Mars remains a true puzzle, but not for the reasons most people would think. Sure, there’s debate over whether it ever had surface water, oceans and life. But Mars’ small mass relative to earth and Venus have been a major conundrum that has plagued planetary scientists for decades.
Because the red planet’s mass is roughly only a tenth that of earth, it prompted Carnegie Institution planetary scientist George Wetherill to dub it the ‘small Mars’ problem. There have been several hypotheses to try and explain why Mars ended up so much smaller than earth and Venus. But when hard pressed, few planetary scientists are willing to go to the mat for any of them.
If there is enough material in the disk to grow something the size of earth and Venus, it’s not clear why there also wasn’t the same amount of material near Mars so that it might grow to be the same size, Matthew Clement, a planetary scientist at Johns Hopkins University’s Applied Physics Lab in Maryland, told me via email.
Is Mars’ small mass the key to why the red planet could never hold on to its water and ultimately never remain habitable?
Mass is the single most fundamental property of a planet, Sean Raymond, an astrophysicist at the Bordeaux Astrophysical Laboratory in France, told me via email.
Mars’ geological history and loss of water both stem in large part from its mass, he said.
Mass Matters
It certainly is a primary factor, along with its distance from the Sun, Stephen Kane, a planetary geophysicist at the University of California in Riverside, told me via email. Mars’ small size severely limited both the longevity of its geological and volcanic activity that replenished the atmosphere, he said. Its small mass also made it susceptible to being stripped of its atmosphere by the solar wind, said Kane.
Mars reached half its mass very early on in the history of our solar system.
Basically, whatever the source of the size difference between the two planets had to be in place very early on in the solar system’s history, probably within just a few million years, said Clement.
Why is Mars so different in mass than earth?
There are three leading hypotheses as to why.
The so-called Grand Tack Model features our very early Jupiter migrating inward toward the Sun before —- somewhat akin to the movement of a sailboat —- tacking back outward in the opposite direction. In the process, our young Jupiter disrupted the planetary building blocks that would have allowed Mars to form a larger mass planet.
Without Jupiter (and its migration), Mars may well have been the size of Venus and earth, or even bigger, Kane said.
The low-mass Main Asteroid Belt model in which Mars had very little material in its vicinity from which to grow larger in mass.
The early instability model in which the early inner solar system was basically such a chaotic, orbital mess that the building blocks for rocky planets beyond the orbit of earth and Venus were too perturbed to coalesce into any larger than Mars.
In truth, each of these hypotheses likely played a role in keeping Mars small. But it will likely take much more sampling of our solar system’s Main Asteroid Belt to pinpoint which, if any, of these ideas holds the most promise.
Why is solving this conundrum important?
The inner solar system contains four vastly different planets, and we still don’t really understand why, said Clement. Is it that something went wrong for Mars, he asked, or did something just fortuitously go right for the Earth?
Forming Planets Out Of Dust
Mars’ small mass is just a byproduct of how the Solar System transformed dust into planets, said Raymond. Learning more about this process — through both theoretical modeling and continued observations of dust around young stars, where planetesimals are forming right now — is the future, he said.
The Bottom Line?
Our solar system’s architecture is rather unusual, particularly as only about 10% of solar type stars have a cold Jupiter, Kane said. It’s therefore possible that most systems do allow the formation of numerous Venus and Earth-size planets beyond that which we see in the solar system, he said. This is potentially good for astrobiology since giant planets are relatively rare, said Kane.
