Pluto used to be the smallest planet, but it’s not a planet any more. That makes Mercury the smallest planet in the Solar System. The second smallest planet in the Solar System is Mars, measuring 6792 km across.
With all the focus and exploration of Mars, you’d think it’s a really big planet, but actually it’s pretty small. Mars has only 53% the diameter of Earth, and about 1/10th the mass. It only has 15% the volume of Earth. In other words, you could fit 6 planets the size of Mars into Earth, and still have room to spare.
Since Mars is relatively small compared to Earth, and it has a fraction of our planet’s mass, the force of gravity on Mars is very low. If you could walk on the surface of Mars, you would experience only 38% the force of gravity you feel pulling you down on Earth. In other words, if you weighed 100 kg on Earth, you’d feel like you only weighed 38 kg on Mars.
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Mars is so small that its core cooled down billions of years ago, and so it doesn’t have a magnetic field any more. Earth’s magnetic field helps push away the Sun’s solar wind, which is trying to carry away our atmosphere. When you match this with its very low amount of gravity, and Mars has lost almost all its atmosphere. The pressure of the atmosphere on Mars is 1% what we experience on Earth.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.