The measured compositions of Uranus and Neptune from their formation on the Co ice line

Mohamad Ali-Dib, Olivier Mousis, Jean Marc Petit, Jonathan I. Lunine

    Research output: Contribution to journalArticlepeer-review


    The formation mechanisms of the ice giants Uranus and Neptune, and the origin of their elemental and isotopic compositions, have long been debated. The density of solids in the outer protosolar nebula is too low to explain their formation, and spectroscopic observations show that both planets are highly enriched in carbon, very poor in nitrogen, and the ices from which they originally formed might have had deuterium-to-hydrogen ratios lower than the predicted cometary value, unexplained properties that were observed in no other planets. Here, we show that all these properties can be explained naturally if Uranus and Neptune both formed at the carbon monoxide ice line. Due to the diffusive redistribution of vapors, this outer region of the protosolar nebula intrinsically has enough surface density to form both planets from carbon-rich solids but nitrogen-depleted gas, in abundances consistent with their observed values. Water-rich interiors originating mostly from transformed CO ices reconcile the D/H value of Uranus's and Neptune's building blocks with the cometary value. Finally, our scenario generalizes a well known hypothesis that Jupiter formed on an ice line (water snow line) for the two ice giants, and might be a first step toward generalizing this mechanism for other giant planets.

    Original languageEnglish (US)
    Article number9
    JournalAstrophysical Journal
    Issue number1
    StatePublished - Aug 28 2014


    • planets and satellites: atmospheres
    • planets and satellites: composition
    • planets and satellites: formation
    • planets and satellites: interiors
    • protoplanetary disks

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science


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