Planetary News: Venus (2006)
First Venus Express VIRTIS Images Peel Away the Planet's CloudsBy Emily Lakdawalla
April 14, 2006
The first image released from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) aboard Venus Express reveals subtly detailed, swirling patterns in the upper levels of Venus' thick and complex atmosphere. "Venus is much more photogenic than we originally thought!" remarked VIRTIS co-investigator Kevin Baines. The image has also, for the first time, shown that Venus has a southern "polar vortex" matching the one discovered at the north pole by the Pioneer Venus mission in 1978.
Baines explains what the two-faced image shows: "The VIRTIS image is a composite of two pieces, all taken together in one single image cube." VIRTIS, being an imaging spectrometer, slices incoming radiation into about 200 wavelength "bands," producing essentially a stack of 200 photos at slightly different wavelengths. Such a stack is referred to by imaging scientists as an image "cube" because it has three dimensions: two spatial and one spectral (wavelength).
Baines continued: "On the left, the bluish image shows a daytime view of cloud top features observed in reflected sunlight near 70-80 kilometers [43-50 miles] altitude. We combined two images: one at 0.33-micron wavelength (that is, a UV image), which we colored both red and green to make yellow. The other is at 0.95 micron [infrared], and is colored blue." These wavelength bands were chosen because of the way they reveal subtle cloud structures at the upper reaches of Venus' atmosphere -- the part of Venus that is visible to our eyes from Earth.
Already, this image has revealed interesting dynamics in Venus' upper atmosphere at the south pole, including the net motion of cloud particles from lower to higher latitudes. "The combination shows that most places look pretty uniformly gray. But there is a distinctive yellow arc of clouds which is offset from the pole; it doesn't follow a set latitude. Instead, it is part of a spiral structure resulting from the net movement of aerosol particles which mostly move east-to-west in the zonal wind but also have a poleward, meridional component," Baines explained. This kind of cloud motion has been observed at Venus' north pole by Pioneer Venus and Mariner 10, but neither of those missions made many observations of Venus' south pole.
But Baines was more excited about the details revealed in the right half of the image. Both sides of the image were captured simultaneously in a single observation. The left half reveals everything that is visible in reflected sunlight (it is a polar view, with the Sun off to the left). But the right view shows what VIRTIS was sent to Venus to do. It is a view in an infrared wavelength of 1.7 microns, which detects thermal energy -- emitted by Venus' overheated atmosphere from an altitude of about 20 to 30 kilometers (12 to 18 miles), Baines said. "As the heat encounters large-particle clouds near the 50-kilometer [30-mile] level, it is attenuating by varying degrees, depending on the cloud opacity. What we see then are clouds silhouetted against this background of thermal glow from Venus' interior." In other words, it is a back-lit view of clouds at a deep level within Venus' atmosphere. Our view of such clouds has always been blocked by the high-level clouds visible on the left half of the image. "Thick clouds then are dark, and thinner clouds are bright. This image reveals these deep-level clouds to be variegated, with dramatic filamentary structures, encompassed within an overall spiral structure that again shows the basic spiral flow toward the pole."
By studying Venus at different infrared wavelengths, VIRTIS will be able to peel away cloud layer after cloud layer in this fashion, building up a three-dimensional view of the complex and multilayered structure of the atmosphere. Not only that, but such observations will be repeated continuously throughout the mission to add the dimension of time, allowing scientists to characterize the short- and long-term motions of Venus' wind patterns over the course of each rotation of its atmosphere and over the course of Venusian days and years. Venus rotates very slowly, so that its surface sees solar days (noon to noon) that are 117 Earth days long; but its "super-rotating" atmosphere blows around the entire planet much faster than that, every 4 Earth days.
Having seen these first VIRTIS images, Baines confirms that VIRTIS will be able to follow all these swirling cloud motions. "I had been concerned that the pole would be completely covered by a 'polar cap' of clouds, as was indicated in [Galileo] NIMS and Earth-based imagery. Such a broad expanse of clouds would have prevented us from using our cloud-tracked wind measurement techniques, where we follow small clouds over time as they move around. Now we know we'll have plenty of small-scale filamentary and discrete structures to follow, so we'll get great wind measurements."
According to Baines, we won't have to wait for long to get some great results from Venus Express. He points to the infrared view of the night side of Venus and a very dark spot at the center, Venus' south pole. "The pole itself has a very dark, thick cloud encircled by a bright ring of thinner clouds. This looks to be part of a polar vortex that Fred Taylor and others (like myself!) have been looking forward to find and characterize. At the North Pole, a strange 'polar dipole' within about 10 degrees latitude of the pole was followed by Fred's VORTEX experiment [on Pioneer Venus] for 3 months in 1978, before his instrument broke down. No one has ever seen the south pole clearly enough to witness a central, polar vortex there. We may be beginning to see one. Over the next day or two, we should see the missing half, which was in sunlight in this image, as it rotates into night, and get a very clear idea of the entire southern polar vortex structure. As well, we'll also see the missing half of the high-altitude cloud tops over the next few days. These first images are incredible!"