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.)

(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.

D-6: Only A While

The following was originally published on the Stellaris forums.


Drink up my darling
Be brave and smile
Death is for ever
Life’s only a while
Hello and welcome to another episode of the astroknowledge series! Today I’m going to talk about a place which I find utterly enthralling. This is not particularly interesting from a scientific point of view, but as a human I can’t get over how evocative it is.

In 1993, Artie Hatzes, an American astronomer living and teaching in the pleasant Thuringian mountains of Germany, developed a hypothesis that there was a gas giant orbiting the nearby giant star of Pollux. At the time there was very little way to tell (even Latham’s Planet was only confirmed in 1991), but Hatzes kept his idea in mind. Technology soon improved and in 2006 he was able to confirm that it existed. The planet has had various designations but in 2015 it was named Thestias.

(Artist’s impression courtesy IAU.)

Thestias is not habitable: it is a gas giant and is far too close to Pollux for its moons to be habitable either. However, it may have been within Pollux’s habitable zone back before she became a giant star. This means that if anyone was living there, they would have had to evacuate.

(For the moment, we’ll ignore the fact that Pollux is too young for Thestias to have developed intelligent life. Earth took ~600 million years to develop even single-celled life, which is more or less Pollux’s entire main sequence lifespan.)

They’d have had millions of years to evacuate, especially if their astronomers were competent enough to spot it coming, so it’s not as if this would have taken them by surprise. Entire civilisations can rise and fall in that time.

However – and here’s the interesting thing – it is likely that where there’s one planet, there’s more. This means that when Pollux became a giant and her habitable zone moved outwards, some planets might have become habitable. The new habitable zone would be somewhere around 6-10 AU, which in our Solar system places Jupiter just outside of it and Saturn just inside it.

It wouldn’t last long, of course. Pollux is 742 million years old and will not live to see her nine hundred millionth birthday. This habitable zone is only temporary But even a hundred million years is a while.

What would it be like to live there?

The evacuation of a planet sounds like a rushed affair, but remember that they have advanced warning and plenty of time. However, the expansion and first dredging of a star is sudden, and Pollux’s habitable zones would not shift gently. The two planets would not be habitable at the same time. Therefore while the evacuation of people from the moons of Thestias may have taken generations, they would have been fleeing to a planet which has no atmosphere and no liquid water. Domed cities and tunnels would be their best bet here. It would not be a pleasant life for the first few generations of colonists, and they may not be pleasant people. There’s work to do here, but most of it consists of waiting and preparing.

It’s only once Pollux swells from a white star to an orange giant that Old Thestias can truly be abandoned and the work on New Thestias can begin. Asteroids can be gathered and melted for air and water. After a few centuries, the biosphere can begin to grow. People can emerge from their tunnels and domes to walk freely under the sky of their new home.

In a few hundred thousand generations they’d have to do it again, of course, and this time they would have to take a much longer journey. All that they’ve gained is time. But time is enough, because life is good at enduring, especially when that life has radio telescopes and supercomputer simulators.

Don’t cry for the people of New Thestias, because you should be commiserating: we’re in the same situation. The Sun will give us perhaps another 5 billion years before Earth becomes uninhabitable. If we survive until then, which I think we will, we’ll have to evacuate the same way – and inconveniently our new habitable zone will fall around Jupiter, none of whose moons are massive enough to hold an atmosphere. We’ll have to take the long journey out to the stars.

We’ve got perhaps two hundred million generations left on this planet. That’s enough time to think in the long term, even if it is only a while.

D-7: The Bride

The following was originally posted on the Stellaris forums.


A big thank you to Murmeldjuret or another fantastically informative weekend! Coming up this week we have all sorts of interesting stuff for you in the astroknowledge series. We’re close to the end now, very close. I’ve been holding back some of the best stuff in space until now, so as to close this series with a bang. As a result, I hope that this week will be as good for you as it is for me.

We’ll also have another guest article from the pen of the delightfully learned Admiral Howe.

Let’s talk about Pollux. This is a star that humans have known about for a very long time; together with Castor it makes up the two bright stars that the Gemini constellation is named after.

(Image courtesy astropixels.com)

Once upon a time, back before humanity existed, Pollux was a white A-class star on the main sequence. Sadly stars of this sort, while very beautiful, don’t live very long. Let’s look at what happened next, to understand what happens to stars when they reach the end of their lives.

