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

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 _m).

PR Photo 21b/01: ISAAC image of Jupiter (Narrow-band 4.07 _m).

PR Photo 21c/01: ISAAC image of Jupiter (Narrow-band 3.28 _m).

PR Photo 21d/01: ISAAC image of Jupiter (Narrow-band 3.21 _m).

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 _m) as
well as several narrow-band filters (3.21, 3.28 and 4.07 _m). 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 _m region, but for
the time being only used for the 3-5 _m 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

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).

Thermal-IR Views of Jupiter

ESO PR Photo 21a/01
ESO PR Photo 21b/01

ESO PR Photo 21c/01
ESO PR Photo 21d/01

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 _m (Photo 21c/01) and 3.21 _m (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 _m; (Photo 21a/01)
and in the narrow-band filter at 4.07 _m (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 _m (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 _m) and
Photo 21d/01 (3.21 _m). 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

The auroral ring

ESO PR Photo 21e/01

Caption: ESO PR Photo 21e/01 shows 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 21e/01 is a false-colour combination of the images presented in PR
Photos 21a-d/01, now showing the full disk after careful correction for
the ‘shadowing effects’ of the chopping process, as explained above.

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

ESO PR Photo 21f/01

Caption: ESO PR Photo 21f/01 shows a small area of an image obtained
through a narrow-band filter centered at 4.07 _m. 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.

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 _m, cf. PR Photo 21f/01. 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.

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.


[1]: ISAAC registers (infrared) electromagnetic radiation at wavelengths
between approx. 1.0 and 5.0 _m which we sense as heat. The human eye
registers electromagnetic radiation (light) at shorter wavelengths, from
about 0.4 to 0.7 _m.

Technical information about the photos

PR Photos 21a-d/01 are based on on-target exposures lasting a total of 30
sec (L-band), 44 sec (4.07 _m), 58 sec (3.28 _m) and 58 sec (3.21 _m),
respectively. The real observing time is twice as much, with half of the
time spent in the off-target chop position. The fields shown measure
72 x 72 arcsec2; 1 pixel = 0.07 arcsec. PR Photo 21e/01 is a colour-coded
combination of these four exposures. North is up and East is left.

Addendum: About the Jovian aurorae and polar haze

Aurorae Borealis and Aurorae Australis (‘Northern and Southern Lights’) are
observed on Earth as well as on Jupiter. They appear as wavy curtains of
light that follow the magnetic field lines at high latitude and they
surround tring of light. The light is produced by the impact of energetic
charged particles (electrons or ions lost by the magnetosphere) onto the top
of the atmosphere where they excite the atmospheric atoms and molecules,
mainly atomic (H) and molecular hydrogen (H2).

While the emissions that are excited directly by particle collisions are
radiated in the visible and the ultraviolet regions of the spectrum, it was
discovered at the beginning of the 1990’s that the auroral rings may also be
detected in the infrared (IR) region of the spectrum. The reasons for this
are also known. The auroral particles excite or ionize the atoms (and ions)
in the atmosphere, creating in some areas large numbers of H3+ ions. They
also generate a huge amount of thermal energy — in fact, the total energy
deposited in the Jovian aurorae is about 10**14 watts, or 1000 times more
than in a typical terrestrial aurora. H3+ ions are capable of radiating
their energy in narrow spectral lines in the infrared part of the spectrum
near 2 – 4 _m wavelength and, as shown by the present ISAAC images, this
radiation can be detected with ground-based telescopes.

The importance of monitoring Jovian auroral emissions is that it allows to
measure the activity of the Jovian magnetosphere and — with the help of
magnetic field models — to map in detail the auroral structures and the
motion of energetic particles in the magnetosphere. The capability to
perform such studies from the ground with a quality approaching that from
space now promises dramatic improvement in our understanding of Jovian
auroral processes and, equally important, the possible to compare them
with those on the Earth.

There are other light emission mechanisms on Jupiter than the aurorae.
Clouds are present in Jupiter’s stratosphere that efficiently reflect the
sunlight and which are responsible for the overall brightness of Jupiter’s
disk. In addition, the polar regions are covered by a ‘haze’ which is
particularly bright in infrared light. The very nature and the origin of
this haze is still quite puzzling, although it is now generally agreed
that it is a by-product of the auroral activity.

It has been suggested that the polar haze may at least partly consist of
heavy hydrocarbon molecules, polymers and/or aerosols that are produced
where incoming energetic particles from the magnetosphere enter the upper
atmospheric layers. Alternatively, the amount of energy deposited in the
auroral atmosphere is so large that violent upward winds are produced that
carry atoms and molecules from the deep atmosphere into the stratosphere.
While discrete narrow emission lines dominate the auroral infrared spectrum,
the polar haze emits at all wavelengths (a spectral ‘continuum’).

Contrary their Jovian counterparts, terrestrial Aurorae are directly related
to solar activity. Since it is now near the maximum in the 11-year cycle,
terrestrial aurorae are unusually frequent and intense, and may also be
visible at lower geographical latitudes than normal.