White spots on Ceres evolving
WHEN the spacecraft called Dawn arrived at the dwarf planet Ceres a few years ago, it astounded astronomers. Certainly, it was pleasing to have the first close-up pictures of the largest of the bodies orbiting between Mars and Jupiter, but a bright white spot, visible even at a considerable distance, was quite unexpected.
Now, analysis of the Dawn observations has led to the conclusion that there was not only a salty ocean beneath the surface in the past, but that it still exists.
Ceres was the first of the asteroids to be detected. It was discovered on January, 1, 1801 — the first day of the 19th century — by Guiseppe Piazzi at the Palermo Observatory in Sicily.
Astronomers had long expected that a planetary object would be found between Mars and Jupiter, because there was a significant gap between the orbits of the two planets. However, as it turned out, there was no major planet within the gap. Ceres was simply the first to be discovered, and the largest, of the rocky bodies in that region of the solar system. With the adoption of the term “dwarf planet” by the International Astronomical Union in 2006, Ceres became one of these, along with Pluto and others.
It was 2015 when the Dawn spacecraft approached Ceres and first showed us a bright white spot. I had a few queries at the Planetarium as to whether it could be a sign of an alien civilisation. However, it turned out that the spot was caused by reflective salts.
These spots are an indication of salty liquid that has been brought to the surface from underneath. There is no liquid actually on the surface, because it would immediately evaporate in the vacuum to which it is exposed.
However, other materials that were in the liquid have been left behind.
Dawn’s measurements have shown that the deposits contain the salt sodium carbonate, in addition to other compounds. (The scientific description of a “salt’’ is not restricted to the salt sodium chloride that we are used to using with our food.)
Key points of interests are how the white deposits got there, and how long they have been there.
It is inside a crater called Occator that much of the research has been focused. The very bright white spots there were the first to be noticed during Dawn’s approach, and it is thought that the salty material was brought to the surface by the impact that caused the crater.
However, it is estimated that this impact occurred about 22 million years ago. If there had been no further activity, the brilliance of these spots would have been much reduced, or perhaps eliminated, by a long period of tiny impacts, gradually removing traces of the salts.
The answer is that the salts we see are not very old, and that processes are still occurring in Ceres that are bringing fresh material to the surface.
In another area on Ceres, pictures from Dawn have shown us an amazing feature called Ahuna Mons, which is called a cryovolcano because it is emitting icy material. Ahuna Mons is about four kilometres tall, which is nearly half as tall as Mount Everest on Earth. This is very significant: bear in mind that the diameter of Ceres is only about 940km, compared with Earth’s diameter of 12,742km.
The sub-surface “ocean” on Ceres is not as large as the ones we have detected in Jupiter’s moon Europa, or Saturn’s moon Enceladus. However, there is no doubt that a salty, “slushy” material is there.
Another piece of evidence is that the information from Dawn shows that there is a variation in the density of material just below the crust, consistent with a volume of liquid. This kind of deduction can be made by carefully monitoring the way in which Ceres’ gravity has affected the motion of the craft.
We could once view the asteroids simply as moving points of light through our telescopes. Now, our spacecraft are able to show them to us in stunning detail. We live in exciting times.
… WHEN THE DAWN SPACECRAFT APPROACHED CERES AND FIRST SHOWED US A BRIGHT WHITE SPOT. I HAD A FEW QUERIES AT THE PLANETARIUM AS TO WHETHER IT COULD BE A SIGN OF AN ALIEN CIVILISATION.