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Filter Problems Hamper Venus Observations

file photo
by Staff Writers
Sunspot, NM (SPX) Feb 13, 2006
Instrument problems have kept the team at the Dunn Solar Telescope from observing the planet Venus as they had hoped."The filter on our infrared camera is leaking thermal radiation and keeping us from observing the planet," said Nancy Chanover, principal investigator from New Mexico State University.

"Basically, we have excess heat shining on the detectors. We have exhausted all possibilities for working around this after a valiant effort by the Dunn Solar Telescope staff."

Chanover said she thinks the planned technique for observing Venus using a solar telescope and adaptive optics is sound, and she hopes to attempt the observations again after resolving the problems with the filter. The current instrument time expires Feb. 15, however, and the team has not yet set dates for another attempt.

The atmosphere of Venus is composed of thick clouds of sulfuric acid at altitudes from 30 to 43 miles (48 to 70 kilometers), plus unbreathable carbon dioxide and nitrogen. The planet has virtually no water, thus inviting contrasts with Earth's weather and climate systems where water is all-important, but dingy yellow-white clouds block the view of the Venusian surface in visible light and even ultraviolet.

The planet's atmosphere has holes, however, and Chanover is using the solar telescope to peer through them.

"Observations of Venus from a nighttime telescope at a single location are very difficult because Venus is so close to the Sun in the sky," she said. "You can observe it for about two hours at most." Then the Sun rises and blinds the telescope, or Venus sets, depending on the time of year.

"Alternatively you can find a telescope designed to be open when the Sun is above the horizon, and observe for several hours," Chanover continued. That is where the Dunn Solar Telescope comes into play. It is part of the National Solar Observatory, and although it has operated since 1969, it rarely has looked at the planets because it is smaller than most nighttime astronomy telescopes.

Since 2004, the Dunn has been equipped with high-order adaptive optics that iron out the wrinkles that Earth's atmosphere introduces into images of the Sun or any object in space. Effectively, the Dunn now sees seven times sharper than an equivalent, uncorrected telescope - something that offers Chanover and her colleagues the opportunity to observe Venus for several hours at a time.

The observations will take place in infrared, particularly around 2,300 nanometers, a wavelength about three times longer than the deepest red (about 770 nm) the human eye can see.

"This is a special wavelength where nightside clouds are relatively transparent and you see thermal radiation from the lower atmosphere peeking through the clouds," Chanover explained. "It's significant in that you can compare what you see in the lower atmosphere with the ultraviolet views at the cloud tops and get a sense of how winds change with altitude."

The first measurements - by NASA's four Pioneer Venus Probes as they plunged through the atmosphere in 1978 - revealed winds of up to 224 miles per hour (100 meters per second). That speed is called super-rotation, because the winds are fast enough to circle the planet in five to seven days, 60 times faster than the planet's 243-day rotational period.

"Something unusual is going on here and it's poorly understood," Chanover said. Then scientists discovered in 1984 that near infrared light can escape from deep inside the atmosphere and even from the surface. In 1991 the Galileo spacecraft's infrared camera produced striking images of Venus' lower atmosphere as the craft flew by in 1990 on its way to Jupiter.

A simultaneous ground-based campaign tracked cloud features for up to 17 hours and revealed complexities - and uncertainties - in the atmospheric circulation.

"With a time series of nightside images you will see cloud motions and you can take wind speed measurements at different latitudes and longitudes," Chanover said. Nightside viewing is an important aspect of the observing plan, because sunlight reflected from the dayside would overwhelm the view. The best observing is when Venus appears as a thin crescent and displays more of its nightside. It also sits in the night sky for the shortest period of time, thereby making a solar telescope attractive.

Chanover will observe at 2,295 nm wavelength using an infrared camera cooled with liquid nitrogen. The camera was developed for infrared polarimetry to explore the relatively unknown infrared region of the Sun's spectrum. A new narrowband filter has been added to concentrate on the 2,295 nm light emitted deep in Venus' atmosphere. Clouds higher in the atmosphere absorb this band, so the research team can track the silhouettes.

The team previously observed Venus with the 3.5-meter telescope at the Apache Point Observatory next to Sunspot. This round with the Dunn is part of a larger campaign involving several telescopes, including Apache Point, but Chanover said she hopes the Dunn's adaptive optics will provide sharper images over a longer time interval each day than other telescopes have achieved.

"With the combination of adaptive optics and longer temporal baselines, we're hoping to increase the number of features we can track and the number of latitude points where we can measure wind speeds," she said.

"I want to emphasize that the importance of the Dunn observations is the combination of high spatial resolution and the extended temporal coverage of the cloud motions," Chanover said. "Observing during the daytime with a solar telescope enables us to track features for two, four, even six hours at a time, whereas most other nighttime facilities are limited to one or two hours. We'll really need the Dunn's adaptive optics to maintain good seeing on Venus throughout the morning as the surface of the Earth starts to heat up."

If the Dunn and its adaptive optics work as planned, Chanover expects the observations of Venus will complement the detailed measurements that will be taken by Europe's Venus Express orbiter, scheduled to arrive in April.

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Peeking Behind The Veil Of Venus
Sunspot, NM (SPX) Feb 13, 2006
The planet Venus is best known for the thick layers of clouds that veil its surface from view by telescopes on Earth. But the veil has holes, and a New Mexico State University scientist plans on using a solar telescope to peer through them to study the weather on Venus.

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