4 ms·

Cassini skims Saturn's atmosphere

saagarjha 9y ago
> Researchers know it is roughly 10-and-a-half hours, but they would like a more precise number. I find it interesting that something this “basic” is still not known. Couldn’t they just take photos of how long it takes certain features, say storms or clouds, to reappear?
- tambourine_man 9y ago
  How do you distinguish wind from planet rotation on a gas giant? Not easy I'd wager
  - saagarjha 9y ago
    I’m not sure it really affects the result. Wind speeds on Saturn top out at around 1,000 miles per hour. Using some back of the napkin math, the rotation “speed” at the equator should be well over 20,000 miles per hour. Is that not enough to get an accurate assessment?
    - svachalek 9y ago
      Perhaps accurate but certainly not precise.
    - InclinedPlane 9y ago
      You think 5% error is "accurate"?
      - nobodyorother 9y ago
        For psychology studies, sure. Not for landing airplanes, unless they're paper ones. Depends on your context, I guess.
  - BurningFrog 9y ago
    I guess the only rotational property you can be sure it has is angular momentum. Various parts of the gas can rotate in different speeds and ways that even change over time. If so, it would be great to learn about that.
- mturmon 9y ago
  The brief quote in the article has left the wrong impression, I think. I don't think the goal is to get a precise value of an arbitrary metric as an end in itself. I think the science here is that there is (1) an optically measurable (latitude-dependent) rotation rate of top-of-atmopshere gas; (2) a magnetically measurable rotation rate (and axis) of subsurface conductive hydrogen; and (3) gravimetrically measurable rotation and density. The close approaches of Cassini to Saturn will allow much better gravimetric measurements. It's surprising what can be pulled out of gravimetric techniques, see, e.g., https://www.jpl.nasa.gov/missions/gravity-recovery-and-climate-experiment-grace/ https://www.jpl.nasa.gov/missions/gravity-recovery-and-clima.... They would like to compare these measurements to get insight about the interior composition of the planet. For a bit more, see this old overview: https://futurism.com/new-spin-saturns-rotational-period/ https://futurism.com/new-spin-saturns-rotational-period/ and http://www.nature.com/nature/journal/v460/n7255/full/460582a.html http://www.nature.com/nature/journal/v460/n7255/full/460582a... * It may be surprising, but groups have been working on general circulation models (GCM) for outer planet atmospheres for over a decade, and the state of the art is pretty advanced. (A good recent presentation with lots of nice plots: https://theoryofclimate.sciencesconf.org/conference/theoryofclimate/Slides_Spiga.pdf https://theoryofclimate.sciencesconf.org/conference/theoryof...) The top-of-atmosphere rotational velocities would be a major constraint on models like these.
Sir_Cmpwn 9y ago
NASA JPL's own website for this is a great place to keep up with developments: https://saturn.jpl.nasa.gov/ https://saturn.jpl.nasa.gov/

> Researchers know it is roughly 10-and-a-half hours, but they would like a more precise number. I find it interesting that something this “basic” is still not known. Couldn’t they just take photos of how long it takes certain features, say storms or clouds, to reappear?

tambourine_man 9y ago
How do you distinguish wind from planet rotation on a gas giant? Not easy I'd wager
- saagarjha 9y ago
  I’m not sure it really affects the result. Wind speeds on Saturn top out at around 1,000 miles per hour. Using some back of the napkin math, the rotation “speed” at the equator should be well over 20,000 miles per hour. Is that not enough to get an accurate assessment?
  - svachalek 9y ago
    Perhaps accurate but certainly not precise.
  - InclinedPlane 9y ago
    You think 5% error is "accurate"?
    - nobodyorother 9y ago
      For psychology studies, sure. Not for landing airplanes, unless they're paper ones. Depends on your context, I guess.
- BurningFrog 9y ago
  I guess the only rotational property you can be sure it has is angular momentum. Various parts of the gas can rotate in different speeds and ways that even change over time. If so, it would be great to learn about that.
mturmon 9y ago
The brief quote in the article has left the wrong impression, I think. I don't think the goal is to get a precise value of an arbitrary metric as an end in itself. I think the science here is that there is (1) an optically measurable (latitude-dependent) rotation rate of top-of-atmopshere gas; (2) a magnetically measurable rotation rate (and axis) of subsurface conductive hydrogen; and (3) gravimetrically measurable rotation and density. The close approaches of Cassini to Saturn will allow much better gravimetric measurements. It's surprising what can be pulled out of gravimetric techniques, see, e.g., https://www.jpl.nasa.gov/missions/gravity-recovery-and-climate-experiment-grace/ https://www.jpl.nasa.gov/missions/gravity-recovery-and-clima.... They would like to compare these measurements to get insight about the interior composition of the planet. For a bit more, see this old overview: https://futurism.com/new-spin-saturns-rotational-period/ https://futurism.com/new-spin-saturns-rotational-period/ and http://www.nature.com/nature/journal/v460/n7255/full/460582a.html http://www.nature.com/nature/journal/v460/n7255/full/460582a... * It may be surprising, but groups have been working on general circulation models (GCM) for outer planet atmospheres for over a decade, and the state of the art is pretty advanced. (A good recent presentation with lots of nice plots: https://theoryofclimate.sciencesconf.org/conference/theoryofclimate/Slides_Spiga.pdf https://theoryofclimate.sciencesconf.org/conference/theoryof...) The top-of-atmosphere rotational velocities would be a major constraint on models like these.

Sir_Cmpwn 9y ago

NASA JPL's own website for this is a great place to keep up with developments: https://saturn.jpl.nasa.gov/ https://saturn.jpl.nasa.gov/