D+2: The Guide

In 2003, a Hispanic, an Anglo and a Jew walk into an observatory. This sounds like the beginning of a poor-taste joke, except that it actually happened. Because all three people involved (Chad Trujillo, Mike Brown and David Rabinowitz respectively) are badass astronomers, it resulted in the discovery of a dwarf planet called Sedna.

Sedna is very far away from the Sun, and is either on the outer edge of the Kuiper Belt or the inner edge of the Oort Cloud, depending upon whom you ask. Like most things that far out, she’s made mostly of ice and she doesn’t have a nice neat round orbit. Trujillo studied Sedna’s movement and concluded that when she was closest to the Sun, she would be 76 AU away (for reference, Neptune is 30 AU away) and at her furthest she would be an astonishing 937 AU, putting her on the near side of the Oort Cloud.

Sedna is a chunk of ice a thousand km across, which travels between two distances: “very far away” (we call this the perihelion) and “very, very, very far away” (we call this the aphelion.) It’s exciting to astronomers, but for the rest of us… well, are you interested? Be honest.

In 2012, Trujillo discovered a further lump of ice out in the void, which has not been properly named yet and so is called 2012VP113. He studied it and, along with Scott Sheppard, published a paper which told the world what its orbital measurements were. Again, astronomers were interested but the rest of us were busy with other things.

Trujillo and Sheppard then changed that: they pointed out that although 2012VP113 has a very different orbit from Sedna, they both have a similar perihelion. They not only both get closest to the Sun at around 80 AU, but both have a tilted orbit which tilts by the same amount at that perihelion point, and does it in the same direction. This might just have been coincidence, except that (as they pointed out in 2014) some Kuiper-belt objects have similar perihelions too. At this point it started to be a pattern, and the astrophysicists started to get interested.

Enter Konstantin Batygin, an astrophysicist. Astrophysicists study orbits, and we usually use computer simulations to do it. Batygin started studying this weird similar-perihelion pattern. What he noticed was amazing. See, most things in the inner solar system tend to have neat circular or nearly-circular orbits, and tend not to have tilted orbits. When you start looking at things that exist out past Neptune this stops being true. However, lots of the smaller Kuiper Belt objects have orbits which have a similar perihelion point to the one shared by Sedna and 2012VP113. Up until now nobody had thought to compare perihelions, but the more he looked the more he found a pattern. The crucial step was comparing a number called the “argument of perihelion.” Lots of the arguments of perihelion came out as around 300.

Space is big, and unlikely things can happen in it, but for arguments of perihelion to be the same is straight-up sorcery. By all rights they should change over the course of millions of years. Even if they were all the same once, the faint gravity from the gas giants in the inner solar system would nudge the outer objects into having different arguments of perihelion. Something had to be acting to make this happen.

Batygin then had a flash of genius: he started looking at the arguments of perihelion of other outer-system objects which are further out than 80AU. Most of them aren’t 300 (which is no surprise) but lots of them are closer to it than random chance would have it. It appears that some mysterious thing is pushing those objects’ perihelions towards that magic point.

The opposite of an argument of perihelion is an argument of aphelion. For objects closer to the sun than 80AU, the same mysterious thing would affect their aphelion instead of perihelion. When Batygin looked at their arguments of aphelion, he saw a similar pattern. One of the objects he looked at was the dwarf planet Pluto.

When Batygin saw Pluto acting funny, he knew that there was only one person he could talk to: Mike Brown. You may remember Brown from earlier in this essay, as one of the discoverers of Sedna. However, most people know Brown as the man who killed Pluto. (He is extremely proud of that, by the way; Brown’s twitter handle is @plutokiller.) Most importantly of all, the two of them are both based at Caltech and would pass each other in the hall every morning.

Batygin posed the question to Brown: there’s this mysterious thing that might be causing a lot of the weird orbits we see amongst the Kuiper Belt objects. It might even be the cause of Pluto’s weird orbit. We should probably check it out. Brown agreed. They managed to get a supercomputer, and started doing maths.

Supercomputer astrophysics is hard. You can’t just come up with the answer: you have to guess at what the answer might be, then check and see if you’re right. This takes time and often doesn’t give you any hints. They tried for a while. Eventually, however, they found a possible answer: it could be a planet. A distant planet with a tilted, eccentric orbit would give enough of a gravitational push to explain not only all the weird perihelion stuff, but also some other weird solar system things they hadn’t anticipated.

