By Paul Rincon
BBC News science reporter, Cambridge
Astronomers have produced a new colour map of Pluto, the most distant planet in our Solar System, using images from the Hubble Space Telescope.
The detailed map shows areas likely to be methane frost and a bright spot perhaps made of frozen carbon monoxide.
And another team has obtained the most precise estimate yet for the size of its moon, Charon, with data gathered during its eclipse of a star.
This figure could be used to calculate a more accurate size for Pluto itself.
The US space agency's (Nasa) New Horizons spacecraft will set off for an encounter with Pluto and Charon next year, but will not arrive until 2015 at the earliest.
Until then, astronomers say they will continue to seek insights into this mysterious world and its lone satellite.
The latest global map was produced using data obtained by Hubble's Advanced Camera for Surveys (ACS) between July 2002 and June 2003.
The telescope worked over 12 orbits and looked through two filters. Producing the map has taken two years of computer processing.
The researchers, led by Marc Buie of the Lowell Observatory, have found dark areas thought to be dirty water-ice and brighter ones indicating nitrogen frost. Red areas indicate methane ice and possibly other organics (carbon-based molecules).
The methane frost seems to be everywhere, running into dark and light areas on "a hemispheric level", said Dr Buie.
An unusual bright spot near the centre of the global map could indicate the presence of carbon monoxide, said Dr Buie. The Lowell Observatory researcher said he had asked members of the New Horizons team to investigate this area with their spacecraft.
An accurate measurement of Charon's radius and density were obtained from observations made during the moon's occultation on 11 July 2005. During this occultation, the moon eclipsed, or hid, a star.
Bruno Sicardy, from the Paris Observatory, France, and colleagues used the data from this event to tie down the radius of Charon to 602.5km, plus or minus one kilometre - the most precise figure yet obtained for its size.
Previous observations had given a lower limit for Charon's size, but could not say how big it might be.
From the new radius, Dr Sicardy's team was able to determine a very accurate density for Charon of 1.73 (plus or minus 0.08) grams per cubic centimetre.
Planet under question
Astronomers can now re-analyse data on Pluto gathered in the 1980s using the new figures for Charon's size and density to better constrain these values for Pluto itself.
Recent discoveries in the outer Solar System have cast doubt on Pluto's status as a planet. Some think it is simply the first historically recorded representative of a larger family of distant bodies known as Kuiper belt objects.
Dr Buie explained that Pluto seemed to be very similar to Neptune's moon Triton, which is thought to be a Kuiper belt object captured by Neptune's gravity. This is despite the fact that the process of capture should have altered Triton's surface drastically through heating.
"I'm surprised Triton and Pluto aren't more different than they are," he told the BBC News website.
The primary launch window for the New Horizons mission runs from 11 January-14 February 2006. If it launches within that window, it will swing by Jupiter for a gravity assist and arrive at Pluto in 2015.