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Atmospheres Another major difference between these planets is their atmospheres, or the layer of gases that surround them. Mercury has no atmosphere, and Mars has only a trace of one. But Venus has a thick atmosphere— much thicker than the one on Earth.

Venus is nearly the same size as the Earth, so its structure is probably very similar. But its core is probably slightly smaller and completely solid. Mercury, on the other hand, seems to have a very big core for its size, and only a thin layer of mantle. Mars has a smaller core but a thicker mantle.

∆ Mars might have a thicker crust than the other planets.

∆ Earth differs from the other planets in having

a core that is partly molten.

∆ The four terrestrial planets are tiny compared with Jupiter.