An eclipse on Mars
Ken Tapping, 18th September, 2013
On 17 August, the controllers of the Curiosity Mars Rover, a robot currently exploring the surface of Mars, commanded the robot to point a camera at the sky. It was to observe something we have never, ever seen before, an eclipse of the Sun, on Mars. This was a really an instance of being in exactly the right place at exactly the right time.
Here on Earth, eclipses of the Sun happen when the Moon passes between the Sun and the Earth. Because the Moon and the Sun look the same size when viewed from the ground, we have to be in precisely the right place to see the solar disc completely covered. This makes the events rarer, but the great benefit is that with the bright solar disc exactly covered, we can see the parts of the Sun usually hidden in the glare: the pearly solar corona and prominences, great loops of red, glowing gas trapped in the Sun’s magnetic fields. This convenient matching in sizes and distances is probably very rare, and may not occur anywhere else in our galaxy. We are very lucky.
Mars has two moons, Phobos and Deimos, Greek for “fear” and “panic”, appropriate companions for the God of War. However, these moons are very small, with diameters of 22 and 12 km respectively, compared with our Moon’s 3,475 km, and they orbit much closer to their planet. Phobos orbits Mars at a distance of 9,380 km and Deimos 23,460 km. Our Moon lies some 384,000 km from us. Because Mars’ moons lie so close to Mars, they don’t take long to complete an orbit. Phobos takes about 8 hours and Deimos roughly 30 hours. Our Moon takes a little over 27 days. From this we can conclude that neither Phobos nor Deimos can completely cover the solar disc, so total solar eclipses like the ones we have cannot occur on Mars. Secondly, with the moons being so much closer to the planet, eclipses will be very common, probably once per orbit, but we have to be in the right place. Eclipses at any single location are probably very rare. Finally, these eclipses will not last long, probably a few seconds. However, on 17 August, Curiosity happened to be in exactly the right place at the right time, and obtained a stunning video of Phobos crossing right through the centre of the solar disc.
The impression we get from the video, which can be seen on the NASA website, is of a jagged rock moving rapidly across the solar disc. Phobos and Deimos are too small for their gravity to be strong enough to pull them into spheres, so they solidified as very roughly rounded rocks. Almost as intriguing is an image taken from a spacecraft orbiting Mars, which shows Phobos’ shadow on Mars’ surface – a dark ellipse on the desert.
For most of our history of studying the universe beyond the Earth, the other planets have been dots or discs seen through telescopes, where seeing details required our sitting patiently at our telescopes waiting for those elusive moments where our atmosphere steadied and we got a detailed glimpse before the image went blurry again. Now, thanks to a few decades of sending spacecraft around the Solar System, and landing on other planets and moons, those dots and blurry discs have become places, with landscapes, geology, geography, weather and possibly biology. We have talked about “worlds like ours” for many years, but now we can actually see them as worlds like ours. I recently downloaded a picture from the Curiosity web site. It showed a landscape with a cliff in the distance, with clearly visible layers in the rocks. I sent it to a geologist friend, and he sent back an explanation of what the rock features were telling us about Mars’ history. This is a good example of how we are now able to study some other worlds the way we can study ours.
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