MIRAGES AND THE NATURE OF PLUTOS ATMOSPHERE

We present model occultation lightcurves demonstrating that a strong t hermal inversion layer at the base of Pluto's stratosphere can reprodu ce the minimum flux measured by the Kuiper Airborne Observatory (KAO) during the 1988 occultation of a star by Pluto. The inversion layer al so forms the occultation equivalent of a mirage at 9 radius of 1198 km , which is capable of hiding tropospheres of significant depth. Pluto' s surface lies below 1198 km, its radius depending on the depth of the troposphere. We begin by computing plausible temperature structures f or Pluto's lower atmosphere, constrained by a calculation of the tempe rature of the atmosphere near the surface. We then trace rays from the occulted star through the model atmosphere, computing the resultant b ending of the ray. Model light curves are obtained by summing the cont ribution of individual rays within the shadow of Pluto on Earth. We fi nd that we can reproduce the KAO lightcurve using model atmospheres wi th a temperature inversion and no haze. We have explored models with t ropospheres as deep as 40 km (implying a Pluto radius of 1158 km) that reproduce the suite of occultation data. Deeper tropospheres can be f itted to the data, but the mutual event radius of 1150 km probably pro vides a lower bound. If Pluto has a shallow or nonexistent troposphere , its density is consistent with formation in the solar nebula with mo dest water loss due to impact ejection. If the troposphere is relative ly deep, implying a smaller radius and larger density, signficant amou nts of water loss are required. (C) 1994 Academic Press, Inc.