by Mark Perew

Posted January 19, 2001

For the second day in a row, astronomers and astrophysicists have released new results which diametrically contradict previous understanding of Earth's nearest neighbor. These results severely question how well we understand a planet with such stark differences, yet so close at hand and so similar in size and mass.

In November of 1999, Dr. Tom Slanger from SRI International used the HIRES spectrograph on the Keck 1 telescope to examine the night side of Venus. The signature of atomic oxygen was expected since the telescope was looking through Earth's atmosphere. A related, but slightly red shifted signature from Venus, came as a complete surprise to him because previous research by planetary missions in the 1970s had shown none.

Data from the NASA Pioneer Venus Orbiter plus the Soviet probes Venera 9 and 10 did find molecular oxygen, but not free oxygen atoms. The data from HIRES showed plenty of molecular oxygen and the amounts agreed with what was seen by PVO and Venera.

"It's hard to understand how you can have such a huge difference," commented Dr. Slanger. "The echelle spectrograph gets very good coverage and resolution. With sensitive CCDs you get results that are very outstanding." Obviously, Slanger believes that his data is solid.

It wasn't just a weak trace of atomic oxygen either. The data shows a green line nearly as intense as the glow from Earth's atmosphere, even after taking that effect into account in the ground based data. Since free oxygen is about 0.1 parts per million in Venus' atmosphere, an infinitesimal fraction of the amount on Earth, the intensity only confounds the puzzle.

One other feature of the data is that the oxygen is strongest in the center of the disk, and drops away as the data nears the limb.

On Earth the atomic oxygen is created from carbon dioxide on the dayside and moved around by the ionosphere. On the night side the atoms recombine to form O2 and ozone, among other molecules. While Venus also has an ionosphere, its lack of a magnetic field means that the ionosphere doesn't operate as smoothly as on Earth. Particles tend to swirl around and flow in a disordered way.

Granted that PVO and Venera were examining Venus during a solar minimum and we are now at the peak of the solar activity cycle. Maybe the atomic oxygen generation is affected by the solar cycle. That's one possibility, but the lack of a magnetic field problem makes that unlikely. What's making the atomic oxygen move around on Venus?

Accompanying the Science paper by Slanger, et al, is a commentary by Dr. David Crisp from the NASA/Caltech Jet Propulsion Laboratory.

"I certainly trust those data," stated Dr. Crisp. "Something weird is going on in the upper atmosphere of Venus."

The first bottom line is that we just don't know what's going on.

The second bottom line is that this calls into question some of the methods we might use to study extrasolar Earth-like planets. When we do find a terrestrial planet orbiting some distant sun and we begin to analyze the spectrum of its light, the data might not being telling us what we think it is. Do we know terrestrial planet atmospheres well enough to understand the spectrum data? The evidence from Venus says we don't.

Next month Dr. Slanger will collect more data using the New Mexico State University Telescope at Apache Point. At the same time, Dr. Crisp will be at Mauna Kea and pointing the instrument from NASA's Infrared Telescope Facility at Venus. Hopefully their new data will turn up some answers instead of erasing the old ones.

Mark Perewfreelance writer, a member of the National Association of Science Writers and a JPL Solar System Ambassador