Dawn VIR mosaic


I haven’t written anything since a long time. I must say that since my trip to Chicago this summer, I didn’t have much new things to show. But data from the Dawn mission have finally been released (download from here). I started to create a mosaic of VIR images (VIR is the spectrometer onboard the probe) to study the composition of asteroid Vesta‘s southern hemisphere.

The preparation of the mosaic required more time than I initially thought. After the initial processing of the data using ISIS3, an USGS software commonly used in planetary exploration, I had first to put together data from both the visible and the near-infrared detectors. Then, before creating the mosaic itself, I also had to change the spatial resolution of every images so that the resolution is the same everywhere. Indeed, the spatial resolution is directly related to the probe’s altitude at the image acquisition time and thus the resolution changes over the mission duration.

I had a hard time for this step as I used some particular IDL/ENVI functions for the first time. And those functions are not always intuitive! I wrote two small programs to automatize the use of ENVI_LAYER_STACKING_DOIT and MOSAIC_DOIT. Everything is available on the programming page of the website. Maybe it will be useful for someone one day….

Mosaic Dawn

Left: Dawn Framing Camera mosaic of Vesta’s southern hemisphere (NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)
Right: Dawn VIR mosaic over the same region

Thanks to that, I was able to produce a VIR mosaic including all the available images in the southern hemisphere (see above figure) and I started to look at the surface composition. It’s relatively easy to get a nice pyroxene signature! And this spectral signature is similar to the one observed for some HED meteorites, consolidating the idea that those meteorites come from Vesta.

Spectre Vesta

Comparison between a VIR spectrum of Vesta (black), an HED meteorite spectrum (red) and two pyroxenes laboratory spectra (pigeonite in blue and augite in green).

For more details on Vesta’s surface mapping, I recommend the paper by Ammanito et al. (2013) in Meteoritics & Planetary Science.