When scientists first saw this structure on the images taken by their camera on the Dawn space probe, they could hardly believe their eyes: from the crater-strewn surface of the dwarf planet Ceres rises an even, smooth and steep-sided mountain, towering over 4,000 metres high. It is the highest mountain on the thousand kilometre-diameter, almost spherical dwarf planet, and one of the most remarkable structures in the entire Solar System. A study involving scientists from the German Aerospace Centre (DLR) has now solved the mystery of how Ahuna Mons, as the mountain is called, was formed, using gravity measurements and investigations of the geometrical form of Ceres. A bubble made of a mixture of salt water, mud and rock rose from within the dwarf planet. The bubble pushed the ice-rich crust upwards, and at a structural weak point the muddy substance, comprising salts and hydrogenated silicates, was pushed to the surface, solidified in the cold of space, in the absence of any atmosphere, and piled up to form a mountain. Ahuna Mons is an enormous mud volcano.
"In this region, the interior of Ceres is not solid and rigid, but moving and at least partially fluid," explains Wladimir Neumann of the DLR Institute of Planetary Research in Berlin-Adlershof and the University of Münster. The geophysicist contributed towards the study, which has now been published in the journal Nature Geoscience. "This ‘bubble’ that formed in the mantle of Ceres beneath Ahuna Mons is a mixture of saline water and rock components." Scientists describe a 'cryochamber' – from the Greek word for ice, kryos – when talking about a magma chamber in volcanoes on earthlike planets. Ceres is a dwarf planet on the outer edge of the asteroid belt. The largest body in this zone between Mars and Jupiter, populated by minor planets, consists primarily of siliceous rocks, but also, to a considerable extent, of ice and presumably layers of water. The scientists are working on the assumption that up to a quarter of the mass of Ceres is ice or water – an even higher proportion than Earth's reserves of freshwater and ice.
The dwarf planet Ceres consists of up to a quarter ice and water
After the asteroid Vesta, the Dawn space probe orbited the dwarf planet Ceres from 6 March 2015 to late October 2018, as the second destination on its mission, which began in 2007. Two identical framing cameras jointly developed by the Max Planck Institute for Solar System Research and the DLR Institute of Planetary Research photographed and mapped Ceres from different altitudes. Many areas were captured stereoscopically so that scientists working in the field of planetary geodesy were able to calculate digital terrain models that could be used to represent the topography of the surface of Ceres.
The interior of Ceres is not homogeneous, but rather, to use geologists' terminology, partially 'differentiated', which means that after the formation of the celestial body, its components have become segregated and separated, at least to a certain degree. Components with a higher proportion of heavy elements, such as magnesium or iron, sank into the centre of the body, while lighter components like rocks with a high aluminium silicate or water content rose. Bubbles and domes are being created due to the heat that is still being generated today, four-and-a-half billion years after the formation of Ceres, by the decay of radioactive elements. The presence of liquids has influenced the inner formation differently to the usual rocky planets. As a result of their lower specific weight compared to the surrounding materials, these bubbles rise and press against the crust from below. The kilometre-high domes deform the crust, and once they break through it, fluid material penetrates the surface.
An 'iceberg' as high as Mont Blanc
When the Dawn mission arrived at Ceres, it captured extraordinary, almost snow-white surfaces on the planets, which we now know is the result of a hydrogenated sodium carbonate or ammonium-containing clays, pale salts that are the result of 'cryovolcanic activity', i.e. via the eruption of aqueous solutions that freeze immediately in surface temperatures of around minus 100 degrees Celsius. Ahuna Mons, named after the harvest festival of the Sumi Naga ethnic group in India, was created through this process in the recent geological past. With a base area of 20 kilometres in diameter and heights of 4,000 to 5,000 metres above the surrounding area, it has similar dimensions to Mont Blanc, the highest peak in the Alps.
"In order to explain the origin of Ahuna Mons, we had to use a new geophysical model that was specially tailored to Ceres, and thus get at the 'hidden' information behind the data from the space probe," explains Antonio Genova of the University of La Sapienza in Rome. Ottaviano Ruesch of the European Space Agency (ESA), who was the lead author of the study, adds, "We were thrilled to be able to find out which process occurring in Ceres' mantle, just beneath Ahuna Mons, was responsible for bringing material to the surface. Of course, Ahuna Mons was also a bit 'dubious' due to its shape as a volcano."
Anomalies in the gravitational field gave away the mud bubble in the interior
The scientists interpreted the data to mean that Ceres' gravitational field was an anomaly, with the result that its gravitational pull, which also exerted itself on the Dawn space probe orbiting Ceres, is a little larger. As a result, the speed of the spacecraft accelerated a little over this area, while also slightly lowering its orbit. This could be measured by the doppler effect on radio communications with the space probe, as wavelengths were compressed or distended depending on the geometrical constellation of the radio link. "We took a closer look at this anomaly, and further modelling revealed that it had to be a bulge in Ceres' mantle," continues Ottaviano Ruesch. "The conclusion was obvious: the mixture of fluid substances and rocks had come up to the surface and piled up into Ahuna Mons."
Cryvolcanic activity is widespread in the Outer Solar System. Traces of this ice volcanic activity have been discovered on moons of Jupiter and Saturn, while some structures on Pluto also appear to have been formed in this way. Ceres is the first body in the asteroid belt where this type of extrusion has been observed. Unlike Jupiter's moons Europa and Ganymede, or Saturn's moon Enceladus, where water is compressed on the surface, the 'magma' in the rising bubble on Ceres is composed of a mixture of saline water and mud or rock particles. Observations of the mineralogical composition of Ahuna Mons using a spectrometer on board Dawn appear to confirm this finding. The result of the study shows that large asteroids or dwarf planets that are made of siliceous rock and ice can form bubbles of saline water and rock constituents within their interior, which can rise to the surface and escape there. Scientists assume that this process may take place in these bodies over long periods of time, possibly billions of years, creating cryovolcanoes on the surface.
The Dawn mission was led by the Jet Propulsion Laboratory (JPL) of the US space agency NASA. JPL is part of the California Institute of Technology in Pasadena. The University of California in Los Angeles is responsible for the scientific part of the mission. The camera system on board the space probe was developed and built under the direction of the Max Planck Institute for Solar System Research in Göttingen, in collaboration with the Institute of Planetary Research at the German Aerospace Center (DLR) in Berlin and the Institute of Data Technology and Communications Networks in Braunschweig. The camera project receives funding from the Max Planck Society, the DLR and NASA/JPL.