Gas falls in
Light comes out
That’s what stars
Are all about

A star is a balancing act. Gravity pulls gas inwards towards the core, and light (and other radiation) that streams out of the core pushes it back. This might seem weird to you if you aren’t used to thinking of light as being something powerful enough to hold up a heavy weight, but this gives you an idea of just how bright the cores of stars are. We call this “thermal pressure.” *

When Pollux exhausted the hydrogen in her core, this light began to fail. Gravity, which had been kept at bay for hundreds of millions of years, is a patient force: the core contracted and enormous amounts of fresh hydrogen poured into it.

What happens when you pour enormous amounts of fresh fuel into a dying fire? It doesn’t just restart gradually: it explodes.

This sudden reignition of the core pushes back against gravity hard. The outer layers of the star are blasted outwards, as is a lot of the still-burning hydrogen that just began to flood into the outer core. We call this the “first dredge-up.”

At this point Pollux became vastly larger, which is where she is today: she’s only twice the Sun’s mass but is almost nine times her radius and thanks to her enormous surface area is a full forty-three times as luminous.

(Image courtesy NASA. That small yellow thing next to Pollux is the Sun, included for size comparison.)

Pollux, which had been a bright white star for a few hundred million years, became a larger, cooler orange giant. Her outer layers are the only part we can see, which create her orange colour. Deep inside, her core has now started to fuse helium instead of hydrogen. This gives her only another few hundred million years of life. When this fuel source ends then it will be like before: her core will contract, bringing in more helium, which will cause a second dredge-up. She’ll get even larger as her outer layers inflate even further, and her surface will be even cooler and redder.

At this point her core will be full of oxygen and carbon, which she isn’t heavy enough to fuse. She’ll continue to glow but this will be from the outer layers rather than the core. Eventually the outer layers will fade and drift away, creating a nebula that slowly cools to black; and a core of oxygen and carbon, still glowing white from residual heat.

Don’t weep for Pollux. Space is a fantastically good insulator, and so she’ll cool extremely slowly. While her life up until this point will have been less than a billion years, her lifespan as a white dwarf is projected to be considerably longer than the current age of the universe. Everything up until this point has really just been her childhood: a brief hydrogen- and helium-burning phase she went through before she settled down to be what makes her truly happy: being small, and beautiful, and white as snow.

Tomorrow we’ll talk about her planet.


* Thermal pressure is a combination of radiation pressure (that is, light) and convection pressure (that is, hot gas rising.) For small cool stars convection pressure dominates; for larger hotter stars radiation pressure dominates.

D-8: I Spy From My Little LEO

The following was originally published on the Stellaris forums by Murmeldjuret as a guest post on my series. Reproduced here with permission. Thanks, Murmeldjuret.


Today I will be continuing TheBeautifulVoid’s astroknowledge series.

In 1728 Newton first described a hypothetical cannonball that would orbit the earth like the moon did. An orbit, like the moon, but artificially constructed. In 1903 Konstantin Tsiolkovsky calculated the exact speeds and dynamics needed for artificial satellites, and proposed using liquid fuel rockets to lift them there. Since the launch of Sputnik, United States Space Surveillance Network has tracked 26000 objects in space, of which now only 8000 remain, the rest succumbing to the fiery grave of reentry. Today, there are roughly 1000 operational satellites orbiting our planet.
There are many types of satellites, the four main being:

  • Communication satellites (satellite tv, mobile phones, internet etc)
  • GPS satellite array
  • Astronomical satellites looking outward towards the stars
  • Earthward satellites looking down on earth
Spoiler: Satellite Orbits


Up until around 600km altitude, atmospheric drag is significant enough to decay orbits and eventually send the satellite into reentry. Therefore most LEO satellites are in the 700-800km altitude region. The most common orbit for earthward satellites is sun-synchronous orbit, in a semi-polar orbit. This means that they are above the same spot on earth at the same time of day. Useful if you want to take images of the ground in broad daylight, when the sunlight is the brightest and you get the best readout on your instruments.

The main method used to monitor things is namely passive remote sensing, using the light of sun reflecting off what you want to see. Our eyes are passive remote sensors. This differs from active sensing that sends out its own signal to be reflected, such as RADAR or laser inferometry. Regardless, the atmosphere between the satellite and the ground has to be filtered out. The same atmosphere that makes ground to space imaging difficult makes space to ground imaging difficult. This is however, an easier problem as we know what we are looking at. Unlike ground to space that are placed above the cloudline, what we want to see here is usually below the cloudline, and we will thus not get any data for most wavelengths.


