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A Scientist's Musings On Chandra And Jupiter

April 9, 2002 ::

This Chandra image of Jupiter shows concentrations of auroral X-rays near the north and south magnetic poles.(Credit: NASA/CXC/SWRI/G.R.Gladstone et al.)

On December 18, 2000, the Cassini spacecraft zoomed past Jupiter en route to its final destination of Saturn. Astronomers, however, did not let this opportunity go unnoticed. Instead, they coordinated some of the most powerful telescopes -- NASA's Chandra X-ray Observatory and Hubble Space Telescope -- to observe Jupiter as it never had been before.

With the fleet of ground-based and space-borne observatories fixed on Jupiter simultaneously, scientists came up with some remarkable results. Chandra, for example, discovered a pulsating "hot spot" of X-ray near Jupiter's polar regions - a discovery that current theories cannot explain.

This composite image displays X-ray data from Chandra (magenta) and ultraviolet data Hubble Space Telescope (blue) overlaid on an optical image of Jupiter. While Chandra observed Jupiter for an entire 10-hour rotation period on December 18, 2000, this image shows a 'snapshot' of a single 45-minute X-ray pulse.
(Credit: X-ray: NASA/SWRI/R.Gladstone et al., UV: NASA/HST/J.Clarke et al., Optical: NASA/HST/R.Beebe et al.)

Randy Gladstone is an institute scientist at the Southwest Research Institute in San Antonio, Texas, who led the team that observed Jupiter with Chandra. (More details of his team's results are available at http://chandra.harvard.edu/press/02_releases/press_022702.html.) He recently answered questions posed by a student about his research as well as the topic of Jupiter as a whole.

What specifically have you studied about Jupiter?
Mostly the chemistry of the upper atmosphere and the aurora and dayglow.

This illustration portrays the Cassini spacecraft with Jupiter in the far distance. (Credit: David Seal)

How have you studied it?
By simulating the chemistry with computer models, and by looking at the aurora and dayglow with space observatories (e.g., HST, Chandra, FUSE, EUVE, ROSAT, and IUE).

What do you consider to be the major breakthroughs in the last 15 years and why?
For the study of Jupiter, the major breakthroughs of the last 15 years have been mainly observational - e.g., the great spatial resolution of HST (and now, ground-based telescopes), the Shoemaker-Levy 9 comet impacts of July 1994 (from which a lot has been learned about the chemistry and dynamics of Jupiter's atmosphere and a lot about impact processes), the Galileo mission, and the Cassini flyby of a year ago. There is such an abundance of great data that it will take many decades to digest it all. Some of the major discoveries are 1) a probable subsurface oceans on Europa (and perhaps Callisto), 2) a magnetic field on Ganymede, 3) ammonia clouds on Jupiter, and 4) lava fountains on Io.

What interested you about Jupiter and why?
The auroras are hundreds to thousands of times more powerful than the ones here on Earth. And, much less is known about them, so they're more interesting.

This image of Jupiter's moon Callisto was taken in 1998 by NASA's Galileo spacecraft. Callisto is approximately the size of the planet Mercury, making it the third largest moon in the Solar System.
(Credit: NASA)

Do you believe there are sub-surface oceans on Callisto and Europa? Why?
Certainly seems likely on Europa, given the "raft" features in the close ups of the surface. I think the argument for Callisto is less compelling.

How many satellites do you believe Jupiter has (16, 28, somewhere in between)?
That depends how small you are willing to allow a satellite to be. If you say they must be bigger than a couple of tens of kilometers in diameter, then there are 16.

Do you believe that Jupiter could have more satellites that we have not discovered?
Sure, but they are probably small and/or far away from Jupiter.

Io ("EYE oh") is the fifth of Jupiter's known satellites and is the innermost of the Galilean moons. Io is slightly larger than Earth's Moon.
(Credit: NASA)

Why do you think that Jupiter's moon Io is so active?
The resonance of Io's period with Europa and Ganymede (they are in the ratio 1:2:4) forces Io's orbit to be eccentric, even though Jupiter is forcing the orbit to be perfectly circular. This tug of war flexes the inside of Io and heats it up, and so this results in a very volcanically active moon.

Do you feel the study of Jupiter is important? Why?
Yes, it's the major planet of our solar system, the prototype for the gas giants, and Jupiter-sized planets seem to be abundant in other solar systems. So if we want to understand planets, then understanding Jupiter is important.

What do you think we should be spending our money on in the future in terms of Jupiter? Why?
A polar orbiter would be great, as it would provide a great view of the auroras and could very accurately measure Jupiter's gravity and magnetic fields (so that we could figure out how the insides of Jupiter are moving around).

What do you believe the focus of study should be on and why?
I have no preference, there are lots of interesting things to study.

The recent observations of Jupiter by the Chandra X-ray Observatory were yet another step in the quest to better understand our Solar System's largest planet. Undoubtedly, there will be many more attempts - some perhaps using the power of X-rays - to probe the gas giant. It is an endeavor that many scientists and members of the public believe is worth the effort.

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