One of my greatest writing pleasures has been getting to know Heidi Hammel when I wrote her biography, Beyond Jupiter for the Joseph Henry Press “Women’s Adventures in Science” series. Among the many things we have discussed is the advance in imaging made possible by adaptive optics, which enables the Keck telescope (and others) to correct for atmospheric distortions.
Adaptive optics uses an artificial “guide star” projected by a laser to measure atmospheric distortion in real-time and thus correct for it. However, an object as large and bright as Jupiter normally overwhelms the artificial star. Fortunately, there are times when Jupiter’s bright ice-covered moon Europa can serve that purpose instead.
That’s why I find the following news release fascinating, not only for the data it describes but also for the use of Europa as an adaptive-imaging guide star.
February 9, 2011
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EUROPA HELPS ASTRONOMERS PEER INSIDE JUPITER’S MISSING BELT
The ongoing turmoil inside Jupiter’s missing — and slowly re-emerging
— South Equatorial Belt can now be seen in unprecedented detail
thanks to the Keck II telescope’s adaptive optics system and the
cooperation of the icy Jovian moon Europa. In this newly released Keck
image, the gas giant is shown as it looks in thermal infrared (IR)
light, at a wavelength of nearly 5 microns (shown in bright red and
yellow), overlaid on a composite image of three shorter, near-infrared
bands (1.21, 1.58, and 1.65 microns).
“The thermal IR senses breaks in the cloud cover,” said astronomer
Mike Wong of the University of California at Berkeley. The thermal IR
data is essentially showing heat from Jupiter’s interior being
radiated into space. The three other IR bands, in contrast, capture
reflected sunlight. Put them all together and compare them to visible
light images and scientists get a picture of a thinning, breaking
layer of high, bright, icy clouds that have obscured the brown-red
South Equatorial Belt (SEB) for about a year, making it look like a
wide white zone.
“We see wispy cloud-free regions at 5 microns in the SEB,” said Wong,
“But they are much less extensive than the near-infrared dark regions
surrounding them. The data show that the change from zone-like to
belt-like appearance is a complex process that takes place at
different speeds in each layer of Jupiter’s atmosphere.”
The four-band infrared image was created using a clever twist on the
Keck II telescope’s adaptive optics, which effectively cancels out
much of the interference of Earth’s atmosphere. Normally astronomers
use a powerful laser to create an artificial guide star. With that
they can monitor Earth’s constantly changing atmosphere and cancel out
the distortions at a rate of up to 2,000 times per second.
But Jupiter is so bright that it hides the laser guide star. The
astronomers needed something much brighter that was also very close to
Jupiter in the sky. On November 30, 2010, the icy Jovian moon Europa
was positioned just right to serve that purpose, explained Franck
Marchis, also of UC Berkeley and the SETI Institute.
Timing observations right so that Europa makes adaptive optics
possible is no easy feat , according to Marchis, and underscores the
technical challenges involved in peering into Jupiter’s clouds.
Despite many other researchers watching Jupiter’s changing clouds with
other telescopes, none, not even the Hubble Space Telescope, is
equipped to peer in the 5-micron band with such high resolution.
Marchis, Wong, and UC Berkeley astronomer Imke de Pater are involved
in a large monitoring program aimed at following up on these rare and
mysterious happenings in Jupiter’s atmosphere. They were assisted in
their Keck II observations by W. M. Keck Observatory support
astronomer Randy Campbell.
Univ. of Calif., Berkeley
Univ. of Calif., Berkeley / SETI Institute
Keck Adaptive Optics
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The W. M. Keck Observatory (http://keckobservatory.org/) operates two
10-meter optical/infrared telescopes on the summit of Mauna Kea. The
twin telescopes feature a suite of advanced instruments including
imagers, multi-object spectrographs, high-resolution spectrographs,
integral-field spectroscopy, and a world-leading laser guide star
adaptive optics system. The Observatory is a private 501(c)3
organization and a scientific partnership of the California Institute
of Technology, the University of California, and NASA.