In January of 2016, Batygin & Brown published their paper, and it dropped like a dubstep bassline. Everyone was really excited.

The plan now is to refine Batygin’s simulations until we have a really good idea of where this mysterious guessed-at planet might be. Then, once we have an idea, we’ll give Brown the biggest telescope we have, point it at that place, and see whether we’re right or not. If we are, then there will be Nobel prizes to hand out.

For those who’ve been following my other column, this is very good news. For a long time we’ve assumed that the strange orbits of the outer objects meant that other systems would also have strange nonsensical orbits. However, if we find out that it’s all because of the influence of a single planet, then it means that space might be a tidier place than we think, which means that other systems might be tidier and thus your calculations will be easier.

Let’s wait and see.

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D-0: The Pixel

The following is the final piece in the Astroknowledge series. It was originally published on the Stellaris forums.

 

Welcome to the final episode of the astroknowledge series. Over the course of the last month, I’ve had a lot of fun and learned a lot; and I hope you have too.

Some time ago, I said that the Crab Nebula was my second favourite thing in space. Admiral Howe asked what my favourite was, and I said that we would cover it on D-0. So here we are.

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(Image is in the public domain, courtesy NASA.)

This photograph was taken in 1990 by the Voyager probe, at a distance of 40 AU. Those vertical bars you see are light pollution from the Sun. The subject of the photo is a single blue pixel, about halfway down the rightmost brown bar. See it?

That’s Earth.

This is a selfie taken by homo sapiens, holding out our camera as far as we can in order to get a good image. Every single human being that was alive on 14 February 1990 is in this picture. If you’re too young to be in this photo then you are descended from people who were in it.

(If you were alive on 14 February 1990, can you remember what you were doing? I was still a child in the Southern Hemisphere. I would like to believe that I was classy enough not to pull a duckface for this selfie, but I suspect I wasn’t even aware of it.)

This picture was taken from a distance which is less than Pluto’s average distance from the Sun. This means that it is still very much inside our Solar system. From another star, we would be so faint as to be impossible to see; they would have to use planet-finding techniques in advance of our own to detect our planet, and the presence of humanity on it would be harder still.

If anything happened to this pixel – and over the course of the last month we’ve discussed a number of those – then we would all be dead and there would be no future generations of humans, with only be a handful of artifacts left to remind the cosmos that we had ever existed at all.

Well, fuck that. Homo sapiens deserves a better ending. We didn’t work as hard as we did and learn as much as we did to go down like a bunch of chumps. We need to get off this pixel, even if it may take hundreds of years to develop the technology and gather the resources. In order to do that, we need to cooperate, to mobilise our society effectively and not to wipe ourselves out first. Every discipline, from physicists to poets, has an equally worthy part to play here. Either all our descendants live and we spread endlessly across the galaxy, or the final generation of humans will die on the same pixel that their ancestors evolved on. We’re all in this together.

This is the final exam paper that the gods set us. One question, pass/fail. Win, we live forever. Lose, it will be as though we never lived at all.

Let’s do this.

I will leave you with the thoughts of my hero, Carl Sagan, who was inspired by the photo above to write the following lines. I hope that his words inspire you as they have me, and I hope that I may have inspired you in turn.

From this distant vantage point, the Earth might not seem of any particular interest. But for us, it’s different. Consider again that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every “superstar,” every “supreme leader,” every saint and sinner in the history of our species lived there – on a mote of dust suspended in a sunbeam.

The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that in glory and triumph they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner. How frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds. Our posturings, our imagined self-importance, the delusion that we have some privileged position in the universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity – in all this vastness – there is no hint that help will come from elsewhere to save us from ourselves.

The Earth is the only world known, so far, to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment, the Earth is where we make our stand. It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another and to preserve and cherish the pale blue dot, the only home we’ve ever known.

D-1: Just Over The Horizon

The following is Murmeldjuret’s final guest column in this series. It was originally published on the Stellaris forums.

 

Now that we are getting close to release, you know what would really suck? Time slowing down or even stopping.

Gravity slows time down, very marginably. The GPS satellites experience time slightly faster than we do at the surface. It is only a few microseconds each day. The closer you are to a densegravitational source, the more spacetime is curved, and the slower time passes. If you follow this to its extreme, you can with a gravitational singularity stop time as we know it.

