Squishier moons are better for life

My more eager readers will have noticed a sudden flurry of submissions to the arXiv since Christmas. I’ll try and bring you up to date with what I’ve been publishing recently, which is hitting a variety of topics.

The first is a return to a favourite area of research for me: exomoon habitability. As you can see from earlier posts, I’ve been looking at this subject for a while now, focusing in particular how an Earthlike object would fare orbiting a giant planet, which in turn orbits a star like our Sun.

I’ve been using a simple, 1D climate model to follow the temperature changes on this Earth copy, and discover what sort of orbital parameters might be needed for it to possess surface liquid water. But we’ve known from the start that these climate models have been overly simple, and in some cases they’ve missed out important physics that might affect our answers.

In our latest work, we took aim at two aspects of the model that we felt were lacking. In the first, we investigated the issue of atmospheric composition. Up until now, we had assumed a fixed composition, but we know that the Earth has adjusted its atmosphere over time. Sometimes, this is due to the presence of life (like the great oxygenation events that are responsible for the good stuff filling your lungs), but other processes play a role.

In fact, we can identify a complete cycle of processes that affect the total amount of carbon dioxide. CO2 is emitted into the atmosphere via volcanic eruptions. It then precipitates into rain, which falls into the sea, and gets incorporated into rocks and seashells on the ocean floor. This ocean floor is eventually sub ducted at a tectonic plate boundary, and returned to the mantle, where the whole cycle begins again.


It’s known as the carbonate-silicate cycle (because the rocks in play are carbonate and silicate rocks). What’s rather clever about this system is that it acts as a thermostat. If the planet starts to warm, then more Co2 rains out of the atmosphere and into the oceans than is expelled by volcanoes, which reduces the amount of Co2 in the atmosphere. As Co2 is a greenhouse gas, getting rid of it allows the planet to cool more easily. If the planet cools, less co2 rains out and is added to by volcanism. A little extra greenhouse gas helps the planet keep its heat. This is why scientists are worried about manmade co2 production, as we’re mucking around with the thermostat, and too much fiddling could break it.

We never considered this process before, so we changed our model to allow the co2 levels of our moon to vary to help keep the temperature warm and stable.

We then turned to the tidal heating of our moon, which until this point was done using a rather simple model. As you may already know, tidal heating is generated by the planet’s gravity stretching and squeezing the moon as it goes around its orbit. Crucially, the amount of heat the planet can generate in the moon depends on the material the moon is made of, as well as what state it’s in. If the heating is intense enough to allow melting, this can reduce the tidal heating, and stop moons from becoming too hot for life.

We found that when we added both effects, the habitable zone for the moon moves further away from the star.  It also gets wider around the planet as well – moons can orbit closer without being roasted, and changing CO2 levels lets the moon stay warm further away from heat sources by boosting its greenhouse effect.

We’re far from the final answer here: one day I hope to be telling you about fully 3D models of exomoon atmospheres.  Even 1D models like ours still need some extra physics, like investigating how changing the spectrum of incoming radiation affects this answer.  But every step is a step forward!

The power myth will soon be all that’s left of modern politics

The latest episode of This American Life has got me thinking about the dynamics of modern politics, and not just the 2016 American presidential elections, but the upcoming referendum on the UK’s place in the EU.

In the episode, we’re introduced to Alex Chalgren, a young Trump supporter with a troubled background. A gay black man adopted by an evangelical, Cruz-supporting Christian family, who loves Alex but cannot reconcile this with his sexuality, his life has been described as an “impotent man [seeking] power”, a person whose circumstances have never been entirely in their control.

That last sentence describes us all, doesn’t it? We are the captains of our souls, but we are not the masters of our fate. Disenfranchisement is the cloud that hangs over all of Western politics, hiding the silent masses of people sick-and-tired of political business-as-usual. The politician that harnesses this swirling vortex of despair, hate and fear has an almost unlimited power source, if they can transmute it into a seemingly positive force.

The SNP have been extremely successful in this practice in Scotland, capitalising on the independence referendum to engage sectors of the electorate who had never seen the inside of a polling booth, and destroy the competition in last year’s elections. Donald Trump is crushing his rivals in the Republican primaries, scooping up voters from almost every demographic, and Bernie Sanders is a strong second to Clinton, an unthinkable socialist in the top tier of US politics.

A common theme to all these successes is empowerment. Alex Chalgren paraphrases Frank Underwood in House of Cards, when he says

a fool goes after money. But someone that really seeks to control goes after power…

The message is pretty simple. You feel powerless.  I have power, vote for me and let me give you some of it. Trump’s power-persona, carefully constructed in simple sentences, spiced with hatred and accented by bullying and insults, is attracting the disempowered vote and dumbfounding the GOP establishment. The SNP’s principal cause of Scottish independence is the ultimate empowerment for a small nation, with historical baggage and a Tory government that very few Scots voted for. Even Sanders’ roaring rhetoric against the 1% imbues him with similar tough-talking attributes as Trump.

What will decide the EU referendum in the UK is the fight for the disempowered. The Out campaigns have a simple message of returning UK “sovereignty” and repealing apparently unending legislation. The In campaign have a much tougher job.  Firstly, they need to prick this “sovereignty” balloon (how can you have true freedom to legislate in a market economy, when you need to satisfy regulations to export goods to Europe? And will being out of Europe really protect us from the dreaded TTIP?).  Michael Gove’s recent statement supporting Out is typical of those made by the Out campaign, and this response is an excellent example of how to completely invalidate them.

