Lovers of the sky, this Monday, November 11th is feast day. Mercury is visiting us. The smallest planet in our solar system will pass very slowly in the afternoon in front of the Sun. In metropolitan France, the show starts at 1:35 pm and ends approximately five hours later. A rare event, visible everywhere where the sky will be cleared.
This "transit," as astronomers say in their jargon, occurs only thirteen to fourteen times a century. The next is for November 13, 2032. Stephanie, a Parisian, attended, taped, on her last visit on May 9, 2016: "It was very disturbing. When you see this tiny black ball progress to a Senator's train between the Sun and the Earth, you really feel angry in the universe, it relativizes everything.
"Do not try to follow the show with the naked eye"
To taste – safely – at this quasi-metaphysical moment, be careful! It is imperative to have a telescope with solar filter or a solarscope to safely observe the sun by projecting its inverted image on the screen. "Do not try to follow the show with the naked eye," warns the Observatoire de Paris, which retransmits the event on its website, "first, because by fixing the Sun without special protections, you risk serious eye damage, then, because you are going to white cabbage. Even with eclipse glasses, you will not see anything. Mercury is way too small.
And it is true that it has everything a chip, this little telluric planet, three times smaller than the Earth. Why are we celebrating this passage so much? Mercury is the closest planet to the Sun – comparatively, the Earth is three times farther away – which has made it very difficult to observe. Too many dazzling!
A small but heavy planet
Among the four rocky planets (Venus, Earth, Mars, Mercury) that account our solar system, it is also the one we know least well. So far two probes of NASA (Mariner 10 in 1974, then Messenger in 2011) risked themselves in the furnace of its orbit. Thus "98% of its surface could be mapped, but the data collected gave us many more questions of answers", explains Océane Barraud at the Observatoire de Paris.
That's all intriguing at Mercury. Normally, given its measurements, it should be "a featherweight". According to the commonly accepted telluric planetary formation scenarios, the smaller a planet is, the lighter it must be. But not at all ! Mercury is a big chump! A real petanque ball, which relegates to the rank of simple tennis ball the Earth, yet much larger!
Still very mysterious
Such a density implies that it has a hypertrophied core very rich in heavy metals, occupying 40% of its volume. "But if that's the case, we should find large amounts flush on its surface, but Messenger has not detected any trace of titanium. It even showed that on Mercury, there is less iron than on Earth, "says the scientist. To this first great oddity, adds another, even more enigmatic: the presence of a magnetic field around Mercury.
Earth, three times farther from the Sun than Mercury, has one too. And fortunately ! Without this shield that protects us from the electric anger of the Sun, all our computers and our electrical networks would fall back, sending us back to the Middle Ages. But Mercury, she should not have a long time.
An open-air laboratory
"It's a planet whose gray surface is very lunar, pitted with multiple craters, in short, everything indicates that we are in the presence of a very old body," says the scientist. It was therefore expected to find a sleeping object, whose nucleus would have solidified for a long time, becoming inactive. But no! ". Where does this magnetic field come from? Is it just an old relic or is it still working? In short, is Mercury still alive?
For scientists, this mystery is a boon. The boom in IT and electrical infrastructure has made us very vulnerable to the effects of solar storms. With Mercury, they found an open-air laboratory, to understand how an anti-radiation shield lives and dies, or not! Parties last year from Kourou, Guyana, two probes, one Japanese, the other European, are en route to Mercury to play detectives. Patience. To reach the goal, they have "only" five years of travel left.
The Paris Observatory retransmits the show on its website and opens the doors of its Meudon site (Hauts-de-Seine) from 12:30 to 18:00. Mobile telescopes equipped with special filters will be installed. National terraces of the observatory, 5 place Jules-Janssen, 92195 Meudon.