Because it was starved during infancy, says Kevin Walsh (Observatoire de la Cote d’Azur and Southwest Research Institute, Boulder). In a paper in Nature, written with four colleagues and published June 5, 2011, he argues that when the planets were first forming, Jupiter migrated inward toward the Sun about as far as the present orbit of Mars, or 1.5 astronomical units (AU). (Earth orbits the Sun at 1.0 AU, the asteroids between 2.0 and 3.2 AU, and Jupiter currently is at 5.2 AU.)
The team’s computer simulations show Jupiter formed within a few million years at a time when the young Sun was surrounded by a gas-rich disk of pre-planetary debris. Over about 100,000 years, however, interactions with the gas in the disk caused Jupiter (and Saturn, Uranus, and Neptune, all lying farther out) to drift inward until Jupiter reached about where Mars now orbits the Sun.
The inward migration of Jupiter quickly reduced the available rocky, dusty, and gaseous material for making Mercury, Venus, Earth and its Moon, and Mars. The team’s simulations show that once Jupiter’s powerful gravity had largely cleared the inner planetary disk, it and the other gas-giant planets reversed their inward drift and moved outward to their current locations over the next half-million years. Left behind was a comparatively sparse and rocky inner planetary disk. Out of such remnants, they say, came the inner planets, a much thinned asteroid belt, and a stunted Mars.
Another report published by a different group shortly before dovetails with this finding. Nicolas Dauphas (University of Chicago) and Ali Pourmand (University of Miami) calculated the age of Mars using the radioactive decay of hafnium to tungsten in meteorites from Mars and from the asteroid belt. They found that Mars formed in only 1 to 2 million years, and is mostly likely a “stranded planetary embryo.”
Says Walsh, “A fast-forming Mars and a small Mars go hand in hand.”