Tuesday, April 13, 2010
Mercury in STEREO H1b
This is Mercury as seen in the STEREO spacecraft H1b imager (8-4-2010 left, 9-4-2010 right), if you think it looks vaguely cometary, you are right. Mercury has a tail. Now the question is, is the tail real, or an imaging artefact. Now Comet Al, myself and a few others have been following these apparent tails in the H1a imager. (the first time I saw it, I thought it was a comet).
We are pretty convinced we're seeing the sodium tail of Mercury, where the solar wind blasts sodium ions off Mercury's surface. However, proving this is hard. You can only see the tail when Mercury is at inferior conjunction (Ie when most of Mercury's surface is dark, and it's light doesn't overwhelm the image), and the orientation of the tail changes as Mercury approaches the Sun, and you can see it im multiple conjunctions over the years. Seeing the tails in the H1b imager, in the correct orientation, makes it even less likely that this is an artefact (first spotted by Jiangao Ruan, congratulations we've been searching for this for a while).
You can download an avi of Mercury showing the tail on 8 April 2010 (caution 1.6 Mb) here.
Animated Mercury with tail in YouTube, unfortunately, compared to the avi, it doesn't work very well.