Mohawk Valley Astronomical Society

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Heat of the Night

by Perry Pezzolanella

Planetary missions provide exciting views of planets and moons sometimes so alien that it is hard to believe they are real. The photos are illuminated by sunlight that casts eerie shadows across the landscape and cloud tops. The night sides of these worlds are almost never shown because except for an aurora or a fleeting lightning flash the scenery is simply black, or too dark to be of any interest. For one planet this is an incorrect assumption.

Venus is nearly earth-like in size but the similarity ends there. Even though it is about 25 million miles closer to the Sun than Earth, but 31 million miles farther than Mercury, it is the hottest of all the planets with the most fearsome environment imaginable. Venus is completely enshrouded in a thick veil of carbon dioxide clouds laced with sulfuric acid, which imparts a yellowish tint to its otherwise white clouds. The 96.5 percent carbon dioxide atmosphere drives a runaway greenhouse effect with temperatures as high as 900 degree F and an atmospheric pressure averaging 92 times Earth's at the surface. The heat is so intense that it is suspected that the rocks in the hotter lowlands glow a dull red at night. Visually this glow would be eerie, but if we had infrared vision, night would be as bright as day! The Galileo spacecraft, flying past Venus on its journey to Jupiter, and the Venus Express orbiter had capabilities of seeing at infrared wavelengths and both revealed that the night side of Venus is not such a very dark place after all. Strong winds over 200 miles per hour create swirling and striated patterns and are capable of splitting and thinning the clouds. Turbulence, updrafts and downdrafts also create thick, convective clouds and dry, thinner areas. It is not possible to see the surface through the thinnest areas, except during the night. How can this be possible?

Infrared spacecraft images reveal that the night side of Venus glows at varying degrees of brightness; the brighter areas are hotter and the darker areas are cooler. The brighter areas are the searing surface of Venus shining almost unobstructed through a thinning in the cloud deck! The images obtained by Venus Express have provided a wealth of detailed information on how the clouds move and evolve at various heights during the night. Clouds of varying patterns and thickness can be seen at different levels far better when backlit by the infrared light of the blazing hot surface below than can be seen during the daytime with sunlight shining straight down from above or even from an angle. Most dramatic are the polar vortices over the poles that can be seen growing, shrinking, splitting in two, and recombining. At even higher resolution it is possible to accurately measure the surface temperature on the night side better than during the day where sunlight would cause interference. Such an instrument was aboard Venus Express but it failed upon arrival; a similar instrument is expected to fly on a future mission. Another instrument was able to measure and map the temperature across a smaller area of Venus at lower resolution and found the range to be from 840 degree F to 890 degree F. In an infrared surface image, the hotter lowlands appear brighter, while the cooler highlands do not shine as bright, but even at a cooler 700 degree F it is no paradise.

It is always fun to follow Venus on each clear evening as its crescent diminishes. Amateur astronomers are now capable of imaging crescent Venus in the infrared and although the images are small, the night side of Venus readily reveals its cooler, dimmer highlands such as Ishtar Terra and Aphrodite Terra against the hotter, brighter lowlands. It is still debated whether the dull glow on the night side of Venus seen at visible wavelengths, known as Ashen Light, is due to the hot surface. Infrared is an entirely new type of imaging that can rewardingly reveal the surface of Venus without the aid of sunlight. Venus, the Roman goddess of love and beauty, is fearsomely beautiful indeed!