Peeking behind the veil of Venus (Forwarded)

National Solar Observatory

For more information, contact:

Dave Dooling, Education and Public Outreach Officer
National Solar Observatory
1-505-434-7015
or
Karl Hill, University Communications
New Mexico State University
MSC 3K, Box 30001
Las Cruces, NM 88003-8001
1-505-646-1885

Feb. 7, 2006

Peeking behind the veil of Venus

Sunspot, NM -- 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.

"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," said Dr.
Nancy Chanover, a planetary scientist at NMSU in Las Cruces, NM. "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's where the Dunn Solar Telescope at Sunspot, NM, comes into play. The
Dunn is part of the National Science Foundation's National Solar
Observatory. While it has operated since 1969, it has rarely 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. That offers
Chanover and Dr. Eliot Young, her colleague at Southwest Research
Institute in Boulder, CO, the opportunity to observe Venus for several
hours, from horizon to horizon and even after the sunrise. Their
observations are scheduled during Feb. 10-15, 2006. Their work is funded
by the National Science Foundation.

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

Infrared is a different matter, particularly around 2,300 nm, 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," she 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 the four Pioneer Venus Probes as they plunged
through the atmosphere in 1978, revealed winds that howl at up to 100 m/s
(224 mph). This is called super-rotation because the winds are fast enough
to circle the planet in five to seven days, 60 times faster than its
243-day rotational period. "Something unusual is going on here and it's
poorly understood," she 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 5 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 explained.

"Nightside" is an important aspect of Chanover's observing plan because
sunlight reflected from the dayside would overwhelm the view. The best
observing is when Venus appears as a thin crescent and we see more of its
nightside (see The Phases of Venus, below). It is also in the night sky
the shortest period of time, thus making a daytime -- 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 Chanover and Young can track the
silhouettes.

Chanover also has observed Venus with ground-based telescopes, including
the 3.5-meter telescope at the Apache Point Observatory next to Sunspot,
on three previous occasions. This round with the Dunn is part of a larger
campaign involving several telescopes including Apache Point. But 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 that
it will add a new tool to those now used by planetary astronomers.
Chanover's technique will also complement detailed measurements by
Europe's Venus Express orbiter, which arrives in April.

SIDEBAR

The Phases of Venus

Observing Mercury and Venus from Earth is more challenging than observing
the outer planets because the orbits of Mercury and Venus are inside the
orbit of Earth. These worlds thus appear smallest and closer to the Sun
when full, and thinnest when larger and closer to Earth.

The orbits of the two inner planets and Earth constantly change their
positions relative to each other and the Sun. This generates phases like
the Moon -- new to waxing crescent, quarter (or half), waxing gibbous,
full, and then back -- but with varying sizes -- unlike the Moon, which
holds a nearly constant distance from Earth.

When either planet appears nearly full it also is on the other side of the
Sun from Earth (called superior conjunction). It also appears smaller and
closer to the disk of the Sun or even passes behind it (occultation). As
the planet swings back from behind the Sun, we see it waning gibbous as
the nightside slowly comes into view.

IMAGE CAPTION:
[http://www.nso.edu/press/venus06/ima...nus_phases.jpg (14KB)]
Schematic of the phases of Venus. (NASA)

Meanwhile, the planet is getting closer to Earth and its apparent diameter
is growing.

The illuminated side wanes until the planet appears like a half-moon,
properly called quarter phase, at greatest west elongation. Earth, Sun,
and planet form a V, not a right angle, because we are looking at the edge
of a circle.

As the planet continues to approach Earth the quarter phase becomes an
ever-thinner crescent. This crescent will always be centered towards the
Sun, and will be thinnest when the planet is between Earth and Sun
(inferior conjunction). The planet also appears steadily larger.

Continuing in its orbit, the planet's phases reverse to waxing crescent,
quarter (rises earliest in the morning, greatest east elongation), waxing
gibbous, and back to full and the apparent size steadily decreases.

Occasionally the orbital planes of Earth and Mercury or Venus line up and
the planet appears to pass directly across the face of the Sun. Such
transits are important in the histories of astronomy and geography.

By comparison, planets from Mars outward can never present less than a
gibbous face to Earth because we are looking away from the Sun and onto
their illuminated faces. (Martians studying Earth, though, will see it go
through the same phases as Mercury and Venus!) And the more distant the
outer planet, the smaller the change in its apparent size because the size
of Earth's orbit becomes a much smaller fraction of the total distance.

Links to Further Information

* Adaptive optics at the Dunn Solar Telescope
http://nsosp.nso.edu/ao/
* Infrared observations at the Dunn Solar Telescope
http://www.nso.edu/press/AAS_0604/AAS04_NJIT2.html
* Dr. Nancy Chanover's home page
http://astronomy.nmsu.edu/dept/html/...nchanove.shtml
* Dr. Eliot Young's home page
http://www.boulder.swri.edu/~efy/
* Transit Venus as observed by NSO's Global Oscillation Network
http://gong.nso.edu/venus2004/
* NASA's Solar System Exploration web page for Venus
http://solarsystem.nasa.gov/planets/...m?Object=Venus

IMAGE CAPTIONS:

[Image 1:
http://photojournal.jpl.nasa.gov/cat...mber=pia00221]
The lower atmosphere of Venus as seen by the Near-Infrared Mapping
Spectrometer (NIMS) instrument carried by the Galileo spacecraft. These
images were taken in 1991 as Galileo flew past Venus in a complex series
of gravity-assist maneuvers to rehape its trajectory outward to Jupiter.
(JPL & NASA)

[Image 2:
http://www.nso.edu/press/venus06/images/may04_venus.jpg (31KB)]
Venus as seen by the Apache Point Observatory during Chanover's 2004
observations. While the sunlit side is washed out, cloud structures are
clearly visible on the nightside. (Nancy Chanover, NMSU)

[Image 3:
http://www.nso.edu/press/venus06/images/PVO_clouds.jpg (83KB)]
The ultraviolet camera on the Pioneer Venus Orbiter revealed striking
details about circulation at the tops of the sulfuric acid-laden clouds.
(NASA)