Dawn at Ceres 
Kamal Lodaya, The Institute of Mathematical Sciences, Chennai

Ceres was discovered by the Italian priest and astronomer Giuseppe Piazzi in 1801. Pallas, Juno and Vesta were discovered in the next four years. All four orbit the Sun between Mars and Jupiter. People realized that these were not planets, but small bodies, and they came to be called asteroids.
Nasa's Dawn spacecraft reached Ceres in March this year and went into orbit around it. Before that it had visited the asteroid Vesta and orbited it for several months, making it the first spacecraft to make an extended visit to two objects in space.
Although Ceres is the largest asteroid, 950 kilometres in diameter, it is less than one-third the size of our Moon. Its surface area is about the same as India's. In comparison, the planet Pluto (see elsewhere in this issue) has surface area about the same as Russia's.
Just like the Moon the surface of Ceres is dull grey and dotted with craters. What makes Ceres more interesting is that when it was photographed by the Hubble space telescope (which is just outside the Earth's atmosphere) it showed a small spot which looked much brighter.
As Dawn approached Ceres, it was found that a small bright spot seen from far was a small bright region with many bright spots, inside a crater (now named Occator) about 90 kilometres in diameter. Other regions were found to have even smaller bright spots. So the big question is: what are the bright spots on Ceres?
There are two popular hypotheses now when I am writing this article. By the time this Jantar Mantar reaches you the answer to the question may be known. There are other peculiar features on Ceres, like a plateau which rises 5 kilometres above the surrounding region and where we can see signs of something flowing. What could have caused this?
The first idea about the bright spots is that they are ice. It is known that Ceres's atmosphere sometimes does have water vapour. At the distance of Ceres, the Sun is not powerful enough to melt ice, so ice can stay on Ceres's surface for a long time. So we guess that below the surface of Ceres there are large underground oceans. Water from there sometimes comes out on the surface, whether by something hitting Ceres and making a crater, or by some other means, we do not know. Once the water comes out and freezes as ice it stays there for a long time, perhaps years. Perhaps the bright spots seen by Hubble were not the same as the ones seen now.
The second idea is that the spots reflect so much because they are made of salt. If you visit the Rann of Kachchh in Gujarat, you will see that the salt deposited there makes the entire landscape appear white. Tourists make trips there at night to see it shine in moonlight. So for Ceres we again guess that there are large underground oceans from where water comes out on the surface. Over time, perhaps after many years, the water evaporates away. What is left behind is the salt. But that salt should stay for a much longer period.
Or there could be a third explanation ...
Dawn is now orbiting Ceres at a height of only 1500 kilometres. Its instruments will soon reveal the answer!