Ionian Web

Io's Nighttime Heat as seen by Galileo's Photopolarimeter-Radiometer

Powerful volcanoes and the previous day's sunshine warm the nighttime surface of Jupiter's moon Io, as seen in this image from NASA's Galileo spacecraft. The left-hand frame shows the best view that Galileo has yet provided of Io's nighttime temperatures. For reference, the right hand frame, based on Galileo camera images, shows the same hemisphere of Io in visible light. The thin bright crescent indicates the only observable portion illuminated by sunlight during the temperature measurements. Several volcanoes are identified on both images: L-K is Lei-Kung Fluctus, L is Loki, Pi is Pillan, M is Marduk, and Pe is Pele.  The temperature map comes from observations by Galileo's photopolarimeter- radiometer instrument during the spacecraft's 27th orbit of Jupiter, in February 2000. Blue indicates the coldest temperatures, near 90 degrees Kelvin (minus 297 degrees Fahrenheit), while oranges and yellows indicate the highest temperatures, in excess of 170 K (minus 153 F). Small areas of the volcanoes are far hotter than this, exceeding 1,500 K (2,240 F). However, in this relatively low-resolution view, which shows no features smaller than about 340 kilometers (210 miles) across, radiation from these small, hot regions is mixed with radiation from surrounding colder regions, so the high temperatures are not detected directly. The observation was made without a filter, so it measures the total heat radiation from Io's night side at all wavelengths.  Much of the heat comes from a few discrete volcanoes. The brightest is Loki, which radiates roughly 15 percent of Io's total volcanic heat, and would appear much brighter still were it not for severe foreshortening due to its position near the edge of the disk. Second brightest is Pillan, where the heat is radiated by extensive cooling lava flows produced largely by an eruption witnessed by Galileo in June 1997 and later. In contrast, although the volcano Pele spouts Io's largest plume, which produces the large orange ring seen in the right-hand image, Pele emits very little total heat. This is because Pele's activity, though vigorous, is confined to a small volcanic crater, where there is no room for large, warm lava flows to radiate heat.  The cooler regions, shown in blues and purples, are dominated by radiation from the surface between the volcanoes, which was warmed by sunlight the previous day and retains some of that heat through the night.  Because high- latitude regions receive less sunlight during the day, we would expect them to be cooler at night, but the image shows that temperature varies little with latitude, which is puzzling. Perhaps excess volcanic heat is radiated at the poles, or the polar regions are composed of surface materials that cool off less at night than the materials at lower latitudes. Also, viewing the poles from this angle preferentially presents slopes that were tilted towards the Sun during the day, and thus received more sunlight than the average surface, and this effect might also contribute to the apparent warmth near the poles. The photopolarimeter-radiometer observations of the polar regions on subsequent Io flybys may clear up this mystery. 

To read the rest of the caption or to obtain a copy of this release, click here.

A bright polar feature is visible on several ISAAC images of Io, obtained through a narrow-band filter at 4.07 µm, 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.

Read the rest of the release and view the new images here.

The Cassini spacecraft, passing through the Jupiter system on Jan. 1, 2001, en route to Saturn, recorded a sequence of images showing Jupiter's moon Io glowing in the darkness of the giant planet's shadow. University of Arizona planetary scientist Paul Geissler presented a color version of this "movie" today at the American Geophysical Union meeting in Boston. The movie shows details of moon's visible aurorae that solve some of the puzzles presented by the Galileo spacecraft observations, Geissler said. Little was known about these dazzling light shows before the Galileo spacecraft arrived at the Jupiter system in late 1995. Galileo pictures of Io in eclipse showed a colorful display of red, greenish and blue emissions bright enough to be seen with the naked eye. These glows are due to various gases in Io's tenuous atmosphere that are excited by electrical currents, much like the Aurora Borealis on Earth.

The Movie and associated caption can be found here
A Crescent view of Jupiter and Io can be found here.
The rest of the news release can be found here.

I have posted the new issue of the Tvashtar Sun. In this issue, you can read the first part of a two part series on Tvashtar and a preview of the ongoing C30 encounter. This issue also includes news from the last two months. Finally, the usual abstracts, meeting announcements, and meeting abstracts are included.

Check it out here.

NASA and JPL recently announced that the Galileo mission has been extended one final time. In this mission, the Galileo spacecraft will flyby the Jovian moons Callisto, Io (3 times), and Amalthea before crashing into Jupiter in September 2003. In 2001, remote sensing is planned. This includes the flyby of Callisto and two of the flybys of Io. However, NASA has refused to allow remote sensing in what many believe to be the most important flyby of Io for remote sensing, I33 (flyby of Io in the 33rd Orbit). In this flyby, Galileo will fly over the projovian hemisphere of Io, an area imaged best by Voyager. Voyager found many interesting features on this hemisphere. Some of these features, however, are still mysterious. Galileo has not imaged this hemisphere very well during its mission and this flyby would allow us the best imaging of this hemisphere. The amount of money needed to fund remote sensing in this orbit would only be .01% of the total NASA budget, or $1,500,000. We believe that remote sensing in I33 is important for understanding features seen by Voyager, at low resolution by Galileo, and of features on the other hemisphere that was well seen by Galileo. We believe that NASA should reverse its decision on remote sensing in I33. 

This is the petition text for Pennies for Pele, a petition drive to try to get NASA to approve remote sensing in I33. The petition will be sent to my representative and senators, as well as head people at NASA. There is a page now up for the petition here. This page includes information on this flyby like objectives for all three of the remote sensing instruments, information on Io, and information on features to be imaged during this flyby. You can also view my press releasehere.