineris were on Earth it could stretch clear across the continental United States!
It’s thought that Mars’ canyon was created as the planet cooled, its crust contracting and splitting apart (as opposed to gradual excavation by a flowing river.) But there are sections of Valles Marineris that appear to have had a watery past, and new maps made by the USGS from data acquired by the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter—which has been in orbit around Mars since March 2006—reveal regions that were once likely covered by liquid water.
“The major finding of this work is that the layers of rock exposed within what is called western Candor Chasma record a past environment where groundwater was abundant and occasionally seeped onto the ground surface, forming pools,” said USGS scientist Dr. Chris Okubo, lead author of the maps. “These pools would have been habitable for life, just as they are on Earth. Dust and sand blown into these pools may have buried and preserved evidence of past Martian life, which would be present as fossils within the rocks that we see today.”
It may be in its 14th year on Mars but Opportunity still has some surprises to show us—like this, a series of images captured on May 3, 2017 showing the Sun as seen from Mars. But that’s not the special part: see the change in brightness along the Sun’s edge near the end? That was a brief transit of Phobos, the largest (and nearest) of Mars’ two moons!
Can’t see it very well? It’s quick, I know—so check out a cropped and enlarged version below:
It’s all downhill from here! (Well not really, but it was for a little while when Opportunity was at the top of that hill!) The image above is a mosaic I assembled from six color-composites, each made from three separate images acquired in near-infrared, green, and near-ultraviolet color wavelengths on April 21, 2017 (mission sol 4707). It’s been adjusted to appear in approximate true color to what the scene might look like to a human standing on Mars. The view shows a ridge called “Rocheport” located on the western rim of Endeavour Crater (the interior of which would be toward the right in this image) which was the final segment of Opportunity’s last target region of exploration, Cape Tribulation. Opportunity’s wheel tracks can be seen at the bottom center, heading back up the ridge and zig-zagging toward the top (detail below).
Mars isn’t a planet well-known for its natural satellites but it actually does have two small moons. The larger, Phobos, is an irregularly-shaped, heavily grooved and cratered world only about 17 miles (27 km) across at its widest. It orbits Mars so closely that it completes 3 orbits every day, and isn’t even visible from some parts of the planet. But Phobos has an even smaller companion in orbit: Deimos, which at the most 7.5 miles across is half Phobos’ size. Deimos orbits Mars much further away as well, taking about 30 hours to complete one orbit.