On June 8 Earth-based solar telescopes will follow a tiny black
orb as it appears to travel effortlessly across a wrinkled, brilliant
sea. Timothy Brown, a scientist at the National Center for Atmospheric
Research (NCR), will not sit idly by as Venus traverses the Sun for the
first time in 122 years at an angle visible from Earth. Peering through
a specialized solar telescope in the Canary Islands, Brown will study
the chemical composition and winds of Venus’s upper atmosphere, a region
poorly observed until now. NCAR’s primary sponsor, the National Science
Foundation (NSF), is funding the research.

An extrasolar planet expert at NCAR’s High Altitude Observatory (HAO),
Brown has been applying a technique known as spectroscopy to piece
together atmospheric data on a planet orbiting star HD209248, located
150 light years from Earth. He found sodium in the planet’s atmosphere
in 2001 and is now searching for water and carbon monoxide. HAO director
Michael Knoelker, who specializes in precision solar spectroscopy, is a
coinvestigator on the Venus project.

During next week’s transit, Brown will apply the same technique to
examine regions of the solar spectrum that are strongly absorbed as they
pass through Venus’s atmosphere between 65 and 85 kilometers (40 and 53
miles) altitude–a region above the planet’s thick cloud layer.

Each wavelength absorbed will indicate the presence of a specific gas
and its height in the atmosphere. Brown will also be able to construct
wind patterns based on the gases’ Doppler shifts, that is, their motion
toward or away from Earth.

“This is Venus’s first transit in front of the Sun since quantitative
astronomical spectroscopy was invented,” explains Brown, “so it’s our
first chance to use the technology to observe close up the transit of a
planet with an atmosphere.” Venus, Mars, and Earth constantly orbit the
sun. Only when the orbital planes and positions align in a particular
way is a transit across the solar disk by Mars or Venus visible from
Earth. Mercury’s transits are visible every few years–its last trip
was in November 1999–but whatever atmosphere it may have is far too
thin to be observed.

Brown will examine the Venusian atmosphere using a vacuum tower
telescope located on Tenerife in the Canary Islands, a Spanish province
near the northwest coast of Africa. The 70-centimeter solar telescope
and its infrared spectrograph are operated by the Kiepenheuer Institute
in Freiburg, Germany. The spectrograph works well for the occasion
since the planet’s atmosphere is composed almost entirely of carbon
dioxide, a gas that absorbs near-infrared radiation.

Scientists have learned much about Venus’s atmosphere since its
discovery two hundred years ago. In recent times, probes from orbiting
spacecraft have measured its chemical composition and temperature at
various heights. These data have enabled scientists to construct
computer models of the planet’s atmosphere. But the region that Brown
will be observing has escaped close scrutiny until now.

“I hope to make a substantial addition to knowledge of an atmospheric
region not well observed in the past,” says Brown. “But also the
experiment will be a source of great inspiration and motivation for
investigating the atmospheres of extrasolar planets.”

Applying spectroscopy to a distant planet too tiny to see directly is a
challenge far greater than observing a neighboring planet in our own
solar system.

“Observations of extrasolar planets as precise and compelling as those
of Venus will not happen in my lifetime or that of my children,” says
Brown. “What we can see on Venus today previews what will be seen on
far distant planets long after I’m gone. You could say it’s cheating on
time.”

Brown believes his own observations of a planet circling the star
HD209248 will reveal whether there are significant clouds, dust, or
particles in that planet’s upper atmosphere, how big the particles are,
and what the dust is made of. His observations using the Hubble Space
Telescope in 2001 revealed the presence of sodium, and he thinks carbon
monoxide may show up soon. Later this year he will continue his search
for water vapor around the same planet, again using the Hubble.

The planet is so close to its star that its annual orbit lasts only 3.5
days, and it is perpetually blasted by ultraviolet radiation and
thousand-degree heat.

“Whatever can withstand those temperatures is what’s there,” says Brown.
“In other words, no little green men.”

Knoelker was a scientist with the Kiepenheuer Institute for many years
before coming to NCAR. He has been using the telescope on Tenerife for
solar spectroscopy, and he was part of a team that observed the impact
of the Shoemaker-Levy comet on Jupiter in 1994.

Brown’s other collaborators in the Venus observations are Wolfgang
Schmidt and Helmhold Schleicher, both of the Kiepenheuer Institute, and
Roi Alonso Sobrino, a graduate student at the Astrophysics Institute of
the Canary Islands. NASA and NSF fund Brown’s extrasolar planet research.