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.

Thermal-infrared image of Jupiter, obtained by the ISAAC multi-mode instrument at the 8.2-m VLT ANTU telescope on Paranal on November 14, 2000. Photo: ESO

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.

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. 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 _m. It is equipped with two state-of-the-art detectors, a Hawaii array (1024 x 1024 pix2; used in the 1.0 - 2.5 _m spectral region) and an Aladdin InSb array also with 1024 x 1024 pix2, 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 arcsec2 (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 RenÈe PrangÈ (Institut d'Astrophysique Spatiale, Orsay, France).

Narrow-band image of Jupiter at wavelength 3.28 µm. Photo: ESO

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, e.g. in the narrow-band (NB) filters.

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; (first photo) and in the narrow-band filter at 4.07 µm.

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 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.

Narrow-band image of Jupiter at wavelength 3.21 µm. Photo: ESO

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 above. 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 deg towards the Earth so that the North Pole is well visible. Moreover, the magnetic axis is inclined 9.6 deg 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
A false-colour combination of the images presented above now showing the full disk after careful correction for the 'shadowing effects' of the chopping process, as explained above.

The Jovian aurorae, in particular the northern ring (here shown in yellow/orange) as well as the "polar haze" (blue). The visibility of the various features has been enhanced by the use of false-colours. The moon Io is visible to the left. Photo: ESO

The auroral oval is well visible all the way around the pole. The visibility on the far side is enhanced because of the grazing angle of view: near the limb, the apparent brightness increases since the line of sight passes along a longer section of the emitting layer, whereby the number of emitting atoms in these directions increases. On the contrary, it more difficult to detect the faint ring at lower latitudes on the day-side disc, where the path length is shorter.

In fact, the front part of the auroral oval has never before been observed from the ground -- so far it was only seen with the Hubble Space Telescope (HST). The present photo therefore highlights ISAAC's excellent image quality and high stability. Note also that it has been possible to resolve two separate arcs on the right side of the ring; this is normally only possible by means of observations from space.

Another interesting property of this image is the extension of the polar haze, here seen in blue colour. A comparison with the rotation (yellow arrow) and magnetic (white arrow) axes shows that the polar haze is centered on the rotation axis whereas its source, the auroral ring, is centered on the magnetic axis.

This observation therefore suggests the following interpretation: the atoms and molecules that make up the polar haze are continuously created at the footprint of the auroral magnetic field lines, i.e., below the auroral rings. They spread over both polar regions, much more so in longitude than in latitude. This bears witness to the important role of the zonal winds in the Jovian atmosphere (blowing along the same latitude) in transporting the haze material, much stronger than that of the meridional winds (along the same longitude), even at the high latitudes of the auroral region. Jupiter's rapid rotation (about 10 hours per revolution) obviously plays an important role in this.

A volcanic eruption on Io
Io, the innermost major satellite of Jupiter is one of the most remarkable bodies in the solar system. Volcanic activity on its surface was first discovered by the NASA Voyager 1 and 2 spacecraft during fly-by's in 1979. This is attributed to internal heating caused by tidal effects between Jupiter, Io and the other Galilean satellites. Apart from the Earth, Io is the only other body in the solar system that is currently volcanically active. The volcanism on this moon is the main source of electrically charged particles (plasma) in the magnetosphere of Jupiter.

A bright polar feature is visible on several ISAAC images of Io, obtained through a narrow-band filter at 4.07 µm, (photo below). In this waveband, the effect of reflected sunlight is negligible and the image resolution is the best. Applying a basic filtering algorithm, the sharpness of this image was further enhanced. The recorded emission is found to correspond to the Tvashtar hot spot that was discovered by NASA Infrared Telescope Facility (IRTF) in November 1999 and observed simultaneously by the Galileo spacecraft during its I25 flyby.

A small area of an image obtained through a narrow-band filter centered at 4.07 µm. The bright object is the Jovian moon Io; its image is further enlarged to the left. A strong asymmetry is evident, with the Tvashtar hot spot well visible in the upper right quadrant. Photo: ESO

Such outbursts normally have a short lifetime, less than 1 month, and a very high temperature, more than 1000 K (700 C). However, the Tvashtar outburst is quite anomalous and has lasted more than one year. The temperature has been estimated at about 1000-1300 K (700-1000 C); this range is typical for silicate-based volcanism observed on the Earth.

The Galileo spacecraft observed the onset of this eruption, and twice again this year. Monitoring of this event by means of ground-based telescopes, as here with ISAAC at the VLT or by the ADONIS Adaptive Optics system on the ESO 3.6-m telescope at La Silla, gives the astronomers a most welcome opportunity to follow more closely the temperature evolution of the eruption and hence provides excellent support to the space observations.

The forthcoming arrival on Paranal of NAOS (the adaptive optics system for the VLT) and CONICA (the connected IR camera equipped with an Aladdin detector) will lead to a significant improvement of the achievable image quality. It will be employed for a large variety of astronomical programmes and will, among others, allow the detection and frequent monitoring of a large number of hot spots on the surface of Io.