When the Dawn probe approached Ceres, it found a striking feature. Against the dim gray of the world was a small patch of brilliant white. While images showed a brilliant almost metallic gleam, that was largely due to the enhancement of the images. Things in space are typically pretty dark, and even the bright spot wasn’t that brilliant. But the extreme contrast between the white region and the surrounding terrain clearly indicated that something unusual was there. Was it water ice? A layer of salt? Or something else? It looks like we finally have the answer.

A new paper in Nature studies the bright region, and finds it’s consistent with magnesium sulfates. The salt is mixed with water ice and other surface material. The presence of salt on Ceres’ surface implies that it contains briny subsurface ice. When a meteor collides with Ceres, it would expose this ice, which would sublime when exposed to sunlight. The bright spots we see would then be the salty material left behind.

This new finding, combined with other research that finds ammonia on Ceres gives support to the idea that Ceres originally formed farther from the Sun that it is now. We know that the location of planets have changed over time, and the formation of water ice and ammonia typically occur beyond a distance known as the ice line, which is generally thought to have been a bit further than Ceres during the early solar system.

But this is just the first of what will be a long line of papers coming from the Dawn mission. Already we’re developing a picture of Ceres as a complex and fascinating world.

Paper: Nathues, A. et al. Sublimation in bright spots on (1) Ceres. Nature 528, 237–240 (2015)

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When you think of a rocket engine, you probably imagine one driven by a chemical reaction, that shoots a powerful flame capable of lifting a large spacecraft from the Earth’s surface. While such chemical rockets are necessary to overcome Earth’s strong gravity, they aren’t as useful in space. That’s because chemical rockets aren’t particularly efficient. So for probes like the Dawn spacecraft now in orbit around Ceres, we use another type of rocket engine known as an ion thruster.

A chemical rocket is a controlled chemical explosion. Credit: Space X

For any type of spacecraft, the key to a rocket engine is how much you can change the craft’s velocity (what’s often known as delta-v). Pound for pound, the more delta-v an engine can give a spacecraft, the more effective it is. That includes both the mass of the rocket and the mass of the fuel. A chemical engine produces thrust by a chemical reaction that superheats the propellent, which is then funneled out of the engine at high speed. Usually the propellent is the exhaust of the chemical reaction powering the rocket, so that the fuel itself becomes the propellent. It is basically a controlled explosion that can provide a great deal of thrust for a short period of time.

An ion engine takes a different approach. Rather than producing a large thrust in a short time, they produce a small thrust over a much longer time. The push given to a probe is about equal to the weight of a few coins in your hand, but it can provide that thrust for days or months at a time. In the lab, ion thrusters have operated continuously for 3 – 5 years without a failure. The only real limitation is the amount of available propellent, which is usually xenon gas.

Ion engines work by giving the gas an electric charge. This allows it to be accelerated to high speed across a potential voltage, after which it is thrust out of the engine. Since the engine is powered by electricity rather than a chemical reaction, it can derive its power by the solar panels of the probe. This, combined with their high efficiency allows ion engines to provide much greater delta-v than conventional engines. Over the course of the Dawn mission, it’s ion engines have provided a delta-v of more than 10 kilometers/second, which has allowed the probe to move into orbit around Vesta, leave that orbit, then reach Ceres and move into orbit around it. This could not have been achieved with chemical rocket engines.

There are plans to use ion engines for crewed missions, such as a journey to Mars, but for now it has allowed us to reach the largest body in the asteroid belt.

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The Dawn spacecraft is on its way to Ceres, and has started sending back new images of the dwarf planet. The images are still pretty pixelated at this point, but we can begin to see features such as craters on the surface. Once considered a planet, then an asteroid, it is now considered a dwarf planet, and Dawn’s study of Ceres is likely to find several interesting surprises.

Possible interior of Ceres. Credit: NASA/ESA/STScI

We already know, for example, that Ceres has water. Spectroscopy of its surface shows that it contains hydrated material, and we’ve observed water vapor within Ceres’ thin atmosphere. Yes, Ceres has an atmosphere, but it isn’t remotely thick like Earth’s or even that of Mars. Because of this, it’s thought that Ceres has a rocky core with a mantle of water-ice. There are even some who speculate that Ceres could have liquid water within its interior, but that isn’t thought to be very likely.

The approach to Ceres marks the second body the Dawn spacecraft has visited. It has already visited and orbited Vesta, giving us a detailed analysis of its geology and history. As it orbits and eventually lands on Ceres, it will likely do the same for the smallest of dwarf planets.

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In 1801 a new planet was discovered in our solar system.  Just twenty years earlier the planet Uranus was discovered beyond the orbit of Saturn, and was the first planet discovered since the dawn of civilization.  The location of Uranus agreed with a (now defunct) model of planetary distances known as the Titus-Bode law, which had correctly predicted the distances of the known planets.  But the Titus-Bode law also predicted the existence of a planet between Mars and Jupiter, which had not been seen until the discovery of this new planet.  The 1801 planet had a distance within 1% of the prediction of Titus-Bode, and it was named after the Roman goddess of agriculture, Ceres.

Until the mid-1800s, Ceres was considered to be a planetary body.  In less than a decade after the discovery of Ceres, three more planets were discovered between Mars and Jupiter, and given the names Pallas, Juno and Vesta.  In 1846, Neptune was discovered beyond Uranus, raising the total number of planets in our solar system to 12.  Within ten years of Neptune’s discovery, dozens of new planets were discovered between Mars and Jupiter:  Astreae, Hebe, Iris, Flora, Metis, Hygiea, and the list continued to grow.

While Uranus and Neptune were similar to the historical planets, these new planets were very different.  They all had roughly the same orbital distance (between Mars and Jupiter).  They were also significantly smaller than the other planets, even much smaller than our moon. Rather, they were more like Ceres, itself a small, rocky body.  It soon became clear that referring to all of these bodies as planets wasn’t very accurate.  So they were put into a new category of Sun-orbiting objects: asteroids.  Thus, Ceres lost its planetary status, demoted to King of the asteroids.  The number of planets in our solar system was thereby reduced to eight.

In 1930 a new planet was discovered, and given the name Pluto.  While Pluto was a small world (smaller than our Moon), it seemed alone beyond the orbit of Neptune.  But in the 1990s more objects were discovered beyond the orbit of Neptune.  By the mid-2000s, trans-Neptunian objects of a size similar to Pluto were discovered, and in 2005 one larger than Pluto was found.  It was named Eris, and for a while enjoyed status as the tenth planet.

But by then it was clear that Pluto was not alone.  Rather there were lots of objects that, like Pluto were small, icy and beyond the orbit of Neptune.  Again it was clear that referring to all these bodies as planets wasn’t an accurate description.  So in 2006 the International Astronomical Union formalized the definition of what constituted a planet.  They had to orbit the Sun, they had to be massive enough to be roughly spherical, and they had to be distinct among objects of their distance.  Pluto and Eris did not satisfy the last condition, being similar to other trans-Neptunian objects.  Like Ceres  before them, they lost their status as planets.

But the IAU also defined a secondary category for objects that satisfied the first two, but not the third.  They were given the name dwarf planets.  Under this definition, Pluto and Eris were categorized as dwarf planets.  Ceres was also promoted to dwarf planet status, as were two other trans-Neptunian objects, Haumea and Makemake.  So currently our solar system has 8 planets, 5 dwarf planets, and countless other small solar system bodies.

As our understanding of the solar system has grown, we have had to refine our naive concept of what makes a planet.  If you mourn Pluto’s exclusion from the solar planets, you should also mourn Ceres, who suffered her loss a century earlier.

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