When Venus is more or less bewteen the Sun and Earth, in the weeks that precede or follow the inferior conjunction, it shows us its night-side face. But far from being uninteresting, the Venus night side is the only way to detect something else than the thick layer of the visible clouds, thanks to the phenomenon of the absorption bands of the carbon dioxid (CO2) in the near-infrared part of the spectrum. At some precise wavelenghts the CO2 is light-absorbing, thus creating on the night side some kind of "windows" from which light emitted by other parts of the planets than the main cloud deck can be detected.

Unfortunately most of those bands are beyond reach of amateurs CCD's whose sensitivity ends near 1,1 micron (1100 nm). For example, a window at 2,3 micron allows to detect an emission coming from lower clouds under the main cloud layer, as can be seen on a few images taken by the Galileo orbiter during its Venus fly-by in 1990 . Other absorption bands may show another kinds of data. One is nonetheless within reach of amateur means, because it is situated just before the en of sensitivity of the CCD cameras (or the webcams) near 1 micron (1000 nm). That window allows some thermal emission to escape into space where it can be recorded, from the surface itself which is hot enough (735 K) to glow at this wavelenght.

A first experience had been successful at the Pic du midi in 1993 (see ) with the help of a coronograph to mask the illuminated part of the disk.

I have made the experience myself in may 2004, thanks to the narrow phase of the planet, because I do not have any coronograph. Thus to detect something it was crucial to wait until the crescent is narrow, or the image would have been totally "burnt" by the spreading of light during a long-exposure... On may 12th, 2004, the phase was only 18 %. I have used an IR 1000 filter at prime focus of the 14" SCT of my astro club, with the ATK-1HS used in the long-exposure mode. To my big surprise the required exposure time was only 10 seconds until the rond, faint image of the dark side become visible !

On this montage the glowing thermal signal from the surface is associated to a normal image of the day-side crescent, taken just before at the same wavelenght.


Images obtained between may 16th and may 21th : a first analisis

Five other observing nights have been secured in may 2004, with a few surprises as a result as this time some zonal differences have been clearly identified in the IR emission. Exposure time have been reduced a bit (8 seconds) in order to limit the effect of poor / fair seeing (all the images have been made with the planet at less than 15 high, because before that moment, the sky background is too bright), and to obtain more raw frames more easily (one hundred each time).

16, 17, 18 May 2004


19, 21 May 2004

It would be nice to explain what are those dark patches in the night side. Two hypotesis come :

1) This is the thermal "footprint" of some high regions in the venusian surface. Higher, they are cooler, and thus they are darker at 1 micron.

2) The thermal emission is blocked by low clouds (at an altitude of 20-30 kms).

It well possible that both explanations are mixed together. No clear correlation with Venus's topography has been established, but it's quite difficult to do so, and the details are not always at the same place night after night. The slow retrograd motion of the globe (243 days), turning here from "left to right", might not as well be the cause, and the "movement" of the dark patches look quite random sometimes. This is why some low clouds are maybe playing a role here.

This idea is supported by some professionnal experiences. This light emission doesn't escape as it is, but it's largely "filtered" and scattered by the thick venusian atmosphere, which is not really transparent at 1 micron but translucent. During Galileo's fly-by at the beginning of the 90's, some images had been taken of the thermal emission, but it was corrected by substracting some images, taken at the same time, of the lower atmosphere only (at a longer wavelenght in the infrared). This out of reach of amateur means.