Introduction

Uranus has a number of important firsts in its history. It was the first planet to be discovered by telescope. On March 13, 1781 German amateur astronomer William Herschel discovered a faint object that appeared to be a disk. What was originally thought to be a comet was indisputably shown to be a planet with a stable elliptical orbit outside that of Saturn. Uranus also played a role in a more modern telescopic first. In the spring of 1976, Bradford Smith, James Janesik, and Larry Hoveland were the first to use the new charge-coupled device (CCD) technology for astronomical purposes, obtaining an image of Uranus in a methane band at 8900 Å.

Uranus has many things in common with the other Jovian planets, large masses consisting almost entirely of H and He gas, rapid rotation, a ring system, and plentiful satellites. However it has one very important difference. While the other three have internal heat flows of the same order of magnitude as the reemitted solar energy, Uranus has only a total energy emitted to absorbed solar energy ratio of 1.060.08 (Pearl et al. 1990). This large internal heat in Jupiter, Saturn, & Neptune is attributed to the release of energy accumulated during gravitational accretion. Either Uranus does not have an internal energy source, or it is being prevented from escaping the lower atmosphere. Uranus is much colder than expected for a Jovian planet with a mean solar distance of 19.2 AU. The troposphere of a planetary atmosphere is defined as the lowest level of temperature inversion. In this region the temperature lapse rate is adiabatic, as energy is being convected upwards to the tropopause where it escapes to space. Observations and chemical arguments point to the formation of many clouds of condensed gas in the tropospheres of the Jovian planets, in increasing pressure being NH, NHSH, and H0. Because Uranus is so cold two additional condensations occur higher up in the troposphere, and HS. Although the methane abundance of Uranus is 30 times greater than the solar value, most of it condenses out before it reaches the stratosphere. As a result methane is not the significant opacity source in the stratosphere that it is in the other Jovian planets. The low effective temperature has one other effect as well, pushing the peak of the blackbody curve farther into the infrared to 50 , which could only be observed by an orbiting infrared telescope or another spacecraft mission which appears unlikely due to the current NASA direction towards small low-cost missions.

The last major observations made in the thermal infrared of Uranus were nine years ago. In 1985 Orton et al. (1987) obtained spectra in both the 8-14 and 17-24 atmospheric window. The IRIS experiment on Voyager 2 in January 1986 measured the planet from 25-50 . Since Uranus has an obliquity of 98 in its rotation each pole receives constant insolation for 42 years out of the 84 year orbital period. This causes the poles to receive annually more sunlight than the equator, theoretically causing a seasonal polar variation of 5K (Wallace 1983). At the time of the Voyager arrival Uranus was at summer solstice for the southern hemisphere, but the data received indicated both poles were at an equivalent temperature. Since the radiative time constant of Uranus is much greater than the orbital period it is clear that some other method must be equilibrating the poles, or it is possible that there is a seasonal lag caused by sluggish thermal transport. Figure 1 shows the sub-solar points of Uranus both at the time of the Voyager encounter (-83) and at the time of my proposed observations (-50) showing the somewhat different face Uranus is projecting towards the interior of the solar system. Interestingly enough I will be seeing almost identically the same face Uranus showed to Smith et al. 19 years previous.

The Voyager Radio Science Subsystem (RSS) experiment obtained a temperature profile of the equatorial stratosphere and troposphere (Lindal et al. 1987). The tropopause (the boundary between the troposphere and the stratosphere) occurs near 100 mbar. This is the boundary between two major forms of energy transport, radiation in the stratosphere, and convection in the troposphere. In the troposphere gas is convecting upwards, heating up at the surrounding atmosphere. Radiation is not significant here because due to high optical depth it is quickly reabsorbed. At the point where the optical depth reaches unity, the gas is effectively able to cool itself by radiating to space. Since H is the prime constituent of the atmosphere it is no coincidence that the tropopause occurs where the collisionally-induced H dipole reaches optical depth of unity. Just above this from 10-100 mbar the stratosphere is unaccountably warm. To account for this heating 15% of the incident solar energy must be absorbed in the stratosphere. It had been postulated that hazes in the upper stratosphere were responsible for this heating (Appleby 1986), but recent modeling by Marley et al. (1995) has shown that these hazes cannot provide the necessary opacity. Recent ground-based infrared data has shown the ethane emission in Uranus to have increased by a factor of 15% from 1985-1991 (Hammel et al. 1992).

All previous ground-based work in the infrared has been disk-averaged because of the poor resolution available, however new technology in the mid-infrared array detectors allows the first spatially-resolved ground-based observations in the thermal IR. I will be putting in proposals for observing time at two telescopes for the first phase of my observations. At the IRTF I will be observing in both the 8-14 and 17-24 atmospheric windows to look at emitted infrared flux, whereas with the 3.5 m at APO I will be observing in the near-IR from 1-2.5 to look at reflected solar light. In addition recent HST images (figure 2) of Uranus show a brightening near the south pole due to reflection off high altitude hazes as well as two distinct clouds. This is the first significant cloud structure seen since Voyager.