Every once in a while, a reminder pops up that our planet is truly unusual in many fabulous ways. These reminders don’t even have to come from far away—they can be as close as our next-door neighbor.
Similar to Earth, Mars experiences times when its moons cast shadows on its surface. But the “eclipses” on Mars, captured by the NASA rovers Opportunity, Curiosity (embedded below) and now Perseverance, are very different from those on Earth.
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Mars’ moons Phobos (“fear” in ancient Greek) and Deimos (“fear”) circle Mars every 7.65 and 30.35 hours respectively, a relative blink compared to the Moon’s 27-day orbit. land They’re also much smaller than the Moon, and considerably lumpier—little moontatoes, rather than the beautiful round disk we see shining so silvery in our night sky.
Technically, these events are not eclipses as we would experience them, but transits that never completely block the light of the Sun. When the lumpy moons of Mars pass between the Sun and observers on the Martian surface, they don’t cover the star as completely as the Moon can from here on Earth.
Check out an example of traffic as seen by Perseverance in the video below.
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It is not difficult to imagine the Sun as a giant eyeball with a peculiar pupil, tracking something unseen by the surface observer.
Fanciful notions aside, scientists have observed a strange effect on the planet with the passage of Phobos’ shadow. The Mars InSight lander, an observatory designed to measure seismic activity, tilts slightly during these events. Scientists have attributed this slight tilt to the deformation of the Martian surface as a result of the very slight cooling effect of reduced solar radiation.
Of the two moons, Phobos has the larger silhouette, blocking up to 40 percent of the Sun’s light, even when the glare is completely engulfed. Deimos, much further away and smaller, sheds far less light, highlighting just how incredibly special our planet is.
During a total solar eclipse on Earth, the Moon’s disk covers the Sun’s disk perfectly, even though the Moon is much smaller.
This is due to a very fascinating coincidence. The Moon is about 400 times smaller than the Sun. It is also about 400 times closer to Earth than the Sun. This means that the Sun and Moon appear to be roughly the same size in our sky.
This does vary a bit, because neither the Earth’s orbit around the Sun nor the Moon’s orbit around the Earth are perfectly circular. So their sizes may appear slightly larger or smaller, depending on the orbital positions. This is how we get annular eclipses, when the Moon does not completely cover the Sun, leaving a ring of light around the Moon’s disc.
Even more interestingly, the emergence of our species seems to have occurred just in time to marvel at such perfect eclipses. The Moon began its life much closer to Earth and is currently moving away at a rate of about 3.82 centimeters (1.5 inches) per year. Give it another 600 million years or so, and total solar eclipses will no longer be possible.
The video below shows Curiosity’s data from an “eclipse” of Deimos.
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Observing solar eclipses from Earth can teach us interesting things about the Sun (and allowed for a critical test of general relativity more than 100 years ago).
On Mars, scientists can also use eclipses to learn things. They can relate the motion of Phobos to its gravitational effect on Mars and use this information to understand the mysterious Martian interior.
Not to mention predicting the final fate of Phobos. The moon is getting closer and closer to Mars, and one day it will grow so close that Mars’ gravity will pull it apart. Then, scientists think, the red planet will temporarily have a ring of Phobos guts.
Tracking Phobos and Deimos across the Martian sky may provide more data that will help map and predict this violent fate.