Pluto gets hot(ter)Posted: April 18, 2011
You know Pluto, a Kuiper Belt object and a previous planet. It’s made of rock and ice, not just water ice but also nitrogen, methane and carbon monoxide ice. Like all planets (and Kuiper Belt objects) Pluto is on an elliptical orbit. However its orbit is much more elliptical than any of the eight planets in the Solar System. This means its distance from the Sun varies by up to 40% during its “year”. As the amount of energy received from the Sun is greater the closer you are to it, large changes in temperature occur over the course of one orbit.
So what happens when Pluto gets closer to the Sun and heats up? It boils. Well actually it sublimates meaning the gases change from solid directly in to a gas. This means that when the ex-planet is near the Sun and hot, it is warmed up and gases are released from the surface forming an atmosphere. When it is far away it gets colder and those gases can freeze back on to the surface.
Luckily for astronomers wanting to study it, Pluto made its closest approach to the Sun in 1989. This means it is still pretty warm and so it has an atmosphere which can be studied. However this is not a simple thing to do.
Pluto is a long way away and pretty small. Hence observing the tiny silver of atmosphere around it is difficult. The simple trick is to wait for Pluto to pass in front of a background star who’s brightness can be monitored. Subtle changes in the star’s brightness provides valuable information about the icy body’s atmosphere. This doesn’t happen often but luckily it is passing through the plane of the Galaxy which means the number of background stars is very high. So this combined with the fortunate timing of its closest approach means the rather jammy folk who study Pluto can look at its atmosphere when it is warm in detail every few years when it passes in front of a star. One such event happened in June 2006. A study based on this found that the atmosphere had warmed by 1.2-1.7 degrees Celsius compared to a previous close encounter with a star in 1988 (although it’s still about 170 below zero). It also detected the atmosphere up to about 135km.
So to a paper which came out today. Astronomers based in St Andrews, Scotland and on the Big Island of Hawai`i used the James Clerk Maxwell Telescope on Mauna Kea to probe the upper atmosphere of Pluto. It’s been thought for a while that there could be part of Pluto’s atmosphere which extends to a few thousand kilometres. This study looks for the telltale traces of carbon monoxide. They have a clear detection of carbon monoxide and use the shape of the spectral line to estimate the temperature and hence the height above the surface this gas would lie at. It turns out to be over 3000km above the surface of Pluto (compared to Pluto’s solid radius of 1153km), much higher than the denser lower atmosphere detected when it passed in front of a star in 2006. The researchers also find a very slight shift in the wavelength of the spectral line, opening the possibility that Pluto may have a comet-like tail.
All this provides an interesting preview of the sort of results NASA’s New Horizons missing which will arrive at the frigid sphere in 2015. While Pluto isn’t significant enough to be considered a planet, it’s certainly still fascinating astronomers.
J. S. Greaves, Ch. Helling, & P. Friberg (2011). Discovery of carbon monoxide in the upper atmosphere of Pluto MNRAS arXiv: 1104.3014v1
Young, E., French, R., Young, L., Ruhland, C., Buie, M., Olkin, C., Regester, J., Shoemaker, K., Blow, G., Broughton, J., Christie, G., Gault, D., Lade, B., & Natusch, T. (2008). VERTICAL STRUCTURE IN PLUTO’S ATMOSPHERE FROM THE 2006 JUNE 12 STELLAR OCCULTATION The Astronomical Journal, 136 (5), 1757-1769 DOI: 10.1088/0004-6256/136/5/1757