You will be aware of the famous line, “Water water everywhere, and not a drop to drink.” In fact it appears that water IS, if not everywhere, it is fairly close to it. The latest discovery, according to Physics World, is that water has been found on two stony S-type asteroids, which have been considered to have been formed dry.
“Hydrated minerals” have been detected on hundreds of asteroids, but not really as water. Water is H2O, and it is possible to remove one hydrogen atom and use the consequent bond potential to join to something else, which generates R – O-H, where R is the something else. One possibility is a silicon atom. In theory, silicon could form silicic acid, which is Si(OH)4, but what happens is the hydroxyls eliminate and all the silicon bonds do is to go to an oxygen atom bound to either another silicon atom, or to divalent metals such as magnesium or ferrous ions, and if they do that we have rocks. Why that is relevant is that the “something else” is in nature invariably at least divalent, which means it joins onto another something else as well, usually a silicon atom, and because there is such a variety of choices you get such a variety of rocks. However, on a surface there is the possibility of no “something else” further out, and we get a hydroxyl group. It is these hydroxyls that give rise to “hydrated minerals”. They are detected because chemical bonds all vibrate, and hydroxyl groups vibrate very strongly with their stretching mode, i.e. where the hydrogen atom vibrates by changing its distance from the oxygen in a vigorous oscillation. If it is warm enough to excite this vibration, it gives off a characteristic infrared vibration with a wavelength of 3 micron or thereabouts. (It is a rather broad signal, usually.) That could be due to water, but equally it could be due to hydroxyl groups. The net result of this is that signals have been obtained from asteroids that have been interpreted as arising from carbonates, hydrides, phyllosilicates (Silicates with a sheet structure, the most common examples are a variety of clays) and sulphates.
So how do you detect water separate from these hydroxyl groups? The answer is that besides the stretching vibration, water also has a bending vibration at a wavelength of about 6.1 microns, however there is a problem recording that from space. That is, you inevitably record signals from water in the atmosphere. The only way to avoid that is the get above the water, and fortunately the air gets very much colder at higher altitudes, so the water ices out. What some astronomers did was to use a Boeing 747SP that flew up to 17 km altitude, and use a 2.7 meter diameter infrared telescope that was looking out a hatch in the plane. They looked at four asteroids. Two of them, 7 Iris and 20 Massalia had water. The signals from the other two, 11 Parthenope and 18 Melpomene, were too weak to tell. As for the amount of water, each cubic meter of regolith contained about a third of a litre of water. Of course, these observations only counted surface water; we have no evidence of the composition of the regolith a few cm below the surface.
The water could be adsorbed onto silicates, or other oxides, thus forming a thin film. Alternatively, the water could be chemically bound to minerals, but that leaves the question, how did it form? These asteroids are considered by some to have formed close to the sun, where it would be too hot for water to be adsorbed. The Physics World article proposed that C-type asteroids mixed with the S-type asteroids, collided, and water from the C-types got added to the S-types. Subsequently, the S-type and C-type separated, the C-type going out and the S-type going to the inner part of the asteroid belt. Personally, I doubt that happened because there is no obvious mechanism by which they could completely separate. It is the same argument that if you mix red and green balls and toss them around, they never spontaneously separate. I am also biased because in my ebook “Planetary Formation and Biogenesis” I have a simple mechanism as to why the C-type asteroids are on the outer part of the belt, and why the S-types are on the inner part of the belt, and also why they should have had water in them when they formed. However, the net result is that water is not impossible to get, even in some unlikely places in space.
ianmillerblog 