Carbon on the Ice Giants

The bulk composition of a giant planet provides insights into the source reservoirs from which it formed, and a window onto the epoch of planetary formation.  For the gas giants Jupiter and Saturn, Galileo and Cassini observations are starting to pin down the abundances of key elements and isotopes (with lots of caveats, especially regarding oxygen).  But for the ice giants Uranus and Neptune, the story is murkier and incomplete.  Until recently, the data seemed to suggest that both the bulk abundances of deuterium and carbon were increasing with distance from the Sun, such that Uranus and Neptune were distinct in terms of their compositional make up.  But more recent observations, from sources like Hubble and Herschel, have shown that this is untrue, and that distinguishing between Uranus and Neptune on the basis of composition is rather more challenging!

On Uranus, the best estimate of the methane mole fraction has been bouncing around a lot over the past few years.  Lindal et al. (1987) measured a 2.3% mole fraction from Voyager radio occultations.  But this was just one of a suite of possible solutions, up to a maximum of 4%.  Baines et al. (1995) inferred a much smaller deep value of 1.6{-0.5,+0.7}% using ground-based spectroscopic observations in the near infrared.  In 2011, Larry Sromovsky combined the Lindal radio refractivity profiles with cloud fitting to HST/STIS data, and found good matches to the data with 3.2-4.5%. Their ‘best compromise’ was 4.0±0.5% at low latitudes on Uranus.

But the problem is that methane appears to be depleted at high latitudes (Karkoschka and Tomasko, 2009; Sromovsky et al., 2011) due to atmospheric subsidence, so the equatorial values have to be taken as indicative of the bulk abundance but could be being redistrubuted by the circulation of Uranus' troposphere.  So there’s a lot of uncertainty out there.  The Baines et al. (1995) value of 1.6% is on the low side, whereas Sromovsky and colleagues seem to favour higher values of around 4% tropospheric methane.

And what about Neptune?  If we look at the study by Karkoschka and Tomasko (2011, Icarus 211 p780-797) using Hubble STIS data, they suggest 4±1% for methane mixing ratio for depths below 3.3 bar, with meridional variations by a factor of three at shallower depths.    So it seems hard to distinguish between Uranus and Neptune in terms of their bulk composition, specifically the abundance of carbon.  Although the values are highly uncertain, both worlds appear to be enriched over protosolar values by a factor of around 90 (Guillot and Gautier, 2014, Treatise on Geophysics).   The same is also true for HD/H2, which has been shown by Herschel (and others) to be the same on both ice giants, indicating that they both formed from similar icy source reservoirs in the distant past.