THEMIS Looks at Phobos

These animations shows three views of the Martian moon Phobos as viewed by the Mars Odyssey orbiter. The apparent motion is due to movement by Odyssey’s camera, Thermal Emission Imaging System (THEMIS) rather than moon.

Each of the three panels is a series of images taken on different dates (from top to bottom): 29 September, 2017; 15 February, 2018; and 24 April, 2019. Deimos, Mars’ other moon, can also be seen in the second panel. These are visible light images, but THEMIS is mainly used for thermal-infrared scans.

Image Credit: NASA

Phobos and Deimos

Mars has two small moons. Phobos, which is about 11.5 km across, was imaged on 26 March, 2019, as in moved across the face of the Sun from Curiosity rover’s post of view. Deimos, which is only about 2.3 km across, was 17 March. Phobos doesn’t completely cover the Sun, so Curiosity saw what could be considered an annular eclipse. Deimos is so small compared to the disk of the Sun, astronomers would say it transited the Sun.

Image Credits: NASA

Bonus GIF—This series of images shows the shadow of Phobos as it sweeps over Curiosity and darkens the sunlight near sunset on 25 March.

THEMIS, Deimos, and Phobos

These images of Phobos and Deimos, the moons of Mars, were taken by the Mars Odyssey orbiter’s THEMIS (Thermal Emission Imaging System) camera using visible-wavelength light. The apparent motion of the moon caused by the camera’s point-of-aimaim being moved during the 17-second span of the observation.

The distance to Phobos from Odyssey during the observation was about 5,600 km. The Deimos was almost 20,000 km away.

Image Credit: NASA

Phobos in UV

phobos-in-uvThis is the Martian moon Phobos as observed by the MAVEN spacecraft’s Imaging Ultraviolet Spectrograph. Orange shows mid-ultraviolet (MUV) sunlight reflected from the surface of Phobos. Blue shows far ultraviolet light which is scattered off of hydrogen gas in the extended upper atmosphere of Mars. Phobos blocks the background far UV light, eclipsing the ultraviolet sky. Comparing MAVEN’s images and spectra of the surface of Phobos to similar data from asteroids and meteorites may provide clues to the moon’s origin–whether it is a captured asteroid or was formed in orbit around Mars. The MAVEN data will also help scientists look for organic molecules on the surface of Phobos. Evidence for such molecules has been reported by previous measurements from the ultraviolet spectrograph on the Mars Express spacecraft.

Image Credits: CU/LASP and NASA

Phobos 360

Images from ESA’s Mars Express orbiter have been combined into a virtual rotation movie showing what the tiny moon would look like from an orbit around it. The rotation is actually a digital illusion: Phobos is tidally-locked with Mars and always keeps the same face toward its home planet as does our Moon. Phobos is losing about of centimeter of altitude a year and is expected to break up and crash onto Mars within the next 50 million years.

Video Credit: ESA

An Annular Eclipse

phobos_eclipseThese three images are of Phobos, Mars’s larger moon, passing directly in front of the sun. They were taken about three seconds apart by the Curiosity rover. Curiosity photographed this eclipse with the telephoto-lens camera of the rover’s Mast Camera pair on 17 August, 2013, the 369th Martian day (or sol) of its mission on Mars. Because this eclipse occurred near mid-day at Curiosity‘s location on Mars, Phobos was nearly overhead, closer to the rover than it would have been earlier in the morning or later in the afternoon. This timing made Phobos’ silhouette relatively large against the sun—as close to a total eclipse of the sun as is possible from Mars.

Image Credit: NASA

The Moons of Mars


moons_apparent-sizesThe Curiosity rover on Mars used its cameras to take the series of pictures stitched together to make this video. These are the first images from missions on the surface which have caught one moon eclipsing the other. The images were taken on 1 August, but some of the full-resolution frames were not downlinked until more than a week later, in the data-transmission queue behind higher-priority images being used for planning the rover’s drives.

The picture on the left shows how big the moons of Mars appear to be, as seen from the surface of Mars, compared to the size that Earth’s moon seen from the surface of Earth.

Image and Video Credits: NASA

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

A Crater on Phobos

Stickney900Stickney Crater, the largest crater on the Mars’ moon Phobos, is named for Chloe Angeline Stickney Hall, mathematician and wife of astronomer Asaph Hall who discovered both the planet’s moons in 1877. At a bit more than 9 km across, Stickney is nearly half the diameter of Phobos itself. It’s so large that the impact that blasted out the crater nearly shattered the tiny moon. This enhanced-color image of Stickney was recorded by the HiRISE camera onboard the Mars Reconnaissance Orbiter at a range of about 6,000 km. Even though the moon’s gravity is less than 0.1 percent of Earth’s, streaks suggest loose material has been sliding down inside the crater walls over time.

Image Credit: NASA


This moon is doomed. In 100 million years or so Phobos will likely be shattered by stress caused by relentless tidal forces, its debris forming a decaying ring around Mars. Mars has two small moons, Phobos and Deimos. Their names are derived from the Greek for Fear and Panic. The larger moon Phobos is a cratered, asteroid-like object in this image taken by the Mars Reconnaissance Orbiter. Click to embiggen the image to 7 m per pixel resolution.

Image Credit: NASA

This Moon Is Doomed

Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These martian moons re probably asteroids from the main belt between Mars and Jupiter or possibly from further out in the Solar System. Phobos, the larger of the two, is revealed as a cratered, asteroid-like object in this image from the Mars Reconnaissance Orbiter. Phobos’ orbit is so close to Mars—about 5,800 km above the surface compared to 400,000 km for our Moon—that gravitational tidal forces are dragging it down. In 100 million years or so, Phobos will likely be shattered by stress caused by the relentless tidal forces, the debris forming a decaying ring around Mars.

Image Credit: NASA