Scientists have long 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 formed about the same time, they might be expected to be more similar – by rights, Mars should be as big as Earth and Venus
But a paper published this week 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, the gas giant Jupiter went wandering in towards the Sun, on the way dragging comets from beyond the asteroid belt as Jupiter moved inwards towards Mars.
It 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 coalesced with Mars, giving it water in abundance 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.
Dr. Kevin Walsh, a research scientist at Southwest Research Institute, in San Antonio, Texas, led an international team performing computer simulations of the early solar system.
They showed how an infant Jupiter may have migrated to within 1.5 astronomical units (AU, the distance from the Sun to the Earth) of the Sun, stripping a lot of material from the region and essentially starving Mars of formation materials.
‘If Jupiter had moved inwards from its birthplace down to 1.5 AU from the Sun, and then turned around when Saturn formed as other models suggest, eventually migrating outwards towards its current location, it would have truncated the distribution of solids in the inner solar system at about 1 AU and explained the small mass of Mars,’ said Dr Walsh.
‘The problem was whether the inward and outward migration of Jupiter through the 2 to 4 AU region could be compatible with the existence of the asteroid belt today, in this same region. So, we started to do a huge number of simulations.
‘The result was fantastic.
‘Our simulations not only showed that the migration of Jupiter was consistent with the existence of the asteroid belt, but also explained properties of the belt never understood before.’
The asteroid belt is populated with two very different types of rubble, very dry bodies as well as water-rich orbs similar to comets.
Water is too volatile to have been present at Earth’s formation and must have been subsequently delivered from outer, colder parts of the Solar System .The water was probably delivered by planetoids and comets thrown out of the asteroid belt by Jupiter.
Walsh and collaborators showed that the passage of Jupiter depleted and then re-populated the asteroid belt region with inner-belt bodies originating between 1 and 3 AU as well as outer-belt bodies originating between and beyond the giant planets, producing the significant compositional differences existing today across the belt.
The collaborators call their simulation the Grand Tack Scenario, from the abrupt change in the motion of Jupiter at 1.5 AU, like that of a sailboat tacking around a buoy.
The migration of the gas giants is also supported by observations of many extra-solar planets found in widely varying ranges from their parent stars, implying migrations of planets elsewhere in universe.
The paper, A Low Mass for Mars from Jupiter’s Early Gas-Driven Migration, appeared in the June 5 issue of the journal Nature, written by Dr Walsh, Alessandro Morbidelli of the Université de Nice, France, Sean N. Raymond of Université de Bordeaux, France; David P.O’Brien of Planetary Science Institute in Tucson, Ariz.; and Avi M. Mandell of NASA’s Goddard Space Flight Center.
The research was funded by the Helmholtz Alliance, the French National Center for Scientific Research and NASA