NASA’s mission to the two largest objects in the asteroid belt is drawing to a close, so what revelations has it uncovered?
As NASA's Dawn spacecraft burns up its last fuel reserves, Jasmin Fox-Skelly looks back at what this 11-year mission has taught us about our Solar System
One of NASA’s most audacious space missions is drawing to a close this month. The Dawn spacecraft has spent the last 11 years travelling to and orbiting around not one, but two of the oldest and most massive residents of the asteroid belt between Mars and Jupiter – the asteroid Vesta and the dwarf planet Ceres.
“Dawn is the first mission to orbit a body in the main asteroid belt, the first to visit a dwarf planet and the first to orbit two extraterrestrial bodies in a single mission,” says Carol Raymond, principal investigator of the Dawn mission at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
The mission examined these two fossils – formed 4.6 billion years ago at the beginning of the Solar System – to shed light on the processes and environments that created the planetary system we know so well. With Dawn coming to the end of its fuel reserves, and the end of its mission, we look back at what the spacecraft has taught us about the formation of our Solar System.
Dawn launched on 27 September 2007, arriving at its first target, Vesta, on 16 July 2011. Scientists
already knew quite a lot about the giant asteroid before Dawn arrived, having previously recorded Vesta’s spectra – the patterns of light given off by an object, which reveal its composition. The results indicated that Vesta was compositionally very similar to hundreds of meteorites found on Earth, suggesting they may have originated on the asteroid. Images from the Hubble Space Telescope showing a gigantic crater at Vesta’s southern pole further backed up this idea; a huge impact could have blasted pieces of Vesta all over the Solar System, with some of those pieces ending up on Earth as meteorites.
Dawn allowed astronomers to zoom into the asteroid and observe its features in more detail. Not only did Dawn’s instruments confirm that the supposed Vesta meteorites had indeed originated on the asteroid, but analysis of the asteroid’s composition showed that it has tectonic features, including a crust and a silicate mantle. Gravity data, which can map the asteroid’s internal structure, also indicated that Vesta has a dense and possibly metallic core. This means that Vesta was once volcanically active, making it more like a planet than the inert piece of rock many other asteroids are thought to be.
On top of this finding, scientists were surprised to see a lot of water-rich materials on Vesta’s surface, particularly in older terrains. That’s odd as it was previously thought that without the protection of an atmosphere, Vesta was too warm to hold on to any water. Meteorites from Vesta are known to be richer in carbon than one would expect. One explanation is that the carbon and water-rich material could have come from outside Vesta, possibly when it was struck by two massive chunks of debris from much further out in the Solar System.
With its mission complete, Dawn left Vesta on 4 September 2012 and headed for Ceres. When it approached the dwarf planet in 2015 the spacecraft spotted hundreds of curious bright spots glinting on its surface. The spots were mostly concentrated in and around impact craters, particularly the large Occator crater. Analysis showed that the bright spots are made from sodium carbonate, which astronomers believe is evidence of an ancient ocean beneath the crust in the process of freezing.
“Ceres once hosted a global ocean until it froze,” explains Raymond, “leaving carbonates in the shallow subsurface that form bright spots when exposed by impacts. The majority of the bright spots are due to these crustal salts, which have been exposed by landslides and small impacts.
“However, those spots found inside the Occator crater have a different origin. They are produced
by the extrusion of briny [salty] liquid from Ceres’s interior. When the water evaporates it leaves behind a salty deposit.”
Scientists think this liquid is being forced up from a magma chamber below, making Ceres volcanically active, though not in the classical sense. Instead of rock, these ‘cryovolcanos’ are made from salt and mud, which erupts as briny water. Only one, Ahuna Mons, is currently active. There could have been several other cryovolcanos in Ceres’s past, but geologists are still interpreting the data.
Another remarkable discovery relates to the origin of Ceres. Its chemistry suggests that it didn’t form in the asteroid belt, but further out in the Solar System, before migrating inwards.
“Its ammonia-rich clays indicate that the dwarf planet actually formed in a colder environment than where it currently resides,” says Raymond. “Ceres is similar in bulk composition to many moons of the giant planets, like Europa and Enceladus. In fact, similar salts have been found on Ceres and in the plumes of Saturn’s moon Enceladus. This means it may be related to those icy moons.”
What’s lurking in the deeps?
So studying Ceres may also help us learn more about these moons, whose structures and internal compositions are hidden by ice. Scientists are particularly interested in learning about the environmental conditions in their deep oceans – and now in Ceres’s too – as they contain two of the main ingredients thought necessary for life: water and organic carbon compounds. However, according to Raymond the chance of life forming
on Ceres is quite unlikely. “Small dwarf planets like Ceres are not expected to have been warm long enough for complex organic molecules to develop,” she says.
But that does not totally discount the possibility of life growing on, or in, the dwarf planet. “The Dawn data suggest Ceres’s early ocean was habitable. That is, it could sustain conditions suitable for life if life were introduced in its interior, for example via material ejected from Earth by large impacts.”
After a decade of discovery, Dawn is fast approaching the end of its mission. Soon the spacecraft will breathe its last breath and run out of the hydrazine fuel that powers the thrusters. But in June the team still had enough left to alter Dawn’s orbit so that it skimmed 34km above the surface of Ceres – 10 times closer than the spacecraft had ever been before. Here the cameras and instruments could send back the most detailed photos of the world yet. This includes recording gamma rays and neutron spectra, which will tell scientists more about the chemical makeup of Ceres’s uppermost layer, including how much water it contains.
But now the mission is drawing to a close. Without thrusters to control its movements and orientation, Dawn won’t be able to point its scientific instruments at Ceres, or direct its antenna toward Earth to communicate. Scientists don’t know exactly when the fuel will run out, but their best guess is August or September. However, the probe will orbit around Ceres for at least 50 years. Perhaps a future mission will return to Ceres and be able to watch Dawn rising up over the horizon.
Around 500 million km from Earth, Dawn will finally run out of fuel orbiting Ceres
NASA informally christened the three craters at the top left of Vesta the Snowman. The bulge at the bottom is the central peak of the Rheasilvia crater which, at 22km tall, just pips Olympus Mons as the tallest mountain in the Solar System
Above top: A view of Vesta’s south pole showing the giant crater, Rheasilvia, which is about 90 per cent the size of Vesta’s diameter
Above bottom: One of the meteorites that has reached Earth which is believed to have originated on Vesta
Vesta imaged by Dawn on 24 July 2011 from a distance of 5,200km. The asteroid is the secondlargest object in the asteroid belt, beaten only by Ceres
The brightest spot on Ceres is Cerealia Facula in the centre of Occator crater. Just to the left is a less bright cluster called Vinalia Faculae
Above left: A close-up of Cerealia Facula. Ceres’s bright spots are created by deposits of sodium carbonate
Above right: Ceres’s volcanic mountain Ahuna Mons. On its steepest side, it is about 3km high, with a diameter of about 120km
Dawn revealed many landslides on Ceres, which researchers believe have been shaped by a significant amount of water ice. The one above has been designated a ‘Type 1’ flow feature, which are mostly found at high altitudes