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Starts With A Bang

NASA Rover Captures What A Solar Eclipse Looks Like On Mars

As captured just this year from Mars’s surface, eclipses of Phobos (L) and Deimos (R) can lead to spectacular annular, but not total, solar eclipses. (NASA/JPL-CALTECH/MSSS/KEVIN M. GILL)

From the right place at the right time, it’s a sight unlike any you’ll find on Earth.


In our Solar System, any planet with moons has a chance for a solar eclipse.

Solar eclipses are possible on Earth, and occur whenever the Moon aligns with the Earth-Sun plane during a new Moon. This same principle applies to any planet with a moon. (FLICKR USER KEVIN GILL)

They occur whenever a moon passes directly between its planet parent and the Sun.

An illustration of the Sun-Moon-Earth configuration setting up a total solar eclipse. When the Moon’s shadow falls on Earth when the nearer-to-the-Sun node aligns, we get a solar eclipse: total if the Moon’s shadow falls on the Earth, annular if the shadow ends before reaching Earth, and partial if the alignment is too imperfect. (STARRY NIGHT EDUCATION SOFTWARE)

From planet Earth, they can appear partial, total, or annular.

From Earth, eclipses can be partial, annular, or total, owing to the large angular size of our Moon as seen from Earth’s surface. From Mars, both of its moons are too small to create total solar eclipses. (© 2013 EXPLORATORIUM)

But on Mars, only partial or annular eclipses occur.

The closer and larger of Mars’s moons, Phobos, makes for a completely alien sight during solar eclipses. Annual eclipses occur frequently on Mars, even appearing perfectly aligned on occasion to the Curiosity Rover. (NASA/JPL-CALTECH/MALIN SPACE SCIENCE SYSTEMS/TEXAS A&M UNIV.)

Mars has two moons: Phobos and Deimos.

The two Moons of Mars, Phobos and Deimos, are both small and in relatively close orbits around Mars, but neither one has a large enough angular diameter as seen from the Martian surface to cause a total eclipse. (NASA / JPL-CALTECH)

Both are too small to completely cover the Sun’s disk.

As viewed from the Martian surface, Phobos (left) and Deimos (right) can block out only part of the Sun; Mars does not experience total solar eclipses. (NASA/JPL/MALIN SPACE SCIENCE SYSTEMS)

Just like Earth’s moon, Phobos and Deimos cast cone-shaped shadows as they orbit through the Solar System.

Within the umbral cone, with apex to the left of Phobos, the Sun is completely obscured. Within the penumbral cone, with apex to the right of Phobos, the Sun is only partially obscured. Since the umbral cone does not reach the surface of Mars at present, all eclipses there are partial. (BRUCE G. BILLS & ROBERT L. COMSTOCK)

However, those cones reach their end before encountering Mars’s surface.

Although it can never completely block out the Sun’s disk, Mars’s moon Phobos can cast a dark spot on the red planet’s surface: its penumbral shadow. Observers within that shadow can see a partial or annular eclipse. (NASA/JPL/MALIN SPACE SCIENCE SYSTEMS)

As a result, Martian solar eclipses never block out the Sun’s disk completely.

The smaller martian moon, Deimos, has features such as craters, composition, and orbital properties that place it in line with a chemical origin similar to Mars itself, rather than with a captured asteroid. (NASA/JPL/UNIVERSITY OF ARIZONA)

Smaller and more distant, Deimos appears tiny and dark, slowly passing between the Sun and Mars.

This enhanced color image of the Martian moon Phobos shows its irregular shape and fascinating surface features. Phobos, as it orbits Mars, creates spectacular annular eclipse views. (NASA / JPL-CALTECH / UNIVERSITY OF ARIZONA)

Phobos, however, is larger, closer, and more irregular, creating a spectacular silhouette against the Sun.

Mars’s moon Phobos, as it passed in front of the Sun on April 4, 2020, was captured by the Curiosity rover’s Mastcam. It’s irregular shape can clearly be seen. (NASA/JPL-CALTECH/MSSS/KEVIN M. GILL)

From NASA’s Mars Curiosity Rover’s Mastcam, humanity learns exactly what Martian solar eclipses look like.

Mars’s smaller, more distant moon, Deimos, appears as barely a dot against the bright orb of the Sun during an annular eclipse. Still, these are frequent and varied enough that the Curiosity rover has had this type of perfect alignment occur numerous times. (NASA/JPL-CALTECH/MSSS/KEVIN M. GILL)

Kevin Gill used that data to construct true-time videos of eclipses for both Martian moons.

The Phobos eclipse occurred on April 4, 2020; Deimos’s occurred on March 28, 2020.

In its ancient past, Mars may have had an innermost, third moon, bringing total eclipses along with it.

Rather than the two Moons we see today, a collision followed by a circumplanetary disk may have given rise to three moons of Mars, where only two survive today. Just as Earth’s moon was formed by a great impact long ago, so, too, were Mars’ moons. (LABEX UNIVEARTHS / UNIVERSITÉ PARIS DIDEROT)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.

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