30 Jun 2011

 

 

 

 

 

How long is the day on Neptune?

 

The answer is 15 hours, 57 minutes and 59 seconds with an uncertainty of a few seconds. Neither the question nor the answer are simple however, as measuring the exact rotation period of a gaseous planet is complicated by the fact that the solid part lies far below the gaseous surface we can see. The length of the day is determined by the rotation of the solid component.
The first gas giant to have its spin period measured with some accuracy was Jupiter, in the '50s, when the first radio telescopes were pointed at the planet. These detected periodic pulses of radio emission, attributed to the magnetic field generated by the rotation of the inner core. For Saturn, Uranus and Neptune however, the same method does not work because at the greater distances of these planets the solar wind prevents radio signals from reaching Earth.
The determination of the spins of these planets had to wait for the Voyager missions, that measured the timing of the radio pulses. So, at least for Saturn it was thought we had an accurate value for the spin period, but the recent Cassini probe actually found a significantly shorter period and even obtained different values for the two hemispheres!
The radio pulses then don't seem to be the most reliable way of determining the rotational period, at least not for all the gas giants. So how can it be measured? The solution to the problem, at least for Neptune, has been found by Erich Karkoschka of the Lunar and Planetary Laboratory at the University of Arizona, and is essentially the same method adopted by Cassini 350 years ago for the Great Red Spot of Jupiter, though based on more solid reasoning.
Basically, Karkoschka looked for, and found, atmospheric features on Neptune that are generated and follow the rotation of the solid core. He searched the Hubble Space Telescope archive for images of Neptune (one enhanced example is shown above) covering a period of over 20 years. Amongst the many atmospheric features he identified, the South Polar Feature and South Polar Wave (similar to Jupiter's Great Red Spot in many ways) turned out to be stable over the entire time period.
From the constancy of their position and regularity of their rotation Karkoschka deduced that there must be a link between these features and the solid core, although there is no clue as to what this connection might physically be. Following these features he deduced a period of 15,9663 hours, compared with the over 16 and up to 18 hours based on other methods.
The immediate consequence of the new value is that Neptune's mass should be more centrally concentrated than previously thought, and this will have repercussions for models that describe the internal structure of this type of planet.

 

by Michele Ferrara & Marcel Clemens

credit: NASA, University of Arizona, Erich Karkoschka