How to Science Your Way Off Mars Ch. 4 Excerpt (Patreon)

Three chapters done, four to go.

I'm not going to be able to put in the print order on this before the beginning of the year, unfortunately. So I'm going to back-burner this project a bit and get some gaming videos made- I haven't really had time for those in half a year.

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Chapter 4 - Geology

Let’s tell a story of a young planet. 

It’s the early days of the Sol system. The gas giants Jupiter and Saturn are slowly retreating out and away from the sun, having cleaned out much of the loose dust and gas in the inner solar system. A small group of rocky planetoids is busily grabbing up what’s left behind, including one in particular with a moderate mix of metals and silicates. Its young, liquid metal core is churning out a strong magnetic field. Its thick atmosphere is fed by the volcanic fires of its surface as its crust cools. 

Meanwhile another planetoid, maybe one-tenth the mass of the hero of our story, is drawing closer. They’re more or less in the same orbit around the sun, and that arrangement simply isn’t stable. As the centuries pass, the two worlds grow closer with each loop around the sun, until one unhappy day...

THWACK.

The two planetoids collide. Much of their planetary atmospheres are blasted away by the energy of the impact. A gargantuan crater is carved out on the larger planetoid’s surface. Debris scatters up into space from the impact, forming a short-lived ring around the planetoid. Most of the debris ends up falling back to the surface, but enough remains in orbit to coalesce into one or more moons. 

Yes, one or more. Did you think I was talking about Earth? 

One of Mars’s most striking features is how different its northern and southern hemispheres are. Its northern hemisphere is lower in elevation- much lower- and much smoother, with very few craters compared to the much more rugged southern hemisphere. Why this is is hotly disputed, but the majority opinion at the moment is because, well, the northern quarter or so of Mars, the Vastitas Borealis, might be the solar system’s largest impact crater.

Now, there are a lot of reasons why, when Earth got hit by Theia in the incident that resulted in the formation of our moon, no giant crater was left behind- for one thing, that was 4.4 billion years again, and it’s rained a LOT since then- but the biggest reason is likely that Theia was roughly the same density as Earth, which meant when it hit, it hit very, very hard indeed- hard enough to liquify anything that was solid on Earth’s surface. There likely wasn’t anything solid left to form a crater, and if there had been the secondary impacts of the crater ejecta falling back to Earth would have taken care of that.

The current Mars impact theory holds that, although whatever hit Mars was about one-tenth its mass, it was likely mostly made of ices instead of rocks. Density matters in an impact of this kind. Furthermore, both Mars and its impactor orbited farther out from the sun, which meant both bodies were moving more slowly, and with both also being an order of magnitude less massive than Earth and Theia, their mutual gravitational attraction was less. Instead of a giant mostly-iron bullet hitting Earth, you had a fast-pitched snowball (with maybe a small rock in the middle, because nature is a bastard) hitting a stone wall. 

Net result: Mars’s crust (which was already thicker than Earth’s) did not entirely liquify. However, the shock waves did fracture the crust on the opposite side from the impact, leading to massive eruptions and crustal formation, raising the surface level of that side of the planet.

This results in Mars becoming a lopsided planet. As a result, Mars’s axis shifts, and the impact zone shifts from near the equator to the north pole. The remaining moons in orbit remain in the same orbital plane they formed in, above the rough line of the initial impact. 

But there’s a problem- a problem of angular momentum.

One of the discoveries of the first steps towards landing on the moon was that the moon- and, indeed, also the Earth and pretty much every other solid planet or moon we’ve yet visited- isn’t uniform and homogeneous in composition. In short words, planets have lumps- not like mountains, but chunks of their interiors which are denser or less dense, depending on all sorts of factors related to their formation and history. And since those lumps exist, that means the gravitational pull of a planet changes slightly depending on what part of it you’re directly above at the time.

What this means in practical terms is, a moon orbiting a planet above the altitude of geostationary orbit (moving slower than the rotation of the planet) will be pulled along and accelerated by the lumps in its parent body, and the parent body in turn will be slowed down by the slower-moving moon. In the absence of other effects, that means that moon will gradually get farther away from its primary, while the planet’s rotation will slow down very slightly in return.

But if a moon is orbiting below the altitude of geostationary orbit, that means it’s moving faster than the rotation of the planet. That means those lumps will be slowing the moon down rather than speeding it up- which means the moon will gradually fall towards the planet.

This continues until the moon in question hits the Roche limit- the point at which tidal forces will rip the moon apart. That limit depends on the mass of both the primary and the moon, but as a shorthand, any moon that’s closer than two planetary diameters to its primary is in very deep trouble. 

