Mars is a weirdo. (Well, as far as planets go).
We’ve got four rocky planets in the Solar System. Earth and Venus are about the same size. But Mars and Mercury are puny. Mercury’s got an excuse — it’s closer to the Sun, has lots of iron and all that. But what’s up with Mars? Why is it so small (just 11% as massive as Earth)?
This “small Mars” issue comes up when trying to understand how our Solar System formed. If we simulate the planets forming from a smooth disk of rocky stuff (animation here), the Solar System doesn’t look right:
Our simulated Marses grow way too big. They’re typically about the mass of Earth, about nine times more massive than the real Mars.
To date there are two viable solutions to the small Mars problem. I’ve written about both before (because I’ve worked on both). The first is the Grand Tack model, which proposes that the inner Solar System was sculpted by Jupiter’s migration. The second is the Low-mass Asteroid belt model, which instead proposes that there was never much rocky stuff between Earth and Jupiter’s orbits in the first place.
Today I’ll present a new (in some ways simpler) solution: the Early Instability model. The scientific paper on the model was led by Matt Clement at the University of Oklahoma and it was just published in Icarus (download it here).
This new solution piggybacks on a different Solar System story. It started in 2005 when a new idea came out, called the Nice model (you can tell it’s a big deal because it has its own Wikipedia page). The key new idea was that the Solar System’s giant planets underwent an instability. Not a super-strong instability as is characteristic of exoplanet systems, but an instability nonetheless.
This animation shows the idea. The giant planets probably formed on a more compact orbital configuration with an outer disk of planetary leftovers called planetesimals. After some time, an instability in the giant planets’ orbits was triggered, causing their orbits to rapidly spread out and change shape. The outer disk of planetesimals was completely cleared out, and one extra ice giant may even have been kicked out of the Solar System (Planet Nine anyone?).
It was first thought that this instability happened late, maybe 500 million years after the Solar System started forming. The idea was to explain the Late Heavy Bombardment, an apparent spike in the bombardment rate on the Moon.
However, our thinking related to the timing of the instability has changed. This has included a re-analysis of different cosmochemical constraints (e.g., from meteorites and Moon rocks). New studies suggest that the Moon’s Late Heavy Bombardment may be an illusion, the tail the bombardment related to clearing out the leftovers of planet formation (see here or here for details).
We still think the giant planet instability happened, but it could have happened much earlier. It may even have happened while the terrestrial planets were forming. The question is: what would the effect be on the growth of the terrestrial planets?
I’m glad you asked! Here is an animation that Matt made of this process. The simulation starts with the building blocks of the rocky planets close to the young Sun, the giant planets (in black), an outer disk of planetesimals (the green dots in the previous animation). There is an extra ice giant in there that is kicked out during the instability.
When the instability is triggered after 10 million years (Myr), the asteroid belt gets really excited and so does Mars’ feeding zone. This has the effect of stunting Mars’ growth and simultaneously depleting the asteroid belt. The instability has little effect on the growth of Earth and Venus, so those planets end up much more massive than Mars, as in our actual Solar System. A nice solution to the small Mars problem.
Matt ran several hundred of these simulations. He found that simulations that match the rocky planets best are those that also match the giant planets best. That is really nice because it’s simple: one event can explain the inner and outer Solar System in one fell swoop.
So now there are three competing models to explain the Solar System: the Grand Tack, Low-mass asteroid belt and Early Instability models. Here is a cartoon comparison:
You can never prove that a model is right, only show that it is wrong. Each of these three models has a potential Achilles heel. For the Grand Tack model it’s the outward migration mechanism. For Jupiter and Saturn to migrate outward they need to maintain a specific mass ratio (Saturn must be between 2-4 times less massive). But the planets are growing as they migrate. Can they maintain the right mass ratio for long enough? For the Low-mass asteroid belt, the issue is simply whether it is plausible for a lot of rocky material to have formed in a ring centered between Venus’ and Earth’s orbits, with almost no big bodies in the asteroid belt. For the Early Instability model the issue is simply the timing of the instability. The model works great if the instability was very early but if it happened after the terrestrial planets were done forming then it could not have shaped them.
