PLANETPLANET

If trees could scream, would we be so cavalier about cutting them down? We might, if they screamed all the time, for no good reason. — Deep Thoughts by Jack Handey

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This series is about how planets die — it is introduced here.

Earth is a cosmic freaking paradise. Just look at our neighboring planets: Venus is a sweltering hellhole and Mars is a barren wasteland.  How did we get so lucky?

Things could easily be different.  The current climate is just one of four possible stable climates for our planet. Only one is good for life:

On the Steamball Earth it is hot — the atmosphere contains much more water vapor and it is way too hot.  The Dry Roasted Earth is even hotter and has lost all of its water (like Venus). The Snowball Earth is the other extreme — a frozen ice ball of a planet.

Earth may actually have passed through several snowball states in its past and it made it out.  But if Earth enters a steamball phase we are doomed because there is no coming back.  And once we lose our water it’s all over.

So what exactly keeps our Earth looking like a cosmic paradise?

Earth’s thermostat: the carbonate-silicate cycle

Earth has plate tectonics.  Crustal plates are continuously being created by material coming up from the mantle, spreading and getting pushed back down.

Plate tectonics drives the carbonate-silicate cycle.  This regulates how much carbon dioxide (CO2) is in Earth’s atmosphere. This matters because carbon dioxide is a greenhouse gas, which means it traps heat and acts like a blanket.  When there is more CO2 in the atmosphere, the surface gets hotter.

The carbonate-silicate cycle keeps Earth’s climate temperate. 

When Earth gets really hot, it rains more.  This removes CO2 from the atmosphere and stores it in carbonate rocks on the seafloor.  This cools the planet back down.

When Earth gets really cold it freezes.  It stops raining but volcanoes don’t stop. This makes carbon dioxide build up in the atmosphere, which heats the planet back up.  (This is how we think Earth escaped its Snowball episodes).

The carbonate-silicate cycle operates on a timescale of about a million years.  It only regulates Earth’s long-term climate.  It is too slow to help with human-driven climate change.

The carbonate-silicate cycle is probably not the only cycle that planets can use as a thermostat.  Other cycles have been theorized, which could be very different.

What can go wrong (from an atmosphere point of view)?

• A planet can lose its atmosphere or its water.
• The greenhouse effect can make a planet overheat and become a Steamball or Dry Roasted planet.
• A planet can freeze over into an Snowball world.

Each of these is a serious bummer for life. Let’s go through them.

Atmosphere and water loss

A planet’s atmosphere is kept in place by gravity. Gas molecules are zooming around at a speed determined by their temperature and their molecular weight.  Heavy molecules like water and Nitrogen are easier for a planet to hold onto than light ones like Hydrogen simply because they are heavier.

A planet loses its atmosphere if its gas molecules start moving too fast for the planet’s gravity to hold on to them.

What makes gas molecules speed up?  Either they heat up or they get lighter. 

Gas heats up by absorbing energetic radiation (ultraviolet and X-rays) from their star.  This process can entirely evaporate away the atmospheres of planets that are very close to their stars. In some cases it can evaporate Saturn-mass planets down to their cores.

Molecules get lighter when they are broken into pieces.  For example, Earth can hold on to water but not to hydrogen.

Here is how many Earth-like planets lose their water: water molecules are broken into their components — two hydrogen atoms and one oxygen atom — and the hydrogen atoms escape to space. This happens way high up in the atmosphere, above the ozone layer, where energetic photons from the Sun can get in.  But most of Earth’s water is much lower down so not much escapes.

In order for a planet to dry out, its water needs to be transported up into the stratosphere.  This can happen when a planet gets really really hot (for example, via the greenhouse effect — see below).

A planet can also lose its water into the ground.  The rocks in a planet’s mantle are like sponges. They can incorporate water into their chemical structure.  Not all rocks are equally-good sponges, but overall Earth’s mantle can soak up at least as much water as there is on Earth’s surface, and possibly up to ten times as much.  We don’t even know exactly how much water is trapped in mantle rocks!

Imagine a planet with a thirsty mantle, with rocks capable of sponging up a lot of water but that are currently mostly dry.  Water that is subducted down into the mantle may never come back out!  The planet could in principle suck back up all of its water!  This remains speculative but it’s plausible.

Heating (and overheating) planets: the Greenhouse Effect

You’ve probably heard of the greenhouse effect. You may have heard that it’s a bad thing. It’s not: Earth needs some greenhouse heating to avoid falling into a deep freeze. Right now we have about 35 degrees of heating. We just don’t need any more than that (which is why humans dumping greenhouse gases into the atmosphere is not good).

The greenhouse effect is a positive feedback loop.  It tries to make the planet hotter, which increases the concentration of greenhouse gases in the atmosphere, which makes it even hotter, and so on.  It looks like this:

Climates have a lot of interconnected processes happening at once. As water evaporates it cools off the surface. Plus, as a planet heats up it radiates away its heat more efficiently, which has a counteracting cooling effect. Clouds also matter, and so do the distribution of continents and the presence of deserts.

