If you stood on Mercury at noon, the ground beneath your feet would be hot enough to melt lead. If you stood in the same spot at midnight, the ground would be colder than anywhere on Earth has ever been. Mercury swings between two extremes that would each kill you instantly, and the swing is bigger than anywhere else in the solar system.
So how does the closest planet to the Sun manage to be both the second-hottest and one of the coldest places we know about? Let's go through it.
How Hot Does Mercury Get?
During the day, Mercury's surface reaches 800°F (430°C). For comparison, the hottest temperature ever recorded on Earth is 134°F (57°C), in Death Valley, California. Mercury's day side is about six times hotter than the worst desert on Earth.
800°F is enough to melt lead. It's enough to melt tin, zinc, and most plastics. It's not quite hot enough to melt iron, gold, or aluminum — but it's getting close.
Mercury gets that hot because it's so close to the Sun. From Mercury's surface, the Sun would look about three times bigger than it does from Earth, and roughly seven times brighter. There's no atmosphere to filter that sunlight. Energy from the Sun — called solar radiation — just hammers down onto bare rock with nothing to slow it down.
How Cold Does Mercury Get?
At night, Mercury's surface plunges to about -290°F (-180°C). The coldest temperature ever recorded on Earth is -129°F (-89°C), at a research station in Antarctica. Mercury's night side is more than twice as cold as that.
That's a temperature swing of nearly 1,100°F (600°C) between day and night. No other planet comes close. Earth, by comparison, swings about 50 to 100 degrees Fahrenheit between day and night, depending on where you live.
Why Such Wild Temperature Swings?
The answer is air — or rather, the lack of atmosphere.
Earth's atmosphere does two big jobs every day. In daytime, it scatters and absorbs some sunlight before it reaches the ground, so we don't get cooked. At night, it traps heat near the surface like a thick blanket, helping us retain heat through the night. Without an atmosphere, Earth's surface would swing wildly between hot and cold every day, just like Mercury's does.
Mercury has almost no atmosphere. What it does have is called an exosphere, and it's so thin it's basically a vacuum. So during the day, sunlight blasts directly onto the surface with nothing to slow it down. And the moment the Sun sets, all that heat radiates straight back into space — nothing there to retain it. There's no blanket.
It's also worth knowing that Mercury's day is incredibly long. Mercury's rotation — one full spin on its axis — takes 59 Earth days, and from sunrise to sunrise takes 176 Earth days. So one side of the planet bakes for almost three Earth months while the other side freezes for the same length of time. There's plenty of time for things to get extreme. Read about Mercury's strange day length.
So Why Isn't Mercury the Hottest Planet?
This is one of the great surprises of the solar system. Mercury is the closest planet to the Sun, but it's not the hottest. Venus is.
Venus is the second planet from the Sun, almost twice as far away as Mercury. But Venus has a thick, heavy atmosphere made mostly of carbon dioxide. CO2 is a powerful greenhouse gas — it traps heat the way a glass greenhouse does. Sunlight comes in, hits the surface, and tries to bounce back out. The carbon dioxide stops most of that heat from escaping.
The result: Venus's surface stays at 870°F (465°C), day or night, all the time. That's hotter than Mercury's hottest noon, and it stays that hot every minute of every day for thousands of years.
Mercury, with no atmosphere to hold heat in, just dumps all its warmth back into space the moment the Sun sets. So Mercury is hotter than Venus during peak daylight in some spots — but Venus stays much hotter on average.
The Coldest Spots on a Burning Planet
Here's the real twist. Mercury's poles are home to some of the coldest places in the entire solar system.
Mercury barely tilts on its axis — only about 2 degrees, compared to Earth's 23.5 degrees. Because of that, the floors of the deepest craters at Mercury's north and south poles are in permanent shadow. The Sun has never shone on those crater floors and never will. The temperature there stays below -280°F (-170°C), all the time.
Scientists call these spots "cold traps," because anything cold that ends up there stays cold forever. And what they trap is water ice. Mercury, the closest planet to the Sun, has frozen water hidden inside its dark polar craters. NASA's MESSENGER spacecraft confirmed this in 2012, and the BepiColombo mission photographed those shadowed craters again in January 2025.
It's one of the most surprising facts about Mercury. Read about water ice on Mercury.
Why This Matters
Mercury's hot/cold paradox is a perfect lesson in how an atmosphere shapes a planet. Earth's air is the reason our days don't fry us and our nights don't freeze us. Venus's air is the reason it's the hottest planet in the solar system. And Mercury, with almost no air at all, shows what happens to a world that has nothing to slow down sunlight or hold in heat.
If you stripped away Earth's atmosphere tomorrow, our planet would behave a lot more like Mercury — baked on the day side, frozen on the night side, sky gone black at noon. The thin shell of air around our planet really is doing a lot of work.
Frequently Asked Questions
Why is Mercury so hot during the day?
Mercury sits very close to the Sun, so sunlight hits the surface much harder than it does on Earth. Mercury also has no atmosphere to filter or absorb that sunlight, so the bare rock heats up to 800°F (430°C).
Why is Mercury so cold at night?
Mercury has almost no atmosphere, so the moment the Sun sets, heat escapes straight into space. Earth's atmosphere works like a blanket that holds heat in at night. Mercury has no blanket, so the surface drops to -290°F (-180°C).
Is Mercury the hottest planet in the solar system?
No — Venus is hotter. Venus has a thick atmosphere made of carbon dioxide that traps heat like a greenhouse. Venus stays around 870°F (465°C) day and night, hotter than Mercury's hottest noon.
How big is the temperature swing on Mercury?
About 1,100°F (600°C) between day and night. That's the biggest day-to-night swing of any planet in the solar system.
Why is the sky black on Mercury during the day?
Earth's sky looks blue because our atmosphere scatters sunlight. Mercury's atmosphere is far too thin to do this, so even at noon the sky stays black with the Sun blazing in the middle and stars visible all around.
Could humans survive Mercury's temperatures?
Not without serious technology. The 800°F day side would cook a human almost instantly. The -290°F night side would freeze them just as fast. Any human visit would need a fully sealed, heavily shielded habitat.