Skip to the timeline
Let's go to Mars
c. 4.56 billion years ago

Mars accretes fast, then stops growing

Hafnium–tungsten dating puts Mars at half its present size within two million years of the solar system's birth. Then it quit growing, at a tenth of Earth's mass.

Illustration of a flat disc of dust and rock around a bright young star, with larger bodies scattered through the plane.
A protoplanetary disc, illustrated. Mars assembled from a plane like this one and stopped early, leaving it a tenth of Earth's mass. · Image: Merikanto

Mars was half its present size before the solar system was two million years old. Hafnium-182 decays to tungsten-182 with a half-life of about 8.9 million years. Tungsten follows iron down into a planet's core while hafnium stays behind in the rock, so the tungsten-182 a mantle gains afterwards dates the core's closing. In 2011 Nicolas Dauphas and Ali Pourmand ran that clock on martian meteorites and got roughly 1.8 million years. Earlier attempts had ranged from 0 to 15 million, because nobody could pin down the martian mantle's hafnium-to-tungsten ratio; they fixed it at 3.51 ± 0.45. Earth was still assembling tens of millions of years later, and the impact that made its Moon landed 50 to 150 million years in.

The planet that resulted masses 6.42 × 10²³ kg, 10.7 percent of Earth, with a radius of 3,390 km. In the standard picture of rocky planet formation, embryos of that size keep colliding and merging until a few large survivors remain. Mars did not: it is an embryo preserved at the size Earth and Venus passed through.

Why it stopped is not settled. Accretion simulations produce a Mars several times too heavy, the "small Mars problem". The best-known fix is the Grand Tack, published by Kevin Walsh and colleagues in 2011: Jupiter migrates inward through the gas disc to about 1.5 AU, reverses when Saturn catches up and locks into resonance, and the round trip leaves the planetesimal disc truncated at roughly 1 AU. Earth's feeding zone survives; Mars's is gutted. The model is contested: it demands finely tuned timing for Saturn's growth, and rival models produce a small Mars from a disc simply depleted at its outer edge, with no giant-planet excursion.

Surface gravity is 3.72 m/s², 38 percent of Earth's, which makes Mars far worse at holding onto gas molecules. A small body also carries much surface area for its volume and sheds internal heat quickly, which sets how long a molten core can keep stirring.