Info

Ices

Mixed with hydroget Mixed with rocks?

. Molecular . Helium + Ices

Ices

Mixed with hydroget Mixed with rocks?

. Molecular . Helium + Ices

Rocks?

1 100 kPa

-0000 K 800 GPa

1 100 kPa

of Science. Copyright

1999. Uranus Neptune much smaller than that observed. The "ices" occurring in the high-pressure, high-temperature interior of Uranus and Neptune would actually exist as a hot molecular fluid, not the solid cold ice that we are more familiar with. Interior models of these planets suggest that their interiors are rather homogeneous and there is no hard evidence for a dense rocky core. This might suggest that the accumulation of these planets was slow, allowing internal heat to radiate away and thus that the ice-rock planetesimals have not been subsequently differentiated. Alternatively, the interiors may have been thoroughly mixed through convective overturning. Either way, the fact that Uranus and Neptune appear to have such similar interiors when their subsequent evolution and current reservoirs of internal heat are so very different is curious.

The high pressure-temperature environments of the internal regions of Uranus and Neptune have been experimentally modeled by "synthetic Uranus" models where a liquid solution of water, ammonia, and isopropanol (with "solar" molar abundances of H, O, C, and N) is subjected to single and double shockwave experiments conducted up to 2.2Mbar and T > 4,000 K (Hubbard, 1997c, d; Radousky et al, 1990). The compression curves derived from these experiments (i.e., the relationship between pressure and fluid density) closely match those required by interior models of these planets to explain the observed density and /-coefficients, again suggesting that the bulk of these planets are made of icy materials. For the outer part of these atmospheres at pressures less than 100kbar, the modeled compression curve matches that of nebular hydrogen and helium. The material in the "mantle" of Uranus and Neptune, if it has a solar composition of heavy elements, should be composed of about 40% water ice, 25% methane ice, approximately 25% iron and rocks, with ammonia making up part of the remaining 10%. The abundance of rocky material would make this material denser than that modeled to exist in the mantle and assumed by "synthetic Uranus" models. Hence, to reduce the density of the mantle material to that required by interior models requires either that the rocks and iron differentiate from the icy material in the mantle and form a rocky core, of which there is currently no evidence, or that sufficient low-density material such as hydrogen and helium is also mixed throughout the mantle to offset the effect of the denser rocks. Roughly 1 Mffi to 2 Mffi of hydrogen/helium would be required to do this (Hubbard, 1997d).

The hydrogen-rich outer layer appears to be rather thin and comprises no more than 15% (3,500 km) of the radius for Neptune (amounting to 0.5-1.0 Me) and 20% (5,000 km) of the radius for Uranus (amounting to less than 3 Me). Uranus and Neptune thus appear to be only slightly evolved from the original embryos that gravitationally swept up the gas in the outer parts of the proto-solar nebula during Phase 2 of formation according to core accretion theory (Section 2.4.1). As mentioned earlier, the longer time required to accrete planets at greater distances from the Sun meant that Uranus and Neptune appear never to have reached Phase 3 of formation before the solar nebula dissipated, either on its own, or when the T-Tauri phase of the Sun swept the solar system clean of any remaining nebula gas.

A small fraction of the hydrogen in the envelopes of these planets may have resulted from the decomposition of hydrogen-bearing ices in the interior. However, the He/H2 ratio in Uranus' atmosphere is found to be very close to the solar value and hence this fraction is probably very small. The high abundance of methane (40-50 times greater than would be expected for a solar C/H fraction) in the observable atmospheres of these planets, however, is good evidence of the presence of an enormous ice reservoir beneath the hydrogen-rich envelope, and considerable mixing between the two which also explains the high D/H ratio in H2 found for these planets. Given the other similarities between Uranus and Neptune, we might then expect Neptune's He/H2 ratio to be similar to that of Uranus. However, initial estimates of the Neptune's He/H2 ratio were supersolar, which was somewhat puzzling. These ratios were determined from the infrared spectra of the planets recorded by Voyager 2 and the analysis made the assumption that there was no N2 in the atmosphere (Conrath et al., 1991). However, the detection at microwave wavelengths of HCN in the stratosphere of Neptune (and not in the stratosphere of Uranus) suggested that there may be N2 in the Neptunian atmosphere. A later study suggested that a mole fraction of only 0.3% of N2 would be sufficient to reduce the He/H2 ratio inferred from the far-infrared spectrum of Neptune to that of Uranus (Conrath et al., 1993). Burgdorf et al. (2003) find from ISO measurements that they can fit the observed spectra with an upper limit on the abundance of nitrogen of 0.7%, which would make the N/H enrichment intriguingly similar to that of C/H. One possibility is that the stratospheric N2 of Neptune comes from nitrogen exospherically lost from the atmosphere of Neptune's moon Triton. However, it would appear that this process has difficulty in matching the amounts of N2 required. An alternative, probably more plausible explanation is that N2 is dredged up from the interior of Neptune via the vigorous convection known to exist on that planet. If the C/N ratio really is approximately solar for Neptune, then it would provide support for the SCIP formation model described in Section 2.6.1.

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