Pluto: evidence for polar caps.

Pluto: Evidence for polar caps

In 1985, for the first time in more than a century, the tilt of the orbit of Pluto's moon Charon reached an angle, as seen from Earth, such that observers could begin to see Charon pass in front of and behind its planet. In other words, the two objects appeared alternately to eclipse each other, as they will for about three more years. Now these shadowings, called mutual occultations, have provided data that one astronomer says may represent "a direct detection" of polar caps on Pluto.

Plutonian caps have been suggested before, but they are not necessarily icy deposits on an otherwise bare surface, as in the case of Earth and Mars. Scientists detected methane in spectral measurements of Pluto more than a decade ago, and Richard P. Binzel of Massachusetts Institute of Technology in Cambridge reports in the Aug. 26 SCIENCE that the mutual eclipses may be showing signs of methane ice topping more methane ice.

But how can observations made from a planet billions of miles away, too distant to show even continent-sized features, reveal an icy cap on top of equally icy terrain? Binzel's measurements, in fact, are not photographs at all but measurements of two colors in the sunlight reflected from Pluto and Charon together.

A methane molecule consists of one atom of carbon and four of hydrogen, and if it lies exposed on Pluto long enough it is regularly bombarded by ultraviolet sunlight that can strip away the hydrogen to leave a tarry, carbon-rich residue. (Another "stripper," points out David Stephenson of California Institute of Technology in Pasadena, would be cosmic rays.)

However, one group of researchers (including Binzel) suggested last year that some of the methane ice in Pluto's polar regions may vaporize to form a short-lived atmosphere, then freeze out again as the planet's elliptical orbit carries it nearer to and farther from the sun (SN: 9/26/87, p.207). This could result in a fresh ice surface, one that has not had as long to darken in the sun.

In 1986, says Binzel, Charon's orbit carried it so that it sometimes passed over Pluto's north-polar region as seen from Earth, and sometimes Pluto itself got in the way. When Charon was hidden from view, he says, the reflected light (measured with the 2.7-meter telescope at McDonald Observatory in Texas) looked slightly redder than when the satellite was blocking the polar region. A year later, however, when the satellite's orbit took it in front of Pluto's equatorial region rather than the north pole, there the light appeared bluer.

The implication, according to Binzel, is that Charon is slightly bluer than Pluto's equatorial latitudes, but similar to the part of the surface closer to the north pole. The redder spectrum would result when Pluto's pole is hidden, as though a spectrally bluer region had been omitted from the reflected sunlight entering the telescope. Alternatively, blocking Pluto's equator would add Charon's "blueness" to the whole.

The bluer color in the measurements when Pluto's pole is visible, Binzel suggests, could be the result of methane ice that was deposited relatively recentely, so that it has not had much time to darken. But how long does that take?

"There may be a significnat redistribution of methane on Pluto in as little as one Pluto year [about 248 Earth years]," says David Stephenson. "The thing I don't know is whether there can be a significant amount of darkening in that year. Another factor is whether there is some way to resupply fresh, still-undarkened methane ice to the surface."

Also, methane molecules presumably can be stripped of their hydrogen atoms only once. When all are reduced to bare carbon (however long that takes--millions of years? billions?), will Pluto's polar caps, if they are indeed there now, disappear?

With future observations of the mutual eclipses, says Binzel, "we hope to get a rough map of an entire hemisphere of Pluto."