Mercury’s sodium glue

Francisco Martin Leon Today 3 min
Image of the tail of Mercury taken by Andrea Alessandrini

Researchers have known for years that Mercury has a huge tail. In the past week, Andrea Alessandrini He photographed her from the balcony of his home in Veroli, Italy.

I took the picture on May 5 using a 66mm (2.5 inch) refracting telescope and a Pentax K3-II camera“says Alessandrini, an amateur astronomer who works by day as an aerospace engineer.”This is a 7 minute exposure at ISO 1000“.

First preceded in the 1980s, Mercury’s tail was discovered in 2001. Its origin is the super-thin atmosphere of Mercury.

Mercury is so close to the sun that the pressure of sunlight can push atoms out of the atmosphere into space. The escaping gas forms a tail more than 15 million miles long.

The key to detecting the tail of Mercury is sodium. There are many elements in the tail of Mercury; sodium is just one. But because sodium is so good at scattering yellow light, it is the best element for tracking the long column of gas. “I use a special 589nm filter tuned to the yellow glow of sodium,” says Alessandrini. “Without that filter, Mercury’s tail would be invisible.”

NASA’s MESSENGER spacecraft spent years observing Mercury’s tail closely. This movie shows how the brightness of the tail varies as Mercury orbits the sun:

For reasons that have to do with the Doppler shift of the sodium absorption lines in the solar spectrum, Mercury’s tail is brightest when the planet is ± 16 days from perihelion (the closest approximation to the sun). Read the investigation in “Final Reference”.

That special date is this week: On May 13, 2021, Mercury will pass 16 days after perihelion and the tail could be up to 10 times brighter than what Alessandrini saw last week. Coincidentally, that same day the crescent Moon will pass by Mercury in the evening sky.


Imaging the sources and full extent of the sodium tail of the planet Mercury. Jeffrey Baumgardner, Jody Wilson and Michael Mendillo. American Geophysical Union/ Geophysical Research Letters.…



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