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Why is Mercury so cold if it is so close to the Sun?


Mercury, with a diameter of 4,879 km (the equivalent of 0.38 Earths), is quite an amazing planet. It is the smallest of the four inner planets in the solar system and also the closest to the Sun (it is 57 million kilometers away). For this reason, we would imagine that the temperature on the surface, given its proximity to our star, would have to be scorching. Well, it is not like that (Venus is the hottest planet in our system).


A wide range of temperatures

It has both hot and cold temperatures. To begin with, its surface temperature can vary from a frigid -180 ºC to a hot +430 ºC and its peculiarities are due, among other things, to the fact that its orbit presents the greatest eccentricity in the entire solar system, since at its most away from the Sun is at a distance 1.5 times greater than at its closest point to the star. Mercury’s temperature changes are the most drastic in the entire solar system.

Impact? Very varied: in the polar areas there are corners where the Sun never reaches and, therefore, they are in a night or permanent darkness. On the other side, as our readers will imagine, there are areas where the Sun never stops giving them; regions, at Mercury’s equator, that are continually exposed to direct radiation from our star when the planet is closest to the Sun.

It is, therefore, the planet that receives more sunlight per square meter than any other in the solar system. While the side of the planet facing the sun can be sweltering, the other side can drop as low as -173 degrees Celsius.

And it is curious that the planet that bears the brunt of the Sun’s energy more directly than any other, harbors ice. But we have already seen that proximity to the Sun does not completely determine planetary heat. Mercury has a really weak atmosphere (composed of oxygen, sodium, hydrogen, helium, calcium, and potassium), but it’s so thin that it’s almost indistinguishable from a vacuum. That of Mars, to give an example, which is quite faint, is still more palpable than that of Mercury.

An atmosphere like the one we have on Earth acts like a bubble, retaining heat and spreading it everywhere. Because it doesn’t have this resource, the front and back of Mercury can have very different temperatures. And there’s no precipitation or storms on Mercury, so it’s very hot and very cold.

And there is ice!

In the dark craters of Mercury, the planet not only houses ice, but makes it. A study published in the Astrophysical Journal Letters showed a feasible way for water to emerge on the planet and accumulate as ice.

Mercury rotates very slowly, so its night side spends a lot of consecutive time in the dark, protected from the sun; hence its temperature now drops to -173°C: it’s an incredibly cold temperature, much colder than any known natural temperature here on Earth.

Referencia: A New In Situ Quasi-continuous Solar-wind Source of Molecular Water on Mercury. B. M. Jones, M. Sarantos, and T. M. Orlando. Published 2020 March 16 • © 2020. The American Astronomical Society. The Astrophysical Journal Letters, Volume 891, Number 2 Citation B. M. Jones et al 2020 ApJL 891 L43

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