Geographic information system (GIS) is the term for mapping the appearance, distance, and topography of the planet’s surface. All have problably seen the various map applications today. Look at google maps of a major city or university town and you can probably tilt the view to see an approximate 3D view inferred from taking multiple pictures at multiple angles. You can actually see anything that is big enough to be above noise including heights of all trees, cars, and hedges. While a more time-consuming method than just flat images, it is remarkably effective. It is even possible to show what the city would look like from the ground, even though it is taken at an altitude of above 600km. Here is the Lund Observatory as approximated in 3D shape as seen from space.

Vegetation Monitoring

Photosynthesis absorbs a specific band in the visible while green leaves reflect a specific band in near-infrared. The ratio between these forms the basis of something called vegetation index. An improved version, the Enhanced Vegetation Index, or EVI is being used more and more to determine the health of plants, farmland, forests, grass, and even plankton. See the top left of image below for Vegetation Index. Bottom left has a simple contrast false colour between absorbed visible and reflected NIR for plants. Satellites are better at telling us what is alive and what isn’t than standing 1m from it and looking at it with only our eyes.
(Image from DigitalGlobe, pretty worthwhile site if you want to see commercial satellite imaging)


With spaceborn interferometers you can measure phase shift from distance between orbit and surface. This is a remarkably exact height measure. It is used to measure glaciers, mountains, and the overall topography. Even more interesting is seeing them vary with time. How much does a glacier shrink every summer to grow back every winter, and how is the mean varying over the years?
The above image is a famous inferometer measurement of the Izmit earthquake in 1999. It measures the total change in height in a wavelike pattern from 0-2.8cm. The line in the middle is the faultline. Since any measurements from the surface would also be from an unknown height it has previously been hard to precisely measure how earthquakes change topography. But a satellite orbit cares nothing for earthquakes, and suddenly we have invariant measurements of earthquake dynamics.

Atmosphere Composition

Just like you can measure atmospheres of exoplanets with transit, you can do the exact same thing with earth. Measure our atmosphere’s spectrum by having it between a satellite and the sun/star. Since we can position a satellite with a specific altitude of atmosphere between it and the lightsource we can determine the atmospheric composition of different altitudes and locations. The ozone hole, the carbon dioxide concentrations over cities, the methane from cow farts, the water evaporation from warm sunny days, it can all be measured with both height and geographic location.

Age of Snow

One of the weirdest data I have seen is a system that measures the age of snowfall. Freshly fallen snow is loosely packed with pockets that can reflect light internally before emitting it to space, whereas older snow loses these pockets. Measuring the ratio of directly vs indirectly reflected light will tell you quite reliably how old snowfall is. Daily maps of ski-resorts telling the quality and type of snow for the resort might actually be a thing in the future.


Can you detect crime from space? Yes you can. Using vegetation index from before, you can track forestry, and thus also illegal deforestation. Both smoke plumes from burning fields and a lack of photosynthesis can be seen from space, and has been used as evidence. Big Brother is watching.


Can you see religion from space? To some degree, you actually can. With muslim attendance day being friday, jewish saturday, and christian sunday, you can map CO2 emissions over denser cities and regions over many weeks, and the statistical deviation on friday-sunday actually corresponds to religious denomination of the region and is statistically significant. (Now I don’t want a religious debate of any kind in this thread so please refrain from starting one.)

There is an apocryphal story that during the recent conflicts in Afghanistan a military satellite tracked the footprints of an ambush group back to their base. While I doubt the credibility it is actually not unthinkable. Satellites are getting to that level, and what they can tell us is surprisingly much. It is cheaper to buy an image of your town from one of several satellite companies than it is to send out a helicopter or plane to take it from the air. For those who dislike agriculture growing dependent on a more and more complicated web, how is satellite imaging as a strand in that web? 100 minerals for +1 food anyone?

D-9: The Iron Age

The following was originally published on the Stellaris forums by Murmeldjuret.


(Today we will only discuss the visible part of the universe or 4.6% of the total mass. So when I use %mass I mean % of the visible mass.)

