Earth and Mars may have formed in fundamentally different ways, new planetary model suggests

Building a planet begins gently and ends violently. Dust and gas stick together, and over time enormous rocks slam into one another at speeds that are hard to picture, until what remains is a new world. Most scientists think Earth and Mars were built this way from rocky material circling the young sun about 4.5 billion years ago.

The two planets are the same age and drew on broadly the same ingredients, yet they ended up very different. Earth is biodiverse and wet. Mars is a cold, irradiated desert. A new study in the journal Nature Astronomy suggests part of the reason may lie in how each one was assembled.

The researchers built what amounts to a game for forging virtual planets. They gathered chemical clues from the rocks of both worlds and ran them through statistical models and computer simulations, repeatedly trying to produce an Earth and a Mars that matched the real ones. Their conclusion is that Mars was pieced together mostly from large, asteroid-like bodies, while Earth relied largely on a swarm of pebbles that gathered themselves together.

Scientists already had a hint that the two planets were put together differently: Mars has only one-tenth the mass of Earth. The usual explanation is Jupiter. Its immense gravity could have snatched many of the rocky building blocks from the path of embryonic Mars and flung them elsewhere.

The new work leans on a second clue, the chemistry of the crust and mantle. Ancient meteorites preserve the composition of the solar system's starting material, and compared with them, both Earth and Mars are short on moderately volatile elements such as sodium, potassium, lithium and zinc. If the planets had different origins, they would have lost those elements in different ways. Colliding rocks generate intense heat that cooks volatiles off into space, though as the colliding objects grow larger, some volatiles buried deep inside may survive. Objects built mostly from gently coalescing pebbles keep more of their volatiles in the early stages.

By that logic, the team calculates that Mars gathered 73% of its mass from collisions among larger, asteroid-like bodies, with the rest swept up as small pebbles. Earth, they say, probably got 75% of its mass from pebbles that clumped into two protoplanets, which then smashed into each other. A bombardment of smaller rocks came later. Anders Johansen, a planetary scientist at the University of Copenhagen and a study author, said the team was "genuinely surprised" that two neighbouring planets could form in two distinct ways.

The finding also touches on where Earth's water came from. Experts generally credit a mixture of comets and damp asteroids, but some have proposed that icy pebbles arriving from the outer solar system added to the supply. If Earth is mostly pebble-made, that idea gains support. Tim Lichtenberg, who leads the Forming Worlds Lab at the University of Groningen and was not involved, said the paper breaks new ground by combining larger objects and tiny stones in one coherent picture.

Outside experts were more guarded about the details. Not everyone agrees on the role of pebbles. Some argue Earth's chemistry makes no sense without them, others that pebbles alone cannot build Earth and Mars, and Jupiter's gravity is often cited as a barrier to pebbles drifting in from the outer solar system. Paolo Sossi of ETH Zurich said the model rests on several simplifications, so pinning down exact proportions "is out of the question," though Earth's volatile pattern strongly hints at multiple types of building blocks and pebbles could be allowed in Earth's formation. Conel Alexander of Carnegie Science called the paper intriguing but stressed that these elements are one piece of the puzzle and many routes could yield the same volatile mixes. "It's certainly not the final answer," he said, though the hybrid model deserves consideration, and he welcomed researchers taking opposing positions and duking it out.