Water ice in the asteroid belt
Herschel detects water vapour on Ceres
Detailed observations of Ceres have revealed clear signatures of water on this asteroid. An international team of astronomers including Thomas Müller from the Max Planck Institute for Extraterrestrial Physics has found the water fingerprint in certain spectra of Ceres taken with the Herschel space observatory. Moreover, it seems that the amount of water vapour varies along the asteroid’s orbit, with an increasing signature if Ceres passes closer to the sun. While this confirms previous hints that there might be water or rather ice reservoirs in the main asteroid belt, this finding raises questions about the origin and distribution of water in our solar system.
On Earth, the so-called "snow line" separates higher regions in the mountains with ice and snow from warmer areas further down. And a similar “snow line” exists in the solar system as well (and indeed in any other planetary system): the inner regions near the sun are too hot for stable reservoirs of water or ice to exist - in the early phases of planet formation at least. This "snow line" lies at a distance of about five astronomical units (~ 750 million kilometres) from the sun and one would expect that water ice can be found only on objects that are created beyond this limit, i.e. on the outer planets, comets, small objects in the Kuiper Belt beyond Neptune and up to the outer edge of the solar system in the Oort cloud.
However, in recent years there has been some evidence that also small bodies in the asteroid belt, only about two to three astronomical units from the sun, contain ice. Some of these objects show sporadic comet-like eruptions; therefore some astronomers talk of the "main-belt comets”. So far, however, no direct proof for water has been found.
An international team of scientists including Thomas Müller of MPE has now obtained detailed observations of Ceres, the largest object in the asteroid belt, and found clear traces of water. With the HIFI instrument on the Herschel infrared space observatory, the scientists were able to record spectra over a period of several months that contain the unique fingerprint of water vapour.
"The intensity of the water line is linked to certain dark regions on the surface; these are either warmer areas or craters where an impact has exposed some layers of ice deeper down," Thomas Müller explains. "Moreover, we were able to track how the amount of water changes along the asteroid's way around the sun: if it passes nearer to the sun the water signature increases and then decreases again in the more remote sections of its orbit."
Overall, Ceres is a dark object, reflecting only about 10 % of sunlight; bright ice surfaces cannot to be found there. Therefore the water and ice have to be hidden under a layer of dust or debris and probably can only emerge at a few places where the ice is relatively close to the surface. This "insulation layer" is probably the reason why ice can survive on Ceres at all: Because of the high intensity of the solar radiation, the lifetime of ice directly on the surface would be very short.
Now that the existence of water on Ceres has been proven successfully, there is still the question about its origin. Did the asteroid form in the early stages of the solar system from a mixture of rocky planetesimals formed locally and icy pieces coming from the outer solar system? In this case, how was the ice able to survive the high temperatures further in? Or was the water ice on Ceres added at a later and very active phase of the solar system in the form of small icy bodies? Traces of this “late bombardment phase" can still be seen as craters on the moon - the water in our oceans was probably delivered to Earth at this late time by comets. Ceres could then have been created "dry" and might have received its water ice about 700 million years later.
The spherical shape and the differentiated inner structure (with core, mantle and crust) tend to favour a rapid development in the early days of the solar system. But there is a third theory based on a formation far from the sun and beyond the "snow line". This, however, still lacks a convincing solution how this asteroid has then migrated to the inner regions of the solar system.
Ice objects will move to the centre of attention in the coming years with the Rosetta mission (going to comet Churyumov–Gerasimenko), the DAWN mission (going to Ceres), and the New Horizons Mission (to the Pluto system). As the ice body, which is closest to the sun, Ceres will play an important role and Thomas Müller is looking forward to these missions: “Ceres could be the key to our understanding of the distribution of ice and water in the solar system. At the same time, this asteroid becomes one of the potential candidates for extraterrestrial life, along with Jupiter's moon Europa and Saturn's moon Enceladus. When the NASA DAWN-mission arrives at Ceres, we expect further insights – maybe even to the origin of water in our solar system.”
The research is part of the Measurements of 11 Asteroids and Comets Using Herschel (MACH-11, PI: Laurence O' Rourke, ESA) programme, which used Herschel to look at small bodies that have been or will be visited by spacecraft.