The presence of water isn’t the only Mars mystery scientists are keen to probe. Another centers around a seemingly trivial characteristic of the Red Planet: its size. Classic models of solar system formation predict that the girth of rocky worlds should grow with their distance from the sun. Venus and Earth, for example, should exceed Mercury, which they do. Mars should at least match Earth, which it doesn’t. The diameter of the Red Planet is little more than half that of Earth (1, 2).
“We still don’t understand why Mars is so small,” says astronomer Anders Johansen, who builds computational models of planet formation at Lund University in Sweden. “It’s a really compelling question that drives a lot of new theories.”