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The Planetary Society Blog

By Emily Lakdawalla

Clouds on Saturn and Venus

Nov. 10, 2006 | 12:53 PST | 20:53 UTC

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The Cassini project released a striking set of images of Saturn's south pole yesterday. Check these out:

Click to enlarge >

Saturn's south pole in many wavelengths
During late 2006 and early 2007, Cassini's orbit was tilted to give it excellent views of Saturn's south pole during the planet's southern summer season. Cassini's cameras and spectrometers found unexpected levels of storm activity there. These six views were captured in different wavelengths of light. The first two images in the top row were captured through broadband color filters: blue (440 nanometers) and infrared (752 nanometers). The upper right and lower left were captured in narrow-band filters centered on infrared wavelengths in which methane gas strongly absorbs light: 728 and 890 nanometers. The lower right images were captured with the VIMS instrument at wavelengths of 2.8 and 5.0 microns. The 5-micron image shows heat radiating from Saturn's interior; higher-level clouds are silhouetted against the radiant heat. Credit: NASA / JPL / SSI / U. Arizona

These images are made possible by the increasing tilt of Cassini's orbit; it's now oriented at a 55-degree angle to the ring plane. The orientation of the elliptical orbit is such that at periapsis (its closest approach to Saturn), it's getting really nice sunlit views of the south pole.

The dominant feature of these south polar views is a polar hole in Saturn's clouds, visible as a big dark spot in most of these images. But I'm more intrigued by all the little freckles that surround this polar storm. The freckles are high-level clouds; you can tell that by looking at the image in the lower right, which was taken by the VIMS instrument at a wavelength of 5 microns. At that wavelength, what you're looking at is heat radiating from Saturn's interior; the brighter the color, the more radiant heat. The south polar hole in the clouds allows more heat to escape so it looks brighter. But surrounding that you can see lots and lots of little dark spots, which are clouds that are blocking VIMS' view of that radiant heat. I'm not surprised to see some kind of feature right at Saturn's pole (though I couldn't have predicted what it'd look like), but I'm very surprised by those hundreds of little bitty clouds.

The news release issued with these images states that the effectiveness with which these clouds block the radiant heat means that these are unusually thick storm clouds extending deep into Saturn's atmosphere and are comprised of relatively large cloud particles, and that their presence is no doubt associated with the summer season -- these parts of Saturn are in constant sunlight. So I'm sure the atmospheric scientists on the Cassini mission will be spending the rest of the mission watching these parts of Saturn to see whether those freckly clouds change in character as summer gives way to fall. (The equinox is on August 11, 2009, which is about a year after the end of Cassini's primary mission.)

If this clouds-blocking-radiant-heat thing sounds familiar to you, that's because I've talked about it before, on a different planet. You can see exactly the same effect in Venus Express VIRTIS images of Venus:

Click to enlarge >

Night side of Venus
This three-frame view of Venus was captured by Venus Express VIRTIS on July 22, 2006 from a distance of about 65,000 kilometers (40,000 miles) at a wavelength of 1.7 microns. At this wavelength, VIRTIS sees heat radiating from clouds at an elevation of 15-20 kilometers (9-12 miles) in Venus' atmosphere. Higher-level clouds blocking this heat radiation appear dark, while brighter areas indicate windows into the lower levels of Venus' atmosphere. Credit: ESA / VIRTIS / INAF-IASF / Obs. de Paris-LESIA

The difference is that at Saturn, we're seeing the thermal radiation at 5 microns, whereas at Venus, we're seeing it at 1.7 microns. Saturn's much colder than Venus, so its thermal radiation takes place at a much longer wavelength.

Cassini was at periapsis yesterday, so it should have gotten another really nice set of data on these south polar clouds. But remember how I mentioned that Mars Global Surveyor is currently having some trouble communicating with Earth? The next step in recovering MGS involves a communication session that will be attempted this afternoon. This communication session wasn't a prescheduled one, obviously, so that means that recovering Mars Global Surveyor is bumping aside whoever else had planned to use the large Deep Space Network dish at Canberra at the same time. As it turns out, that was Cassini. So because of a sticky wicket at Mars, we won't be getting data from Saturn as scheduled. But never fear; the Cassini mission, like all other deep space missions, has contingency plans in place for the loss of planned communication sessions, so hopefully they'll get another chance to get that data down.

There you have it -- Saturn, Venus, and Mars all in one blog entry. It's a small solar system.