Carl Sagan once said we are all made of stardust. There is truth in it. 90% of our mass has been forged in the fusion reactors that power stars. Though it is equally inspiring that the 10% that is hydrogen in us is the same hydrogen big bang created during the recombination 13.8 billion years ago. 10% of your mass is 13.8 billion years old. A lot of (note understatement) hydrogen and helium was created during this recombination, and it is still the same hydrogen that powers the stars.

For most of a star’s life it fuses hydrogen into helium, but as helium content grows and hydrogen content shrinks, it will eventually start fusing helium. This is usually the end of a star’s regular life as the core process that powers it and keeps it in balance falters. The results depend on startype. The new stellar reactor begins fusing helium into heavier elements. See chart below. The heaviest naturally occurring element is plutonium, though it is possible heavier elements have formed earlier in the universe but have decayed to plutonium or lighter by now.

Spoiler: Nucleosynthesis table



Above is a chart of the different types of elements in the solar system. The same ratio is about correct for the universe in general, except hydrogen andhelium are even more common. Hydrogen is about 74% by mass and helium is about 24% of the mass in the Milky Way. That leaves 2% to be shared by all the other elements. We thus say the Milky Way has a metallicity of 2%. Chemists will probably not consider carbon a metal, but since anything heavier than helium has to have been made after big bang it is a more clear distinction from an astronomical perspective.

Interestingly enough, the third most abundant element is oxygen, with a much higher concentration than any element heavier than helium. In fact, a full 1% of the galaxy is oxygen. This is why ice is not uncommon in the universe, it being a stable bond of hydrogen and oxygen, the most and third most common elements. The fourth most common element is carbon, so score one for carbon lifeforms on watery worlds being the most abundant.

Another interesting thing to note from the curve is that even numbers are much more common. This is due to helium being the main source of heavier elements and helium is two protons. There is no way to get the odd numbered elements without something to alter the regular process. Of the ten most common elements by mass, only nitrogen and hydrogen are odd numbered.

The next thing to note is the peak of iron (Fe) and nickel. Iron is a relatively heavy element, but it is still the sixth most common element by weight. 0.1% of the visible Milky Way is iron by mass. All planetary cores are iron and/or nickel based, and iron can be found everywhere. This is strange, element nr 26 beats 21 preceding elements in abundance. The reason is actually quite straightforward. Iron is the most stable nuclei in the universe. It has the lowest binding energy of all nuclei. All the lower elements give net energy when you fuse them, and all the heavier elements give net energy when you split them. There is no way to change iron without adding energy, both splitting or fusing iron costs energy. When the core of star turns to iron the process that powers a star turns to a process that takes energy from the star.

Fissile elements usually decay to lead, see its peak (Pb). The heaviest stable element lead-208 is still radioactive but with a halflife of 10^19 years. Compare that to the 10^10 years the universe has existed. Everything heavier than iron might be radioactive but with halflives unimaginably long.

The hydrogen created at big bang is limited, and it is being burnt in every star, in every galaxy, in every cluster, every second. For each atom of iron it creates, there is less fuel left to power the universe. If nothing else ends the universe as we know it, the hydrogen and helium will run out, and all that would be left is a universe sized cloud ofiron glittering from the light of long gone stars.

D-10: The Cloud World

The following was originally published on the Stellaris forums. No pictures today, sadly.


Once again, a big thank you to Admiral Howe for terrifying us with an explanation of how safe it is to stay on Earth. He’ll be back next week to share more knowledge.

Let’s talk about a planet that we might flee to if and when the Siberian Traps recur and all multi-cellular life needs to find a new home. (Good luck, archaea! Let us know how it turns out!) We’re going to discuss Kepler 442b.

You know the drill by now: first we ask what the star is like. Kepler 442 is K-class, which means she’s orange and slightly smaller than the Sun. Through an Earthlike atmosphere her light would look yellow and there’d be about two-thirds as much of it. We’ve seen worse: Gliese 667Cc, for example, only gets reddish light at 20% of Earth levels. Kepler 442 is, however, quite metal-poor. Because planets and stars are normally formed from the same material, this means that the planets in the Kepler 442 system would be too.

Kepler 442b is a little larger and a little heavier than Earth. The extra size helps to offset the increased pull of gravity, meaning that if you stood on her surface you’d probably only feel 30% heavier.

(We cover a lot of larger-than-Earth planets on the astroknowledge series. This is because they’re easier to spot. There are probably lots of smaller-than-Earth planets too, but we haven’t found them yet.)

