The dwarf planet Ceres was discovered 220 years ago – but this celestial body in the asteroid belt is still a mystery today. Researchers may now have solved one of them: They have found out why the subsurface around some fresh impact craters on Ceres has such a striking bluish shimmer. The cause therefore lies in a combination of the layered silicates that occur there and the water ice ejected from deeper layers. When this ice evaporates on the surface, it literally foams the silicates and leaves behind a porous, bluish shimmering material.
On the night of January 1 to January 2, 1801, Giuseppe Piazzi, director of the Palermo observatory, noticed a faint point of light on the shoulder of the Taurus constellation, the position of which changed from night to night. He had discovered Ceres, the largest object with a diameter of 1,000 kilometers in the asteroid belt between the planets Mars and Jupiter. But only the recordings and measurement data of the NASA space probe, which reached Ceres in 2015 and orbited until 2018, brought closer to this dwarf planet. From their data we know, among other things, that Ceres probably has abundant water ice under its crust, that whitish salts swell to the surface in some places and that there may also be ice volcanoes on this dwarf planet.
What causes the blue spots on Ceres?
However, one feature of Ceres has remained a mystery so far: on the images taken by the probe, some areas were noticeable due to an unusual blue coloration of the underground. Spectral measurements showed such spots with a high blue component mainly in the vicinity of some younger impact craters, including an area several thousand square kilometers at the only two million year old Haulani crater. However, what led to this bluish coloration of the sputum around the crater was not immediately apparent. However, the spectral data indicated that so-called sheet or phyllosilicates were present at these locations. Sheet silicates are widespread on earth in the form of rock-forming minerals such as mica, biotite or muscovite. Some layered silicates can absorb water into their structure and then swell.
How these silicates could be related to the bluish colouration of the subsurface has now been investigated by researchers led by Stefan Schröder from the German Aerospace Center in a laboratory experiment. To do this, they simulated what happens when water ice is thrown up from the deeper subsurface and melted and then deposited in the layered silicates. They put such a water-containing layered silicate in a vacuum chamber at minus 100 degrees Celsius and observed what then happened. “Ceres has no atmosphere, which is why water ice is not stable on the surface and sublimates quickly, so it changes directly from the solid phase to the gaseous phase,” explains Schröder.
Foamed mineral reflects more blue light
The same thing happened in the experiment: The water stored in the layered silicate first froze on the surface, then it sublimed. However, as it turned out, this sublimation also changed the structure of the mineral – it was virtually foamed: the escaping water vapor created countless cavities with bars less than a micrometer thin. This vesicular, almost foam-like structure of the mineral sample also changed the spectral properties, as measurements showed: While the mineral had previously reflected almost the entire spectrum of sunlight, blue light components were now preferably reflected. “This is comparable to the phenomenon that the sky on earth appears blue to us,” explains Schröder. With this so-called Rayleigh scattering, the shorter-wave blue light components are scattered more strongly than the green and red wavelengths.
“This effect takes place in a very similar way in the cavities of the layered silicates on Ceres, from which the water escaped,” says Schröder. “Presumably, it is primarily the tiny cavities and the filaments that are less than a micrometer in size that connect them to one another that enable Rayleigh scattering.” With that, he and his team could have solved the mystery of the blue spots on Ceres: The Sublimation of water from the layered silicates triggers the structural changes that leave a finely porous, almost foamy layer of dust with a bluish reflection. Because these pores with their delicate trabeculae erode and are destroyed over time, these blue areas are only found on fresher craters.
Source: Stefan Schröder (German Aerospace Center, Berlin) et al., Nature Communications, doi: 10.1038 / s41467-020-20494-5