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Peek-a-boo! NASA released a striking video this week taken by the Perseverance Mars rover of the Martian moon Phobos crossing the face of the sun. The images are the latest and best solar eclipse imagery ever made on the Red Planet since the twin NASA rovers Spirit and Opportunity took the first time-lapse photos of Phobos during a 2004 solar eclipse.
Curiosity, the other active Mars rover, took the first video with its Mastcam camera in 2013, but Perseverance's Mastcam-Z camera is a major upgrade. It has the ability to zoom in, shoot at a much higher frame rate and photograph in color through a special solar filter. The result is a far more dramatic and life-like view of the eclipse. And, oh, the detail. You can clearly see ridges and concavities in the spud-shaped moon as well as several groups of sunspots on the sun. Pretty amazing.
“I knew it was going to be good, but I didn’t expect it to be this amazing,” said Rachel Howson of Malin Space Science Systems in San Diego, one of the Mastcam-Z team members who operates the camera.
On Earth, a solar eclipse can last for several hours with totality from a few seconds to 7 1/2 minutes long. This particular Phobos eclipse lasted a little more than 40 seconds. Some are even shorter.
Phobos orbits much closer to Mars — just 3,700 miles (6,000 km) away — so it moves much faster across the Martian sky compared to the moon, located 239,000 miles (385,000 kilometers) from Earth. That's why it zips by so quickly.
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To give you an idea of how fast Phobos orbits, both it and the moon move eastward as they circle their respective planets. We don't normally see the moon's eastward motion during the night because our planet spins fast enough to overcome that movement and "force" the moon to rise in the east and set in the west, just like the stars. But make no mistake — it's moving east. Only we have to wait until the following night before it's noticeable.
Not so with Phobos. It travels eastward so fast that it rises in the west and circles Mars three times a day. Crossing the sky in four hours, it moves visibly during the night and changes its phase just as quickly, something the moon takes days to do.
Because it distance from Mars varies around its orbit, Phobos also changes in apparent size during the night, from about one-fifth as big as our moon when most distant to one-third as big when closest. Also, because the moon orbits above the Martian equator and so close to the surface, it's below the horizon and invisible from latitudes greater than 70.4 degrees north or south.
Orbiting so close to Mars, the little moon will ultimately face dire consequences. The gravitational interaction between the two bodies slightly deforms the planet's crust and mantle while also shrinking Phobos' orbit at the rate of seven-tenths of an inch (1.8 centimeters) a year, or 6 feet (1.8 m) per century. Within 30-50 million years it will either collide with Mars or break apart to form a planetary ring.
Eclipse observations help astronomers better understand the moon's death spiral while also offering a fresh and striking perspective on events in the Martian sky.
Speaking of which, calling this an eclipse may be something of a misnomer. In a broad sense it resembles an annular solar eclipse , when the moon covers all of the sun except for a narrow "ring of fire". However, because Phobos is so much smaller than the sun this is better described as a "transit." In a transit, the nearer object (Phobos) appears smaller than the object it's passing in front of.
Just thought you'd like to know.
"Astro" Bob King is a freelance writer for the Duluth News Tribune.