Physics – Optics
Scientific paper
Jun 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001eso..presp..21.&link_type=abstract
ESO Press Release, 06/2001
Physics
Optics
Scientific paper
Thermal-IR Observations of Jupiter and Io with ISAAC at the VLT
Summary
Impressive thermal-infrared images have been obtained of the giant planet Jupiter during tests of a new detector in the ISAAC instrument on the ESO Very Large Telescope (VLT) at the Paranal Observatory (Chile). .
They show in particular the full extent of the northern auroral ring and part of the southern aurora.
A volcanic eruption was also imaged on Io , the very active inner Jovian moon.
Although these observations are of an experimental nature, they demonstrate a great potential for regular monitoring of the Jovian magnetosphere by ground-based telescopes together with space-based facilities. They also provide the added benefit of direct comparison with the terrestrial magnetosphere.
PR Photo 21a/01 : ISAAC image of Jupiter (L-band: 3.5-4.0 µm) . PR Photo 21b/01 : ISAAC image of Jupiter (Narrow-band 4.07 µm) . PR Photo 21c/01 : ISAAC image of Jupiter (Narrow-band 3.28 µm) . PR Photo 21d/01 : ISAAC image of Jupiter (Narrow-band 3.21 µm) . PR Photo 21e/01 : ISAAC image of the Jovian aurorae (false-colour). PR Photo 21f/01 : ISAAC image of volcanic activity on Io . Addendum : The Jovian aurorae and polar haze. Aladdin Meets Jupiter
Thermal-infrared images of Jupiter and its volcanic moon Io have been obtained during a series of system tests with the new Aladdin detector in the Infrared Spectrometer And Array Camera (ISAAC) , in combination with an upgrade of the ESO-developed detector control electronics IRACE. This state-of-the-art instrument is attached to the 8.2-m VLT ANTU telescope at the ESO Paranal Observatory.
The observations were made on November 14, 2000, through various filters that isolate selected wavebands in the thermal-infrared spectral region [1]. They include a broad-band L-filter (wavelength interval 3.5 - 4.0 µm) as well as several narrow-band filters (3.21, 3.28 and 4.07 µm). The filters allow to record the light from different components of the Jovian atmosphere (mostly greenhouse gases and aerosols) and the appearance of the giant planet is therefore quite different from filter to filter.
At the time of these observations, Jupiter was 610 million km from the Earth and 755 million km from the Sun. The angular size of its disk was 48 arcsec, or about 40 times smaller than that of the full moon. The ISAAC instrument
The ISAAC multi-mode instrument is capable obtaining images and spectra in the near-to-mid infrared wavelength region from 1 - 5 µm. It is equipped with two state-of-the-art detectors, a Hawaii array (1024 x 1024 pix 2 ; used in the 1.0 - 2.5 µm spectral region) and an Aladdin InSb array also with 1024 x 1024 pix 2 , and sensitive over the entire 1 - 5 µm region, but for the time being only used for the 3-5 µm region.
Observations in the thermal-IR wavelength region with the Aladdin array rely on the 'chopping' technique. It consists of tilting the telescope's lightweight 1.1-m secondary mirror back and forth ('tip-tilt') about once per second. This basic technique allows to subtract the strong infrared emission from the sky by also observing an area adjacent to the object area - the difference is then the radiation from the object.
Without this method, the strong and rapidly variable sky emission - that originates in all layers of the terrestrial atmosphere - and also the thermal emission from the telescope would render infrared observations of faint celestial objects impossible. 'Chopping' is further combined with 'nodding' , i.e. moving the telescope in the direction opposite to the direction of the 'chop' in order to achieve better cancellation of residual sky emission.
Thanks to the very good stability provided by the VLT tip-tilt system and excellent seeing conditions, the image resolution obtained on these images is about 0.39 arcsec in the L-band. The field-of-view is 72 x 72 arcsec 2 (1 pixel = 0.07 arcsec) - this corresponds to 1.5 times the size of Jupiter's disk in November 2000. No other infrared astronomical instrument working at these wavelengths is capable of producing so sharp images over such a large field-of-view.