Greenhouse gases — molecules like carbon dioxide, water and methane — act like a blanket. They don’t create heat, they just slow down the escape of radiation from a planet’s surface.

If the concentration of greenhouse gases in a planet’s atmosphere goes up, then so does the temperature at the planet’s surface. The planet simply retains heat more efficiently. Earth’s global warming is caused by the increasing levels of carbon dioxide and other greenhouse gases.

As of the writing of this post (Feb 2019), Earth’s carbon dioxide level is 409 parts per million. Let’s imagine that Earth’s carbon dioxide level continues to increase. The temperature on the surface increases with the carbon dioxide(CO2) level, warming Earth by a few degrees when the CO2 level doubles.  This is the IPCC’s dire prediction for 2100.

Some climate models ask how much Earth would heat up if the CO2 level were much higher (see here for one scientific paper). Earth’s surface would indeed heat up more and more as the CO2 level increased.  If the CO2 concentration reached a crossover point it would get hot fast.  Earth’s surface temperature would jump above 100 degrees Celsius (the boiling point of water) and it would all be over for animal life.

It’s hard to figure out the exact level of greenhouse gases that causes this jump in temperature.  It depends on exactly how water is transported in the atmosphere and feedbacks between different processes. It’s tricky to pin down exactly.

In the worst case scenario, Earth’s surface may get super hot when the CO2 level reaches about 12 times the current value.  At that point the surface temperature would jump from an already balmy 32 degrees C (about 90 degrees Fahrenheit) above the boiling point of water.  Ouch.

Under more optimistic assumptions the surface won’t overheat until the atmosphere contains 100 to 1000 times the current level of CO2.

Venus’ atmosphere is 90 times thicker than Earth’s and is almost entirely made of CO2 (for a concentration ~200,000 times higher than ours). Venus is thought to have undergone a runaway greenhouse effect in its past (that transported water into its stratosphere, causing it to dry out almost completely).

It’s scary to think that Earth has a huge stockpile of carbon dioxide in its mantle.  All of Venus’ CO2 is in its atmosphere.  There is good reason to think that Earth has as much CO2 in mantle rocks (like the carbonates produced by the carbonate-silicate cycle) as Venus has in its atmosphere.  Let’s try to keep it in the mantle, shall we?

Sadly, Earth is doomed anyway.  As the Sun slowly increases in brightness Earth will cross over the inner boundary of the habitable zone, and the greenhouse effect will become super duper strong.  Earth’s planet’s surface will get hotter and hotter until the oceans start to boil.  The atmosphere will get steamier and steamier, which will make the greenhouse effect stronger and stronger (a Steamball). The stratosphere will eventually contain enough water to be broken apart and hydrogen to be slowly lost to space.  Our Earth will then be Dry Roasted like Venus.

What can trigger an increase in greenhouse heating on a planet?

The main trigger is an increase in the concentration of greenhouse gases. This can come from burning fossil fuels (a la humanity), a big increase in volcano-driven release of CO2 (perhaps triggered by tides), or shutting down the burial of CO2 as happens during a Snowball Earth.

An increase in incoming heat or radiation will also crank up the greenhouse heating.  For example, an increase in internal heat created by tidal dissipation, the impact of an asteroid or comet (although that would have some big side effects), or the slow increase in brightness of a star like the Sun.

Greenhouse heating can get weaker if the concentration of greenhouse gases drops, for example due to an increase in the speed of plate recycling that is not balanced by increased volcanism.  That could lead to a…

Global freeze

Have you ever gotten sunburned from being out in the snow?  I’ve had a bright red nose and cheeks after a day of skiing.  It sucks.

The albedo measures how much light something reflects.  A perfect mirror has an albedo of 100%.  A black hole has an albedo of zero because no light is reflected.  Everything else is in between.

The open ocean is dark, with an albedo of about 6%. Ocean ice, on the other hand, is very reflective, with an albedo of 50-70%. And snow is super reflective, with an albedo close to 100%. So if an area of open ocean freezes over it absorbs much less energy from the Sun.

Like the runaway greenhouse, this creates a positive feedback loop.  When the ocean gets colder it freezes.  That makes it more reflective, which makes it colder, and so on.

While ice is concentrated near the poles, the ice-albedo feedback does not have a big effect on the planet as a whole. But it becomes more and more important as ice starts to creep toward the equator.

At a certain threshold (about 30 degrees from the equator), the ice-albedo feedback goes runaway and pushes the planet into a Snowball state.  At this point the planet is so reflective that it can’t use sunlight to heat up.  It’s frozen over!

There is pretty strong evidence that our planet went through a handful of Snowball Earth episodes.  Evidence comes from rocks: low-latitude glacial deposits (basically, lots of ice near the equator).

How did our planet escape from the Snowball Earth? 

With the carbonate-silicate thermostat, of course! It stops raining on an ice-covered planet. So carbon dioxide from volcanoes would have piled up in Earth’s atmosphere.  The greenhouse effect slowly grew in strength until it finally overwhelmed the Snowball and thawed out the Earth!

