Since we just turned a page I'll bring forward some stuff as a reminder of some of what we are watching: ==quote post#806== http://dawnblog.jpl.nasa.gov/2015/12/31/dawn-journal-december-31/ DECEMBER DAWN JOURNAL IS OUT! This journal entry is unusually visually rich with many stunning and fascinating images of Ceres' surface. Do check it out! But it also has this about the GRaND data being taken: ==quote Rayman== ... With the spacecraft this close to the ground, it can measure two kinds of nuclear radiation that come from as much as a yard (meter) deep. The radiation carries the signatures of the atoms there, allowing scientists to inventory some of the key chemical elements of geological interest. One component of this radiation is gamma ray photons, a high energy form of electromagnetic radiation with a frequency beyond visible light, beyond ultraviolet, even beyond X-rays. Neutrons in the radiation are entirely different from gamma rays. They are particles usually found in the nuclei of atoms (for those of you who happen to look there). Indeed, outweighing protons, and outnumbering them in most kinds of atoms, they constitute most of the mass of atoms other than hydrogen in Ceres (and everywhere else in the universe, including in your correspondent). To tell us what members of the periodic table of the elements are present, Dawn’s gamma ray and neutron detector (GRaND) does more than detect those two kinds of radiation. Despite its name, GRaND is not at all pretentious, but its capabilities are quite impressive. Consisting of 21 sensors, the device measures the energy of each gamma ray photon and of each neutron. (That doesn’t lend itself to as engaging an acronym.) It is these gamma ray spectra and neutron spectra that reveal the identities of the atomic species in the ground. Some of the gamma rays are produced by radioactive elements, but most of them and the neutrons are generated as byproducts of cosmic rays impinging on Ceres. Space is pervaded by cosmic radiation, composed of a variety of subatomic particles that originate outside our solar system. Earth’s atmosphere and magnetic field protect the surface (and those who dwell there) from cosmic rays, but Ceres lacks such defenses. The cosmic rays interact with nuclei of atoms, and some of the gamma rays and neutrons that are released escape back into space where they are intercepted by GRaND on the orbiting Dawn. ==endquote== It also has the latest word on the BRIGHT SPOTS on some of the crater floors. They are not pure white, many have a slight blueish tinge. The best known, the Occator crater spots, have a faint reddish tinge. It might take an instrument more sensitive than the human eye to detect this--we might say it was just some tone of white. But the variation in color gives clues as to what salts are there. Rayman discusses this and how the spots might have formed. Magnesium sulfate (a compound somewhat akin to "epsom salts") is considered as a likely candidate. Rayman also discussed the indicated presence of AMMONIA-BEARING minerals on Ceres' surface. Ammoniated clays---this was not expected and is very interesting. Also an important advantage is Ceres ever becomes a site for habitation and chemical industry. Things seem to be going OK. This is a great mission! The targets (especially Ceres) and the solar powered ion propulsion technology were forward looking. I was worried Dawn might not make it down to this final nearest orbit and am very glad it seems so far to be operating as planned. ==endquote== There is an interesting interactive page of graphics that explains in part how the GRaND instrument aboard Dawn is able to give numerical estimates or in some cases lower limits on the abundances of a dozen or so elements in Ceres surface material. This was discussed some in post#801, which gave a link.