A new paper in Elements Magazine about the importance of the mineral apatite in extraterrestrial rocks is co-authored by group member Rhian Jones with colleague Francis McCubbin from the University of New Mexico.
Apatite is an important mineral because it contains much of the phosphorus present in many different kinds of rocks. It is an extremely useful mineral for planetary scientists, because in extraterrestrial rocks it also contains most of the halogen elements, fluorine and chlorine, as well as hydroxyl (OH) groups that represent water. Because water and the halogens are volatile, we can use apatite to investigate the behaviour of volatile elements during the formation of the solar system. In turn this helps us understand more about the source of these elements on Earth, which is important for understanding big-picture science questions like the origin of life.The paper in Elements is a summary of several different applications of apatite studies in extraterrestrial rocks, including meteorites and lunar samples. Meteorites come in different varieties. Chondrites are a mix of dust particles that formed in the solar nebula about 4.6 billion years ago. The dust clumped together to form asteroids. The first apatite grains in the solar system formed within the first few million years of the solar system’s beginnings, when water and other fluids flowed through asteroids warmed by radioactive decay. Some asteroids were heated more strongly and melted: apatite in these rocks was formed from the last drops of molten rock in cooling lavas. Apatite now records the geologic story of melting and crystallization of basaltic rock on asteroids such as 4 Vesta that was visited recently by NASA’s Dawn mission.
Apatite from the Moon has been at the centre of a lot of discussion about how much water is present in the Moon’s interior (for example see here for an overview of the debate). Lunar apatite also formed from the last drops of molten rock when lava cooled. It is possible to work backwards from a measurement of the amount of OH in apatite, to calculate the amount of water in the entire Moon, but this calculation makes many assumptions and the answer is not yet definitive. We know that the Moon is very dry, but the question is, how dry? Knowing the amount of water in the Moon allows us to test models for the Moon’s origin from a giant impact of a Mars-sized body with the Earth.
Meteorites from Mars are an important source of information for understanding the behaviour of volatiles and assessing the possibility that life could have originated and been sustained in a martian environment at some point in its history. Apatite from martian meteorites shows us that Mars is quite different from the Earth and asteroids. There is a lot of chlorine available in the martian crust, and the reason for this is not well understood. Apatite shows that water, phosphorus, and the heat from igneous intrusions clearly came together throughout the history of Mars, and this provides an argument that habitable environments have existed on that planet.
Access the article online here
Full citation to the paper is Francis M. McCubbin and Rhian H. Jones (2015) Extraterrestrial Apatite: Planetary Geochemistry to Astrobiology Elements Vol. 11, no. 3 pp. 183-188. DOI 10.2113/gselements.11.3.183