Venus Above the Clouds

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 (NCAR), 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.

Image Credit: Mitchell, ©

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 Knölker, 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.

The Mercury transit in perspective with sunspot
Credit: ESO


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.



Credit: NASA Workshop, Pale Blue Dot

"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.


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

Knölker 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.

Related Web Pages

Soviet Exploration of Venus
Dr. David Grinspoon’s
Lonely Planets
Magellan Image Server
Magellan Mission Home
Fact Sheet on Venus
Past Missions to Venus
Atmosphere and Weather on Venus
Chemical Weathering Reactions on Venus