And guess what? Right above those low-latitude glacial deposits there are thick carbonate rocks.  Those rocks appear to be the rain starting up again.  Earth probably got pretty hot after it finally thawed and had some massive rainstorms, which removed carbon dioxide from the air and buried it in carbonate rocks.

Without a robust thermostat, a planet that stumbled into a Snowball state would just stay frozen over.

It would be like living on Hoth:

All three climate catastrophe processes are a little different for planets orbiting red dwarf stars.  (Remember, most stars in the Galaxy are puny red dwarf stars so most habitable planets orbit red Suns, not yellow ones.)

• Atmospheric and water loss. Red dwarf stars remain active and continue to flare for much longer than Sun-like stars.  It has been speculated that this could act to more efficiently strip their atmospheres and water, as compared with planets orbiting Sun-like stars.  It is debated whether this is really the case.
• Greenhouse heating. Red dwarf stars take a couple hundred million years to settle down after they are born.  During that interval, they are brighter and their habitable zones farther away.  A planet in the habitable zone of a mature red dwarf therefore had an earlier period during which it was interior to the habitable zone and may have been vulnerable to strong greenhouse heating and water loss.
  
• Global Freeze. The ice-albedo feedback is much weaker for planets orbiting red dwarf stars.  They are cooler than the Sun and give off most of their light at infrared wavelengths. And snow and ice aren’t very reflective in the infrared. When a patch of ocean becomes ice it doesn’t become much more reflective.  So planets orbiting red dwarf stars are less likely to freeze over.

How many potentially life-bearing planets are at risk of a climate catastrophe? 

ALL OF THEM!

These processes are basically universal and should happen on any planet with an Earth-like atmosphere.  Given that something around 10-50% of stars are thought to have Earth-sized planets in the habitable zone, that makes hundreds of billions of planets in our Galaxy alone!

Let’s put these climate catastrophes on our planetary death scale. 

Atmospheric and water loss are definitely sterilizing events.  But we don’t know exactly how often they happen, and they are probably not the bottleneck for life in many cases.  For example, Venus has a thick atmosphere that keeps its surface a balmy 462 degrees Celsius (864 Fahrenheit)!

Greenhouse heating is a fact of life for an atmosphere, and desirable to some degree.  But things can get out of hand. With enough greenhouse gases in a planet’s atmosphere its surface can become too hot for life. But the line between a normal climate and a climate catastrophe is not very clear.

For a planet like Earth a runaway freeze is bad on the short term, as it likely causes mass extinction of any life that can’t handle the cold.  However, on the long term a Snowball Earth phase is no big deal because our climate thermostat comes to the rescue. It may even be a good thing: the Cambrian explosion, a massive diversification of life, came immediately after one snowball Earth episode.

But wait! Earth’s ability to host life depends on its already having life.

Our climate thermostat — the carbonate-silicate cycle — depends in large part on the presence of life.  Biology plays a key role in both removing carbon dioxide from the atmosphere and in burying it in carbonate rocks on the ocean floor.  Without biology our atmosphere would have a *lot* more CO2 and our planet might not be habitable.

This is a chicken-and-egg situation.  If life is needed for a planet to be livable, then how does a planet ever get life in the first place?

Of course, there is the famous Gaia hypothesis, which proposes that life itself can help a planet maintain livable conditions. For example, consider the Daisyworld, an imaginary planet whose surface is covered with daisies, some of which are black and some white.  By adapting the relative number of black and white daisies the planet can regulate how much of its Sun’s energy is absorbed and reflected (its albedo).  If a process exists to select for black daisies when the planet gets cold (in order to absorb more energy) and white daisies when the planet gets hot (to reflect away more energy) the planet can regulate its temperature to maintain ideal conditions.

Of course, real climates are far more complex than the Daisyworld.  But it provides hope that some planets can avoid being destroyed by climate catastrophes!

Finally, climate catastrophes offer a ton of settings for science fiction stories. Here are four off the top of my head:

• An advanced civilization has engineered its own Gaia-like feedback system to maintain climate stability.  But what happens when a rogue technician with a political chip on his back tries to take down the system?
• A scientist discovers that her planet’s water is being sucked down into the mantle at a dangerous.  How will she persuade the government to react?  (Also, she has a pet tiger!).
• An ice world like Hoth starts to warm up.  As the seasons get hotter and hotter, the natives feel an approaching apocalypse. Will they adapt or die?
• A gas giant planet has multiple habitable moons.  A dangerously strong greenhouse effect on one planet triggers an inter-moon space war!

Seems like the right time for some dramatic music!

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Additional resources

• Introduction to the how planets die series
• The “chicken and egg” problem of life on planets
• The Snowball Earth hypothesis
• How orbital oscillations can create periodic Snowball Earths
• The greenhouse effect and runaway greenhouse
• The ice-albedo feedback
• My friend and colleague Franck Selsis, who helped me with several of the concepts in this post
• Planet death by tidally-driven volcanoes, which can in some circumstances trigger climate catastrophes