October 2, 2008
A record 2-hour observation of Jupiter using a superior technique to remove atmospheric blur produced the sharpest whole-planet picture ever taken from the ground. The series of 265 snapshots obtained with the Multi-Conjugate Adaptive Optics Demonstrator (MAD) prototype instrument mounted on ESO's Very Large Telescope (VLT) reveals changes in Jupiter's smog-like haze, probably in response to a planet-wide upheaval more than a year ago.
For decades, scientists and engineers dreamed of being able to correct wide field images for atmospheric distortions. The new images of Jupiter prove the value of MAD's advanced technology, which uses two or more guide stars instead of one as references to remove the blur caused by atmospheric turbulence over a field of view 30 times larger than existing techniques.
"This type of adaptive optics has a big advantage for looking at large objects, such as planets, star clusters or nebulae," says lead researcher Franck Marchis, from the University of California at Berkeley, and the SETI Institute in Mountain View, California. "While regular adaptive optics provide excellent correction in a small field of view, MAD provides good correction over a larger area of sky. And in fact, were it not for MAD, we would not have been able to perform these amazing observations."
MAD allowed the researchers to observe Jupiter for almost 2 hours August 16-17, 2008, a record duration, according to the observing team. Conventional adaptive optics systems using a single Jupiter moon as reference cannot monitor Jupiter for so long because the moon moves too far from the planet. The Hubble Space Telescope cannot observe Jupiter continuously for more than about 50 minutes, because its view is regularly blocked by Earth during Hubble's 96-minute orbit.
Using MAD, ESO astronomer Paola Amico, MAD project manager Enrico Marchetti and team member Sébastien Tordo tracked two of Jupiter's largest moons, Europa and Io — one on each side of the planet — to provide a good correction across the full disc of the planet. "It was the most challenging observation we performed with MAD, because we had to track with high accuracy two moons moving at different speeds, while simultaneously chasing Jupiter," says Marchetti.
With this unique series of images, the team found a major alteration in the equatorial haze brightness, which lies in a 10,000-mile- (16,000-kilometer) wide belt over Jupiter's equator. More sunlight reflecting off upper atmospheric haze means that the amount of haze has increased, or that it has moved up to higher altitudes. "The brightest portion had shifted south by more than 4,000 miles (6,000 km)," explains team member Mike Wong.
This conclusion came after comparison with images taken in 2005 by Wong and colleague Imke de Pater using the Hubble Space Telescope. The Hubble images, taken at infrared wavelengths very close to those used for the VLT study, show more haze in the northern half of the bright Equatorial Zone, while the 2008 VLT images show a clear shift to the south.
"The change we see in the haze could be related to big changes in cloud patterns associated with last year's planet-wide upheaval, but we need to look at more data to narrow down precisely when the changes occurred," declares Wong.