Uranus radiates at most only 6% more energy than it receives from the Sun and its internal heat flux is estimated to be less than 0.04 Wm~2. Hence, to a first approximation it has almost no internal heat source at all, although by the Kelvin-Helmholtz mechanism as much as 1Wm~2 would currently be expected. This low value of Uranus' internal flux is extremely puzzling, especially when it is considered that Neptune, which is otherwise so similar to Uranus, has a very strong internal heat source. In fact Neptune's total thermal emission to space is almost equal to that of Uranus' even though Neptune is much farther from the Sun. Uranus is estimated to include as much as 4 Mffi of rocky materials, and assuming that this rock is heated by radioactive decay at the same rate as terrestrial rocks, then a flux of 0.02 Wm-2 is calculated (Hubbard, 1997b), which is a substantial fraction of that estimated.
There is no consensus on why Uranus' flux is so low. One suggestion is that chemical composition gradients, such as that at 5,000 km depth at the ice/hydrogen-helium interface (Figure 2.10) may act as a convection barrier and thus inhibit the transport of heat from the planet's hot interior. However, why this might happen in Uranus' atmosphere and not in Neptune's is unclear. Another possibility is that towards the end of its formation, Uranus suffered a cataclysmic off-center impact with another planet-sized body. Such an impact would account for Uranus' abnormally high obliquity and compact, equatorially aligned satellite system formed out of collision debris. It might also have had the effect of greatly accelerating the release of internal heat by effectively turning the planet inside out! Neptune, on the other hand, may have suffered much more centered impacts and thus rather than turn the planet on its side, the collision energy was converted into additional internal heat which would explain Neptune's high internal heat source. It should be remembered that these "giant collision'' theories are highly speculative and indeed Uranus' high obliquity could have been imparted by several off-center collisions with much smaller bodies during formation and not necessarily a single giant collision.
Although methane and water in the deep interior should dissociate at the high temperatures and pressures found there, it does not appear that the hydrogen this releases permanently escapes the interior and mixes with the nebular hydrogen in the outer hydrogen-helium envelope since the observed He/H2 ratio is close to solar. There is, however, good evidence of considerable mixing of nebula hydrogen with this dissociated hydrogen since the (D/H)hj ratio is observed to be high (as was discussed in Chapter 2). The case for Neptune is similar although initial estimates of the He/H2 ratio were supersolar, which was very puzzling. It may be possible that the vigorous convection in Neptune's atmosphere, consistent with its high internal heat flux, brings substantial quantities of N2 to the observable atmosphere, which has a substantial effect on the He/H2 ratio detection technique employed by Voyager 2. As little as 0.3% abundance of N2 reduces the estimated He/H2 ratio to that of Uranus (as we saw in Chapter 2). However, an alternative explanation is that the supersolar estimate of the He/H2 ratio was based on far-IR absorption data that have recently been revised for the low temperatures found in Uranus' atmosphere (Orton et al., 2007b). Using these revised data, the far-IR spectrum of Uranus is consistent with the expected solar He/H2 ratio.
Although Uranus' internal heat source appears very low, and thus vertical convection is presumably sluggish, the atmosphere is still very dynamic and the circulation efficiently redistributes relatively warm polar air over the planet. During the Voyager 2 flyby, Uranus was close to northern winter solstice with the South Pole permanently in sunlight and the North Pole permanently in darkness. If there were no atmospheric motion then we would have expected the South Pole to become very much warmer than the North by as much as 7 K. However, Voyager 2 found almost no temperature difference at the 0.5 bar to 1 bar pressure level indicating a very dynamic circulation. The dynamics of Neptune's atmosphere were observed by Voyager 2 to be even more vigorous with winds approaching speeds of 400 m/s, as we shall see in Chapter 5. Subsequent observations of these planets from the ground and from Earth-orbiting telescopes have confirmed the dynamic nature of these planetary atmospheres.
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