I’ll come out and say it directly: unless she had a thick atmosphere, Kepler 442b would be cold. Her surface temperature on the equator during the day would be 233K, which is -40C. However, if she had a thicker atmosphere then she could retain much of this heat and nurture a greenhouse effect.

Too much greenhouse effect, of course, is a bad thing. It’s all about balance.

Greenhouse effects are caused by triatomic gases in the atmosphere: that is, gases which have three atoms per molecule. While there are many gases which are triatomic, the most famous and probably most important are carbon dioxide and water vapour. Climate scientists have modelled atmospheric carbon dioxide extensively, and we’re just starting to learn about what water vapour does in the atmosphere. While it’s not my field, it’s my understanding that water vapour is much better at causing a greenhouse effect than carbon dioxide is.

However, if the planet is below 0 C, there’s going to be no water vapour in the atmosphere because it would be frozen. This means that we’ll need enough carbon dioxide to get the greenhouse started and get the temperature above 0 C, at which point water vapour can start to amplify the process.

In other words, if Kepler 442b doesn’t have enough carbon dioxide then she’ll be an arctic planet. If she does have enough, however, then she could be quite warm.

How warm?

Answer: pleasantly, the answer is “not too warm.” As water vapour increases in the atmosphere it forms clouds. Clouds are white [citation needed.] White things reflect heat. Therefore, the hotter the planet gets the more clouds it has and the more of Kepler 442’s light it reflects away. We call this albedo.

James Lovelock pointed out that this creates a self-balancing system: a hotter planet has more clouds and so has a higher albedo, which means it receives less light and so cools. A cooler planet has a lower albedo, so it receives more light and so warms up. He then got very weird and mystical about it but the principle itself is sound.

If Kepler 442b has a greenhouse effect dominated by water vapour (like Earth does) then her temperature will stabilise between 0 C and 100 C, which is good news because that’s what humans find comfortable.

Considering Kepler 442b’s higher gravity, her atmosphere will be denser than Earth’s, which means that she may well be more humid. This, in turn, means that where Earth has deserts she’ll have jungles, and where Earth has plains she’ll have swamps and forests. Plant life loves humidity, and even if it isn’t hotter it feels hotter and more unpleasant.

What would it be like to live there?

The very densest forests are the sort known as cloud forests: they grow where the air is saturated with water, so that plants can feed directly from it rather than needing deep roots. In the cloud forests, trees grow to immense heights and are covered with secondary vegetation. The humid air of Kepler 442b might lead to this being common.

Innately fertile regions often have extremely fragmented government. If every glade can fend for itself without the help of the next, and if it’s possible to escape a lord’s rule simply by moving elsewhere, then the lord has less ability to enforce their will on the people. In historical Europe, lords tried many elaborate schemes to prevent people “voting with their feet” in this way, from serfdom to border controls, but none of it worked very well. Society naturally fragments into lots of little states, each with their own unique slant on government, philosophy and religion.

Humid, fertile regions are also cauldrons of disease. This would encourage primitive peoples to spread out rather than concentrating in any one region, and so would naturally limit the size of cities.

Both of these factors can be overcome with technology, of course, but their effects upon culture would linger. The people of Kepler 442b might grow up suspicious of foreigners, taking pride in their petty differences, and fearful of unified government. When they went into space this might cause a profound dislike of aliens, but also an attitude to space colonisation as being less about spreading their culture than about creating new cultures. They wouldn’t have colonies so much as alternate home planets, owing little allegiance to Kepler 442b itself.

D-11: It Came from the Core

The following is another guest post, by a gentleman known as Admiral Howe on the Stellaris forums, who will talk to us about geology. It was originally published on the Stellaris forums as part of this series. Republished here with permission. Thanks once again, Admiral Howe.


Greetings all. Today I’ll be your guest host at the incredibly gracious request of TheBeautifulVoid. What I’m going to discuss today is a bit of planetary mechanics, or why spreading your species to the stars is a good idea.

Specifically, we’re talking about volcanos and the existential threat they can pose to the inhabitants of a planet.

First I’d like you to think about what, exactly, is a volcano? Your impressions will vary a bit based on each of your experiences but, ultimately, all of you should have about the same idea. And, that idea is overly complicated because what a volcano really is simply is a method the planet is using to dissipate heat. (A recurring theme in the geosciences actually). And they’re incredibly effective at their job – maybe even too effective if you live near one. That such a simple goal could manifest into a mechanism that has been inspiring fear and awe for the whole of history can be attributed to volcanoes having the best special effects budgets in nature.

