Dwarf planet boasts big weather, and fossil fuels too

Last updated 11:09 23/03/2010

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From 1930 until 2006, Pluto was considered to be the ninth major planet orbiting our sun. This description started to be questioned in the 1970s as scientists learned more about comets and asteroids and discovered other planet-like small objects near Pluto and beyond.

A few years ago, a team led by Professor Mike Brown discovered an object (named Eris) calculated to be 27 per cent more massive than Pluto and considered this to be a possible 10th planet. Scientists then had to look for a more rigorous definition of the term "planet".

Currently, theory holds that there are only eight major planets in the system. Pluto and Eris have been classed as (demoted to) dwarf planets, and more recently, approximately 40 dwarf planets have been observed in our solar system through telescopes.

Listeners to Radio NZ last Monday night were treated to an interesting interview with Prof Brown, who discussed whether Pluto could be seen to be a planet or not (podcast available at radionz.co.nz).

Weather changes on Pluto are, in relative terms, bigger than on any other planet or planet-like object.

The Hubble space telescope has shown that during the past decade, dark patches have grown larger, and light patches brighter.

During this time, Pluto has come a lot closer to the Sun (it has a very elliptical orbit) than for most of its approximately 250 year orbit.

This means Pluto was at its hottest during the past decade, in the region of -230 degrees Celsius. Contrast this with being "cold", which is five to 10 degrees colder. At temperatures so close to absolute zero, a 10 degrees Kelvin temperature change is relatively large and about 25 per cent lower in energy on the temperature scale. In Earth terms, a similar energy change would occur if the world's average temperature of 15C (288K) dropped to -60C (213K).

In addition, the seasons fluctuate more widely, due to the very large tilt of Pluto's axis (122 degrees), unlike the Earth's small 23.5 degrees tilt. As a consequence, methane and nitrogen are frozen on Pluto's surface.

When astronomers observe Pluto with their telescopes, they see about as much detail as we can see when looking at Earth's moon with an unaided eye. Before telescopes, we did not know what the light and dark areas on the moon were.

"That is where we are now when observing Pluto," said Prof Brown. Bright spots appear to be a nitrogen frost. The red areas are generally believed to be frozen methane, the simplest of hydrocarbons. Exposure to sunlight causes the methane to lose a hydrogen atom, which forms hydrogen gas, and the remaining part of the methane reacts with another fragmented methane to give ethane and larger compounds producing an oil or dark tar-like hydrocarbons that are intermediates for tar – "creating fossil fuels without fossils", according to Prof Brown. This is not conclusive scientifically, but spectral and chemical information supports this view.

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From density studies, we know that Pluto is made of 70 per cent rock and 30 per cent water ice, with a small layer of methane and tar on the surface, the dark regions where methane conversions have continued for a long time.

"We do not need to study only planets to find interesting things happening in our solar system," Prof Brown says.

Our apparent fascination with Pluto is not over: a mission is scheduled to leave for Pluto in 2016 and is expected to reveal much more exciting and unexpected information. In the interim, I am reminded of a T-shirt that said simply: "Don't worry, Pluto, I am not a planet either."

David Shillington is Head of the School of Applied Health Sciences at UCOL.

- © Fairfax NZ News

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