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Scientists have long puzzled over why Mars is only about half the size and one-tenth the mass of Earth. 

As next-door neighbors in the solar system formed about the same time, they might be expected to be more similar  - by rights, Mars should be as big as Earth and Venus

But a paper published this week provides an explanation and also reveals why the asteroid belt is such a strange assortment of rocks and ice.

Family portrait: Composite image of Venus, Earth and Mars shows how small the fourth planet from the Sun is in comparison with its larger siblings

While the solar system was forming, a process which began approximately 4.6 billion years ago, the gas giant Jupiter went wandering in towards the Sun, on the way dragging comets from beyond the asteroid belt as Jupiter moved inwards towards Mars.

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