Scientists from the Southwest Research Institute (SRI), in San Antonio (Texas), have developed a new model to understand the formation processes of planets that explains the difference in size between Earth and Mars, whose mass is only 10% of that of our planet. And it is that according to the theories to use on the formation of the Solar System, Mars should have had a size similar or even greater than that of the Earth. Now, researchers at the Southwest Research Institute believe that planets grow from tiny bodies called pebbles (pebbles, in English), and thus to develop would be the reason that Mars is much smaller than Earth. This same process also explains the rapid formation of gas giant planets like Jupiter and Saturn.
Traditionally, the fact that the mass of Mars is equivalent to only 10% of the mass of Earth has been a problem for Solar System theorists. According to the standard model of planetary formation, everything would have started from objects of similar sizes that come together and mix through a process called accretion : rocks incorporate other rocks and create mountains; in turn, mountains join to form ridges and so on.
This idea works well if the Earth and Venus are applied, but it fails if we think of Mars, which theoretically should be similar in size or even larger than Earth. Another problem with this theory is that it overestimates the global mass of the asteroid belt. However, SRI scientists Hal Levison, Katherine Kretke, Kevin Walsh and Bill Bottke have shown that the structure of the inner Solar System is the natural result of a different process of planet growth . They believe that early cosmic dust grows rapidly to form pebbles just a few inches in diameter, some of which are pulled together by gravity to form objects the size of asteroids.
Under certain conditions, these primordial asteroids can feed efficiently on excess pebbles as aerodynamic drag pushes them into orbit, spiraling downward, and merging with the growing planetary body. This is why some asteroids reach the size of planets on relatively short timescales . But not all primordial asteroids would be equally well placed to grow from pebbles.
For example, an object the size of Ceres (about 1,000 km in diameter), which is the largest asteroid in the belt, would have grown very rapidly near the position that Earth currently occupies, but would not have been able to grow efficiently near the current position of Mars or further away as the aerodynamic drag is too weak to capture pebbles. For Kretke, “that means that very few pebbles collide with objects near the current position of Mars and explains why the red planet is so small.”
