A Search for Waves in the Atmosphere of Venus (Part I)

I’m a physics major, but my real passion lies in astronomy.  My honors thesis is on the atmosphere of Venus, and I was recently able to go on a trip to New Mexico, with the help of the Charles Center and Physics Department, to observe the planet with a state-of-the-art telescope.

A bit of background on the atmosphere of Venus: it’s hellish.  At nearly 900°F at ground level, it’s hot enough to melt lead.  The pressure at ground level is 90 times higher that on Earth, so high that the carbon dioxide atmosphere ceases to be a gas and behaves as a supercritical fluid.  Higher up in the atmosphere, there are clouds of sulfuric acid and other toxic compounds.  These factors ensured that the few Soviet spacecraft to visit the surface – robust probes built like submarines – only survived a few hours.  Venus itself is about the same size as earth, but rotates in an opposite direction to all other planets in the Solar System, and does so slowly, rotating once every 243 days (a year on Venus is 225 days, so a Venusian year is shorter than a Venusian day).

My research relates to an poorly-understood phenomenon called “superrotation.”  Although the planet rotates slowly, the upper atmosphere at the equator whips around the planet in about four days, 600% faster than the rotational speed.  On Earth, the strongest hurricanes only blow 10-15% of the planet’s rotational speed.  To sustain the superrotation against friction from the terrain, there must be a mechanism that drives it.  Recent models have suggested that equatorial Kelvin-like waves (a type of wave that balances the centrifugal force against a boundary, like the equator) could power the superrotation.

Venus has been observed through telescopes since 1610, but all that could be seen until fairly recently was a featureless, white crescent.  In the 1950s, ultraviolet observations revealed complex patterns of clouds, which sometimes showed an enormous dark zone that often looked like a letter Y turned on its side.  This so-called Y-feature seemed to change shape, disappear, and reform over time.  It is now thought that this phenomenon is the manifestation of equatorial Kelvin-like waves.  Thus, the Y-feature could be intrinsically linked to superrotation.

The Y-feature, as captured by NASA's Pioneer Venus probe.  The feature can look like a U, V, Y, or Ψ depending on how long it has been visible.

The Y-feature, as captured by NASA’s Pioneer Venus probe. The feature can look like a U, V, Y, or Ψ depending on where it is in its cycle.

My research seeks to put this hypothesis on more firm observational footing.  Under Dr. Kunio Sayanagi at Hampton University and one of his graduate students, Ryan McCabe, I have been modifying cloud-tracking programs to tease out wave-like motions from the photos of churning clouds returned by the Venus Express probe.  While these data have excellent resolution, usually only a small portion of the planet’s surface is visible due to the orbit of the spacecraft.  Therefore it is difficult to tell when and where the Y-feature is present.

Venus as seen from ESA's Venus Express.  Less than 1/4 of the planet is visible in this image.  (courtesy of Ryan McCabe)

Venus as seen from ESA’s Venus Express. Less than 1/4 of the planet is visible in this image. (courtesy of Ryan McCabe)

Ground-based telescopes can image the entire planet, at least the portion visible from Earth, albeit at a lower resolution due to the Earth’s atmosphere.  In addition, combining data from the ground with data currently being collected by the Japanese Akatusuki probe will help refine future measurements.  Dr. Sayanagi and Ryan got a grant to observe Venus using the 3.5 meter Telescope at the Apache Point Observatory in southern New Mexico, and I was invited along to help.  This first observing run, five days, was to determine the optimal setup of filters and cameras to observe Venus and get preliminary data.  I have done hundreds of hours of observing and tinkering with William and Mary’s observatory, but I was excited to learn what it was like to use a telescope so large that it had its own building.  In going to New Mexico, I could further my research while also gaining valuble experience in the operation of real scientific observatories.  I would be closer to being a real astronomer.

Continued in Part II