In the coming years, we should be able to poke at these issues and figure out if they are true Achilles heels that might kill off some of these models. But then again, maybe we’ll come up with some new ones to replace them!
A final philosophical note. You might ask yourself: if there were already two good solutions to the small Mars problem, what was the point of making another model? Well, remember that we are trying to figure out what happened almost 5 billion years ago. It’s like trying to solve a crime that happened in the 1300s using as evidence just a couple of used tissues from that period and some faded paintings of the crime scene (that may themselves have been tampered with). No easy task. So it’s worth considering every plausible explanation. In fact, two of these models (and, quite possibly, all three) must be wrong.
On that note, I’ll leave you with a …. boom!
Questions? Comments? Words of wisdom?
Resources and related stuff
- The scientific paper: Mars’ growth stunted by an early giant planet instability
- Matt Clement‘s website
- The Grand Tack scenario
- The Low-mass (or no-mass) asteroid belt model
- Where did Earth’s water come from?
- A MOJO video of me explaining all of this:
PLANETPLANET 
A couple of recent papers discussed a possible difference in the composition of Earth and Mars and how it might make the formation of Mars via the Grand Tack less likely:
https://arxiv.org/abs/1805.09449 https://arxiv.org/abs/1704.00184
Looking at the animation it appears that Mars ends up with a composition in this model.
A different composition that is.
I like those Mars composition papers. I think the early instability model should be able to match Mars’ composition. The biggest uncertainty right now is whether it remains consistent with Xenon isotopes, which are different in the mantle vs the atmosphere. The “missing” atmospheric Xe signature was detected in comet 67P by Rosetta (Marty et al 2017) but if the instability was early it’s hard to explain why the mantle signature is different. But, this is somewhat controversial and not super well-constrained…
In this paper https://arxiv.org/abs/1610.04251 the population of icy objects controlled by Planet Nine declines when they return to Neptune crossing orbits, could be an alternate source of comet impacts?
Do you mean impacts on Earth? No. Comets have such a tiny probability of impacting Earth that you need a zillion of them to get a handful of impacts (after the gas disk is gone, that is).
Until a few years ago I had the impression that the Late Heavy Bombardment caused by the late instability in the Nice Model was the consensus view, particularly among dynamical astronomers.
There have been a number of papers recently discussing problems with a late instability (the most recent hidden in this https://arxiv.org/abs/1804.08735 ) or using an early instability.
Does this “However, our thinking related to the timing of the instability has changed.” indicate a shift to favoring an early instability or just to we don’t know?
There are several new studies that disagree with the LHB spike interpretation from lunar craters. Current models favor an earlier instability but exactly how early is up for grabs… And not everyone has changed their viewpoint — some still favor the late (~500 Myr instability)
If you use early or late instabilities with the first two models you would have five models to choose from.
While early vs. late may not change much using the Grand Tack, combining an empty asteroid belt and an early instability has some interesting possibilities…
Hmm — what do you have in mind with the LMAB with an early instability?
I was thinking if the asteroid belt was initially empty and the terrestrial planets hadn’t yet formed two reasons for Jupiter to encounter an ice giant during the instability would be lost. There would still need to be some explanation for its irregular satellites but the constraints on the Nice model becomes somewhat more flexible.
That is a really good point that I hadn’t considered. You’re right — the whole jumping jupiter version of the Nice model is designed not to destroy the terrestrial planets or mess up the asteroid belt. But if the belt is barely there (and still getting mixed up with the growing terrestrial planets anyway) then… you’re totally right.
Would the orbital distribution of the icy asteroids scattered in during the Grand Tack be different enough from those scattered in as Jupiter and Saturn form to distinguish between the models, or would the instability mix them up too much to tell?
Well, the scattering during Jup and Sat’s growth definitely happened whereas the Grand Tack may or may not. But scattering during giant planet growth only reaches the terrestrial planets when gas drag is weak: when the gas density is low or for large planetesimals.
In terms of telling the models apart there is no obvious test to distinguish between the models — at least not yet.
I suppose if Jupiter captured stony asteroids as irregular satellites during the Grand Tack they wouldn’t survive the later encounters.
