Why are Venus and Earth about the same size, as shown in the NASA image above, but Mars is definitely smaller than both?
"Planetary scientists have long wondered why Mars is only about half the size and one-tenth the mass of Earth. As next-door neighbors in the inner solar system, probably formed about the same time, why isn’t Mars more like Earth and Venus in size and mass? A paper published in the journal Nature this week provides the first cohesive explanation and, by doing so, reveals an unexpected twist in the early lives of Jupiter and Saturn as well."
The authors of the paper did hydrodynamic simulations that best reproduced the relatively small mass of Mars by assuming a starting disk of material whose outer edge was about where the Earth's orbit is now. They attribute this to an inward migration of Jupiter to about 1.5 times this distance early in the Solar System's history followed by a later outward migration to about 5 times the Earth's orbital radius. It also explains some of the properties of the extant asteroid belt.
The paper is available on the Nature website.
