This month there was a relative flood of interesting information. First, as readers will know, Enceladus, a small moon of Saturn, is unusual in that it has icy eruptions, and the cause of these has led to a lot of speculation. Two papers (Science 344: 78 – 80; Icarus 235: 75 – 85) concluded that these were due to the presence of a subsurface sea that experienced periodic heating of about 1.5 GW due to tidal forces. Further, a low melting temperature of around 175 ^oK is required, which implies relatively large amounts of ammonia. Such large amounts of ammonia (and methanol) are required in the Saturnian system by my mechanism of icy body formation, so these results are pleasing, at least to me. Provided there is ammonia and methanol present, these may be chemically converted to methane and nitrogen, and the conversion produces further energy, but still not enough to power the eruptions. However, the clathration of such gases in ice would help generate the pressure and store the energy, which would support the periodicity.
 
The issue of water on the Moon remains unclear: did it accrete with water or was the rock that formed it anhydrous? The issue is important because some models of lunar formation have the Moon accreting from what is essentially the vapour of silicaceous species, in which case and water with them would be expected to be lost to space. The presence of hydroxyapatite has long been considered to be a marker for the presence of relatively high concentrations of water, however one report showed that the presence of hydroxyapatite is a poor means of determining the water content of the lunar magma because the ease of forming hydroxyapatite also depends on the concentrations of chloride and fluoride, and hence there are too many unknowns. (Science 344: 400 - 402) On the other hand, there are apparently samples of olivine and plagioclase that show that some water must have been present (Science 344: 365 – 366), although it should be emphasised that neither of these rocks will absorb very much water. This issue only indirectly affects my theory, which argues that the impactor that created the Moon (Theia) probably started from the Lagrange points L4 or L5. (Some form of giant impact is required to generate enough heat by which the separation of a hydroxyapatite phase could occur so early.) Any body forming at these Lagrange points should have the same composition as Earth if composition is determined by disk heating, so it is not necessary now to generate so much energy on impact, and the Moon may have accreted around what was essentially a major fragment of Theia.
 
The final piece of relevant news is that an absorption spectrum of carbon monoxide has been recorded from a gas giant around β Pictoris (Nature 509: 63-65). This is a relatively young star, and the reason the giant gives a carbon monoxide signal is that its temperature is about 1600 ^oK, due to gravitational heating as it has accreted. The planet has a mass of about 11 times that of Jupiter, it seems to be in a circular orbit, and it has a spin velocity, determined by the Doppler signal broadening, of about 50 km/s. They also show a graph showing that as planetary mass increases, so does equatorial spin rate. Most of the points are from our solar system, and while Earth is on the graph, it probably should not be there because its spin now is accidental and was affected by lunar formation. However, the fact that this extrasolar gas giant fits the graph suggests a causal relationship. In my view, this is to be expected. In the accretion disk, gas slows below Keplerian velocity and falls towards the star. Accordingly, the planet, which is in Keplerian motion, accretes more gas from its leading face, because the pressure there is greater, and since that gas is falling starwards, it drags the planet into prograde rotational motion. The more gas accreted, the more rotational angular momentum is picked up. Convincing? Hopefully, more data will come in. Of course, only data from planets in near circular orbits are relevant. Some with very high eccentricity have probably had massive gravitational of even collisional experiences, and then the rotation could be anything, depending on the nature of the collision.