You’re right — the instability would jumble that all up!
I liked how the animation smashed a couple of similar mass objects together to make Venus and Earth. Of course the really tricky bit is how one ends up spinning quickly with a moon, and the other spinning slowly without. Not enough physical detail for this model to tell, but there’s all sorts of fun possibilities that can be explored. My favourite is that a retrograde moon of Venus would inspiral and its ultimate infall would cause the “Great Transition” from Warm Ocean World to Hot-as-Hell World, maybe c.0.8 Gya. But no real way of testing such scenarios without more data from Venus’s surface…
While it makes sense to get alternative models to try and explain it without locking yourself into one solution but how do these things interact with each other? After all if other areas of science with competing models tell us anything they don’t need to be mutually exclusive. As scientists we try and break things down to the simplest pieces but the natural world never conforms to such simple and clean elements so has there been any serious efforts to try and mix these models and see if they can compound in a way that produces a better solution that escapes some of the flaws?
Now one open question that I’ve been reading comments regarding planet ages that differ yet I don’t see any valid reputable sources for these numbers.
For instance Mercury is claimed to be 4.503 Ga, the Earth is claimed to be 4.543 Ga, and Mars 4.603 Ga but these all as far as I can tell lack actual SOURCES. Moreover some people say their ages are “roughly the same” while others contradict that and it is driving me mad.
Now if you took these dubious ages at face value then you get Mars being at least 60 Ma older than earth which is a significant time during the early chaotic planet formation. If such measurement differences are real they could help constrain these models. However for that to work we would require reliable and independent age estimates for each of the rocky planets to a sufficient degree of accuracy. And of course there is the whole lack of surface samples for Venus which is essential to narrow things down.
The other bit that these don’t address is why our solar system is so different from what we are discovering around other stars. An ideal planet formation model should probably be able to account for any planetary system around any star at least on a statistical level. For instance why are there no “super earths” in our solar system despite them being one of the most abundant planet types in the galaxy?
First, you are right that models are rarely “all or nothing” and it’s possible the truth lies somewhere in between. It’s useful to start from relatively extreme versions of models in the hopes of being able to falsify at least certain pieces.
Second, there are tons of sources about the ages of the terrestrial planets. Most of the data is from isotopic chronometers that have half-lives similar to the typical formation times of the planets (~tens of millions of years). The Hafnium-Tungsten system is particularly useful. If you’d like some references, see, e.g., Kleine et al (2009; http://adsabs.harvard.edu/abs/2009GeCoA..73.5150K) for a review of Earth’s formation, and Dauphas & Pourmand (2011; http://adsabs.harvard.edu/abs/2011Natur.473..489D) for Mars. There are no strong constraints from Venus or Mercury. To put things simply, we know that a) Earth’s formation did not finish for at least 40 million years (and likely 60-100 million years) after the start of SOlar System formation (generally calibrated using “CAI” inclusions in meteorites). However, Mars’ growth was essentially done within 5-10 million years. There’s a lot more information out there, of course…
Third, the issue of putting the Solar System in a broader context is indeed super important! The review paper I alluded to in this post is actually entitled “Solar system formation in the context of extra-solar planets”. It’s not a simple matter, but here is one little crumb: we think Jupiter is the reason we don’t have super-Earths. If Jupiter formed relatively fast, it would have blocked both the inward drift of both small pebbles and the inward migration of larger planetary “embryos” (see http://nautil.us/blog/give-thanks-to-jupiter-our-little-planets-big-protector). There is a lot more to the story but that gives you a taste of how we’re trying to explain things.
“For the Low-mass asteroid belt, the issue is simply whether it is plausible for a lot of rocky material to have formed in a ring centered between Venus’ and Earth’s orbits, with almost no big bodies in the asteroid belt.”
Indeed, I wonder if the 4 Earth-mass planets on prograde orbits (in the HZ) mentioned in the “Ultimate Retrograde Solar System” article would be possible to form naturally on their own. (The outcome might influence my headcanon rearding planetary systems of the BSG’s Twelve Colonies – because the official info map isn’t good enough.)