Determining how volatile elements essential to life, such as nitrogen and carbon, were introduced to Earth is critical in evaluating the potential for life on other rocky planets. Currently, many scientists speculate that these elements were brought to Earth via primitive meteorites, called carbonaceous chondrites, which originated in the outer solar system. Alternatively, it is possible that these elements were derived from a currently unrecognized reservoir that existed in the inner solar system early in its history. This possibility is supported by chemical evidence showing that Earth’s interior is similar to the magnesium-rich enstatite chondrites, which are thought to have formed in the inner solar system.
Isotopes, atoms of the same element that have different atomic masses, can serve as a fingerprint (isotopic signature) that identifies the potential source region in the solar system from which these volatiles originate. In a recent study led by Damanveer Grewal at Rice University, scientists compiled nitrogen isotope measurements from iron meteorites. Iron meteorites are thought to have formed as the metallic cores of the earliest differentiated protoplanets, and thus their nitrogen isotopic signatures can help to better understand the source and distribution of volatile elements in the early solar system. Importantly, some iron meteorites appear to represent inner solar system bodies, while others (the so-called carbonaceous irons) represent outer solar system bodies possibly related to carbonaceous chondrites. The study found that carbonaceous iron meteorites had distinct isotopic signatures when compared to non-carbonaceous iron meteorites. Combining these results with the timescales of iron meteorite parent body formation led the authors to conclude that the protoplanets forming in the inner solar system acquired their nitrogen from both inner and outer solar system reservoirs.
These results imply that present-day Earth represents a mixture of these inner and outer solar system components, because the modern nitrogen isotopic signature of Earth falls between the isotopic signatures of the two reservoirs delineated by iron meteorites. The authors suggest that mixing could have occurred during accretion, i.e., Earth originally formed from an inner solar system reservoir, but acquired additional volatile elements, including nitrogen, carbon, and water, in the final stages of accretion from impacts by bodies originating from the outer solar system. Finally, they suggest that other rocky, Earth-like planets might have acquired these elements in a similar fashion, implying that volatile elements essential to life are not strictly derived from the outer solar system. READ MORE