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(The black hole from Interstellar)

The most common singularity is what we call a black hole. We call it black because the most used description is that light can’t escape it. This is a simplification. Nothing escapes a black hole. The direction of out no longer exists. Just at the event horizon or surface of the black hole, the spacetime paths that light can take all are bent into leading further into the hole.

For an outside observer, anything falling in will slow down and be redshifted. The moment of it arriving at the event horizon, from the view of an outside observer, it will have stopped entirely. The redshift is now infinite as the time it will take the light to leave is infinite. That last light that never can leave is the event horizon. The event horizon is simply put a stitched together canvas of all the things that have fallen in. The surface is the last event of anything that has fallen in spread out for all future to witness. If a million years passed, that same event would be on the surface, still trying to send out the same light.

Now here is the weird bit. Black holes gain mass when things fall in. Yet nothing ever falls in, as time has stopped on the surface for everything outside the black hole. The weight is from all future events. So in a million years, its mass is felt outside, but its past is still stuck on the horizon.

So how does it look from the perspective of anyone falling in? For them time passes normally. It is like any gravitational free fall. Eventually the gravitational difference will be so great matter gets torn apart. Probably. For us on the outside, nothing happens in a black hole. It just happens to be a heavy collection of futures with a surface of pasts.

So what happens when to black holes merge?
This is a simulation of how the gravitational lensing would look like.
Oj287 is probably a supermassive binary black hole and it will merge sometime within 10000 years. PG 1302-102 is a very likely candidate for a supermassive binary black hole and if so it has already merged and the event will reach us in just shy of a million years. It will shine brighter than a supernova even though it is billions of lightyears away. We know that black holes merge, we have even detected the gravitational waves now, so even if black holes are still weird, logic defying things, we have found them.

We know what happens when two black holes collide from the outside. What happens on the inside? What happens when two events that do not occur collide? Does it even happen if nothing can observe it? What is even the inside of a black hole?

Quantum effects on the black hole surface create Hawking radiation, often called black hole evaporation. Entropy suggests black holes shrink, and it does indeed seem they can do so. Freeing up the events that have already happened as radiation. The universe truly is weird.

D-2: The Gift of Giving

This piece was written by Murmeldjuret as part of the Astroknowledge series and is reproduced here with their permission. It was originally published on the Stellaris forums.

 

Today I will be continuing The Beautiful Void’s astroknowledge series.

Right now you are emitting radiation. So are your walls. Don’t worry it is perfectly normal and won’t kill you. You should actually be glad the walls radiate back to you, if they wouldn’t you would start freezing. The same reason the walls radiate heat to you is the way we find the temperature of suns millions of lightyears away.

All light and heat is created when an electron loses potential energy. It is how lightbulbs, LEDs, x-ray cathodes, and forest fires emit light. Whenever an electron moves from a high energy state to a low energy state it emits the difference as electromagnetic radiation. Short jumps become low energy radio waves, and the longest jumps become gamma rays.

X-ray cathodes work by sending high velocity electrons at a target material, knocking out electrons from their orbit. Higher energy electrons fall into these holes, and emit X-rays while doing so.

Atoms and molecules also have kinetic, rotational, and vibrational energy levels. When they interact with each other, they change their internal energy levels. This is what we see as temperature. Hotter objects have more kinetic, rotational, and vibrational energy. Temperature is their ability to give off this energy to other things. When they touch another colder object, they will lose heat energy and the colder will gain that heat energy. Temperature equalizes because something that is better at giving than something else gives back will lose. This seems natural and is something humans notice quickly. Touching cold objects lowers your temperature, while nobody wants to touch hot coals because they love to give.

Heat has another way of transferring than simple touch, namely as radiation. The side facing a hot fire will become hotter than the side facing away, which is not due to the heat transfer via the air between. The vibrational energy can be transferred via photons, as well as normal matter. All things above absolute zero has vibrational energy and as soon as it interacts with another electron it will change its vibrational energy. Any loss here is emitted as electromagnetic radiation. Much of it is contained inside the object, but anything close to the surface has a fair chance to emit it out of the body.

This is then also related to temperature, or willingness to lose energy. The heat you radiate is absorbed by the walls and their radiation is absorbed by you. Heat radiation is for most parts marginal inside earth’s atmosphere. Air is a better conductor for heat than radiation. This is not the case in space. In space, all heat is exchanged through radiation.

