Apparent six minute slowdown in Saturn’s day length challenges current magnetosphere theory

Posted: July 29, 2014 by tallbloke in Astrophysics, solar system dynamics, waves


From NASA:

Cassini took readings of the day-length indicator regarded as most reliable, the rhythm of natural radio signals from the planet. The results give 10 hours, 45 minutes, 45 seconds (plus or minus 36 seconds) as the length of time it takes Saturn to complete each rotation. Here’s the puzzle: That is about 6 minutes, or one percent, longer than the radio rotational period measured by the Voyager 1 and Voyager 2 spacecraft, which flew by Saturn in 1980 and 1981.

[Tallbloke notes that Venus has also slowed by an unexplained six minutes since 1997]

Cassini scientists are not questioning Voyager’s careful measurements. And they definitely do not think the whole planet of Saturn is actually rotating that much slower than it did two decades ago. Instead, they are looking for an explanation based on some variability in how the rotation deep inside Saturn drives the radio pulse.

The radio sounds of Saturn’s rotation, which are also the first sounds from Saturn studied by Cassini, are like a heartbeat and can be heard by visiting and

“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.”

The first hint of something strange about that type of measurement at Saturn was in 1997, when a researcher from Observatoire de Paris reported that Saturn’s radio rotation period differed substantially from Voyager.

Dr. Michael D. Desch, Cassini Radio Plasma Wave Science team member, and scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., has analyzed Saturn radio data collected by Cassini from April 29, 2003, to June 10, 2004. “We all agree that the radio rotation period of Saturn is longer today than it was in during the Voyager flyby in 1980,” he said.

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.

“I think we will be able to unravel the puzzle, but it’s going to take some time,” said Gurnett. “With Cassini in orbit around Saturn for four years or more, we will be in an excellent position to monitor long-term variations in the radio period, as well as investigate the rotational period using other techniques.”

Cassini, carrying 12 scientific instruments, is just two days from its planetary rendezvous with Saturn. On June 30 it will become the first spacecraft to orbit Saturn, when it begins a four-year study of the planet, its rings and its 31 known moons. The spacecraft recently flew past Saturn’s cratered moon Phoebe, where it captured spectacular images as well as data on its mass and composition.

The Cassini-Huygens mission 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 mission for NASA’s Office of Space Science, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. For the latest images and more information about the Cassini-Huygens mission, visit

Carolina Martinez (818) 354-9382
NASA’s Jet Propulsion Laboratory

Gary Galluzzo (319) 384-0009
University of Iowa, Iowa City

  1. tallbloke says:
  2. oldbrew says:

    ‘He suggests the solution may be tied to the fact that Saturn’s rotational axis is nearly identical to its magnetic axis’

    Possibly, but no logic is offered. Something to do with the solar wind perhaps?

  3. acckkii says:

    Reblogged this on acckkii.

  4. tchannon says:

    The article NASA link points at a 2004 article.

    Then 10 years ago…

    2006, Cassini, “Saturn’s rotation puts astronomers in a spin”

    There again actually believing what is a flea in a haystack
    “A day on Saturn just got a few minutes shorter, if new calculations are correct.

    Using data collected by NASA’s Cassini, Pioneer and Voyager spacecraft, scientists have revised the ringed planet’s rotation period to 10 hours, 32 minutes and 35 seconds—about 15 minutes shorter than an estimate made only last year.”

    1990 paper abs.

    There again, the hexagon in there too
    Epoch Period Reference
    1980-81 10 hr 39 min 24.3 s 1, 3
    1980-81 10 hr 39 min 25.2 s 4, 6
    1990-91 10 hr 39 min 25.2 s 5
    1990-95 10 hr 39 min 23.8 s 4, 6
    1980-95 10 hr 39 min 49.4 s Combined (*)
    2013 10 hr 39 min 20.6 s This work
    (*) Using the NPS for the long-term identification.

    Click to access EPSC2013-170-1.pdf

    And so on.

  5. oldbrew says:

    This NASA report says:

    ‘The different rotation rate observed by Cassini was determined to be due to the latitude at which those waves emanated.’

    Are they implying differential rotation or just a measuring problem?

  6. TLMango says:

    When Jupiter is catching up with Saturn like it is now, the two planets get caught up in
    each others gravity. This led a lot of early astronomers to believe that Saturn’s orbital
    period was slowing down and Jupiter’s period was speeding up (‘the great inequality’).
    When Jupiter passes Saturn the situation should reverse itself. But… many believe
    that Saturn is actually slowing down for reasons of its own. It all gets very confusing
    sometimes. Its my great talent to spread hopelessness wherever I go. Maybe ‘the J/S
    great inequality’ is somehow related to Saturn’s LOD.