12. January 2021
Laboratory experiments following the Dawn mission

Why do some re­gions on the dwarf plan­et Ceres ap­pear blue?

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Blue surface material on Ceres
Blue sur­face ma­te­ri­al on Ceres
Image 1/6, Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI; S. Schröder et al.

Blue surface material on Ceres

This false colour im­age of the en­tire sur­face of Ceres (ex­tent at the equa­tor is 3030 kilo­me­tres) em­pha­sis­es the vari­a­tions in the spec­tral pro­file of the sun­light re­flect­ed from the dwarf plan­et's sur­face. For this pur­pose, three monochrome im­ages from the ra­tio of wave­lengths are su­per­im­posed on the chan­nels usu­al­ly used for a colour im­age – red, green and blue. From these three 'colours' we per­ceive white sun­light in an ad­di­tive colour mix­ing. In this case, the ra­tio of near in­frared to red light (965 nanome­tres/749 nanome­tres) is shown in red, a ra­tio of green light to red light (555 nanome­tres/749 nanome­tres) is shown in green, and a ra­tio of blue light to red light (438 nanome­tres/749 nanome­tres) is shown in blue. This rep­re­sen­ta­tion makes it eas­i­er to iden­ti­fy re­gions that re­flect a high­er pro­por­tion of blue in sun­light. Haulani stands out as a re­cent im­pact crater with a par­tic­u­lar­ly high pro­por­tion of blue light, in con­trast to Er­nutet, which has the high­est pro­por­tion of red, in­di­cat­ing or­gan­ic hy­dro­car­bon com­pounds there.
Haulani in contrast-enhanced colours
Haulani in con­trast-en­hanced colours
Image 2/6, Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Haulani in contrast-enhanced colours

Haulani is one of the youngest craters on Ceres, pos­si­bly formed just two mil­lion years ago. Land­slides on the steep rims of the 34-kilo­me­tre-wide crater, as well as the smooth sur­faces that are free of small­er craters, are ge­o­mor­pho­log­i­cal clues to the crater’s ge­o­log­i­cal­ly young age and the role played by wa­ter and ice in the sub­sur­face. The im­age, tak­en from an al­ti­tude of 1470 kilo­me­tres by the Ger­man Fram­ing Cam­era on NASA's Dawn space­craft, is a con­trast-en­hanced im­age in which the rays of ma­te­ri­al eject­ed when the crater was formed are more vis­i­ble, al­so high­light­ed by the bluish tint. The im­pact that cre­at­ed Haulani caused dust and rock to mix with ice in the sub­sur­face and be eject­ed. The ice lat­er sub­limed, leav­ing be­hind lay­ered sil­i­cate min­er­als that, due to their struc­ture, scat­tered short-wave blue sun­light par­tic­u­lar­ly strong­ly.
The Haulani crater on Ceres
The Haulani crater on Ceres
Image 3/6, Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

The Haulani crater on Ceres

This black and white mo­sa­ic im­age of the 34-kilo­me­tre-wide Haulani crate near Ceres’s equa­tor (11 de­grees east/6 de­grees north) was tak­en by the Ger­man Fram­ing Cam­era on NASA's Dawn space­craft. The strongest ab­sorp­tions in wave­lengths of blue light with­in Haulani were mea­sured spec­tro­scop­i­cal­ly. The name ‘Ceres’ was sug­gest­ed by its dis­cov­er­er Giuseppe Pi­azzi in 1802, af­ter the Ro­man god­dess of fer­til­i­ty and agri­cul­ture. Fol­low­ing a de­ci­sion by the In­ter­na­tion­al As­tro­nom­i­cal Union, craters and ge­o­log­i­cal struc­tures on Ceres are named af­ter sim­i­lar mytho­log­i­cal fig­ures and fer­til­i­ty cel­e­bra­tions in all cul­tures. There is one ex­cep­tion – the crater Pi­azzi. Haulani is names af­ter the Hawai­ian god­dess of plants.
Sample container before and after the experiment
Sam­ple con­tain­er be­fore and af­ter the ex­per­i­ment
Image 4/6, Credit: S. Schröder et al.

Sample container before and after the experiment

The im­age on the left shows the sam­ple con­tain­er filled with a mix­ture of ice and Ceres ana­logue ma­te­ri­al at the be­gin­ning of the sub­li­ma­tion ex­per­i­ment in the Cold Sur­face Spec­troscopy (CCS) cham­ber at the Uni­ver­si­ty of Greno­ble. It was filled with a mix­ture of Ceres ana­logue ma­te­ri­al de­vel­oped at the In­sti­tute of Space As­tro­physics and Plan­e­tol­o­gy in Rome and cooled to a tem­per­a­ture of mi­nus 100 de­grees Cel­sius (173 Kelvin). The ex­per­i­ment was con­duct­ed un­der a high vac­u­um with a gas pres­sure of one mil­lionth of a mil­libar for 133 hours. The ice sub­limed com­plete­ly, leav­ing be­hind a col­lapsed, high­ly porous, dust-like sub­stance (right im­age). The cav­i­ty of the sam­ple cylin­der has a di­am­e­ter of 4.8 cen­time­tres.
Microscopic fine structure after the experiment
Mi­cro­scop­ic fine struc­ture af­ter the ex­per­i­ment
Image 5/6, Credit: S. Schröder et al.