The type of radiation is not dependent on the object, or the shape, or the substance. It is only dependent on temperature, the willingness to give energy. This is Planck’s Law, and describes how much and of what type of radiation an object emits. It is always shaped this way, as the total number of atoms is so incredibly large any oddities get marginalized.
[IMG]
We can from this estimate the peak temperature of emission, and it follows the very simple Wien displacement law. Wavelength = constant/Temperature. It is a good approximationexcept for really cold temperatures.

So what does this mean?
It means that whenever we look at an object in its spectrum, we can accurately give its exact temperature. Below is the sun:
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We can say that the surface temperature is 5778K within a few degrees of error. Similarly, we cantake the temperature of any distant object that we can spectrally resolve. It also shows why hot suns are blue, as their peak is to the left of the visible spectrum, and why cold stars are red, as their peak is to the right of the visible spectrum.

For those who wonder why we often call it black-body radiation, it is because the actual formula includes an emissiveness term, as radiation from object to medium isn’t perfect. Low emissiveness works like heat mirrors. The heat is never taken up by the object to be re-emitted. If emissiveness is max, it follows the curve exactly and this is called black-body radiation. On earth, almost nothing has true black-body radiation, but in space everything is close to true black-bodies.

If you look at the Planck law curves above you can see that things at room temperature (300K) would be far to the right of the visible (400-700nm). This places it in infrared, which is why we often talk of infrared as heat radiation. Like any radiation wecan see it. IR cameras can photograph things in room temperature, but this is always tricky as the camera itself gives off radiation.

As things grow hotter, their wavelengths become shorter, and eventually what is normally considered heat radiation in infrared turns into visible radiation. When things grow even hotter than the sun they begin emitting their peak in UV, and eventually the hottest things at millions of Kelvin emit X-rays from heat alone. As things grow colder they become redder to eventually be invisible to our eyes. Below is a piece of iron at my guess around 1300K, or 1000C. You can see the hottest part appear white, and as the metal grows colder it grows redder to eventually be outside thevisible spectrum entirely. Light and heat is also reflecting off the hammer above and the anvilbelow.

Spoiler: Ironworking

[IMG]

So this is how we can say how hot a sun is, regardless of distance, because it is the shape/colour/spectrum of the light, not only thestrength that depends on temperature. The lightfrom distant and nearby stars include an indicator of heat and total luminosity in their light.

Heat is wonderful thing and all it wants to do is give. So when a superhot sun is melting your ships, just know that all it is trying to do is share some of its warmth. And your cold metal shell is unable to give the same warmth back.

D-3: The Tyrant

The following was written after many requests, and was originally published on the Stellaris forums.

Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.

– Dylan Thomas
Throughout this series I’ve had a lot of requests. Some of them are for things I was going to cover anyway, like Gliese 667Cc. Some were for things I hadn’t planned to cover, like Pluto. Some I initially said no to and then changed my mind, like the Great Attractor.

None of them attracted as many requests as this one did. So let’s do it. Let’s talk about Betelgeuse.

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(Image courtesy NASA)

The Crab Nebula is deadly, and if you go there you will die far too quickly for the death to be horrible. Sagittarius A* is beyond deadly and you won’t even get close to it without dying horribly. Wolf-Rayet 104 has a very slight chance of being exceedingly deadly. Pluto is deadly in its own way, as is the gravity on Latham’s Planet and the icy cold of the dark side of Zarmina. Even the vast dust fields around Fomalhaut will kill you pretty quickly if you go there. But friends, these are weaksauce compared to Betelgeuse. Betelgeuse is the gold standard of things in space that hate you. When death came for Betelgeuse she wrestled him to the floor, kicked his ass and stole his crown.

Once upon a time, Betelgeuse was a main sequence star like any other, born in the enormous hydrogen nebulae of Orion’s Shoulder, wheremany stars are born and die. She was a big star, bright and blue and short-lived; and for reasons we’ll never know she decided that the best way to spend the few million years of her lifespan was to be violently catapulted out of Orion’s Shoulder and go wandering.

By “wandering” I mean she’s moving so fast that she’s got a bow wave. Space is mostly empty, but not quite: there are a few molecules scattered here and there across the vaccuum, and Betelgeuse is going fast enough that these are offering resistance. O-class stars don’t live long, but Betelgeuse didn’t plan to spend that time amongst her peers.

Not that she has peers.

Eventually, Betelgeuse burned through all her hydrogen. When that happened she swelled up into a giant, started fusing helium, and we can imagine that the galaxy breathed a sign of relief. Surely they were safe now? But no, death just made Betelgeuse angry. She didn’t even lose her blue colour, that’s how hot she was.

