Saturday, October 6, 2007

Did an ancient impact bowl Pluto over?

Pluto and its large moon Charon may have been bowled over when they were struck by wayward space rocks in the past, a new study suggests. If so, NASA's New Horizons spacecraft may find evidence of these rolls when it arrives at the distant worlds in 2015.

Jay Melosh of the University of Arizona in Tucson, US, first suggested about 30 years ago that the basins gouged out by impacts would redistribute the mass of planetary bodies, causing them to roll over to re-stabilise themselves.

This mechanism may have caused Earth's Moon– which boasts the biggest impact crater in the solar system – to roll over so that the crater moved from the equator to the south pole. Similarly, a low-mass region of geysers on Saturn's moon Enceladus may have caused that icy world to rotate, as well.

The two distant bodies may have been especially prone to this reorientation because they spin relatively slowly, rotating once every 6.4 days. That is slow compared to many other satellites, Saturn's moon Enceladus spins once every 1.4 days. Slow spinners are more unstable on their axes than fast spinners.

Their surfaces have not yet been imaged in very good detail, so no impact craters are currently known on either body. But in the early solar system, before the planets settled into their present orbits, planetary building blocks are thought to have knocked into each other frequently – leaving behind large impact basins.

Assuming Pluto and Charon have basins as big as those on Saturn's moons Tethys and Rhea and Uranus's moon Titania, the researchers calculate that Pluto probably tipped over by 10° and Charon by 20°. Charon should have rotated farther because a given impact would have had a larger effect on the 1200-kilometre-wide body than on Pluto, which is about twice as wide.

Glittering star cluster is galactic heavyweight

One of the galaxy's most massive young star clusters is revealed in a stunning new image from the Hubble Space Telescope.

The cluster of thousands of stars lies 20,000 light years from Earth in the Carina spiral arm of our galaxy. It is embedded in a star-forming nebula called NGC 3603, a cloud of gas and dust with enough material to form 400,000 stars like the Sun. Watch a video zooming in on the star cluster's location in the sky by clicking on the image at right.

Most of the bright stars in the image are very hot and massive. Their radiation and stellar winds have blown out a large cavity in the nebula around them.

The three brightest ones at the heart of the cluster had previously appeared to be more massive than theory allows. But the Hubble investigation, hints that each of these objects may actually be a blurring of light from two or more individual stars that are too close together to be observed as separate objects.

Previous measurements by Hubble and the Very Large Telescope (VLT) in Chile had come to the same conclusion for two of these star systems, indicating that the heaviest star involved is as massive as 114 Suns, which is at the borderline of what some theoretical models allow.

The new investigation also indicates that the most massive stars have gathered at the cluster's centre, something that has previously been observed in more massive groupings called globular star clusters. Globular clusters behave like cosmic sorting machines. Over time, interactions between the stars cause the most massive ones to settle near the centre of clusters, while less massive stars stay farther out.

Also appearing in the image are some dark and extremely cold "Bok globules" at top right. Bok globules are dense clouds of dust and gas with between 10 and 50 times the mass of the Sun. Among the coldest objects known in the universe, with temperatures just a few degrees above absolute zero, they are thought to be condensing and on their way to forming new stars.