Our story would have ended there were it not for a scientific development at Mars in the years to follow. In 2008, John Moores (then of the University of Arizona’s Lunar and Planetary Laboratory) adapted and developed a technique for the Phoenix Lander that had previously been pioneered by Mark Lemmon of Texas A&M University for use with the Mars Exploration Rovers. This technique was able to discern fine details in pictures of the sky, not visible to the naked eye, by acquiring several images over several minutes.

The technique, dubbed “mean frame subtraction” relies on the high quality of the raw images returned by spacecraft. While often rendered as JPEGs, there are more than just 256 shades of grey in these images. By removing the parts of the image that stay the same over the entire sequence of frames, it is possible to observe only those features which change from frame to frame—including dust devils, cosmic-ray strikes, and especially very thin clouds.

This was a bonanza for the Phoenix Mission, showing thin cloud features even before the first thick clouds were identified on sol 94 (for upward pointing movies, see here; for movies aimed at the horizon, see here). Later the same technique was successfully applied to the clouds of Gale Crater, which are exceptionally thin (see the complete set of movies here).

Being planetary scientists interested in atmospheres across the solar system, we saw the potential to observe changes in the sky over that hour or so that Huygens continued taking images and wondered what might be lurking in this old data. We thought there was, perhaps, an opportunity to extract lower level clouds from the surface, complementing the stunning imagery of clouds that Cassini was obtaining from orbit. While no change was obvious to the naked eye, we thought we could try to extract more from the pixels above the horizon using our mean frame subtraction technique which had been so successful on Mars.