Jupiter's aurorae, volcanic eruptions on Io revealed
EUROPEAN SOUTHERN OBSERVATORY NEWS RELEASE
Posted: June 12, 2001
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.
Aladdin meets Jupiter
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
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 Ren»e Prang» (Institut d'Astrophysique Spatiale, Orsay, France).
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.
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
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
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.
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.
The Hubble Space Telescope's majestic view of the Eskimo Nebula. This spectacular poster is available now from the Astronomy Now Store.