The Neptune Lab

Procedure

1. First, let's look at two images of Neptune taken 16 hours and 3 minutes apart (that is the rotation period that Voyager measured for the magnetic field). Remember that these images were taken from a spacecraft moving through the Neptunian system, so the resolution changes between the pictures. North is at the top of the page. Recall that Neptune has prograde rotation.
   • Plate 1A
   • Plate 1B
   Here we can use the Great Dark Spot as a landmark to determine how fast the atmosphere is moving and compare that to the magnetic field, which rotates with the speed of the interior that generates it.

2. Now, we want to examine the behavior of the Great Dark Spot itself. To do this, you need a little background about atmospheric physics first.
   There are two factors that govern the direction of spin of a storm.
   • CORIOLIS FORCE: An object (like an atmosphere) moving on a rotating body (like a planet) will tend to veer towards the right in the Northern Hemisphere, and towards the left in the Southern Hemisphere.
   • STORM PRESSURE: Gas in an atmosphere will tend to flow away from higher pressure and towards lower pressure. Low pressure centers are called cyclones, and spin counter-clockwise in a northern hemisphere (like hurricanes Luis and Iris). But since the Coriolis force is opposite in the southern hemisphere, a cyclone there would spin clockwise. A high pressure storm (an anti-cyclone) would have the opposite direction of flow: clockwise in the north, counterclockwise in the south.

   Now take a look at Plate 2 and see if you can figure out whether it is a high or low pressure system.

3. We can also learn more about the atmosphere by looking more closely. In Plate 3 we can see the shadows of high clouds and use geometry to determine how high they are above the main clouds. In this case, 60 kilometers above the main cloud layer. How does this compare to Earth's atmosphere?

4. What about the moons of Neptune? Most of them are small and have never been photographed closely, but Neptune has one large moon: Triton. These are two images (4A and 4B) of Triton. Triton is roughly the same size as our (extensively studied) Moon (Plate 4C), which makes a comparison of the two somewhat inevitable.

   Note the unusual terrain (Plate 5) found only on Triton. This picture reminded the scientists who first saw it of a cantaloupe, and so it has acquired the name "cantaloupe terrain". The best explanation for its formation seems to have been formed as Graben (much like the fractures that we saw on Mercury due to contraction of the crust) and have been altered by the icy volcanism present on Triton.