Images and Movies of Venus Options

[Bjorn Jonsson]

Recently I have been taking a fairly detailed look at the Venus Express (VEX) Venus Monitoring Camera (VMC) data set. Apparently this is a little known data set here at UMSF but this is a really interesting data set that isn't too difficult to process and many of the images show interesting atmospheric features. So without further ado here are some results of my processing efforts, followed by more detailed information below.

Two 3x3 montages of VMC images:



I also assembled a movie from 58 UV images obtained during orbit 1640. It is available at YouTube.

A sharpened, tweened version of the movie is also available at YouTube.

I selected the montage images to show various types of features and to show interesting stuff: Global views, atmospheric waves, features that appear dark in the UV, a bright south polar band, thermal emission from the surface etc. So far I have taken a quick look at only a few thousand images (maybe about 10,000) but this is a huge data set. I don't know the exact number of images but it is probably more than 100,000. For comparison, Pioneer Venus obtained ~1500 images (I don't know the exact number). VEX has been monitoring Venus continuously for 6 years now and this is a really fascinating data set that seems to be almost unknown among the image processing specialists here. Movies like the one above are relatively easy to make and for anyone interested, it is definitely possible to make hundreds of them if one has the time. Even though the movie above covers a period of just 8 hours this is sufficient to reveal that Venus' atmosphere can change very quickly - significant changes can occur in just a few hours. I haven't done any mosaics yet but I know they are possible. It is probably possible to make atmospheric movies as well. 'Color' composites are possible but not interesting.

Most of the images above are UV images; their name ends with _UV2.IMG. There are also three IR images; their name ends with _N22.IMG. Since I was experimenting, the processing varies from frame to frame in the montages, some of the images have been contrast stretched and some have also been processed with an unsharp mask.


I became interested in knowing more about some of the cloud features visible in the images and managed to relatively quickly find a lot of interesting and useful information, far more than I was expecting - I now almost 400 (!) 'new' pages of stuff to read. Venus is a really interesting planet. Below is a summary of a few things I found interesting but there is a lot more - I haven't read all 400 pages yet ;-).

The VMC observes Venus in four channels: A UV channel, a visible channel and two IR channels. I didn't flat field the images above so some artifacts are visible, especially in the sharpened movie. One problem with the VMC is that the spots and filaments that need to be removed using flat fielding change relatively rapidly so you really need one flatfield image per channel per orbit but this is not always possible. However, images are frequently obtained showing the north polar region at close range and some of these can be used for flat fielding.

One of the IR images shows haze above the limb. The other two IR images appear to show thermal emission from Venus' surface as they appear very dark without any enhancement (night side images).

A few of the images (V1584_0011_UV2, V1584_0015_UV2 and V1585_0010_UV2) show interesting wavy features. The 'waves' were also seen in earlier data but VEX has been able to image them at much higher resolution. Three types have been observed, long straight features, short wave trains and irregular wave fields. The montages above include images of all of these types. The waves are seen in all VMC channels. This means that they are not due to variability in the UV absorber distribution; they are probably caused by variations in the solar illumination angle near the cloud tops. The waves mainly occur in the high north and they may be associated with the Ishtar Terra highland region. This implies a possible orographic origin, i.e. these would then be mountain waves. This is far from certain though, in particular the waves might simply be easier to detect in the north due to the lower solar illumination angle. Interestingly, the VIRTIS instrument which can penetrate deeper in the atmosphere than VMC also detected waves at a greater depth, i.e. in the lower cloud layer.

The variable brightness in the UV images is caused by the presence of an UV absorber. Conversely, there is evidence that the much lower contrast variations in the visible and IR channels are caused by horizontal variations of total cloud opacity. Some features are observed in all filters but the patterns are not totally correlated.

Venus' appearance can change rapidly over a period of a few days. Changes include global brightness, spatial contrasts and the extent of the bright polar band. The polar band boundary varies and sometimes shifts to as low as 30° latitude. In January 2007 the brightness of the south polar region increased by ~30% over a period of four days. One of the montage images (V0267_0018_UV2.IMG) shows the south polar region when this was happening.

In addition to the VMC images, data from VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) might be interesting to process. VIRTIS observes at many wavelengths and is able to penetrate deeper into the atmosphere. Some of the publicly released VIRTIS images show interesting and spectacular atmospheric features.

The VMC data set isn't very difficult to process (IMG2PNG converts the IMGs to PNGs withoout problems) so if anyone does anything interesting with it please post here!