After the European Space Agency's Mars Express orbiter made some pictures of the crater in 2004, some... people decided that the images are evidence for a Martian civilization. The claim is based on the pattern of regular features seen in an ESA publicity image of the crater, which conspiracy theorists interpret as, well, anything from crop fields to buildings. Unfortunately, the rectangular pattern is caused by image compression artefacts.
The image that is most usually cited as evidence of the claim is not a direct photograph of the surface, but a computer-generated image: it's a computer 3D-graphics rendering showing a "perspective view" of the crater, where the underlying 3D model is elevation data reconstructed from stereographic images and the texture mapped onto it is an image of the surface. Thus, the compression artefacts in the original image got propagated to the several renderings of the 3D model and were rotated and distorted in the process. And yes, the HRSC camera does compress its images to transmit them to Earth:
The data are compressed on-board on-line by JPEG-based discrete cosine transform (DCT) compression with average compression factors of 4–40 and with throughput rates of up to 450 lines per second in four parallel signal chains (each serving a few of the nine CCD lines). Bypassing of the compression is possible. The compression factor cannot be selected directly but is controlled by a quality factor. Therefore compression is dependent on texture, macro-pixel format and illumination conditions. Increased compression rates are also typically encountered in cases of high atmospheric optical depth as a result of dust and haze. The corresponding loss of surface contrast due to the increase in atmospheric light scattering can lead to significant reductions in the local dynamic range, increasing the compression rates. This phenomenon cannot be exactly predicted. High compression ratios may affect images by increasing the 8 × 8 pixel compression block pattern in low contrast areas of the image. (bold by RationalWiki)
Similar artefacts can be observed in Google Earth's images of areas with low contrast, for example this image of a river.
- Crater Hale in Argyre basin, Mars Express News, ESA, 24 November 2004. Clicking on the images opens a sub-page that provides information about them.
- HALE CRATER CIVILIZATION EVIDENCE, "Report #084", Mars Anomaly Research, March 2005
- Threads at the former Bad Astronomy and Universe Today Forum:
- "Unfortunately" because if evidence for a Martian civilization was uncovered, ESA, NASA and space exploration in general would receive a mighty
kick in the ass push forwardbudget increase. And because it would be cool.
- The high-resolution stereo camera (HRSC) experiment on Mars Express: Instrument aspects and experiment conduct from interplanetary cruise through the nominal mission, too many authors to list, Planetary and Space Science 55 (2007), pages 928–952 (full text PDF). Section 2.4. "Data compression".