Earth and Mars, despite being neighbors, may have grown from materials assembled in very different ways.
Rocky planets formed about 4.5 billion years ago in the disk of gas and dust surrounding the young Sun. But researchers are still debating the size of the objects that supplied most of their material. Two main mechanisms are being considered.
Earth and Mars to scale.
In the first, bodies several kilometers across called planetesimals collide and gradually assemble. In the second, small fragments, often called pebbles, are slowed by the disk's gas and then captured by planetary embryos. The two mechanisms can also work together.
To distinguish between them, an international team studied the volatile elements of Earth and Mars. These elements evaporate relatively easily when a material is heated. Their abundance and isotopic composition thus preserve a record of the conditions under which a planet's materials were assembled.
The researchers built a model combining planetary growth with the evolution of these chemical signatures. According to their results, more than 75% of Earth's mass could have come from protoplanets that themselves grew mainly by capturing small pebbles. Collisions between large bodies would then have completed the assembly of our planet.
Mars would have a nearly opposite history. The model attributes about 73% of its mass to the accretion of planetesimals. The Red Planet would therefore have retained more of the signature of growth through collisions between relatively large objects, without going through the same dominant phase of pebble capture as Earth.
This difference could notably stem from the location and pace of formation of the two planets. The available materials, their temperature, and the movements of the gas varied with distance from the Sun. Two neighboring rocky worlds could therefore draw from different reservoirs and grow at different rates.
The researchers argue for hybrid accretion, in which pebbles and planetesimals both contribute, but in different proportions depending on the planet.
This method can now be applied to other bodies in the Solar System. Meteorites from asteroids, lunar samples, and future rocks returned from Mars will make it possible to multiply chemical comparisons and test whether each rocky planet has its own combination of formation mechanisms.