“The Tungurahua volcano spills incandescent rocks, ash and black smoke in a photo taken from Huambalo, Ecuador, March 6, 2016.”

All jesting aside, volcanoes are a fact of life on any body that has internal heat. Earth (my presonal favorite), Venus, Io, Enceladus are all volcanically alive today while the Moon, Mars, and (potentially) a few of the outer moons were alive in the not too distant (geologically speaking) past. And this poses a problem for any civilization as ours has had a number of major events/setbacks linked to these major eruptions. The classical Dark Age around the collapse of the Minoans and covering the whole eastern Med – likely the eruption of Thera in the Aegean. Fall of Constantinople – final year of the siege coincided with a volcanic winter brought on by Kuwae. I could write several articles on that alone, but that’s not the focus here.

Before you think we’re less vulnerable than our ancestors, I ask you to think again. In many ways we’re more vulnerable as volcanic ash shorts out electrical lines better than just about anything we’ve found and chokes engines with amazing efficiency. And in our world of “just in time” supplies, a region losing shipments can quickly spawn health and hunger issues of immense proportions. Could humanity survive? Sure, but only depending on the scale of the event.

What’s the scale we’re talking about here then?

So how do we talk about the scales of eruptions? Best way I’ve found it to use real world examples, so to start that’s where I’ll go. If you mention a volcano to someone in North America and it’s likely they’ll think of Mount Saint Helens in 1980. In that eruption the northern face of the mountain disintegrated and blasted sideways while the core of the mountain erupted vertically in an ash column 24km in height. Ultimately, the energy released was equivalent to 1600 Hiroshima bombs…and was released in a bit under 15 minutes.

It also didn’t hurt that it was the first big eruption of the television era in a nation awash with cameras.

Mount St. Helens before and after the 1980 eruption.

Ash fall from Mount St. Helens.

I know all that may sound impressive but if you didn’t live in the Pacific Northwest you likely didn’t notice more than a slight dip in temperatures for several months and some glorious sunsets until the ash cleared out of the upper atmosphere. After all, we’re only talking about 1.1 cubic km of rock-turned-to-ash here (and another 5 cubic km or so of trapped gasses released).

So, then…what’s the big deal?

The big deal, or why I started here perhaps, is that St. Helens – for all the destruction it caused – came in only as a 5 on the scale we use to rate volcanic eruptions. About the same as Vesuvius actually, but far better studied because of its recentness. See, volcanoes of that power happen about every 12 years around the world so while they’re big news in the region where they happen they’re common enough overall to be background noise on a planetary scale.

And yes what I’m calling background noise is still a staggering 8 cubic km of rock and ash, 40 cubic km of gases, and roughly 13,000 Hiroshima bombs worth of heat injected into the atmosphere every century. Thankfully, a planet is a big place and absorbs it quite easily. No, the reason I started here and mentioned this is because Mount St. Helens is a 5 on the scale…a scale that goes to 8. And is logarithmic.

Volcanic Explosivity Index

Wait then, what’s a 6?

Well the most recent one was Pinatubo in the Philippines. It erupted some 10 cubic km of material and injected enough particulates into the upper atmosphere to lower global temperatures 0.5C for nearly three years. I could go into depth about it but I’d rather talk about another recent 6…Krakatoa (Krakatau).

Krakatoa/Krakatau showing outline of pre-1883 island.

Krakatoa, an island in the Sunda Straight, was known to be unstable for centuries. Dutch travelers remarked in 1680 that the lush, green island had been burned crisp…the vegetation quite dead. Dutch traders reported local history recording at least 9 eruptions between 400 and 1600. And a good translation of Krakatoa comes across as “The Fire Mountain”. So, yeah, people knew not to trust this island.

Which is all well and good, give it space and it won’t bother you. Right? Well, that largely worked until the night of August 26th, 1883. In the span of four vast eruptions the island vanished with a force in excess of 13,000 Hiroshima bombs (so yes, the average century output of level 5 volcanoes in one event). Ships were found miles inland, washed inland on massive tsunamis, some 600 villages and towns vanished. This eruption still holds the record for most-distantly heard sound…people in Perth Australia 3100km away heard it as did those near Mauritius 4800km to the west.

