by Ioana Boian (edited by Andrew Blance)
How did the Solar System form? How did water originate on Earth? These are some of the questions that researchers studying distant icy objects in the outskirts of our Solar System are trying to answer. By investigating Ceres, a small dwarf planet in the asteroid belt, they hope to shed some light on these mysteries.
Ceres arose naturally as a target for observation because of its unique status: it is the largest object in the asteroid belt and the sole dwarf planet in the inner Solar System. Remarkably, it is also known to have an icy mantle and may even have an internal ocean of liquid water. It has even been proposed that under a slightly different set of circumstances Ceres could have evolved into a planet similar to Earth and Mars. Dawn, a spacecraft sent to study dwarf planets, approached this fascinating place in 2015 and sent images home, inspiring further research into the dwarf planet. The images revealed two bright spots on its surface which have proven difficult to explain.
Dawn was launched in 2007. It is not only the first spacecraft to orbit a main belt asteroid but also the first to orbit two extra-terrestrial bodies. The Dawn mission targets both Ceres and Vesta, rocky bodies in the asteroid belt which may hold answers about the early Solar System and its formation. Following the assumptions that Vesta is rocky and Ceres contains large quantities of ice these two bodies provide a bridge between the formation of the inner rocky Solar System and its outer icy parts. Vesta was reached in 2011 and the mission results revealed important new information about its craters and their formation, chemical composition and gravity. The landscape of Vesta was even shown to be surprisingly similar to Mars, or even Earth. Although there are theories that, like Ceres, a pocket of ice exists under the surface, these ideas are still under debate. While a significant amount of data was gathered about Vesta, the more fascinating mysteries involve Dawn’s other target, Ceres.
In 2003 a single shining spot was observed on Ceres by the Hubble Space Telescope, but the discovery was dismissed, believed to be caused by the poor resolution of the image. Later in 2015, Dawn provided more detailed images that proved the spot to be real. It was revealed that the one spot was actually many, and since then several of these groupings of spots have been detected.
The main theory proposed by NASA was that sunlight was hitting reflective material found in the crater where the spots were located. The high albedo of the planet pointed towards ice or salt deposits being responsible and since Ceres is known to have an icy mantle, ice was considered to be the likely cause. However, this theory has been recently discredited. Information made available as the Dawn spacecraft continued its orbit towards Ceres has allowed researchers to say with more accuracy what may be causing the reflective patches. Recent proposals suggest these spots are indeed caused by salt deposits. The liquid water hidden inside Ceres would have allowed these salts to grow. Then, when an asteroid collided with the planet, this solution would have been ejected from the planet’s interior and into the crater. Here the water would evaporate away leaving only the reflective salts seen in the Dawn mission images. It is debated however if liquid water still runs under the surface or whether it was only there when the salt formed in the craters tens of millions of years ago.
This is still not the end to this mystery, though! While the spots have been shown to be made from sodium carbonate, it is unknown how such a large amount of the compound could exist on Ceres. Studies recently have suggested that though Ceres has a large amount of underground ice it is not as common as originally thought. Estimates predict that no more than 35% of its substructure can be composed of it. This is a problem as usually this carbonate forms in large bodies of water. Currently the only other place it is found, aside from Earth, has been Enceladus, a moon of Saturn layered in ice. It is a mystery how a planet as small as Ceres could have enough water needed to form this salt. It is hoped that answering this question and understanding the origin of Ceres water supply will allow us to have an insight into how water formed here on Earth. Currently it is suggested that the water in the inner solar system may have originated from the Kuiper Belt, a disc of icy rocks beyond our planets, but exactly how this was transferred to Earth is a mystery. Ceres has the potential to be an intermediate step between the water on Earth and the ice of the Kuiper Belt. Therefore research into Ceres will hopefully allow us to understand the process of how, and if, the Kuiper Belt is responsible for the formation of water on Earth.
The Dawn mission has provided a huge amount of information about the dwarf planet Ceres and the asteroid Vesta. Many mysteries regarding Ceres and Vesta have been solved, but there are still many questions left unanswered, predominantly on the nature of Ceres’ water supply. Dawn however is continuously bringing new results, and it is expected to provide answers to this and other long standing questions about the origins of the Solar System. In particular, it is hoped that understanding Ceres will give us insight into how water formed here on Earth and the rest of the inner Solar System, an important and consequential result.
The most recent images NASA has received of Ceres’ spots:
Further discussion of what the likely causes of Ceres’ spots are:
Nature article regarding levels of underground ice on Ceres
Nature article regarding salt formation and water levels on Ceres