Scientists have long been puzzled over why Mars is only about half the size and one-tenth the mass of Earth. As next-door neighbors in the solar system that formed around the same time, they might be expected to be more similar - Mars should be as big as Earth and Venus.
Researchers had created a computerized model of the formation of the solar system. Test results correlated with the sizes of Mercury, Venus and Earth -- but Mars consistently showed up much more massive than what it really is. There had to be another force at play.
A recent study provides an explanation and also reveals why the asteroid belt is such a strange assortment of rocks and ice.
While the solar system was forming, a process which began approximately 4.6 billion years ago, scientists believe that the gas giant Jupiter went wandering in towards the Sun, getting as close as within 1.5 A.U distance from it (1 Astronomical Unit or A.U is the distance from Sun to Earth). It dragged comets from beyond the asteroid belt as it moved inwards towards Mars.
Jupiter then moved out again to gravitate towards another gas giant, Saturn, as it formed, this time dragging dry rocks from inside the belt as it moved back outwards again. All this material would otherwise have combined with Mars, giving it an abundance of water and the mass to retain it with its gravitational pull. Instead it ended up as the small, dry and probably dead planet we see today.
The movement of gas giants is not new and has been observed in planets outside our solar system and their varying distances from their parent stars.
Asteroid Belt: Now we know!
This theory also explains the finding of two kinds of rocks in the asteroid belt between Mars and Jupiter -- a combination of water-rich bodies as seen in the outer solar system, and dry bodies of rocks from the inner solar system. Scientists have long believed that Earth could not have had water during the time of its formation -- any water would have evaporated from the intense heat. Water would have been brought to Earth by colliding asteroids as we wrote here previously.
Today, Jupiter resides at 5.2 A.U from the Sun and the asteroid belt lies between 2 to 4 A.U. All the gaseous giants are between 5-30 A.U, and beyond Neptune lies the Kuiper belt -- a belt of icy orbs made up of water, methane and ammonia.
With this one finding, scientists have now been able to explain the size of Mars, the strange composition of the asteroid belt and how icy rocks from the Kuiper belt may have made it into the asteroid belt!