Sharpening Up Jupiter
- From: baalke@xxxxxxxxxxxxx
- Date: Fri, 3 Oct 2008 15:24:09 -0700 (PDT)
http://www.eso.org/public/outreach/press-rel/pr-2008/pr-33-08.html
ESO 33/08 - Photo Release
2 October 2008
For Immediate Release
Sharpening Up Jupiter
New image-correction technique delivers sharpest whole-planet
ground-based picture ever
A record two-hour observation of Jupiter using a superior technique to
remove atmospheric blur has 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) reveal changes
in
Jupiter's smog-like haze, probably in response to a planet-wide
upheaval
more than a year ago.
Being able to correct wide field images for atmospheric distortions
has
been the dream of scientists and engineers for decades. The new images
of Jupiter prove the value of the advanced technology used by MAD,
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 thirty
times
larger than existing techniques [1].
"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 UC Berkeley and the SETI Institute in
Mountain View, California, USA. "While regular adaptive optics
provides
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 two hours on
16 and 17 August 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 the Earth during Hubble's 96-minute orbit.
Using MAD, ESO astronomer Paola Amico, MAD project manager Enrico
Marchetti and Sébastien Tordo from the MAD team 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 brightness of the equatorial haze, which lies in a 16 000-
kilometre
wide belt over Jupiter's equator [2]. 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 6000 kilometres," 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.
Notes
[1] Telescopes on the ground suffer from a blurring effect introduced
by
atmospheric turbulence. This turbulence causes the stars to twinkle in
a
way that delights the poets but frustrates the astronomers, since it
smears out the fine details of the images. However, with Adaptive
Optics
(AO) techniques, this major drawback can be overcome so that the
telescope produces images that are as sharp as theoretically possible,
i.e., approaching conditions in space. Adaptive Optics systems work by
means of a computer-controlled deformable mirror that counteracts the
image distortion introduced by atmospheric turbulence. It is based on
real-time optical corrections computed from image data obtained by a
'wavefront sensor' (a special camera) at very high speed, many
hundreds
of times each second. Present AO systems can only correct the effect
of
atmospheric turbulence in a very small region of the sky - typically
15
arcseconds or less - the correction degrading very quickly when moving
away from the central axis. Engineers have therefore developed new
techniques to overcome this limitation, one of which is multi-
conjugate
adaptive optics. See ESO 19/07 <../pr-2007/pr-19-07.html> for more
details on the Multi-Conjugate Adaptive Optics Demonstrator (MAD)
prototype instrument.
[2] The haze, which could be the nitrogen compound hydrazine - used on
Earth as a rocket propellant - or possibly frozen crystals of ammonia,
water or ammonium hydrosulphide from deeper in the gaseous planet, is
very prominent in infrared images. Because visible light can penetrate
to deeper levels than light at the infrared wavelengths detected by
MAD
(around 2 microns), optical telescopes see light reflected from
deeper,
thicker clouds lying beneath the haze. The haze behaves somewhat like
particles in the tops of thunderheads on Earth (known as cumulonimbus
anvils) or in the ash plumes from large volcanic eruptions, which rise
into the upper atmosphere and spread around the world. On Jupiter,
ammonia injected into the upper atmosphere also interacts with
sunlight
to form hydrazine, which condenses into a mist of fine ice particles.
The hydrazine chemistry in Jupiter's atmosphere is similar to that
occurring in the Earth's atmosphere after a volcanic eruption, when
sulphur dioxide is converted by solar ultraviolet light into sulphuric
acid.
Contacts
Franck Marchis, Michael Wong
UC Berkeley, USA
Phone: +1 510 642 39 28, +1 510 207 22 36
Mobile: +1 510 599 06 04
E-mail: fmarchis (at) berkeley.edu, mikewong (at) astro.berkeley.edu
Enrico Marchetti
ESO, Germany
Phone : +49 89 3200 64 58
E-mail: emarchet (at) eso.org
Paola Amico
ESO, Chile
Phone: +56 2 463 3103
E-mail: pamico (at) eso.org
.
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