Volatile mystery: the rocky planets Mercury, Venus, Earth and Mars. (Courtesy: NASA)

Planetary scientists in France have reviewed and analysed recent research on the origins of “volatile” elements in the inner solar system and concluded that these elements have several different origins. They point out that the mechanisms involved in delivering volatiles to rocky planets such as Earth probably play a crucial role in a planet’s habitability. As a result, a better understanding of the origins of volatiles in the inner solar system could inform our search for life on other planets.

Today, the Earth has an abundance of the volatile elements hydrogen, nitrogen, carbon and oxygen, which are all crucial for life as we know it. Planetary scientists, however, do not understand why these elements are so common on Earth and other rocky planets. Scientists believe that the solar system was formed by a protosolar nebula (PSN) of gas and some dust. The PSN then condensed to form the Sun, planets, asteroids, and comets. The problem is that the elemental and isotopic makeup of the volatiles in the inner solar system does not match that predicted for the PSN. This suggests that these elements did not come directly from the PSN but were instead delivered by more complicated processes.

Three delivery processes

In their recent research, Michael Broadley and colleagues at the University of Lorraine looked that three separate processes that could have been involved in delivering volatiles to the inner solar system. First, they look at how volatiles are incorporated within solids that formed early in the PSN. Then, they looked at how these volatile-bearing solids were distributed within the PSN. Finally, the team considered how these solids would accrete to form the rocky planets.

An important part of their work is an analysis of volatile distributions is the role of “chondrites,” solid bodies that contain a large proportion of the solar system’s volatile elements. Chondrites can be made of the mineral enstatite, can be more carbonaceous in composition, be “ordinary” stony bodies, or are comet-like with a mostly icy makeup. Comets contain more water and carbon than any of the other three types of chondrites, so from this we can conclude that volatiles are not evenly distributed throughout the solar system.

In their review, Broadley and colleagues establish that volatiles are present in chondrites and comets, contained within the microscale structures of carbon-based organic compounds and water-containing hydrated silicates. The authors confirm the presence of volatiles in these celestial bodies through analysis of the isotopic signatures in their resident organic and silicate compounds. Given that certain isotopes can be found in the primitive extraterrestrial materials of some space objects and not others, it is possible to determine which objects contain the same volatiles that were formed by the PSN. This radioactive signature of volatiles is unequivocally distinct from the composition of the PSN, which is known to have formed the terrestrial planets. This means that volatiles come from a different cosmochemical reservoir than other elements in the solar system.

Ultimately, there are many unknowns in planetary science, including the origin of volatiles throughout the solar system. Broadley and colleagues’ work codifies our understanding of the distribution of volatiles in chondrites, comets, and terrestrial planets, by using diagnostic criteria to evaluate the properties of so-called “primitive matter.”

The research is described in Nature.

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