IRTF Images of Jupiter
<<These near-infrared images of Jupiter were taken as a part of a campaign to observe changes in Jupiter's atmospheric conditions using instrumentation at NASA's Infrared Telescope Facility, located at the summit of Mauna Kea, Hawaii. North is toward the top in these images; Jupiter rotates from left to right. These wavelengths sample cloud reflectivity (1.58 microns), haze and cloud particles in the upper troposphere (3.8 microns) and lower stratosphere (2.3 microns) and thermal emission from cloud tops (4.85 microns). Auroral emission is also seen near the poles at 3.8 microns.
At a wavelength of 1.58 microns, we sense sunlight reflected from cloud particles with virtually no absorption by Jupiter's gases. Thus, the variations from one region to the next indicate changes of the reflectivity (albedo) of cloud particles. The appearance of the planet at 1.58 microns is the closest near-infrared analogue of its visible (red) appearance.
At a wavelength of 3.8 microns, we sense two sources of radiation. One is sunlight reflected from cloud particles in Jupiter's upper troposphere which has not been extinguished by weak methane (CH4) gas absorption. The Great Red Spot is seen as a high cloud on the right, just below the center; the brightest reflection - and probably the highest cloud particles in this region - are detected in discrete plumes just north of the equator, close to the latitude in which the Galileo Probe will enter on December 7. The other source of radiation comes from emission by
H3+ from an airglow which covers the planet and is most pronounced at the edge of the disk (the limb) and from auroral emission near the poles. This image has captured oval-shaped
H3+ auroral emission near the south pole.
At a wavelength of 2.3 microns, we sense sunlight reflected from cloud and haze particles higher up - in Jupiter's lower stratosphere - which have not been extinguished by methane (CH4) and molecular hydrogen (H2) gas absorption. The most prominent features are the particulate caps covering the poles (in this image, the dark spot in the middle of the south polar cap is an artifact of the NSFCAM optics at the time). From mid-latitudes toward the south pole, the moderate reflecting particulates are mostly remnants of particles remaining from impacts of Comet Shoemaker-Levy 9 fragments near 45 deg.S lat., now distributed rather uniformly over longitudes and migrating far to the south and somewhat to the north of the impact latitude.
At a wavelength of 4.85 microns, we are sensing heat from Jupiter, which we indicate by the false red shading. The darkest regions correspond to emission from temperatures near 185 Kelvins (-190 deg. F) and the warmest regions to temperatures near 255 Kelvins (-64 deg. F), indicating emission from 1 - 5 bars of atmospheric pressure in Jupiter. The absence of strong gaseous absorption at this wavelength means that we are sensing temperatures near the tops of clouds in the atmosphere. The brightest spots are discrete regions which are locally clear of obscuring clouds, known as ``hot spots''; they are one of the special targets planned for the atmospheric investigations by the Galileo Orbiter instrument teams. Note that there is a relatively bright (clear) but broken band to the north of the Great Red Spot. There are also bright rings around several of the visibly white oval features south of the Great Red Spot.
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Science News
Really Big Planets: When Do Gas Giants Reach The Point Of No Return?
ScienceDaily (Dec. 7, 2007) — <<Planetary scientists at UCL have identified the point at which a star causes the atmosphere of an orbiting gas giant to become critically unstable, as reported in this week's Nature (December 6). Depending upon their proximity to a host star, giant Jupiter-like planets have atmospheres which are either stable and thin, or unstable and rapidly expanding. This new research enables us to work out whether planets in other systems are stable or unstable by using a three dimensional model to characterise their upper atmospheres.
Tommi Koskinen of UCL's Physics & Astronomy Department is lead author of the paper and says: "We know that Jupiter has a thin, stable atmosphere and orbits the Sun at five Astronomical Units (AU) - or five times the distance between the Sun and the Earth. In contrast, we also know that closely orbiting exoplanets like HD209458b - which orbits about 100 times closer to its sun than Jupiter does - has a very expanded atmosphere which is boiling off into space. Our team wanted to find out at what point this change takes place, and how it happens.
"Our paper shows that if you brought Jupiter inside the Earth's orbit, to 0.16AU, it would remain Jupiter-like, with a stable atmosphere. But if you brought it just a little bit closer to the Sun, to 0.14AU, its atmosphere would suddenly start to expand, become unstable and escape. This dramatic change takes place because the cooling mechanism that we identified breaks down, leading to the atmosphere around the planet heating up uncontrollably."
Professor Alan Aylward, co-author of the paper, explains some of the factors which the team incorporated in order to make the breakthrough: "For the first time we've used 3D-modelling to help us understand the whole heating process which takes place as you move a gas giant closer to its sun. The model incorporates the cooling effect of winds blowing around the planet - not just those blowing off the surface and escaping.
"Crucially, the model also makes proper allowances for the effects of
H3+ in the atmosphere of a planet. This is an electrically-charged form of hydrogen which strongly radiates sunlight back into space and which is created in increasing quantities as you heat a planet by bringing it closer to its star.
"We found that 0.15AU is the significant point of no return. If you take a planet even slightly beyond this, molecular hydrogen becomes unstable and no more
H3+ is produced. The self-regulating, 'thermostatic' effect then disintegrates and the atmosphere begins to heat up uncontrollably."
Professor Steve Miller, the final contributing author to the paper, puts the discovery into context: "This gives us an insight to the evolution of giant planets, which typically form as an ice core out in the cold depths of space before migrating in towards their host star over a period of several million years. Now we know that at some point they all probably cross this point of no return and undergo a catastrophic breakdown.
"Just twelve years ago astronomers were searching for evidence of the first extrasolar planet. It's amazing to think that since then we've not only found more than 250 of them, but we're also in a much better position to understand where they came from and what happens to them during their lifetime.">>
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