Astronomy on Maunakea is predominantly nighttime activity. Those visiting the summit during the day see the domes of the telescopes closed until just before sunset. Exceptions exist; astronomers observing comets and other solar system objects in the daytime will occasionally use the NASA Infrared Telescope Facility (IRTF).
However, if the dome at Canada-France-Hawaii Telescope is open during the day, one can safely bet we are observing Venus for a French astronomer named Thomas Widemann.
Daytime Venus observations are scheduled very carefully. Usually they occur when Venus is at its maximum elongation from the sun. In 2015, Widemann and his teamed observed at CFHT in conjunction with another set of daytime Venus observations on the summit at IRTF. The opportunity to obtain simultaneous spectra or rainbows from Venus in multiple wavelengths of light was too perfect of an opportunity to pass up, even though Venus was closer to the sun than we prefer.
Why mention these 2015 observations? In March 2017, the team released a paper with some fascinating insights into the winds of Venus using the data obtained at CFHT in 2015. The paper provides the first scientific evidence on Venus of wind between the equator and the poles. This wind — knows as a meridional wind — was discovered using data from Espadons at CFHT and ESA’s Venus Express spacecraft.
The team analyzed reflected sunlight off the cloud tops on Venus. They identified wind traveling perpendicular to the equator. This wind, with an average velocity of 81 km/hr, is similar to the Hadley cells on Earth. Solar heating is greater at the equator than the poles resulting in the circulation of warmer air away from the equator. This wind reduces the temperature difference between the equator and poles.
On Earth, the Hadley cells exist on either side of the equator. The cells span the Earth between the equator and 30th latitude. As the air moves poleward in the tropopause, the Coriolis effect turns the air eastward — creating the subtropical jet streams. At the surface, the Coriolis effect turns the winds westward, creating the trade winds — well known and beloved in Hawaii for keeping the islands a pleasant temperature year round.
According to Pedro Mechado, a collaborator of Widemann and frequent CFHT observer, “this detection is crucial to understand the transfer of energy between the equatorial region and the high latitudes, shedding light on a phenomenon that for decades has remained unexplained and which is the super-rotation of the Venus atmosphere.”
Astronomers are searching for a physical model to explain the super-rotation on Venus. The newly released data adds to the model by studying the wind parallel to the equator and how these winds change with time and latitude.
The team’s success comes in part from a method they designed using visible light to measure the instantaneous speed of the wind on another planet from Earth-based telescopes. The method is based on the Doppler effect that the clouds cause on the reflected sunlight.
Understanding the weather of different planets is challenging, but important work. As astronomers learn more about other planets, we gain a greater understanding of Earth.