And what would that be?
First predicted in the 1980s, Mercury’s tail was discovered in 2001. Its source is Mercury’s super-thin atmosphere. Mercury is so close to the sun, pressure from sunlight itself can push atoms out of the atmosphere and into space. The escaping gas forms a tail more than 24 million km long.
The tail is rich in sodium–a substance sputtered from Mercury’s surface by solar wind and micrometeorite impacts. That’s why [Dr. Sebastian] Voltmer’s yellow sodium filter did such a good job revealing the gaseous stream. [spaceweather.com]
There’s a lovely backyard picture of it at the above link, but I shan’t steal it.
It makes sense, of course, but that with the addition of a yellow filter your telescope can see it caught me by surprise. And pleases me.
