Mars is dead. How the corpses go is quite interesting: volcanoes so large that they could change the rotation of the planet; enough atmosphere to support seasonal weather patterns; a cleft in the surface that makes the Grand Canyon look like a ditch by the roadside; polar ice caps; painful, suggestive geological features. Interestingly enough, humanity has spent little time and effort getting to know him.
As of this writing, there is a small plutonium-powered robot crawling on the floor of one of Mars’ craters, getting to know the planet even more. Curiosity is the latest in a series of celebrated Martian expeditions, which began with the Vikings in the 1970s and continue to be delivered, with intermittent success, to this day. Our robots, however, are only a precursor. Eventually someone will travel to an alien world and it is incomprehensible to think that their destination will be anything other than a rusty corpse that is Mars.
Why does the Red Planet intrude so much on our scientific imagination? First, it’s a matter of convenience. It is close to certain definitions of the term “easy” and easy to reach. Researching elsewhere would be flawed. But Mars’ dream is not just about efficiency. Percival Lowell probably did not think about orbital mechanics when he mapped the (completely illusory) channels of the planet back in the 19th century.
Neither was HG Wells when he wrote The War of the worlds. However, he considered death for both Mars and the unfortunate inhabitants of England in the 1880s, for whom an ecological collapse on a distant planet meant an invasion. The connection between Mars and war and death and blood, of course, stretches back thousands of years. It is not because of us that the name comes down to us as the Roman God of War.
Now we are interested in the death of desiccation. Over the last few decades, it has become increasingly apparent that our favorite red planet used to be blue. There was water everywhere on ancient Mars. What we see now is the spirit of a once dynamic world.
There is an important difference between “dead” and “inert”. You would never say a rock is dead (even if that rock is literally made of dead plankton!). You would never say Mercury is dead. Death means life. But it turned out that on Mars the language was quite precise. There is no evidence that biological life ever existed there, but everything else suggests that billions of years ago the planet could boast a denser atmosphere, water cycle, and everything we might call a world inhabited. So what happened between then and now?
Mars died, and Jupiter probably killed him.
As the planets go, Mars is pretty stuck. It is about 4,200 miles wide and weighs about 15 percent of the Earth’s mass. Now that Pluto has been kicked out of the club, it is the second smallest planet in the solar system, defeating only Mercury.
It turns out that being small is harmful for long-term survival on a planetary level. Strong magnetic fields are needed to defend against solar radiation that takes lighter elements out of the planet’s atmosphere. On Earth, this magnetic field is fed by convection in its semi-liquid outer core of iron, which creates what is known as a planetary dynamo. And Mars used to have a dynamo. You will be shocked to hear that he died.
Small-mass planets lose heat faster than larger ones, and although Mars still has a liquid iron core, it no longer seems hot enough to create the convection needed to power a significant magnetic field. Definitely uses that we have it – you can recognize it by the magnetized rocks laid while the planet was young – but no one can find any evidence of an active dynamo later about 3.7 billion years ago. Without a proper dynamo and a protective magnetic field, most of Mars’ water has long since been ejected from its atmosphere by solar radiation, leaving us with a very dead world.
What has Jupiter got to do with this? Good question!
The last few decades have been quite exciting for planetary scientists, who finally have more solar penetration systems than ours. When all we needed to continue was our neighborhood, it seemed pretty reasonable. Rocky planets, some asteroids, gas giants, ice giants. Nice and tidy. But when we started looking at exoplanets, it turned out that the structure of the solar system was actually quite unusual. Where are the super-Earths? Where are the Hot Jupiters?
It is important to understand that we have now strayed into the realm of the hypothetical. Although we have some geological evidence of a Mars dynamo and we can indeed determine that water once flowed across the surface of the Red Planet, we are now trying to elicit mysteries from the dawn of the solar system. This is difficult, and obvious is the correct answer will not be marked. I should also note that I am in no way qualified to assess these hypotheses in anything other than in terms of “is it interesting”.
In any case, the most interesting hypothesis, at least when it comes to Mars, is the Grand Tack. These include Jupiter, which roamed the early solar system like an absolutely huge orb of destruction. Jupiter was the first planet to form and the most important. It massages far more than any other planet combined and has an amazing gravitational impact on its peers. And according to the Grand Tack hypothesis, it once decided to fall on the Sun.
Early Jupiter did not quite reach its destination, but from its point of formation it was about 3.5 AU * spiral to only 1.5 AU. This leads Jupiter well into the orbit of Mars, where it would cannibalize almost all the building material of young Mars. You can also imagine what this little trip would do to the asteroid belt. The reason Jupiter turned (i.e. jumped) and retreated to the outer ends of the solar system is Saturn, which ended up in orbital resonance that slowly pulled it to about 5 AU. And a good thing, because Jupiter, when it gets close to Earth’s orbit, would mean I’m definitely not typing this now.
* Astronomical unit is the current average distance from the Earth to the Sun.
If the Grand Point hypothesis is correct – there are reasons why it might not be, as you would expect from this type of research – Mars’ small size is easily explained: Jupiter ate most before Saturn could break through (I always think of resonances as o involves drumming; I don’t know why) to help. And so, thanks to this complex development of events, Mars’ dynamo faded, water from the atmosphere was taken away by solar radiation, and the planet dried up and died.
There is another hypothesis about what happened to the Mars dynamo, although the latest evidence of Martian magnetism messes up with time. Sufficiently large shocks through sufficiently short time frames could, in theory, be able to stop a planetary dynamo. Mars has seen its share of influences, and indeed there is a period (again, at least partially hypothetically) of the history of the solar system that took place approximately 4 billion years ago known as the Late Heavy Bombing. The LHB, faithful to ‘HB’, included inner planets that were absolutely knocked down by asteroids, comets, etc., and there could have been big enough blows to shut down the Martian dynamo. Maybe.
That, incidentally, would still be Jupiter’s fault. Indications are that LHB would have formed if interactions with Jupiter had ejected Neptune into its current orbit, which would disrupt bodies in the Kuiper belt and throw a good portion of them into the inner solar system.
In other words, Mars could be a shitty pile of rust, but that’s probably not his fault. Guilty Jupiter.