Feature

Pluto Has Mumps?

Recent work at Carter Observatory in Wellington suggests that Pluto has surface features which swell and contract, changing the amount of light reflected from the tiny planet.

By Tim Banks

Of all the planets, Pluto has the most eccentric orbit, swinging in past Neptune towards the Sun and out again during its 248-year orbit. This results in a three-fold variation in the level of sunlight reaching the surface. Research at the Carter Observatory has indicated that there may be seasonal variations as well.

Imagine shining a light on a slowly spinning spinning sphere, which is white on one side and dark gray on the other. As the gray side rotates into view, the amount of light reflected off the sphere will decrease. As the white side comes into view, the amount of reflected light will get progressively greater. By following these cyclic variations, the time taken for the sphere to rotate can be determined.

For 25 years, we have known that the light reflected by Pluto varies on a 6.4 day cycle. It was not until recently that computer techniques were developed to model this cycle on the basis of surface variations in reflectivity. The models are fairly crude, due to the low information content of the data.

Researchers at the Observatory have found that the light variations could be explained by the presence of two circular regions darker than their surroundings. They were surprised to find that, for these features to adequately model the variations observed over the last 30 years, the regions had to have grown over those years.

The average brightness of Pluto has decreased over this time. Astronomers think that this is due to a global thinning of a surface layer of methane, allowing a darker sublayer to peek through as the planet approaches the sun. The dark spots represent regions where the sublayer has become completely exposed.

As Pluto starts to recede from the Sun, the model predicts that the atmosphere will freeze back onto the surface. This will eliminate the dark spots and cause Pluto to become more reflective once again. It will take some time to test the model -- although its closest approach to the Sun was in 1989, Pluto's progress along its orbit is very slow.

Tim Banks is a PhD student at Carter Observatory and Victoria University