Microscopic fine structure after the experiment

Up­on com­ple­tion of the ex­per­i­ment with the Ceres ana­logue ma­te­ri­al, the sub­limed wa­ter ice left be­hind a net­work of tiny cav­i­ties in the clay min­er­als held to­geth­er by fine fil­a­ments. These fil­a­ments are so small and del­i­cate that in­ci­dent sun­light re­sults in Rayleigh scat­ter­ing in which the short­er-wave­length, high­er-en­er­gy blue com­po­nent of sun­light is scat­tered more than oth­er wave­lengths, lead­ing to the blue colour of the younger craters on Ceres.
Sequence of the sublimation process on Ceres
Se­quence of the sub­li­ma­tion pro­cess on Ceres
Image 6/6, Credit: S. Schröder et al.

Sequence of the sublimation process on Ceres

This schemat­ic il­lus­tra­tion shows how the sub­li­ma­tion pro­cess of sur­face dust mixed with ice on Ceres ap­pears in the sim­u­la­tion with ana­logue ma­te­ri­al in the ex­per­i­ment. In each of the spher­i­cal ice par­ti­cles pro­duced ex­per­i­men­tal­ly, lay­ered sil­i­cate platelets formed con­nec­tions be­tween the wa­ter ice crys­tals. Af­ter sub­li­ma­tion of the ice, the dis­ap­pear­ance of the ice crys­tals left cav­i­ties that in­ter­con­nect­ed a high­ly porous net­work of phyl­losil­i­cate fil­a­ments. In some cas­es, the ice-free struc­ture re­tained the spher­i­cal shapes of the orig­i­nal ice par­ti­cles. More of­ten, how­ev­er, the ice spheres frag­ment­ed in­to a dis­or­dered, foam-like struc­ture of platelets and fil­a­ments.
  • Water evaporating from clay minerals leaves behind finely porous, bluish rock dust.
  • NASA's Dawn spacecraft explored Ceres between 2015 and 2018, during which time it observed puzzling blue patches in recent impact craters.
  • The dwarf planet's crust may contain up to 25 percent water ice.
  • Focus: Space exploration, Solar System exploration, laboratory experiments

This month marks the 220th anniversary of the discovery of the first asteroid. During the night of 1–2 January 1801, Giuseppe Piazzi, the director of Palermo Astronomical Observatory, noticed a 'star' on the shoulder of the bull-shaped constellation, Taurus. But the position of the star was changing each night. Piazzi had discovered Ceres, the largest body in the vast space between the planets Mars and Jupiter. Between 2015 and 2018, Ceres – now ‘promoted’ from an asteroid to a dwarf planet – was the second target of NASA's Dawn mission, after the asteroid Vesta. In the images of Ceres acquired by the German instrument on board the Dawn orbiter, as well as in the spectral measurements from the spacecraft, some regions stood out due to their unusual blue colour. The origin of this colour remained a mystery until today. Laboratory experiments carried out by a team led by DLR Planetary Scientist Stefan Schröder have now solved the mystery. Impact events experienced by Ceres during the recent past caused material mixed with ice to rise to the surface. The water ice embedded in the crystal structure of the clay minerals then sublimed. What remained was a finely porous dust that reflects sunlight with a blue colour due to its 'fluffy' structure.

"Ceres has no atmosphere, so water ice is not stable on the dwarf planet's surface and sublimes rapidly. This means that it passes directly from the solid phase into the gaseous phase," explains Stefan Schröder from the DLR Institute of Planetary Research. “In the laboratory, we have now been able to simulate what happens when water ice, such as that initially introduced into the crystal structure of very specific minerals by impacts on Ceres and then transported to the surface, escapes from there into space. What remains on Ceres is a finely porous, almost fluffy layer of dust, which is responsible for the bluish, shimmering surfaces of some recent impact craters." Schröder and his colleagues from the University of Grenoble and the Institute for Space Astrophysics and Planetology in Rome came to this conclusion following their experiments. Over the course of just under a week, they observed material containing water ice – which corresponds to the material present in the strikingly blue patches on Ceres – in the laboratory under vacuum conditions and temperatures similar to those in the outer asteroid belt. The researchers report on their findings in today's issue of Nature Communications.

