the stars, but did not have enough observations to identify it as a new planet.

Neptune's mass is 25.2 times that of the Earth, similar to that of Uranus. From occupations, we can tell that its radius is 3.88 Earth radii. From these numbers, its density turns out to be 1.6 g/cm3, slightly greater than that of Uranus. The acceleration of gravity on its surface is slightly greater than on Earth at 1.14g. Its rotation period is also hard to determine, with published values ranging from 17 to 26 hr. The currently accepted value is 25.8 hr. We can deduce the presence of an atmosphere by the rate at which starlight dims during occultations. Neptune has two larger and six smaller moons.

Jupiter's atmosphere contains hydrogen and helium in the same proportion as in the Sun's atmosphere. This suggests that Jupiter has its original atmosphere. With its large mass, it has been able to hold even the lightest atoms. A number of minor constituents have been identified as well. NH3 (ammonia) and CH4 (methane) are the most prominent. In addition, C2H6 (ethane), C2H2 (acetylene), H2O, PH3, HCN (hydrogen cyanide) and CO (carbon monoxide) have been identified.

The temperature distribution is shown in Fig. 25.6. The temperature is 125 K at the cloud tops. As you go down from there, the temperature increases by about 2 K for every kilometer that you drop. Above the cloud tops, the pressure increases as you go up. The emitted radiation is approximately the same at all latitudes. This is true despite the fact that the solar heating is greatest at the equator. This may mean that winds are effective at distributing heat from the equator to the poles. Such large winds would require large temperature differences to drive them. We do not see these differences in the upper atmosphere. This means that the transport must take place in the lower atmosphere. Another possibility is that Jupiter has an internal heat source that supplies more heat to the poles than to the equator.

Example 25.1 Energy from Jupiter Compare the energy given off by Jupiter with the energy it receives from the Sun. Assume that Jupiter radiates like a 125 K blackbody, the temperature of the cloud tops.

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