Neptune’s Clouds

Voyager's look at clouds on Neptune.  Credit: NASA Jet Propulsion Laboratory / NASA Planetary Photojournal

Voyager’s look at clouds on Neptune. Credit: NASA Jet Propulsion Laboratory / NASA Planetary Photojournal

The bit of an interlude in the ESA’s Comet watch blog is a good time to look at some of Voyager 2’s images of Neptune. This is one of my favorites. I don’t really know if there is more than coincidence that the New Horizon’s spacecraft crossed the Neptune orbit 29 years almost to the day after Voyager started its Neptune encounter.

There is a lot of comparisons being drawn between the New Horizon’s and Voyager missions. Hey I’m on board with it. If I had my way there would be a “Le Verrier” or “Galle” spacecraft, a Neptune analog of the Cassini spacecraft in orbit right now.

In case you were wondering what was going on with Rosetta, everything is fine. Mission managers are looking at images from as close as 50 km trying to select the best landing spot. New images will be posted shortly.

This image comes from NASA’s Solar System Exploration (and Planetary Photojournal) site:

This Voyager 2 high resolution color image, taken 2 hours before closest approach, provides obvious evidence of vertical relief in Neptune’s bright cloud streaks.

These clouds were observed at a latitude of 29 degrees north near Neptune’s east terminator. The linear cloud forms are stretched approximately along lines of constant latitude and the Sun is toward the lower left. The bright sides of the clouds which face the Sun are brighter than the surrounding cloud deck because they are more directly exposed to the sun. Shadows can be seen on the side opposite the sun. These shadows are less distinct at short wavelengths (violet filter) and more distinct at long wavelengths (orange filter). This can be understood if the underlying cloud deck on which the shadow is cast is at a relatively great depth, in which case scattering by molecules in the overlying atmosphere will diffuse light into the shadow.

Because molecules scatter blue light much more efficiently than red light, the shadows will be darkest at the longest (reddest) wavelengths, and will appear blue under white light illumination.

The resolution of this image is 11 kilometers (6.8 miles per pixel) and the range is only 157,000 kilometers (98,000 miles). The width of the cloud streaks range from 50 to 200 kilometers (31 to 124 miles), and their shadow widths range from 30 to 50 kilometers (18 to 31 miles). Cloud heights appear to be of the order of 50 kilometers (31 miles).

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