Tidal gauges in places like the English Channel recorded the shock wave pass…seven times as it echoed around the world over the next five days. And world weather took five years to recover from the – though the brunt of the 0.3C drop in world temps only lasted about a year.

And the craziest part, at least to me, is reappearance of the volcano’s core in 1927. It’s already back to 300m in elevation and slowly recharging, waiting for its next turn. And since tier 6 volcanoes happen once or twice a century, who knows, Krakatoa’s turn may be sooner rather than later.

But that’s only a six, the list goes to eight.

So, ladies and gentlemen, I’d like to introduce our exemplar for a type 7 volcano…Tambora, Indonesia.

Tambora, Sulawesi Indonesia

Oh, don’t mind the picture…the upper third of the mountain has been missing since 1815 when it erupted over 100 cubic km of ash and rock over the span of a few days. The eruption column has been estimated to have reached 43 km into the atmosphere, think about that compared to modern jet cruising altitudes for a moment. This eruption led to the “Year without Summer” in northeastern United States where the summer of 1816 saw temperatures struggle to get above freezing. And there was even a great “fog” covering the sky for most of 1815 and 1816 that dimmed and reddened the sun – so much so people could view sunspots directly.

All that from one seven.

Fortunately for us, sevens happen only once or twice per millennia. Which is really for the best, I don’t think we’re all that well positioned to deal with them even as infrequently as they happen.

And for those curious, the energy released here was roughly 10^20 joules, or the equivalent of about 2.2 million Hiroshima bombs.

So, thinking of moving off world yet? Terrestrial planets are nice but they have their downsides…and we’re only to seven on our eight point scale.

So then, what’s an eight?

Doomsday, as one of my professors put it.

Looking across the Yellowstone caldera. Yes, everything in this picture is part of the volcano.

The category 8 volcanoes, like Yellowstone pictured above, are massive complexes where the eruptions release rock and ash in the thousands of cubic km (on the small eruption end) from their calderas. Yellowstone is particularly interesting to me both in its proximity and in how reasonably well it’s mapped and understood. Unlike the smaller ones listed above Yellowstone is actually tied to a hot spot – essentially a river of slightly-hotter magma extending upwards (we think) from the core/mantle boundary.

Cut-away showing an artists interpretation of the magma-chamber complex below Yellowstone.

And this river essentially blow torches its way through the overlying crust – sometimes gently in the case of Hawaii and other times with violence like here. If you know what to look for you can actually trace the motion of the overlying plates across these seemingly-fixed plumes which, while good for research, doesn’t change the problem presented here. Yes, the hot spot creates the volcano but it also is constantly refreshing its magma…Yellowstone has a recharge and release timer of about 700,000 years.

So how bad can it be, really? Well Tambora was around 100 cubic km. Two of Yellowstone’s last three have been 1000 cubic km and 2500 cubic km. One of those today would largely destroy the ability of North America to host modern life for decades…and there’s nothing to be done to prevent it. At least with modern technology.

And as for an energy-released estimated, let’s be honest, they’re really irrelevant at this point.

Space doesn’t sound as scary, does it?

It should, really. Especially with nice, shiny FTL ships. Oh and perhaps because I’ve left one little problem off this discussion until now. I mentioned that the index we use goes up to 8. Well, volcanos are illiterate and haven’t actually read the list…there are things worse than 8.

Folks, to wrap this up I present the Siberian Traps.

Estimated extent of Siberian Traps based on surviving rocks.

The Siberian Traps, and eruptions like her, are often called large igneous provinces or flood basalts. Traps, here, is taken from Swedish trappabecause of the step-like formations left behind often leave distinctive step-like layering making them easy to spot with some training.

Now, the output of eruptions like these are measured in the millions of cubic km and they can have durations pushing a million years. Imagine, having two or three Mount Saint Helens going off per year, every year, for a million years. I’d board a colony ship at that point.

These massive events don’t have a well understood mechanic set yet and it’s doubtful we would be able to image a nascent one in the mantle. At least with any degree of certainty. Not that it matters, if one began there’d be nothing we could do to stop it.


And all this goes on just so the planet can get rid of a little heat.

Thanks for reading, it was fun to dig through notes and texts too rarely read nowadays. And than you again TheBeautifulVoid for letting me guest a spot and help explain why heading into space isn’t such a bad idea.