Neptune's Spots:

Now You See 'em,
Now You Don't

from Lake Afton Public Observatory


New Hubble images have caused planetary scientist to rethink some ideas. To see an old Neptune image from Voyager and a new image from HST click here.

In August of 1989 Voyager 2 gave us our closest look at the planet Neptune. Voyager found a stunning variety of cloud formations, including high altitude cirrus-like clouds, dark cloud belts and two dark spots. The larger of the two dark spots was named the Great Dark Spot. Planetary scientists drew obvious comparisons between it and Jupiter's Great Red Spot. Both spots lie at similar latitudes (about 20o south of the equator). Each spot is large in size (Neptune's large spot is nearly Earth sized and Jupiter's is much larger than Earth) and rotates in a counterclockwise direction.

Jupiter's Great Red Spot has been observed continuously for over 300 years. This fact, coupled with laboratory and computer simulations, has led to the idea that these features are essentially permanent. Just when everybody got comfortable with the theory of giant spots on giant planets, Neptune pulled a surprise.

Spot Remover

Astronomers using the Hubble Space Telescope photographed Neptune in 1994. To their surprise, the Great Dark Spot had vanished. Although the Hubble pictures are not as high a resolution as those of Voyager 2, they are detailed enough to easily reveal features like the Great Dark Spot. Astronomers also found that Neptune's second, smaller spot has disappeared as well. Yet other features of the planet's atmosphere remain unchanged. The bright cirrus-like clouds and a dark southern belt were seen by both Voyager and Hubble. Images of Neptune were taken at different dates and times with different portions of Neptune facing us. So there is no way that the Great Dark Spot is lurking on the planet's far side. What made Neptune shed its spots is a mystery.

Further Studies

No spacecraft are currently planned to return to Neptune. This means that any long term monitoring of the changes in Neptune's cloud features will have to be done with telescopes. Hubble's famed mirror flaw was fixed in December of 1993. The flaw prevented the Space Telescope from seeing fine detail and kept us from seeing Neptune's weather change. Now that it has been restored to full working order the telescope can be used to monitor any future long term changes in Neptune's weather.

The Hubble studies undertaken in June and October of 1994 will be complimented by additional observations of Neptune in the summer of 1995.

Suggested References

Neptune: The Big Blue Suprise

Sky and Telescope Magazine February, 1995.

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