Some of these images are shown below. They were prepared and analysed by Jean Gabriel Cuby (ESO-Chile), Franck Marchis (CFAO/University of California, Berkeley, USA) and Renée Prangé (Institut d'Astrophysique Spatiale, Orsay, France). Thermal-IR Views of Jupiter
ESO PR Photo 21a/01
ESO PR Photo 21a/01 [Preview - JPEG: 400 x 448 pix - 136k] [Normal - JPEG: 800 x 895 pix - 280k]
ESO PR Photo 21b/01
ESO PR Photo 21b/01 [Preview - JPEG: 400 x 448 pix - 136k] [Normal - JPEG: 800 x 895 pix - 280k]
ESO PR Photo 21c/01
ESO PR Photo 21c/01 [Preview - JPEG: 400 x 448 pix - 144k] [Normal - JPEG: 800 x 895 pix - 320k]
ESO PR Photo 21d/01
ESO PR Photo 21d/01 [Preview - JPEG: 400 x 448 pix - 128k] [Normal - JPEG: 800 x 895 pix - 264k]
Caption : ESO PR Photos 21a-d/01 show a series of thermal-infrared images of Jupiter, obtained by the ISAAC multi-mode instrument at the 8.2-m VLT ANTU telescope on Paranal on November 14, 2000; the Universal Time (UT) of each exposure is indicated. They demonstrate the dramatically different appearance of Jupiter's disk and the aurorae when viewed through different thermal-IR imaging filters (see the text). Note also the motion of the moon Io (left). The contrast in these and the following photos have been enhanced to better show the faint details in the aurorae. Technical information about these photos is available below.
The above images were obtained in different wavebands. The appearance of the planet depends on whether the filter corresponds to a spectral band in which auroral emission lines dominate over the polar haze continuous emission (for details, read the Addendum), e.g. in the narrow-band (NB) filters at wavelength 3.28 µm ( Photo 21c/01 ) and 3.21 µm ( Photo 21d/01 ).
In the filter bands where this is not the case, the contrast between the auroral ring and its surroundings is less prominent, as in the broad-band L-filter that covers the wavelength interval 3.5 - 4.0 µm; ( Photo 21a/01 ) and in the narrow-band filter at 4.07 µm ( Photo 21b/01 ).
There is also a dramatic difference in the brightness of Jupiter's atmospheric clouds. This effect is linked to the degree of absorption of the sunlight by a methane layer that varies very much with wavelength. For instance, the spectral band at 3.28 µm ( Photo 21c/01 ) is at the edge of a strong methane absorption band and the disk therefore appears very dark at this particular wavelength.
As explained above, the chopping technique must be applied to perform these observations. It is achieved by moving the 1.1-m secondary mirror of the ANTU telescope in the direction perpendicular to Jupiter's axis of rotation. The dark circles that cover the right part of the images of the planet are due to the fact that the chop throw is limited to 30 arcsec only. While this is quite sufficient for observations of other, smaller objects, it is less than Jupiter's angular diameter at the time of these observations (48 arcsec). For that reason, the image of the planet is subtracted from itself at the right edge.
The bright spot to the left of the planet is Io , the innermost of the large moons. Its shadow on Jupiter is well visible on Photo 21b/01 (4.07 µm) and Photo 21d/01 (3.21 µm). The dark spot to the right on the images is a 'negative' image of Io , caused by the chopping and image subtraction.
Note that Io is moving towards the right during the observations. At the time of the observations, the rotation axis of Jupiter was tilted about 3° towards the Earth so that the North Pole is well visible. Moreover, the magnetic axis is inclined 9.6° to the rotation axis. Thus the northern auroral ring is fully on the Earth-facing hemisphere, while the coresponding southern ring is barely visible at the lower limb of the planet. The auroral ring
ESO PR Photo 21e/01
ESO PR Photo 21e/01 [Preview - JPEG: 400 x 409 pix - 136k] [Normal - JPEG: 800 x 818 pix - 288k]
Caption : ESO PR Photo 21e/01 shows the Jovian aurorae , in particular the northern ring (here shown in yellow/ora
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