This update covers two months and focuses on some compositional issues. Why is composition important? In my theory, initial accretion is driven by chemical interactions, hence material that accretes at different temperatures may have different compositions. The mechanism of initial accretion in standard theory is undefined, but is usually considered to be due to gravitational interactions, in which case there should be no compositional differences, apart from outer bodies being icy. Unfortunately, the following does not show much light on this issue.

One paper involved the formation of the Moon (Nature 504: 27 - 29) The problems here are reasonably simple. Collisional dynamics suggest that which is flung off Earth comprises mainly material from the impactor, and this should have different isotope compositions from Earth, since it appears that certain isotopes varied in relative concentration by some radial function of their location in the accretion disk. However, isotope evidence indicates both bodies came from the same source. Thus the oxygen, chromium, titanium, tungsten and silicon isotope compositions of the two bodies are indistinguishable, which suggests common origin. The answers to this usually invoke extra processes, such as extensive mixing or a later gravitational resonance with the sun, but the feasibility of any of these as explanations is unclear. There are differences in composition between the Earth and the Moon. The Moon has less than 10% iron, and is poorer in volatile elements. The collision theory explains the former in terms of the iron core of the impactor merging with the earth's core, while the lack of volatile elements is consistent with these being lost from a hot disk.  It appears that refractory elements have similar abundance in both bodies. Seemingly, either the Moon formed from material from Earth's mantle, or that the Moon and the silicate portion of Earth each formed from an identical mix. My explanation is that both formed at the same radial distance and hence formed the same way from the same material, the Moon having come from either one or two bodies that grew at the Lagrange positions L4 or L5, and were dislodged when they became too big to remain in those positions. That concept is not original, but my theory makes accretion of solid bodies much more probable at our solar distance. Further, a late-forming body at L4 or L5 would have less iron, because the body, and the outer part of Earth, would form from material that started further from the sun (because it is the last part of material moving inwards).

The second major set of publications was the collection of papers in vol 343 of Science relating to results from Curiosity in Gale Crater, Mars. These results have already been announced, and as far as theories of planetary formation are concerned, these were not very interesting. One point that was of interest was the evidence of water flow and of aqueous leaching. The rover found sedimentary rock, smectites (clays, both Fe and Al rich being present) and calcium sulphate that had been precipitated from water. The evidence is in excellent agreement with the theory put forward some years ago that when an impactor struck Mars and formed a crater, it would also heat the ground beneath it and liquefy any ice. Calculations indicated this could remain in the liquid state for perhaps thousands of years. The water at Gale Crater was estimated to have been liquid for a minimum time of hundreds to tens of thousands of years. Such a short time is consistent with this impact theory, but of course since the measurements were taken inside an impact crater, it may not be relevant to Mars in general.

Finally localized sources of water vapour were detected by far infrared spectroscopy on the Herschel Space Observatory on the dwarf planet (1) Ceres (Nature 505: 525 – 527). This water vapour appeared to have been emitted from localized mid-latitude sources. The cause of the water evaporation could not be determined, but it could be due to either comet-like sublimation or to cryo-volcanism. The amount of water on Ceres is of interest because it might indicate that Ceres did not originate in the asteroid belt. More will be known about Ceres when the Dawn space craft reaches it.