As mentioned, the resolution and coverage of the mapping instruments - MVIC, LEISA and LORRI - considerably exceeds the Pluto mission's Category 1 requirements. These call for a 500 meter resolution black-and-white photographic map of one side of Pluto and Charon, a 1.5 to 6 km resolution color map of them, and a 10-km resolution near-IR spectral map of them. For Pluto, the spacecraft should actually be able to obtain two black-and-white maps with 300-meter resolution, four 4-color maps with 800-meter resolution, and four near-IR maps as sharp as 6.5-km spatial resolution and a spectral resolution considerably better than that required. And for Charon, it should obtain 3 black-and-white maps with resolutions as sharp as 400 meters, four-color maps 1.2 km resolution, and two near-IR maps at 5.6 km resolution.

The Category 2 mapping requirements will also be more than met - for instance, half of both Pluto and Charon will be imaged in stereo at 500 meters resolution for Pluto and 2 km resolution for Charon. And large areas of both worlds will be imaged at considerably higher resolution - as sharp as 50 meters for Pluto and 150 meters for Charon - and near-IR mapped as sharply as 300 meters for Pluto and 1 km for Charon. The instruments will even be able to photograph the nightside of Pluto and Charon --using the light reflected from the other world - well enough to map frost patterns. (Although New Horizons lacks the mapping IR radiometer of POSSE, such frost-pattern mapping should provide a good deal of detailed data on Pluto's and Charon's surface temperatures - along with some microwave measurements of their overall temperature made by the REX experiment's radio receiver.)

Mapping the surface of Pluto and Charon

All this is crucial both to study the physical shape of Pluto's and Charon's surface features and map their compositions. Pluto is remarkably patchy in appearance - it has more overall contrast variation between its light and dark-colored areas than any other world in the Solar System except Saturn's two-faced moon Iapetus, and far more than its counterpart, Neptune's moon Triton (which is thought to be a similar Kuiper Belt world captured into orbit around Neptune which then underwent dramatic tidal heating while its initial lopsided orbit gradually became circular).

The light areas are various forms of ice - apparently mostly water ice and frozen nitrogen, with much smaller amounts of frozen methane and carbon monoxide mixed in - but we don't know their exact mixture - but which is a vital question as they are left over from the initial nebular cloud out of which the Solar System formed. And the dark areas are thought to be organic compounds - some of them, perhaps, quite complex - formed out of methane ice by the UV light of the distant Sun; but we don't understand why they are so blotchily distributed. (One theory even suggests that Pluto may have a layer of organic compounds deeply buried between its thick outer water-ice shell and its rocky core.)

Similarly, the cratering on Pluto and the more blandly colored Charon - whose surface composition is intriguingly different from Pluto - may reveal how the size distribution of the icy "planetesimals" that make up the Kuiper Belt - the last fossil remnants of the original solar nebula from which the planets formed - has changed down the eons. And Charon is thought most likely to have formed for the same reason Earth's Moon did - another large world crashed into Pluto early in its history, splitting off a cloud of debris that then recoalesced into a moon - but we would like to look for evidence confirming this, and see how the impact changed both the physical structure and the chemical composition of the two worlds resulting from it.

Meanwhile, the ALICE and REX experiments will analyze Pluto's extremely tenuous atmosphere - and look for any trace of one around Charon - more sensitively than required. Using airglow, one measurement of sunlight during a solar occultation for each world, and two occultations of stars by Pluto and one by Charon, ALICE will measure a wide variety of expected or possible gases (including nitrogen, carbon monoxide, methane, noble gases, and traces of hydrocarbons and hydrogen cyanide) - providing still more data on both Pluto's original composition and the extent to which radiation-triggered chemical reactions have modified it over the eons.

ALICE and REX will also profile the temperature, density and haze layers of Pluto's atmosphere - which is a bit of a puzzle; Earth-based measurements indicate that it is a good deal warmer, and thus balloons up to higher altitudes above Pluto's surface, than expected. Despite its extreme rarefaction - at most a few hundred-thousandths as dense as Earth's air - it also seems to contain a surprisingly high amount of haze.

Indeed, Pluto's atmosphere seems to be a totally unique case in the Solar System - intermediate between the atmosphere of a normal planet and the extended gas-cloud comas of comets. It may extend in faint traces for thousands of kilometers away from Pluto - perhaps even being to some slight degree tidally exchanged between Pluto and Charon - and it is apparently being swept away from Pluto by the solar wind in a way unlike that of any other present-day planetary atmosphere, but very much like the way the super-intense "T-Tauri" solar wind after the Sun's nuclear reactions first switched on, swept away much of the initial dense atmospheres of the inner planets.

Determining the rate at which Pluto's atmosphere is thus being stripped away is another important goal of New Horizons - and it will be studied not only by ALICE and REX, but by the plasma and ion measurements of the PAM experiment, which should be able to set a firm boundary on just how far Pluto's super-rarified outer traces of atmosphere do extend from the planet.