Mars is dead. When corpses go, it’s quite interesting: volcanoes so big they could have altered the rotation of the planet; enough atmosphere to support seasonal weather patterns; a crack in the surface that makes the Grand Canyon look like a roadside ditch; polar ice caps; pungent, suggestive geological features. Interestingly enough, humanity has put in some time and effort to get to know it.
At the time of writing, a small plutonium-powered robot is scurrying around on the floor of one of Mars’ craters, getting to know the planet even further. Curiosity is the latest in a series of celebrated Mars expeditions, which began with the Vikings in the 1970s and continues with occasional success to this day. However, our robots are only the vanguard. Eventually someone will make the journey to an alien world, and it is incomprehensible to think that their destination will be anything other than the rusty corpse that is Mars.
Why does the Red Planet hold our scientific imaginations so much? First, it is a matter of convenience. It’s close by and, for certain definitions of ‘easy’, easy to get to. Exploring elsewhere would do things that are out of order. But the dream of Mars is not all about efficiency. Percival Lowell probably didn’t think about orbital mechanics when he mapped the planet’s (wholly illusory) channels in the 19th century.
Neither was HG Wells when he wrote The War of the Worlds. However, he contemplated death, both for Mars and for the hapless inhabitants of 1880s England, whose ecological collapse on a distant planet meant an invasion. The association between Mars and war and death and blood, of course, stretches back thousands of years. It is not without reason that the name comes down to us as the Roman god of war.
It is the death of dehydration that interests us now. Over the decades, it has become increasingly clear that our favorite red planet was once blue. Water was everywhere on ancient Mars. What we see now is the spirit of a once-dynamic world.
There is an important distinction between ‘dead’ and ‘inert’. You would never say a stone was dead (even if that stone was literally made of dead plankton!). You would never even say that Mercury was dead. Death implies life. But it turns out at Mars that the language was fairly accurate. There’s no evidence that biological life ever existed there, but everything else suggests that billions of years ago, the planet had a thicker atmosphere, a water cycle, and everything we might need to call a world habitable. So what happened between then and now?
Mars died, and Jupiter probably killed it.
As planets go, Mars is pretty dinky. It is about 4,200 miles wide and has a mass about 15 percent that of Earth. Now that Pluto has been thrown out of the club, it is the second smallest planet in the solar system, beating only Mercury.
It turns out that being small is detrimental to long-term survival on a planetary level. Strong magnetic fields are necessary to prevent solar radiation from removing the lighter elements from a planet’s atmosphere. On Earth, that magnetic field is driven by convection in the semi-liquid iron outer core, which creates what is called a planetary dynamo. Mars also had a dynamo. You will be shocked to learn it died.
Low-mass planets lose heat faster than larger ones, and while Mars still has a liquid iron core, it appears that it is no longer hot enough to generate the convection needed to generate a significant magnetic field of current. to provide. It sure used to have one – you can tell by the magnetized rocks laid out when the planet was young – but no one can find any evidence of an active dynamo later than about 3.7 billion years ago. Without a working dynamo and a protective magnetic field, most of Mars’ water was blasted from the atmosphere long ago by solar radiation, leaving us with a very dead world.
What’s Jupiter got to do with this? Good question!
The past few decades have been pretty exciting for planetary scientists, who finally have more solar systems to poke around in than ours. When we had to continue alone, our own neighborhood, it looked reasonable. Rocky planets, some asteroids, gas giants, ice giants. Nice and orderly. But when we started looking at exoplanets, it turned out that the structure of the solar system is actually quite unusual. Where are the super-Earths? Where are the Hot Jupiters?
It is important to understand that we are now wandering far into the realm of the hypothetical. While we have some geological evidence of the dynamo of Mars, and can indeed establish that water once flowed on the surface of the Red Planet, we are now trying to unravel mysteries from the dawn of the solar system. This is difficult, and of course it is is a correct answer, it will not be indicated. I should also note that I am in no way qualified to judge these hypotheses in anything other than the sentence ‘is this interesting’.
Regardless, the most interesting hypothesis, at least as far as Mars is concerned, is the Grand Tack. This means that Jupiter wanders through the early solar system like an absolutely gigantic wrecking ball. Jupiter was the first planet to be formed, and the most important. It’s much more massive than any other planet put together, and has an incredible gravitational impact on its peers. And according to the Grand Tack hypothesis, it once decided to fall into the sun.
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Early Jupiter did not quite reach its destination, but it spiraled from its formation point about 3.5 AU * to only 1.5 AU or so. This puts Jupiter well within Mars’ orbit, where it would have cannibalized almost all of the building material of young Mars. You can also imagine what this little foray would have done with the asteroid belt. The reason Jupiter turned around (i.e. tacked) and retreated to the outer reaches of the solar system is Saturn, which ended in an orbital resonance that slowly dragged it back to about 5 AU. And a good thing, because if Jupiter got closer to Earth’s orbit, I certainly wouldn’t be typing this right now.
* An astronomical unit is the current average distance from the earth to the sun.
If the Grand Tack hypothesis is correct – there are reasons it might not be, as you would expect from this kind of research – the small size of Mars is easy to explain: Jupiter ate most of it before Saturn could pound (I always think to resonances as with thumping; I don’t know why) to save. And so, thanks to this complicated turn of events, the Mars dynamo faded, the water was stripped from the atmosphere by solar radiation, and the planet dried up and died.
There is another hypothesis about what happened to the Martian dynamo, although the most recent evidence of magnetism on Mars disrupts the timing. Enough big consequences in a short enough time frame could theoretically stop a planetary dynamo. Mars has seen its fair share of impacts, and indeed there is a period (again, at least partially hypothetical) of the solar system’s history that happened about 4 billion years ago, known as the Late Heavy Bombardment. The LHB concerned, true to the ‘HB’, that the inner planets were absolutely hammered by asteroids, comets, etc., and there may have been enough major impactors there to shut down the Mars dynamo. Perhaps.
By the way, this would still have been Jupiter’s fault. There is some evidence that LHB might have been caused by interactions with Jupiter tossing Neptune into its current orbit, disrupting bodies in the Kuiper Belt and throwing much of it into the inner solar system.
In other words, Mars might be a rust mess, but that’s probably not to blame. Blame Jupiter.