[extropy-chat] ASTRO: Length of a day on Saturn

[Amara Graps]

(Spike: it is the radio emissions that appear to have changed in period.)

http://saturn.jpl.nasa.gov/news/press-releases-04/20040628-pr-a.cfm

"The rotational modulation of radio emissions from distant
astronomical objects has long been used to provide very accurate
measurements of their rotation period," said Dr. Don Gurnett,
principal investigator for the Cassini Radio and Plasma Wave Science
instrument, University of Iowa, Iowa City. "The technique is
particularly useful for the giant gas planets, such as Jupiter and
Saturn, which have no surfaces and are covered by clouds that make
direct visual measurements impossible."

[...]

Gurnett said, "Although Saturn's radio rotation period has clearly
shifted substantially since the Voyager measurements, I don't think
any of us could conceive of any process that would cause the rotation
of the entire planet to actually slow down. So it appears that there
is some kind of slippage between the deep interior of the planet and
the magnetic field, which controls the charged particles responsible
for the radio emission." He suggests the solution may be tied to the
fact that Saturn's rotational axis is nearly identical to its magnetic
axis. Jupiter, with a more substantial difference between its magnetic
axis and its rotational axis, shows no comparable irregularities in
its radio rotation period.

"This finding is very significant. It demonstrates that the idea of a
rigidly rotating magnetic field is wrong," said Dr. Alex Dessler, a
senior research scientist at the University of Arizona, Tucson. In
that way, the magnetic fields of gas giant planets may resemble that
of the Sun. The Sun's magnetic field does not rotate uniformly.
Instead, its rotation period varies with latitude. "Saturn's magnetic
field has more in common with the Sun than the Earth. The measurement
can be interpreted as showing that the part of Saturn's magnetic field
that controls the radio emissions has moved to a higher latitude
during the last two decades," said Dressler.

-- 

Amara Graps, PhD
Istituto di Fisica dello Spazio Interplanetario (IFSI)
Istituto Nazionale di Astrofisica (INAF), Roma, ITALIA
Amara.Graps at ifsi.rm.cnr.it