A pulsating hot spot of X-rays has been discovered in the polar
regions of Jupiter’s upper atmosphere by NASA’s Chandra X-ray Observatory.
Previous theories cannot explain either the pulsations or the location of
the hot spot, prompting scientists to search for a new process to produce
Jupiter’s X-rays.

“The location of the X-ray hot spot effectively retires the existing
explanation for Jupiter’s X-ray emission, leaving us very unsure of its
origin,” said Randy Gladstone of the Southwest Research Institute in San
Antonio and lead author of a paper on the results in the Feb. 28, 2002,
issue of the journal Nature. “The source of ions that produce the X-rays
must be a lot farther away from Jupiter than previously believed.”

Chandra observed Jupiter for 10 hours on Dec. 18, 2000, when NASA’s
Cassini spacecraft was flying by Jupiter on its way to Saturn. The X-ray
observations revealed that most of the auroral X-rays come from a pulsating
hot spot that appears at a fixed location near the north magnetic pole of

Bright infrared and ultraviolet emissions have also been detected
from this region in the past. The X-rays were observed to pulsate with a
perioý@¤f 45 minutes, similar to the period of high-latitude radio
pulsations detected by NASA’s Galileo and Cassini spacecraft.

An aurora of X-ray light near Jupiter’s polar regions had been
detected by previous satellites. However, scientists were unable to
determine the exact location of the X-rays. The accepted theory holds that
the X-rays are produced by energetic oxygen and sulfur ions that become
excited as they run into hydrogen and helium in Jupiter’s atmosphere. Oxygen
and sulfur ions (originally from Jupiter’s moon Io) are energized while
circulating around Jupiter’s enormous magnetosphere. And some – the
purported X-ray producers – get dumped into Jupiter’s atmosphere when they
return to the region of Io’s orbit.

Chandra’s ability to accurately determine the location of the X-rays
proved this model incorrect, as ions from regions of Jupiter’s magnetic
field near Io cannot reach the high Jovian latitudes where most of the
X-rays were observed.

This result has its own problems. At the large distances required
for the source of the ions — at least 30 times the radius of Jupiter —
spacecraft measurements have shown that there are not nearly enough
energetic oxygen and sulfur ions to account for the observed X-ray emission.

One possibility is that heavy ions among the particles flowing out
from the Sun as the solar wind are captured in the outer regions of
Jupiter’s magnetic field, then accelerated and directed toward its magnetic
pole. Once captured, the ions would bounce back and forth in the magnetic
field from pole to pole in an oscillating motion that might explain the

The High Resolution Camera used for the Chandra observations was
built by the Smithsonian Astrophysical Observatory in Cambridge, Mass.
NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra
program, and TRW, Inc., Redondo Beach, Calif., is the prime contractor. The
Smithsonian’s Chandra X-ray Center controls science and flight operations
from Cambridge, Mass.

Images and additional information about this result are available