Rapid Formation May Have Stunted Mars’ Growth


Somewhere between two and four million years after our solar system formed, a rocky little runt went through a rapid growth spurt. In its embryonic stage, it was much like Earth. But it didn’t end up being terrestrial. Earth ended up being twice its size through collecting other rocky bodies as they passed by. But not Mars… Oh, no. Not Mars.

“Earth was made of embryos like Mars, but Mars is a stranded planetary embryo that never collided with other embryos to form an Earthlike planet.” said Nicolas Dauphas at the University of Chicago. “Mars probably is not a terrestrial planet like Earth, which grew to its full size over 50 to 100 million years via collisions with other small bodies in the solar system.”

The latest study of Mars just released in Nature puts forth the theory that the red planet’s rapid formation helps explain why it is so small. The idea isn’t new, but based on a proposal done 20 years ago and heightened by planetary growth simulations. The only thing missing was evidence… evidence that’s hard to come by since we can’t examine firsthand the formation history of Mars because of the unknown composition of its mantle – the rock layer beneath the planetary crust.

So what has changed that gives us a new view of how Mars came to be the runt of the solar system litter? Try meteorites. By analyzing Martian meteorites, the team was able to pick out clues about the mantle composition of Mars, but their compositions also have changed during their journey through space. This debris left over from the genesis time is nothing more than a common chondrite – a Rosetta stone for deducing planetary chemical composition. Dauphas and Pourmand analyzed the abundances of these elements in more than 30 chondrites, and compared those to the compositions of another 20 martian meteorites.

“Once you solve the composition of chondrites you can address many other questions,” Dauphas said.

And there are many, many questions left to be answered. Cosmochemists have intensively studied chondrites, but still poorly understand the abundances of two categories of elements they contain, including uranium, thorium, lutetium and hafnium. Hafnium and thorium both are refractory or non-volatile elements, meaning that their compositions remain relatively constant in meteorites. They also are lithophile elements, those that would have stayed in the mantle when the core of Mars formed. If scientists could measure the hafnium-thorium ratio in the martian mantle, they would have the ratio for the whole planet, which they need to reconstruct its formation history. When the team of Dauphas and Pourmand had determined this ratio, they were able to calculate how long it took Mars to develop into a planet. Then, by applying a simulation program, they were able to deduce that Mars… Oh, yes. Mars. Reached its full growth only two million years after the solar system.

“New application of radiogenic isotopes to both chondrite and martial meteorites provides data on the age and mode of formation of Mars,” said Enriqueta Barrera, program director in NSF’s Division of Earth Sciences. “That is consistent with models that explain Mars’ small mass in comparison to that of Earth.”

And still there are questions… But fast formation seems to be the answer. It might explain the puzzling similarities in the xenon content of its atmosphere and that of Earth’s. “Maybe it’s just a coincidence, but maybe the solution is that part of the atmosphere of Earth was inherited from an earlier generation of embryos that had their own atmospheres, maybe a Marslike atmosphere,” Dauphas said.

Mars? Oh, no. Not Mars.

Source: University of Chicago, AAS

15 Replies to “Rapid Formation May Have Stunted Mars’ Growth”

  1. that is to bad that it not became much larger. Maby than there could be life.

  2. that is to bad that it not became much larger. Maby than there could be life.

    1. If Mars were larger, Earth might have been smaller. It would have been more effective at clearing its orbit.

      Likely Mars is smaller because Jupiter thinned out its region. Just a thought.

    2. Imagine that Mars would have been larger and that we’d have two blue planets with life in our solar system, and that someday we’d see the life there for the first time. That would have been the most incredible and awesome thing in history.

      1. And if Venus and Mars switched places we could have had three habitables. Assuming the strangulation of impactor flow is due to Jupiter early growth, it would have taken a “hot Jupiter” position (without knocking away the terrestrials in the process).

      2. But then they would have invaded us last century and we would have wiped them out with microbes. Or vice versa.

  3. What’s “Mars? Oh, no. Not Mars” about?

    How do we know the causal relationship between formation speed and size? It would seem that there are a lot of ways to form, shatter, and re-form a planet depending on the density, size distribution, orbital characteristics, etc of the starting planetesimals.

  4. Apologies but I still hold the belief that Mars was full of its own form of tropical life until the planet-killing asteroids hit it. Water, plantlife, sea-life, perhaps even land-living life. Please don’t attack me, I’ve been studying Mars since childhood and find its evolution to hold a strong probability of this much. Save attacks for after visits to Mars by actual humans that have engaged in Mars-local archeology. I don’t think I’m being unreasonable.

    1. You mean the Late Heavy Bombardment? Mars would have cooled faster and developed life earlier?

      Indeed, there’s only one way to find out. Likewise with geothermal vents inside the moons of Jupiter and Saturn.

    2. If what is expected to occur does so, the world is too predicable — if what unexpectedly occurs is a disappointment, the world is pragmatic and retains hope for a different future than what is expected to occur next and next…

      Insanity is doing the same thing and expecting a different outcome.

      When we say “I expected ________ from that occurrence” and feel dashed at times, we still record it and move on, taking our baggage with us where ever we go. The baggage might be what keeps some of us sane enough to keep on keeping on.


    3. There’s reason to believe Mars never had much of a magnetic field (for this it would require a molten core, which in turn would rely on a larger planet and abundant radioactive material), in which case surface life would be susceptible to molecular damage from cosmic rays and UV. This leads me to believe life could not have survived on the continents, but I like to imagine it did develop in its seas.

      Are there any good books that analyse the possibilities of past life on Mars?

    4. I think the main model is that Mars could indeed have been habitable (at least in the seas, as squidgeny points out).

      But that the main problem was its drying out, loss of atmosphere, freezing from atmospheric loss and weaker greenhouse effect.

      Lately it has been estimated that, at least for Earth, the early solar system impactor “rain” was survivable. Cells proliferate and spread faster globally than impactors sterilize locally. Impactors even supply food in the form of volatiles and heat & chemical energy in such a scenario. (Though I think the cells would prefer it not to, there was plenty of that around already. =D)

      Even putative “crust breakers” which could temporarily sterilize the global surface could be survivable in a crust Goldilock zone some km down (moderate temperature and pressure).

      Mars would have its own unique impact history, so I don’t know how the results transfer. I only wanted to point out the possibility that life started early and concurrently independently on both planets, possibly Venus too, but only Earth lucked out in the “having just so much atmospheric and water loss” department.

      The difference is that if it happened, Mars could still harbor significant amounts of life in the crust, Venus life could instead be cells drifting high in the atmosphere.

  5. Ah, makes sense. A stunted growth, likely that then fast growing Jupiter bully again – it cut off the food supply early by growth/wandering.

    Now if anyone can explain Mercury to me. A Mars style runt, a great impactor or a series of large impactors making away with most of the crust, or a combination of both? Anyway unless they tried it, the authors could test the model there too (with or without access to Mercury material).

    1. Is there any chance Mercury could be the remains of a gas giant whose gaseous mass has been siphoned off by the sun? I’ve wondered this for some time and never seen anything to prove it wrong. Since we continue to find many hot gas giants in close proximity to their stars and no one knows what lies at their cores, it seems plausible to me. What do you think?

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