How Long Is A Day on Saturn?

http://www.pparc.ac.uk/Nw/Saturn42.asp

How long is a day on Saturn?
Particle Physics & Astronomy Research Council
May 3, 2006

Measuring the rotation period of a rocky planet like the Earth is easy,
but similar measurements for planets made of gas, such as Saturn, pose
problems. Researchers from JPL, Imperial College London and UCLA
present
new results in this week's Nature (4th May 2006) that may solve the
mystery. Using the magnetometer instrument on Cassini, they have found
a
clear period in the magnetic field of the planet that they believe
indicates a day of 10 hours and 47 minutes.

This is a whole 8 minutes slower than NASA Voyager results from the
early 1980s, and slower than previous estimates from another Cassini
instrument. The magnetometer results provide the best estimate of the
Saturn day to date, because it can see deep inside Saturn.

Planets rotate around their "spin" axes as they orbit about the Sun.
Rocky planets like the Earth and Mars have rotation periods that remain
quite constant and are easy to measure because we can see the surfaces
rotate.

Gaseous planets do not have a solid surface to track and are not as
rigid as rocky planets. Thus, their periods may change more than those
of rocky planets while being less easy to measure. Scientists have
sought to use proxy measures such as the repetition rate of radio
signals or the period of the rotation of the direction of the magnetic
axis of the planet. However for Saturn this has proved difficult
because
previous missions could not detect a period in the magnetic field
measurements and whilst radio data have shown a period - it has changed
in the time between previous missions and Cassini.

Since the Voyager days scientists have been seeing changes in the
period
of radio observations. They knew that it was virtually impossible to
slow down or speed up a mass as large as Saturn. As Cassini's
measurements of the rhythms of natural radio signals from the planet
continued to vary, scientists began to realize these signals were
probably not a direct measurement of the internal rotation rate.
Suddenly the length of Saturn's day became uncertain. Measurements of
the magnetic field help scientists "see" deep inside Saturn and may
have
finally solved this puzzle.

Professor Michele Dougherty of Imperial College London, says "Making
this measurement has been one of team's most important science goals.
Finding a period in the magnetic field rotation helps us to understand
the internal structure of Saturn's magnetic fields and from that, of
Saturn itself, which will help us understand how the planet formed.
After almost two years of collecting data, we are starting to get
fascinating insights in Saturn, but we still have more questions than
we
do answers."

According to Dr Giacomo Giampieri, a researcher at the Jet Propulsion
Laboratory (NASA) and lead author of the paper, Saturn's rotation posed
a great challenge to scientists in the past. In fact, Saturn's internal
magnetic field is almost perfectly aligned with the rotation axis. To
explain the consequences of this alignment, Giampieri says to consider
a
Compact Disc in a CD player.

"Imagine you want to check whether the CD is playing" Giampieri says
"If
your CD has a label it is easy to see at a glance that it is spinning
very fast in the CD player. But if the CD has no label, you would not
be
able to tell whether the CD is moving or not because it would look
static". Giampieri explains that "Saturn's magnetic field is similar to
a blank CD: if you just look at it, it seems that it is not rotating at
all."

In the past, Pioneer 11 and two Voyager spacecraft encountered Saturn
during brief fly-bys and collected data, but no clear periodic signals
were found in their magnetic field data. In July 2004 the Cassini
spacecraft was inserted into orbit about Saturn and it now has
completed
many orbits around the gaseous planet. Thanks to the extent of data
collected over this extended period of time and the use of appropriate
algorithms, a small but regular periodic signature in the magnetic
field
close to the planet has been detected, with a period of 10 hours 47
minutes and 6 seconds (plus or minus 40seconds). This discovery is like
finding a small spot on a CD that allows you to measure how fast it is
spinning.

The result is somewhat surprising. Giampieri explains "the period we
found from the magnetic field measurements has remained constant since
Cassini entered orbit almost 2 years ago, while radio measurements
since
the Voyager era have shown large variability. By monitoring the
magnetic
field over the rest of the mission, we will be able to solve this
puzzle".

The periodic signal of Saturn's magnetic field does not fit simple
models for planetary magnetic fields. Giampieri explains "Saturn's
periodic magnetic field differs from that found at Jupiter, which can
be
modelled as a dipole field tilted with respect to the rotation axis."
This study opens a new perspective on the internal structure and
dynamics of Saturn, and how it affects the source of the magnetic
field.
"We now know that the internal rotation of Saturn and its connection to
the external magnetic field is very complex. Our study is the first
step
in breaking the code" Giampieri says.

Notes for Editors

The article is titled: "A regular period for Saturn's magnetic field
that may indicate its internal rotation." In addition to Giampieri the
other authors are Michele Dougherty from Imperial College, London,
Edward Smith from JPL, and Christopher Russell from UCLA.

Images

Images of Saturn taken by Cassini can be downloaded from JPL's
Cassini-Huygens website
http://saturn.jpl.nasa.gov/multimedia/images/images.cfm?categoryID=1.

Contacts

* Prof. Michele K. Dougherty (in the
USA at the time of this release)
Imperial College London
Mobile in the USA: 0016262417606
* Dr Giacomo Giampieri
Jet Propulsion Laboratory
California Institute of Technology
Mobile: +1 626 807 7025
* Julia Maddock
PPARC Press Office
Tel +44 1793 442094

The Cassini-Huygens is a cooperative project of NASA, The European
Space
Agency, and the Italian Space Agency. The Jet Propulsion Laboratory, a
division of the California Institute of Technology in Pasadena manages
the Cassini-Huygens for Nasa.