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Thread: Measuring the days of gas giants

  1. #1
    Join Date
    Jun 2009

    Measuring the days of gas giants

    The question here has come up on a BA blog post on this thread :

    #14. Just me Says:
    May 7th, 2010 at 5:00 am
    Kind of related question, and something I’ve been wondering for a long time: how is the rotational period for the gas giants determined? I mean, they all have high winds, storms and such, but I assume that the rotational period is not determined by the cloud movements, right? Any thoughts?
    How do you tell the length of day (rotation period) of a gas giant planet such as Saturn?

    Also how do you measure "surface gravity" on a planet without any solid surface?
    Last edited by Messier Tidy Upper; 2010-May-07 at 12:50 PM. Reason: To include the original post asking the question. Add link to thread, fix typo.

  2. #2
    Join Date
    Jun 2009
    BTW. My reply to that there # 15 is :

    15. Messier Tidy Upper Says:
    May 7th, 2010 at 6:31 am
    @ ^ Just me :

    but I assume that the rotational period is not determined by the cloud movements, right? Any thoughts?

    I’m not sure & I could well be wrong – please correct me if I am – but I think cloud patterns *are* actually used along with perhaps magnetic fields to judge the gas giants rotation periods.

    Its a good question and pretty hard to work out given these planets are gas and fluid rather than solid bodies.

    I vaguely recall reading somewhere that Saturn’s rotation period has actually changed between the time Voyager II last flew past the planet in 1980-83 ish (?) to when Cassini arrived in 2004 ~ish (?) – although maybe it was just a case of a more precise measurement being made. I’ll have to research this further myself now.

    You might try asking this question on the BAUT forum too :

    if you haven’t done so already.

    Otherwise, if anybody can explain or elaborate further on this it’d be great.
    & I've directed the original questioner here too via a link in my next comment there.

    Also yes - a quick Wikipedia check has confirmed that Cassini began orbiting Saturn in 2004 & that Voyager II fly past Saturn in 1981 & the Voyager I fly by occurred in 1980.
    Last edited by Messier Tidy Upper; 2010-May-07 at 01:00 PM. Reason: Added exploration dates checked.

  3. #3
    Join Date
    Jul 2005
    Cloud features on gas giants are long-enough lived, and zonal winds consistent enough in direction and magnitude, that one can measure a rotation period for a particular cloud zone. One can also measure the rotation period of the magnetic field, which is presumed to be anchored to the core of the gas giant. So for gas giants you may see several rotation periods quoted, for different zones and for the core, usually with names like "System I", "System II", etc.

    "Surface" gravity is conventionally calculated for some standard height in the atmosphere, often defined by the barometric pressure.

    Grant Hutchison

  4. #4
    Join Date
    Jun 2009
    Thanks for that -much appreciated. :-)

    Can anyone elaborate any further please?

  5. #5
    Join Date
    Jan 2005


    To elaborate:

    Surface Gravity: As mentioned, one usually takes the 1-bar level to be the "surface", though of course there are other ways, such as just taking the mean radius (tricky for Jupiter and Saturn, with their significant oblateness) and plugging in the numbers same as for any planet.

    Longitude: As GH wrote, gas giants (Jupiter and Saturn anyways) these days typically have at least three different rotation definitions. For Jupiter, there's System I (for latitude 10N-10S, covering the equatorial zone), System II (For all latitudes north and south of these), and System III (the "internal" rotation worked out by timing the activity of the planet's magnetic field). For a long time, the equatorial System I was considered the "default" rotation rate. Movement of all atmospheric features was measured relative to each other; later, arbitrary--but ultimately authoritative--central meridians were determined for each System, so longitude could be worked out for various features.

    The result is that any one feature on Jupiter or Saturn has three different longitudes, though typically only I or II are important for terrestrial observations. Literally, you can observe a feature crossing Jupiter's central meridian (simply the imaginary line bisecting the center of the disk from north to south), and calculate its "true" longitude by referring to a set of tables, and thus determine the drift rates for any conceivable atmospheric phenomenon. It sounds more unreliable than it is; such measurements allowed us to discern powerful jet streams on Jupiter long before the Voyagers gave us the ground truth, such as it is.

    For more info, I recommend the following:

    The Giant Planet Jupiter, by John Rogers. This is an expensive tome, and my copy is a little dated (1995, though a more recent edition is out), but IMO it's the definitive work on the planet, summarizing atmospheric observations from the 17th century through the Space Age.

    Galileo's Planet, by Thomas Hockey, which covers telescopic observation from Galileo's time through 1900.

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