While Mars has only 10 per cent of the mass of the Earth, conventional formation models generate analogs to Earth and Venus, but predict that Mars should be of similar-size to Earth. Image Credit: NASA/JPL/MSSS
A study into planet formation may have solved a longstanding question as to why Mars is so much smaller than Earth.
The theory operates on a model called Viscously Stirred Pebble Accretion (VSPA), in which space dust assimilates and grows into pebbles a few inches in diameter, some of which then gravitationally collapse to form asteroid-sized objects.
These bigger bodies continue to grow as the remaining pebbles are pulled into their orbit via aerodynamic drag and eventually spiral into planetary body’s surface.
In this model, some asteroids can grow into the size of planets relatively quickly.
This model is contrary to the ‘accretion’ method of planetary formation, in which space rocks fuse with other space rocks, eventually forming mountains and growing into the size of planets.
However, according to the accretion theory, Mars should be of a similar size to Earth and Venus.
The VSPA theory, its authors say, solves this conundrum.
"Understanding why Mars is smaller than expected has been a major problem that has frustrated our modeling efforts for several decades," says Hal Levison, an Institute scientist at the SwRI Planetary Science Directorate and author of a new study on the model.
"Here, we have a solution that arises directly from the planet formation process itself."
The VSPA model shows that not all asteroids involved in the process can grow at the same rate: rather, it depends on the planetary body’s position.
The study says that an object the size of Ceres, a dwarf planet in the asteroid belt, would have grown much quicker if it had been positioned near Earth, but would not have been able to grow as quickly near Mars because the aerodynamic drag is too weak.
"This means that very few pebbles collide with objects near the current location of Mars.
That provides a natural explanation for why it is so small," says co-author Katherine Kretke.
"Similarly, even fewer (pebbles) hit objects in the asteroid belt, keeping its net mass small as well.
The only place that growth was efficient was near the current location of Earth and Venus."
The team behind the study says the discovery, if true, has implications for theories behind the formation of the Kuiper Belt on the outer fringes of the Solar System.
Previous models predicted that the belt originally contained a couple of Earth-masses' worth of material and that planets began to grow there.
However, this new model suggests that the belt never contained much mass in bodies like the currently observed asteroids.
"As far as I know, this is the first model to reproduce the structure of the solar system -- Earth and Venus, a small Mars, a low-mass asteroid belt, two gas giants, two ice giants (Uranus and Neptune), and a pristine Kuiper Belt," says Levison.