The top image shows Neptune in visible light, much as our eyes would see it. The bottom image was captured at infrared wavelengths. The bright spot (bottom) in the IR image shows that Neptune's South Pole is the warmest place on the planet!
Click on image for full size
Images courtesy of NASA/JPL (visible light) and VLT/ESO/NASA/JPL/Paris Observatory (infrared).

The Poles of Neptune and Its Moons

The South Pole of the planet Neptune is unusual in several ways. Triton, Neptune's largest moon, also has interesting features at its poles.

Like Earth, Neptune's spin axis (which defines the locations of its poles) is tilted a moderate amount. Neptune's axial tilt is 28.32, slightly more than Earth's 23.45. That means that Neptune has seasons in a fashion very similar to Earth. However, since Neptune takes roughly 164 years to orbit the Sun once, each season on Neptune lasts more than 40 Earth years! It has been summertime in Neptune's Southern Hemisphere for the last few decades, so that part of the planet has been receiving more sunlight and has been warming up for many years. In 2007 astronomers captured infrared images of Neptune that showed a brighter, relatively warm spot in the planet's atmosphere above its South Pole. Neptune's South Pole is the warmest place on the planet!

Since Neptune is not a solid object, but rather a ball of gas and ice, not all of it spins at the same speed. Areas near the equator rotate once every 18 hours, but regions close to the poles go around every 12 hours. This difference in rotational rates generates strong winds; at 70 S latitude they blow at speeds of 1,080 km/hr (671 mph)! The composition of Neptune's atmosphere also varies with latitude. Methane, ethane and acetylene are 10 to 100 times less abundant at the poles than they are near the equator.

Neptune's magnetic field is tilted with respect to the planet's spin axis. Unlike Earth's mild difference of 11 between magnetic and geographic poles, Neptune's magnetic field is tilted a whopping 46.9. If Earth's magnetic field was tilted that much, the North Magnetic Pole would be at a latitude somewhere south of Paris.

Last modified April 17, 2009 by Randy Russell.

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