Ceres, largest and first of asteroids
Ceres was the first asteroid discovered in the Solar System. This object is both a planetesimal and a protoplanet, but is not large enough to be considered a minor planet. Ceres straddles the line between an asteroid and a planet.
In 1772, the German mathematician Johann Elert Bode elaborated and refined a pattern in the Solar System first described by Johann Daniel Titus, another German mathematician. According to the pattern, every subsequent planet in the Solar System was twice the distance from the Sun as the previous one. This pattern was widely known as the Titius-Bode law, and sparked interest in the discovery of a planet between Mars and Jupiter, where a world should have existed.
Instead of a world, astronomers discovered many minor-planets at the expected distance, between the orbits of Mars and Jupiter. The first of these was also the largest, discovered by the Italian mathematician Giuseppe Piazzi, who spotted a faint object moving through the constellation of Taurus on 1 January 1981. Piazzi conducted follow-up observations of the object over many weeks, but was cautious on announcing the discovery of a planet. We now know that Ceres is simultaneously the largest asteroid in the Solar System, as well as a dwarf planet.
An Enigmatic World
Ceres is a relic from the infancy of the Solar System, measuring 940 kilometres wide. The object challenges our assumptions on the type of objects that can potentially harbour life. Ceres, named after the Roman goddess of agriculture, is not a barren world. There are signs of geological activity, including ice volcanoes and mysterious bright spots that provide tantalising hints of a subsurface ocean. Close observations of the object have elevated it from a footnote in astronomy textbooks.
The internal structure of Ceres. (Image Credit: NASA).
When Giuseppe first spotted the dim world, he suspected that it must have been a comet. However, the object bizarrely lacked a tail, and its steady orbit indicated that it was not a comet. For over two centuries, it was considered a large asteroid that had assumed a spherical shape under the influence of gravity, because of hydrostatic equilibrium. In 2006, the International Astronomical Union came up with a controversial and frankly senseless definition of a planet, and while Pluto got a demotion, Ceres got a promotion. The designation of a dwarf planet, not to be confused with a minor planet, distinguished it from the eight ‘true’ official planets.
Close approach
NASA’s Dawn spacecraft made a close approach in 2015, and began to reveal the true nature of Ceres. Launched in 2007, Dawn was a pioneer, the first spacecraft to orbit two extraterrestrial worlds, first Vesta, the fourth asteroid to be discovered, and then Ceres, the first. The spacecraft was powered by an innovative ion propulsion system. When Dawn settled into orbit around Ceres, the images that it beamed back from the surface left scientists slack-jawed. This was not a featureless grey expanse, but a dynamic surface that was actively being shaped.
The most dramatic of these features were bright spots, nestled within the 92 kilometre wide Occator Crater. These light spots became an immediate mystery, and scientists speculated that they could be ice or salt, or something even more exotic. The answer emerged after years of analysis, they were deposits of sodium carbonate, the type of salt likely to have been dredged up to the surface by cryovolcanism, or volcanic activity driven by ice rather than igneous rocks. Ahuna Mons, one of the cryovolcanoes on Ceres has a four kilometre high peak, towering over the surrounding landscape.
Ahuna Mons on Ceres. (Image Credit: NASA).
Ahuna Mons likely spewed a slushy mixture of ice, water and salts millions of years ago, as indicated by its smooth sides. Scientists believe that the activity is driven by a global subsurface ocean of liquid water or at least briny sludge. The ice world of Ceres, is driven by an unexpected dance of geology and chemistry, unexpected from an iceball in the asteroid belt.
Ceres accounts for about one-third of the total mass of the asteroid belt. It is the only dwarf planet in the inner Solar System. While most asteroids are rocky and metallic, Ceres has a differentiated interior, with a core of rock, a mantle of ice, and potentially a thin crust of dust. Water is estimated to make up 25 per cent of the mass of Ceres, which is more than all the freshwater on Earth. There is evidence to suggest that there is a subsurface ocean or at least pockets of water on the rocky core, beneath the ice mantle. Ceres is a unique object that blurs the line between an asteroid and a planet.