During the transit of Venus next Tuesday, the Hubble Space Telescope will be looking at … the Tycho crater on the Moon. It will be trying to detect the atmosphere of Venus.
The idea is to measure the transit of Venus reflected off the surface of the Moon, to place ourselves in the same conditions as when we observe transiting exoplanets. The intensity of the sunlight will drop by about a thousand during the transit, and a small part of this will be filtered through the atmosphere of Venus and leave a spectroscopic signal.
Catherine Huitson here has been preparing some of the “Phase II” for these observations. In HST jargon, Phase II means setting the details of the observing sequence, things like exposure time and instrument setup. Earlier this year, on January 12, the Space Telescope team also took some images of the Moon to make sure these observations were feasible.
The actual observations will not be images, but spectra with three different instrument (STIS, ACS and WFC3), to span the wavelength range from ultraviolet to infrared.
To be honest, the project is a very tall challenge. The projected area of the atmosphere of Venus compared to the whole planet is similar to that of hot Jupiters: the scale height of the atmosphere of Venus is 16 km for a radius of 6000 km, compared to 200 km for 90’000 km for a typical hot Jupiter (about 0.2 % in both cases). But the transit depth is much smaller, one part per thousand instead of one percent or more. As a result, the main atmospheric features will cause flux differences of one part per million or so in the flux reflected from the Moon. The best accuracy in precision transit photometry with the HST is about twenty parts per millions. The Moon is brighter than exoplanet host stars of course, so there will be more signal to work with. On the other hand, at the part per million level, the Sun is variable and the surface of the Moon quite rugged. We’ll probably require very large spectral features in the atmosphere of Venus to be able to detect anything, for instance some narrow atomic lines formed very high in the atmosphere.
The programme is lead by Alfred Vidal-Madjar at the Institut d’Astrophysique de Paris and David Ehrenreich at the Institut d’Planétologie et d’Astrophysique de Grenoble.
[FEATURE IMAGE: Space Telescope image of the Moon in preparation for the Venus transit programme. Taken on January 12, 2012 with the Wide Field Camera 3, using the narrow-band filter F502N and an exposure time of half a second (amusingly, the gap between the two parts of the camera has been filled for the press image. Spot the fuzzy band).]
NASA Press Release on the preparation of the Moon observations
Our outline of a paper on the expected transmission spectrum of Venus by Ehrenreich et al.
A sketch of the geometry of the HST Venus transit observations: