Pluto’s new face

After years of work, pictures from the Hubble space telescope are changing our way of looking at this far-off world.

 

The surface of Pluto as shown by the Hubble space telescope. This is the most accurate map of this dwarf planet situated on the borders of the solar system. The pictures were taken in 2002 and 2003. Credit: NASA/ESA/M. Buie (Southwest Research Institute)

Discovered in 1930 by American astronomer Clyde Tombaugh, Pluto was considered to be a proper planet until 2006, the year when the International Astronomical Union downgraded it to a “dwarf planet”.

There are seasons on Pluto!
Too small and not having sufficiently “cleaned” its orbit (there are too many objects of a comparable size close to its orbit for the planetologists), this dwarf planet with its diameter of 2,306 km is smaller than the 3,474 km diameter Moon. Relegated to the borders of the solar system, it moves between 4.4 and 7.3 billion km from the Sun. To make things clearer, our good old Earth is on average 39 times closer to our celestial body of the day than Pluto. And this was enough to give this small far-off world the image of a frozen, dead object... But, in actual fact, this is not true and there are even seasons on Pluto! Talking of which, the Hubble space telescope has just confirmed the previous works. In 1994, the European FOC (Faint Object Camera) had already made it possible to draw up a full, relatively detailed map (given the distance) of the surface of this small planet. A few years later, in June 2002 and June 2003, Hubble repeated the operation with the ACS camera (Advanced Camera for Surveys). Very subtly, the space telescope operators slightly shifted the pointing every time a photograph was taken: by combining the images obtained in this way, it was then possible to artificially increase the resolution and therefore the details captured. A massive task to process the data was necessary and it was just yesterday, 4 February 2010, that the Space Telescope Science Institute in Baltimore released the new map of Pluto. The video below makes it possible to go the whole way round the dwarf planet.


The north pole, lit by the Sun is obviously more brilliant than in 1994. It is a bit like the beginning of the summer... If it can be called that as the temperatures there range from -240°C to -218°C. The dark and light areas (yellow-orange coloration) confirm, however, the existence of a seasonal cycle on this dwarf planet, predominantly composed of rock and nitrogen, methane and water ice. The Sun could sublime the nitrogen (immediate transformation from a solid to a gaseous state) with the season change and thus modify Pluto’s appearance. But other phenomena are at work as the overall tint of certain regions has reddened. Astronomers are putting forward the theory of an action of the Sun on the methane. Once sublimed, the ultraviolet radiation would dissociate it and would leave the carbon residues responsible for the orangey coloration on the surface of this far-off world. And the more of these deposits that were to form, the more the areas in question would change from yellowish to orange, the famous “reddening” that has been noted.

Comparison of the global maps of Pluto in 1994 (top) and in 2002/2003 (bottom). It is possible to note obvious differences, sign of a seasonal cycle at work. The light area at the top (2002/2003 image) is situated close to the north pole of this dwarf planet. In 2003, Pluto was 30.6 times further away from the Sun than the Earth. It is still moving away (31.8 at the moment) and will reach its maximum in 2130. Pluto takes 248 years to orbit the Sun. Credit : NASA/ESA/M. Buie (Southwest Research Institute)

Appointment in 2015
This new face of Pluto means therefore an amazingly active world where seasons are at work. The map created in this way, will also remain the most accurate for several years even though the “finest” details are but hundreds of km. But this will all change at the beginning of 2015 when NASA’s New Horizons probe will be sufficiently close to take better pictures. The month of July of the same year is when the probe will carry out the much awaited flyby, 14 being the date it is to fly closest. The New Horizons’ teams are already working on Hubble’s pictures as they will help them to further refine the exposure settings for the different onboard cameras: there will not be a second chance so it would be better to avoid any under or over exposed pictures.

Launched on 19 January 2006, the New Horizons probe is to fly past Pluto on 14 July 2015 (illustration). Credit: NASA

This date, 14 July 2015, will also mark a highly symbolic meeting as the probe is carrying some of Clyde Tombaugh’s ashes. Pluto and its discoverer will never have been so close...

Published on 5 February