More Evidence for Liquid Water on Comets Robert B. Sheldon and Richard B. Hoover NASA/MSFC/NSSTC/VP62, 320 Sparkman Dr, Huntsville, AL, USA ABSTRACT It has been seven years since we presented evidence for liquid water on comets and the wet comet theory that comets melt and undergo an irreversible phase change on their first passage through the inner solar system. Since then there have been three more comet flybys and analysis on returned cometary material. We review the wet comet model and discuss the new data, showing that the model not only has been further vindicated, but explains several more independent observations. Not only do comets show evidence of some melting, they show evidence of complete melting. 1. INTRODUCTION Whipple Dirty Snowball Model There are many differences between long and short period comets which Whipple’s model does not explain, and even between comets and asteroids which do not have easy explanations. In Table 1 we list some of the strange properties of comets which the Whipple model does not address. The old paradigm of comets was developed primarily by the late Fred Whipple, who suggested that the dust and gas of telescopically observed comets could be explained if they were an amalgam of ices and dust formed out of the left-overs of the proto-solar nebula now found out beyond the orbit of Pluto in what is known as the Oort Cloud. Since these ices have never been close to the Sun or any star, they are composed of CO, CO 2 , CH 4 and H 2 0 ice grains loosely cementing together interstellar dust. The Oort cloud objects, with mean radii of 1 lightyear from the Sun, are very loosely bound to the gravitational well of the Sun, and can experience significant perturbations by passing stars, which occur regularly whenever the Sun in its orbit about the galactic center passes through the plane of the galaxy. In addition, events such as nearby magnetar gamma ray bursts can heat a layer of ice several centimeters below the surface and blow off chunks of ice that supply enough delta-v to send the comet into the inner solar system. As a pristine comet enters within the orbit of Jupiter, the CO ice will begin to evaporate, and within the orbit of Mars, the H 2 O will begin to evaporate forming a tail. These “long-period” comets have apogees out beyond the orbit of Pluto, and typically are very bright, very dusty, and very large. According to Whipple’s model they have rather high albedo, reflecting most of the sunlight that falls on them, and their outgassing keeps their surface cool. Gravitational interactions with Jupiter, the largest gravitational cross-section planet in the solar system, can trap their aphelion, causing them to have 5-6 year orbits with aphelion near Jupiter’s orbit. These Jovian or “short-period” comets not only are periodic but they typically are smaller, have less dust and are not as bright. There is a third category of Kuiper-belt objects that have aphelia near the orbit of Neptune, which may include comets such as Halley’s comet with periods of 75 years. Review of Wet Comet Model In our earlier papers on comets, Hoover et al. 2004, Sheldon & Hoover 2005, 2006 and 2007 (HEA04, SH05, SH06 and SH07) we presented evidence that comets must go through a phase transition as they enter the orbit of Mars. The outgassing of comets provide random torques that spin the comet up. At low spin rates, the Rayleigh-Taylor instability causes the comet to act as a refrigerator. The gravity vector points down, and the Sun’s radiant heat is applied at the top of the atmosphere. Since heat rises, the heat does not penetrate in toward the comet. But as the comet rotates to point away from the Sun, the atmosphere radiates to the cold night sky and colder gas sinks down. Thus heat is pumped out of the comet. But as the spin rate increases and exceeds the local force of gravity so that a pebble will just barely lift off the equator, then the Rayleigh-Taylor instability begins to operate in heat-pump mode. The gravity vector points upward, and as the sun-facing side applies heat at the top of the atmosphere, the heat convects toward Further author information: E-mail: rbs@rbsp.info, Telephone: 1 256 653 8592