What would happen to temperature on Mercury during the day if a sunspot was directly in line with its surface?

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If a sunspot were pointed right at Mercury in this way, the planet would become even more mind-bogglingly hot. Sunspots look dark in our visual spectrum, so we might suspect that they are colder (or at least less hot) than the rest of the Sun; but in fact they are...

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If a sunspot were pointed right at Mercury in this way, the planet would become even more mind-bogglingly hot.

Sunspots look dark in our visual spectrum, so we might suspect that they are colder (or at least less hot) than the rest of the Sun; but in fact they are hotter. They emit even more energy than "normal" Solar surface; it's simply in the form of UV radiation and charged particles that our eyes cannot see.

We still don't really understand what causes sunspots or how to predict them; but we do know that several major climate events in Earth's history have been triggered by particularly high or low levels of sunspots.

Mercury already reaches temperatures over 700 Kelvin (290 Kelvin is comfortable, 310 Kelvin is hot---700 Kelvin is approaching the melting point of aluminum), so with a sunspot pointed right at it, it might well exceed 735 Kelvin, making it for a brief time even hotter than Venus.

But since Mercury has no atmosphere, it cools off very quickly, and by midnight that same spot on the planet would actually become extremely cold---as cold as 100 kelvin at the surface, which is cold enough to free CO2 into dry ice.

Mercury's temperature swings wildly; at noon you can melt aluminum, but at midnight you get dry ice. Of course, that's partly because the day is longer than the year.

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