Bright, blue patches on Ceres puzzled researchers

The almost spherical dwarf planet Ceres is just under a thousand kilometres in diameter. It orbits the Sun on an elliptical orbit near the outer edge of the asteroid belt, at distances between 382 and 445 million kilometres. Unlike the almost exclusively rocky asteroids orbiting closer to the Sun, the small bodies at the outer edge of the asteroid belt contain a significant amount of water ice. Ceres's crust could store considerable amounts of it, with estimates ranging from a tenth to half of its volume. Ice could therefore lie just a few metres below the surface.

On the surface, Ceres is not very different to other cratered bodies. Its appearance resembles the far side of the Moon or that of the numerous icy moons of Jupiter or Saturn. For this reason alone, exceptionally bright patches in recent impact craters that strongly reflect sunlight, as well as the blue regions in their vicinity, have been among the most discussed phenomena on Ceres since the arrival of Dawn. Bright patches, such as in the Occator crater, are caused by mineral salts. However, this explanation does not apply to the blue regions. A region measuring several thousand square kilometres at Haulani crater for example – believed to be just two million years old – exhibits strikingly blue spectra. It is clear that any impact of a body on the surface of Ceres causes ice in the crust to melt and mix with the minerals in the regolith, the dusty layer on the surface of the body.

Spectral measurements acquired from orbit by instruments on board Dawn have shown that layered silicates (or phyllosilicates, from phyllos, Greek for leaf) must have been present at these sites as essential rock-forming minerals. Salts are also likely to have risen in aqueous solutions from melted ice. Layered silicates such as mica, black biotite or brightly shimmering muscovite in granite rock, are widespread on Earth. As basalt (the most common volcanic rock) is weathered, it produces clay minerals such as the phyllosilicate group of smectites (the mineral montmorillonite is a more widely known representative) as it interacts with water. Such phyllosilicates can expand due to the water molecules they contain, meaning their volume increases – that was the approach for the laboratory experiment of the planetary scientists.

From grey to blue – evaporated water changes mineral structure

The researchers filled a sample container with a smectite preparation that is very similar to the material on Ceres's surface, both chemico-mineralogically and in terms of its spectral properties (colour and brightness). In the experiment, the sample was exposed to a high vacuum and low temperatures of minus 100 degrees Celsius – conditions representative of those on Ceres – for 133 hours in the laboratory of the Institute for Planetary Sciences and Astrophysics at the University of Grenoble. As expected, the water ice sublimed and escaped from the sample. However, the finely porous phyllosilicates was preserved, leaving behind a skeletal, pore-rich residual substrate. Due to the microscopically small cavities, the volume of the blistered, almost foam-like structure of the mineral sample even increased considerably. In the process, its spectral properties also changed. The spectrum that was previously approximately continuous – corresponding to white sunlight with its blue, green and red components up to the near infrared – now showed significant reflections of blue light.

"This is comparable to the phenomenon that causes the sky on Earth to appear blue to us," explains Stefan Schröder. "The comparatively white, long-wave sunlight is scattered by tiny molecules in Earth's atmosphere to a greater or lesser extent depending on the wavelength. The higher frequency parts of the light – the blue wavelengths – are scattered more strongly than the lower frequency parts – the green and red wavelengths. As a result, the sky appears blue. Similarly, this effect, called 'Rayleigh scattering', takes place in the cavities of the phyllosilicates on Ceres from which the water has escaped." The substance in the laboratory reflected about 40 percent more light in total, which explains the striking brightness of these surfaces on Ceres. There is also a much higher proportion of blue light in the reflected wavelengths. "Presumably, it is primarily caused by the tiny filaments, less than one micrometre in size, that connect the phyllosilicates to each other. They allow Rayleigh scattering and therefore we see a higher proportion of the more energetic blue light reflected," says Schröder.

The regions on Ceres with an increased proportion of reflected blue light are not as bright as the white regions, whose origin can be traced back to the upwelling of mineral salts in water ice mixtures called brines. The researchers’ experiment that simulated sublimation processes in the surface material of recent craters on Ceres has shown that the evaporation of water from clay minerals is the microscopic mechanism that creates the tiny structures in the regolith that cause the blue colouration. At the cavities and the filaments that connect them, which are much smaller than wavelengths of visible light and the near infrared, Rayleigh scattering leads to the blue colouration of the mineral dust.

Contact
  • Melanie-Konstanze Wiese
    Cor­po­rate Com­mu­ni­ca­tions, Berlin, Neustre­litz, Dres­den, Je­na and Cot­tbus/Zit­tau
    Ger­man Aerospace Cen­ter (DLR)

    Pub­lic Af­fairs and Com­mu­ni­ca­tions
    Telephone: +49 30 67055-639
    Fax: +49 30 67055-102
    Rutherfordstraße 2
    12489 Berlin-Adlershof
    Contact
  • Ulrich Köhler
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Rutherfordstraße 2
    12489 Berlin
    Contact
  • Stefanus Schröder
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Plan­e­tary Ge­ol­o­gy
    Rutherfordstraße 2
    12489 Berlin
    Contact

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