The gas planets (Jupiter, Saturn, Uranus, and Neptune) have no surface to speak of, so how was it determined how long a day is?
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The gas rotates. You measure that rotation.
On earth, it’s not as if only the solid, rocky surface rotates but not the atmosphere, or we’d have 1000mph winds! The atmosphere rotates in unison with the ground.
A planet entirely made of gas would rotate the same way.
You can measure the average rotation rate of all the particles. It’s like looking at how long it takes the great red spot to go around Jupiter. Each individual but of it moves at a different rate, but overall it balances out.
An early attempt was to do some storm watching. Jupiter is constantly buffeted by atmospheric storms, so the theory was that you could locate the center of a storm and get some idea of the length of a day. The problem scientists encountered was that the storms on Jupiter are very fast moving, making them an inaccurate source of rotational information. Scientist were finally able to use radio emissions from Jupiter’s magnetic field to calculate the planet’s rotational period and speed. While other parts of the planet rotate at different speeds, the speed as measured by the magnetosphere is used as the official rotational speed and period.
However, Saturn is different. Its unique magnetic field is nearly perfectly aligned with its rotational axis. This is why the rings finding has been key to homing in on the length of day. During Cassini’s orbits of Saturn, instruments examined the icy, rocky rings in unprecedented detail and used the data to study wave patterns within the rings. Earlier findings had determined that the rings respond to vibrations within the planet itself, acting similarly to the seismometers used to measure movement caused by earthquakes. The interior of Saturn vibrates at frequencies that cause variations in its gravitational field. The rings, in turn, detect those movements in the field.
https://solarsystem.nasa.gov/news/814/scientists-finally-know-what-time-it-is-on-saturn/
So we’re all agreed then?
They rotate exactly the same as any planet with a surface… so exact same way we determined how long a day is on earth.
For Jupiter, the clouds near the equator rotate almost 1% faster than clouds nearer the poles. Radio observations allow the rotation period of Jupiter’s magnetic field to be determined and this is only 0.03% faster than the polar rotation. It’s the radio period that’s given as the standard measurement of Jupiter’s day.
Gaseous planets do not experience uniform rotation rates at their cloud tops across all latitudes. You can take an average for the rotation of a section of gas as it rotates about the center of the planet (For example, identifying a storm system and watching it rotate around the planet), but usually the rotation rate of the planet’s internal magnetic field is used to define a sidereal day for them. If you look at the wikipedia page for planetary rotations, you can see that several different figures can be used for different latitudes.
https://en.m.wikipedia.org/wiki/Rotation_period
IIRC you map the magnetic field of the planet and use that. No idea what you would do if you found a planet where that field was also independently rotating.
They really don’t have surfaces? I thought that gas planets just meant they had a ton of gas on a planetary scale. I didn’t know that meant they LITERALLY have no surface.
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