Smallest planet
Mercury is only a little wider than Earth's Moon, but it is a full planet because it orbits the Sun and is round from its own gravity.
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Planet lesson
Mercury
Rocky planet
Mercury is the smallest planet and the closest planet to the Sun. It has a rocky surface covered with craters.
Kid fact
Mercury has no moons and no thick air to protect it.
On the Edge of Mercury
Day
58.6 Earth days
Year
88 Earth days
Moons
0
Temperature
-173 to 427 C
Fast year
Mercury is closest to the Sun, so it has the shortest path around it. A Mercury year lasts only 88 Earth days.
No air blanket
Mercury has almost no atmosphere to hold heat or slow space rocks. That is why its surface has many craters and huge temperature changes.
What spacecraft saw
NASA's MESSENGER spacecraft mapped Mercury and found long cliffs, smooth plains, and signs that ice can hide in shadowed polar craters.
Look closely
In images, look for gray crater circles. Ask: which side would be hot in sunlight, and which places might stay cold in shadow?
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