There is plenty of water ice on the dwarf planet Ceres

Scheme of water molecules falling into cold traps on Ceres (Image NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)
Scheme of water molecules falling into cold traps on Ceres (Image NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

Two articles published in the magazines “Science” and “Nature Astronomy” describe two studies also presented at the American Geophysical Union meeting taking place in San Francisco that reported new evidence of the presence of water ice below the surface of the dwarf planet Ceres. The researchers used the data collected by NASA’s Dawn space probe to find two sets of evidence that in Ceres’s subsoil there’s more ice than expected and that it can exist for a very long time.

Ceres seemed water-poor, a dark body with some white patches made bright by the presence of salts that reflect sunlight. However, the various research carried out thanks to the data collected by the Dawn space probe after it started studying the dwarf planet from its orbit, are showing a different situation and the research just presented suggest that there’s actually an abundance of ice water.

The team led by Thomas Prettyman of the Planetary Science Institute, Tucson, Arizona, which produced the article published in “Science”, used the GRaND (Gamma Ray and Neutron Detector) instrument to determine the concentrations of hydrogen, iron and potassium on Ceres’ surface layer and measure the energy generated by gamma rays and neutrons emitted by the dwarf planet.

The neutron measure is important because those particles are produced by cosmic rays interacting with the surface of Ceres. Hydrogen slows them down so detecting those that escape the dwarf planet allow to assess the amount of this element near the surface, most likely combined with oxygen in water that in those conditions is frozen.

A confirmation of the possibility that water ice exists near the surface of Ceres a few billion years after its formation is important because it opens up interesting possibilities concerning asteroids. The elements detected on the surface of Ceres indicate that liquid water is rising from its interior by altering the dwarf planet’s top layer. It’s possible that some process had warmed the water, perhaps the chemical one called serpentinization of olivine or the decay of radioactive materials.

According to the authors of a study published in the journal “Nature” in June 2016 it’s possible that even today there’s liquid water in Ceres’ underground. Even if today it was all frozen, it would constitute an explanation of the difference between the rocky interior and the surface layer containing ice with different chemical compositions.

An interesting comparison is the one with the meteorites called carbonaceous chondrites, which were altered by water. They probably originate from bodies smaller than Ceres, which contains more hydrogen and less iron than them. One explanation is a hypothesis already offered by other researchers, namely that Ceres formed in a region different from the meteorites. The presence of ammonia may indicate that it formed in the outer solar system.

The team led by Thomas Platz at the Max Planck Institute in Göttingen, Germany, who produced the article published in “Nature Astronomy”, focused on what have been called “cold traps”, craters where temperatures drop to 110 Kelvin (-163° Celsius, -260° Fahrenheit).

In at least 10 craters deposits of reflective materials were identified and in one of them partially illuminated by sunlight the Dawn space probe’s VIR (Visible and infrared spectrometer) instrument confirmed the presence of ice. Scientists think that at those conditions very little ice can sublimate over billions of years.

Cold traps have also been found on the Moon and even on Mercury there are icy areas because some craters are always in the shadow. On Ceres the situation is different and ice could have come from the dwarf planet’s icy crust or from space. In any case, water molecules can move from warmer regions and fall back into cold traps: a lot of vapor gets lost in space but a part falls on the surface where it freezes.

The Dawn space probe began its extended mission in July and at the moment its orbit is elliptical at a distance of over 7,200 kilometers (4,500 miles) from Ceres. Its data collection continues but research already confirmed a presence of water greater than expected. It’s another proof of the abundance of water in the solar system.

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