Cassini skims Saturn's atmosphere | Hacker News Reader

Cassini skims Saturn's atmosphere

bbc.co.uk

NASA JPL's own website for this is a great place to keep up with developments:

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?

How do you distinguish wind from planet rotation on a gas giant? Not easy I'd wager

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?

Perhaps accurate but certainly not precise.

You think 5% error is "accurate"?

For psychology studies, sure. Not for landing airplanes, unless they're paper ones. Depends on your context, I guess.

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.

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-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/

and

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/theoryof...) The top-of-atmosphere rotational velocities would be a major constraint on models like these.

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