Eventually she burned through her helium too. Surely now she would go? Nope. You underestimate just how much Betelgeuse likes burning things. You probably can’t light ashes and smoke on fire: similarly, most stars can’t fuse oxygen and carbon. In this, we see that most stars just aren’t as committed to burning stuff as Betelgeuse is. She swelled up even further and started fusing oxygen. When she ran out of oxygen, she swelled up even further and started fusing yet heavier gases.

Around 100 BC, a Chinese astronomer noticed Betelgeuse and remarked that she was a beautiful yellow colour; so beautiful, in fact, that he proposed that she be used as the standard reference point for a yellow star.

A millenium later, Arab astronomers noticed Betelgeuse and remarked on her beautiful red colour. Not having read the work of their Chinese predecessor, they suggested that she be used as the standard reference for a red star. Her name comes from them: “Betelgeuse” is a corruption of the Arabic word for “a reddish colour.”

In between these two sightings, you might think that Betelgeuse cooled down: red stars are cooler than yellow stars, after all. However, you would be mistaken.

The Arabic astronomers noticed another puzzling thing, which also puzzled Europeans when we finally got our act together and stopped being so backwards: Betelgeuse changes luminosity drastically. She’s traditionally catalogued as the ninth brightest star, but she sometimes goes ashigh as seventh brightest and at other times diminishes to tenth. Many stars change like this, but they normally do so predictably and on a strict pattern. Betelgeuse is utterly unpredictable, sometimes changing in just days.

[IMG]
(Image courtesy NASA)

This isn’t because she hates us. Well, not just because of that. Most variable stars vary because they’re rotating and one side is darker than the other. Betelgeuse, by contrast, is doing it because she’s surrounded by a shroud of her own blasted-off, cooling outer layers. These extend out up to 30AU, which means that if she was in the Solar system they would reach the orbit of Neptune.

Because of her bow wave, sometimes this shroud of gas gets a little thinner, and then we see the hotter, angrier star below it, and we realise that Betelgeuse still burns with a hatred and a hunger that never went away.

[IMG]
(Image courtesy ESA / Herschel)

Even more frighteningly, occasionally a bubble of hot gas comes up from the layers close to her core and reaches the surface; and when this happens we realise that Betelgeuse is still blue on the inside. This whole “giant red star” thing is a costume she’s donned, a ruse to fool us into complacency.

Because of her unpredictability, there is no such thing as a safe distance from Betelgeuse. If you are orbiting her then she might kill you at any moment. If you don’t want to be killed at any moment, then stay away from her. Stay light years away from her. This is the only warning that you will get.

D-4: Waterbending

The following is Admiral  Howe’s second (and last) guest post on the series. Originally posted on the Stellaris forums.

 

Hello again! Admiral Howe here for a repeat performance. Thanks once more to The Beautiful Void for being gracious enough to offer me a turn in the rotation for his awesome series. He, single handedly, has made this period of super-hype go far more easily. Especially for my hype-addled mind. :D

So, last time I left you with doom, gloom, and a whole ‘die in fire’ motif. It wasn’t my intention to be such a gloom-bearer so, today, I’ll balance my firebending with a bit of waterbending.

Water? Really?

[IMG]

Yes. Free surface water makes Earth the crown jewel of Sol’s system – a fact we all raise a drink to I believe. This gleaming azure orb spins through space draped in a mantle of 1,400,509,000 cubic kilometers of liquid water. That’s enough water to mantle Mars in oceans averaging 10km deep. (Take that Kim Stanley Robinson!)

[IMG]

But that water is here and it’s vital to the biology we all rely on (and really, it would suck to try to move it). This information is nothing shocking to moderately well informed folks. Yet, like with volcanoes, that’s only a small portion of the story…for water plays a vital role in Earth’s geology. And that role is likely repeated on any water-bearing terrestrial planet.

You see, water runs this show.

Take these two winning folks here:

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and

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First you have anhydrite, also known as calcium sulfate, or CaSO4. Second you have gypsum, also known as calcium sulfate dehydrate. Yes, CaSO4*2H2O. Same chemistry as anhydrite but with a tiny amount of water added. In fact, anhydrite will spontaneously change to gypsum when exposed to water in normal circumstances – something we exploit in plaster of Paris.

