An international team of cosmochemists has discovered some material in a meteorite that could have only formed in a very specific process. This is the first time that such stardust material has been identified and it provides some new insight into the formation of our planet and in particular its water.
The study, published in Science Advances, reports the discovery of a higher proportion of strontium-84, an element expected to form in the p-process, in some of the oldest material known. There is a theoretical description of this p-process but it is unclear in which catastrophic stellar event (like a supernova or collision) this happens.
The team found the unusual levels of strontium-84 in the famous Allende meteorite, among stellar grains that formed before the solar system. Studying this allows scientists to look back in time, as the material forming at the very beginning of the solar system eventually accumulated in the first planetesimal that coalesced together as the Sun began to shine. Other stable isotopes of Strontium (with a higher number of neutrons in their cores) also formed via this p-process.
"Strontium-84 is part of a family of isotopes produced by a nucleosynthetic process, named the p-process, which remains mysterious," co-lead author Caltech's François L. H. Tissot, assistant professor of geochemistry, said in a statement. "Our results points to the survival of grains possibly containing pure strontium-84. This is exciting, as the physical identification of such grains would provide a unique chance to learn more about the p-process."
So what has Strontium-84 got to do with water on Earth? Well, it’s important to know the amount of this particular isotope to work out how old certain materials are. Just like carbon-dating is used to work out the age of an archeological find, to work out the age of a cosmic rock you can do rubidium-dating. Rubidium-87 has a half-life three times longer than the age of the universe, so it is ideal to date things that might be older than the Universe.
Rubidium-87 turns into strontium-87, so by studying the ratio of these two elements in a certain material, one can work out how old it is. Rubidium is also quite volatile, so if the system is heated it will be lost just like you’d expect water would be lost.
So Earth might have formed in two ways. Either from water-rich material that then lost most of its water or from water-poor planetesimal. Just by measuring the proportion of strontium-87, you should be able to tell that. If Rubidium-87 was there but then evaporated away with the water you shouldn’t have much strontium-87.
Traditionally, researchers used the oldest known stellar grains trapped in meteorites. These are called calcium- and aluminum-rich inclusions or CAIs. The problem is that the ratios of all the strontium atoms in these appear to be off, having a bit too much strontium-84.
The team took some of these CAIs and used acids to remove more and more elements. After many steps, they ended up having a sample of almost pure strontium-84. Now, this was surprising because the typical sample would contain 0.56 percent strontium-84.
"Step-leaching is a little bit of a blunt instrument because you are not entirely sure what exactly it is you are destroying at each step," explained co-lead author Bruce L. A. Charlier of Victoria University of Wellington. "But the nub of what we've found is, once you have stripped away 99 percent of the common components within the CAIs, what we are left with is something highly exotic that we weren't expecting."
Using the strontium values in CAIs, the team has been able to say that both Earth and volatile-pure meteorites appear to have a higher ratio of strontium-87. And this suggests that they once had a lot more water that evaporated away within the first few million years of their formation.