Today Mars has two extremely tiny moons, Phobos and Deimos. Both are basically potato-shaped rubble piles, very loosely packed together, with such a low density that it’s estimated that as much as thirty percent of both moons are just empty space between boulders covered by a fine coating of dust. Phobos appears about one-fifth the size of our moon in Mars’s night sky, and Deimos maybe one-tenth that large. 

Deimos, the smaller moon, orbits Mars about once every thirty-one hours. That’s slower than Mars’s rotation, which means it’s gradually rising away from the planet.

Phobos, on the other hand, takes less than eight hours to orbit Mars. It’s falling in. Its current average orbital altitude is about one and a quarter planetary diameters of Mars. In roughly fifty million years, give or take, it’s going to break up, forming a short-lived ring around Mars. Ninety percent of it will end up falling to the Martian surface, while the remaining tenth or so, flung into a higher orbit by gravitational interaction with the other fragments, will coalesce into another tiny, doomed moon.

There is a theory being discussed right now, with recent evidence to support it from evidence of tides in the sediments of Gale Crater, that says this has happened at least four or five times already, and that Mars once had at least one moon forty to fifty times the mass of Phobos. That means, on at least four or five occasions, a moon has ripped itself apart in the Martian skies and rained death down on anything and everything under its former orbital path, albeit with each iteration being less destructive than the last by at least an order of magnitude.

But let’s say five times, which means the great-great-granddaddy of both Phobos and Deimos would have been a moon roughly one thousand times the mass of both combined- or roughly 0.1% the mass of Mars itself. (That’s hardly stunning; Earth’s moon is about 1% the mass of Earth. Assuming proto-Phobos had a similar origin to the moon, the remaining 9/10 can easily be accounted for if the impactor was mostly water and other volatiles, similar to the dwarf planet Ceres. The missing material would have been blown away like a cometary tail over the eons.)

In any case, between the great Martian THWACK, the occasional destruction of a moon orbiting far too close, and the general inner system bombardment of what is called the Late Heavy Bombardment, a lot of space rocks hit Mars, leaving behind millions of craters ranging from modest divots in the planet to giants like (leaving aside the great polar basin) Hellas. Hellas, in particular, was a strike so powerful that the shock wave ripped through the planet and, most likely, jump-started the volcanism that created one of Mars’s most famous features, the Tharsis volcanic zone, which includes Olympus Mons, the largest known volcano in the entire solar system.

But, once Mars cooled and settled down from the great THWACK, it became, for a while, a warm, wet place. At that time Mars would still have had a thick atmosphere of carbon dioxide and other volcanic gases, producing a strong greenhouse effect that would offset both its distance from the sun and the fact that Sol, in the early days of the solar system, burned with only 75% the heat it produces today. And, thanks to the THWACK, it had just received a massive delivery of water to go with whatever it already had.

It rained. It rained a LOT.

In its earliest days Mars was an ocean world. Rain fell on highlands, eroded the mostly volcanic basalt surface, and flushed the sand and mud down rivers and into the Great Boreal Ocean, where it eventually formed mass sediments. We can still see evidence of alluvial fans where sediments were deposited at the mouth of places like Ares Vallis, which opens up into the southernmost part of the boreal lowlands, Acidalia Planitia. Lesser sediments formed in places like Hellas, Gale, Jezero, and Schiaparelli. 

There was so much water that Mars even flirted, for a geologically brief period of time, with plate tectonics. We know of one places where the Martian crust split and shifted, because we can find the magnetic anomalies from when the crust cooled, and we can see how they misaligned. That zone eventually froze in place, eroded away, and became the solar system’s largest known canyon, Vallis Marineris. 

Unfortunately, these good things couldn’t last. One side effect of the THWACK was that it provided Mars’s internal heat a faster route of escape. Mars, being a small planet, would have cooled fairly rapidly anyway, but with thin spots allowing easy heat transfer through small volcanoes and geysers (and later mud volcanoes), it cooled quicker than it would have otherwise. 

The magnetic field shut down. The atmosphere began blowing away in the solar wind. Ultraviolet radiation and cosmic rays began splitting apart water molecules, with the hydrogen scattering out into space and the oxygen either following or oxidizing the iron-rich basaltic dust of the surface. 

By a billion years after the THWACK, Mars was a cold, dry world, with little geological activity beyond the occasional eruption of its mega-volcanoes. 

And this, with minor changes, was the planet Mark Watney came to explore, and also the planet five adorable castaways barely survived a crash-landing onto.

Well, that’s the planet in general, anyway. Now let’s examine the details.