But, more importantly here, the two crystals are different size – gypsum is considerably larger because of the absorbed water. The difference is enough that adding water to even the hardest rock rich in anhydrite will cause the rock to disintegrate in minutes as the gypsum crystals push apart the rock’s matrix.

Now imagine siting an important piece of infrastructure on bedrock containing anhydrite. Thank goodness no one would ever make such a mistake.

Mineralogical cycles and processes change dramatically when water is added in a vast number of contexts. Venus may be our twin in mass and density but the rocks and processes we find on her surface will never look like Earth. And Mars may have once been much warmer and wetter but in its desiccation even the rocks themselves changed as their water bled away.

Where this is most dramatic, and most impactful for us, is with the igneous rock known as basalt. I touched on these before when I mentioned the Siberian Traps as those Large Igneous Provinces are uniformly basalt and thus can flow for great distances until they cool.

[IMG]

Once they do that water begins facilitating rapid weathering as the iron and magnesium rich basalt will react with most chemistries mobilized by water – especially if the planet also has oxygen available. In fact, the longer basalt is exposed to water – and water gets a chance to percolate through the cracks and fissures in the cooling rock – the more it can weather and alter the basalt into new minerals. And this is vitally important because the seafloor is uniformly basalt.

[IMG]

This map shows the seafloor with the ages marked (key is on the bottom left). You can clearly see the globe-spanning Mid Ocean Ridge, the single greatest mountain range in the solar system, and you’ll notice the seafloor is brand new right at the ridge.

[IMG]

We know these ages of the seafloor because of yet another mineral, an iron mineral named magnetite. This mineral earned its modern name because it is naturally magnetic (historically it is known as lodestone) and when it crystallizes it does so in alignment with the Earth’s magnetic field.

[IMG]

Every reversal is recorded, world-wide, precisely on the seafloor because of this and – when combined with land-based research – lets us pin down the ages of most of the floor. It’s actually amazing that once we discovered this (another surprise from WW2) we not only rapidly established plate tectonics as our working theory of Earth’s crust, developed a magnetic-field calendar for earth to let us age-date large portions of the world, and also could establish spreading rates for the seafloor current and past (and find they’re not uniform).

While this seems like a neat aside, bear with me a bit longer…because water is responsible for this too. If you look back up three images and take a look at the ages of the seafloor something should jump out at you.

Go ahead, take a moment.

I’ll still be here.

Really, where would I go before Monday?

Yeah, you should have noticed it…on our 4.6 Gy planet the ocean floor is no older than about 200 million years. You’d think losing 4.4 Gy of ocean floor is embarrassing but, it’s not. It was a revelation.

What happens is after hundreds of millions of years of cooling and mineralogical alteration by water basalt reaches a point where it is now denser than the mantle beneath it. When this threshold is reached, even if there isn’t any plate collision pushing against the basalt, the seafloor falls away and begins sinking back into the Earth.

This is known as subduction and the pull of subduction is what powers the whole tectonic engine, from driving the continents across the globe to pulling open the Mid-Ocean Ridges and leading to new crust formation.

Just that by itself would be enough to keep me fascinated for a whole career…cold, wet seafloor diving into the abyss while causing new seafloor to be born thousands of kilometers away. A whole planet’s face constantly rearranging simply in response to this endless conveyor belt.

But wait, there’s more.

One more bit about water here and that’s what happens when the old seafloor begins its death dive. Increasing pressure and heat begins to force the water in the seafloor plate out and into the mantle. And there, like the anhydrite->gypsum reaction above, dramatic changes in the minerals take place.

[IMG]

The first effect, and perhaps the more obvious one, is the water entering the mantle lowers the melting point of the surrounding rock. That melted rock is, naturally, more buoyant than the solid rock around it and it rises only to erupt in long chains of volcanic mountains mirroring the subduction off shore. It’s why the Ring of Fire is where it is and why, someday, a new one will form around the Atlantic when it’s that ocean’s turn to disappear again.

The second effect is the more profound one. As the melted magma rises it carries with it the water from the subducting plate’s water. This water changes the nature of the melt and thus the rocks being formed by the magma as it reaches the surface. Instead of basalt erupting the new melt – a combination of water, mantle, and melted crustal rocks passed through on the way to the surface – you get andesite and rhyolite. Or, more commonly, continental rocks.

