The Measured Compositions of Uranus and Neptune From Their Formation on the Co Ice Line

Abstract

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 had deuterium-to-hydrogen ratios lower than the predicted cometary value, unexplained properties 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 iceline. 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 and Neptune's building blocks with the cometary value. Finally, Our scenario generalizes a well known hypothesis that Jupiter formed on an iceline (water snowline) for the two ice giants, and might be a first step towards generalizing this mechanism for other giant planets.

ICE GRAIN COLLISIONS IN COMPARISON: CO2, H2O, AND THEIR MIXTURES

“…Planet formation in general, but especially collisional outcomes, are tied to these icelines and the physics of the prevailing kind of ice. (Aumatell & Wurm 2011Ali-Dib et al 2014;Blum et al 2014;Deckers & Teiser 2016;Musiolik et al 2016).…”

Constraints from Comets on the Formation and Volatile Acquisition of the Planets and Satellites

“…The scenario of clathration of volatiles also matches the observational data of Uranus and Neptune (Gautier and Hersant 2005) but the known constraints on these two planets are too scarce and can also be explained by alternative scenarios. For example, it has been proposed that Uranus and Neptune could have formed between the CO and N 2 ice lines, assuming that the disk was stationary (Ali-Dib et al 2014). This model explains the D/H value found in both planets, matches well the heavy C enrichment in the two planets and predicts that N is protosolar in the envelopes.…”

“…The exact nature of these ices is still not determined, although most recent results seem to argue for full clathration of volatiles to explain the enrichment of volatiles in the giant planet atmospheres and the 14 N/ 15 N in Jupiter's (Mousis et al 2009c(Mousis et al , 2012) and Saturn's atmospheres (Mousis et al 2014a). The formation scenarios of Uranus and Neptune are not as well constrained and could also be explained by formation in a region between the CO and N 2 ice lines (Ali-Dib et al 2014). Questions remain as to the later contribution of comets to the volatile inventories of the giant planet atmospheres.…”

The EChO science case

“…The reason for this enrichment is unclear: is it due to upward mixing, early or late delivery of planetesimals, or because they formed at the CO ice line [6]? EChO would guarantee these measurements in many planets, thereby providing observations that are crucial to constrain models.…”

“…(ii) The target wavelength coverage of 0.55-16 μm guarantees that CO 2 and C 2 H 6 can be detected in temperate planetary atmospheres. It also offers the possibility of detecting additional absorption features for HCN, C 2 H 2 , CO 2 and C 2 H 6 for all other planets and improves the retrieval of thermal profiles [13].…”