[IMG]

Oceanic crust never gets more than 5-10km thick, as shown above, but continents can grow over 70km thick – with parts of Eurasia potentially 120km thick. The reason for this is buoyancy once again. Continental crust is significantly less dense than the mantle and cannot be subducted baring incredibly specialized circumstances. Once you form continental rocks they remain for the rest of the planet’s life gradually accreting into larger and larger continents. And there lies what I think is the the biggest magic water has on the Earth…without the plate tectonic system we wouldn’t have the very continents as we understand them. Their diverse terrains, soils, climates, and resources are a byproduct of the tectonic system pushing and pulling them all over the globe.

All because a little water got into the system.

And somewhere out there, maybe Kepler 186f, this same process is building and reshaping the continental faces of unfamiliar worlds our descendants may one day lovingly call home.

Thanks for reading.

I would like to thank The Beautiful Void one more time for giving this rambling author a chance to guest spot his wonderful series.

D-5: The King

The following was originally published on the Stellaris forums.

 

Welcome to another episode of the astroknowledge series! We’re approaching the end now, and I’ve been saving the best for you.

The search for habitable Earthlike planets is full of hype. NASA and ESA are both guilty of feeding people’s imaginations with (often absurdly) overoptimistic portrayals of every new planet we discover. Some astronomers like Steve Vogt, the discoverer of Gliese 581g, are inspiringly optimistic and hopeful people who see only what could be. Scarcely a month seems to go by without a lurid “Most Earthlike Planet Ever” headline. For the rest of us, particularly for pessimists like myself, this often gets annoying.

Therefore, it is nice to be able to discuss a planet which is unquestionably as good as it seems. This is a planet which Guillermo Torres (a profoundly realistic man) rates as a good prospect, and evenAndrew LePage cannot find any issues with.

Let’s talk about Kepler 186f. In 2014 she was discovered by the transit method which means we know her radius but not her mass, along with five other closer planets, orbiting a small, cool red star. The optimists portrayed her as the very twin of Earth. The pessimists have tried very hard to disagree, but we can’t.

Kepler 186f is probably habitable.

Throughout this series I have been trying to paint the rosiest picture possible, saying “may” instead of “probably does not” whenever possible. Here, I don’t have to. Even I can breathe out a sigh and say that after nearly two years of intensive study, Kepler 186f is a greater than fifty percent bet. In fact, she’s so good that SETI have been listening intently for radio transmissions from her. (So far nothing.)

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(Artist’s impression courtesy NASA)

Let’s talk about the king.

Kepler 186f has a radius 1.1 times that of Earth; if she’s Earthlike then her mass will be about 1.4 times that of Earth, meaning that her effective surface gravity will be about 17% stronger. I’m 70kg; on Kepler 186f I would move like someone who’s 82kg. This isn’t even that noticeable.

Kepler 186f is on the outer edge of the habitable zone, but is still inside what’s known as the “conservative habitable zone”, meaning that we don’t need to invent fanciful scenarios for how she’ll be warm enough. A reasonable level of carbon dioxide will be enough to put her above 0 C. This will allow liquid water on her surface and water vapour in her atmosphere.

The problem with planets close to their stars is that they’re often tidally locked. In the case of Kepler 186, the first five of her six known planets are almost certainly tidally locked. Kepler 186f, however, is probably not tidally locked yet.

(Tidal lock is a matter of time: unless a planet has one or more moons large enough to prevent it, or unless the planet or star dies first, the planet will eventually become tidally locked. The bigger the planet is and further out it is the longer it will take.)

What would it be like to live there?

She’ll probably be on the way to a tidal lock, meaning that her day/night cycle will be slow, with each “day” taking weeks to months. This will likely cause intense weather as temperatures on the day and night side seek to equalise.

On the plus side, we think that her orbit is extremely regular and she has hardly any axial tilt, so she’ll have extremely mild seasons if she has any at all.

It is not only possible, but probable – and it feels amazing to type that – that if there is water on Kepler 186f, humans will be able to live there. The only thing stopping us is that 182 parsecs (500 light years) is a long way.

The fact that we know of 5 other planets in the system means that there will probably be lots of other stuff orbiting: asteroids, comets, stuff like that. The planets around Kepler 186 are close enough to the star that moons will have to be very small, but they might still exist. Moons would act to prevent tidal locking and make her even more habitable.

Oh, and she’s probably within a billion years of Earth’s age, which means that if she had a biosphere, it may have developed to similar levels. If life exists outside of Earth, this is our best shot at finding it.

This is it. This is real. We should go there.

Hail to the king.