Secondly, the In campaigners need to convince us that regulations exist to facilitate fair and free markets while protecting our social, human and natural capital.  A post-Brexit Britain would be far more prone to lobbying resulting in deregulation, destroying the environment and eroding UK citizens’ rights.  In fact, studies indicate it would have to do this to mitigate losses to GDP on Brexit.

We bought into the EEC ideal in the 70s (and the EU latterly) because we thought it would empower us to create a peaceful and prosperous Europe, free of continental war at last, with a cleaner environment, healthier and happier citizens and a better future for our children. The world has changed a lot since then, and we’re facing unprecedented migration, climate change and dangerous extremism, challenges on a scale we haven’t faced since the Second World War.  It’s not surprising that the EU is showing signs of strain. It may have its failings, but to leave it is to ignore its great successes and its potential for further success, throw the baby out with the bathwater, and step back from our closest allies when they need our support the most.

The challenge now for the In campaign is to win the disenfranchised – I wish them the very best of luck.  But it leaves the future of political discourse looking fairly bleak. It feels like the old practice of debating individual policies is rapidly disappearing, and the election victories are going to the man who looks best when they don the purple (and yes, I use the word “man” deliberately).  And if it finally goes, we’ll just be voting for the “strongman” – and history has taught us where that will lead.

Galactic Habitable Zones Are Messier than You Think

In the search for life in the Galaxy, an important question to ask is: Can we predict which parts of galaxies are more likely to have habitable planets? If we knew the answer, we could target our search at the right parts of the sky, and we could assess the odds for life appearing in galaxies throughout the cosmos.

In an attempt to answer this question, the concept of the Galactic Habitable Zone was established. This is built from considering two factors that affect habitable planets: the availability of raw materials for planet construction (what astronomers refer to as metallicity, or “everything that isn’t hydrogen and helium”), and the chances of being hit by high levels of damaging ultraviolet radiation.

The Earth’s atmosphere screens the worst of the Sun’s ultraviolet rays, but if we were swamped by radiation from say a nearby supernova, that radiation could deplete the ozone layer, and cause a mass extinction. Some astronomers and geologists think there is some evidence that Earth’s geological and biological history bears the scars of such supernovae, but this is far from clear.

These two factors have a common link: star formation. Most of the heavy elements which make up planets like Earth, and all the life-forms along for the ride, are forged in the cores of stars. The elements heavier than iron are born in supernovae and other massive explosions that occur at the end of a massive star’s life. So local star formation is a blessing and a curse for habitable planets: we need several generations of star birth and death to give us the ingredients for planets, but vigorous star formation results in supernovae which can be deadly if too close.

In the classic galactic habitable zone picture, this means that the interior of galaxies are ruled out because the local radiation is too hazardous, and the outer parts are too metal poor to form habitable planets. Somewhere in the middle, things are best, a Goldilocks zone that is reminiscent of the Goldilocks zone around stars (although they exist for quite different reasons).  The Galactic Habitable Zone is therefore an annulus, a ring around the Galaxy’s centre, which the Sun happens to reside in.

The classic galactic habitable zone.  In the centre, there is too much hazardous radiation.  On the outskirts, there isn't enough heavy elements to form habitable planets.  Note the Sun nestled inside the zone
The classic galactic habitable zone. In the centre, there is too much hazardous radiation. On the outskirts, there isn’t enough heavy elements to form habitable planets. Note the Sun nestled inside the zone

However, this classical picture assumes galaxies are nice geometric shapes. And they ain’t. They grow through collisions with other galaxies, and tear up satellite galaxies into streams of stars and gas before gobbling them up entirely.

So in our latest paper, we tried to look at how this messy picture of galaxy formation affects the galactic habitable zone. We took a cosmological simulation of the Local Group, our intergalactic neighbourhood which includes The Milky Way, Andromeda and Triangulum galaxies.  These simulations model the dark matter distribution on large scales, and the gas on small scales, with some of the gas becoming dense enough to form stars, which can then enrich the surrounding gas with heavy elements when they die.  We took the data from these simulations, and used standard galactic habitability criteria to measure the GHZ.

The cosmological simulation of the Local Group we used for our habitability calculations. The orange-red colour indicates high levels of gas density.

And it’s quite a mess! The GHZ isn’t nice and symmetric at all. Those big streams of stars and gas I mentioned? Quite suitable for habitable planets apparently.  As are those satellite galaxies before they’re gobbled up.  The best places to live in galaxies shifts quite a bit over time, as the whole edifice is assembled through all those collisions and mergers.  Each galaxy has its own path to tread towards producing habitable planets.

The habitability of Triangulum (M33).  The redder the colour, the more habitable planets present.  The centre looks very red, but this is only because the stellar density is high.
The habitability of Triangulum (M33) over time. The redder the colour, the more habitable planets present. The centre looks very red, but this is only because the stellar density is high.

In the animation above, it looks like the very centre is the most habitable, but really the centre has the highest concentration of stars.  Each star has far less habitable planets, but this makes a lot of habitable planets if we cram enough stars together.  The best spots are further away from the centre typically, but this changes quite a bit as the galaxy is built.

But this is only really the beginning.  What we discovered in this work is how inadequate our understanding of the problem really is.  We still have lots of questions about how supernovae can sterilise a planet, and we haven’t made a big enough census of exoplanets to really nail down the relationship between metallicity and producing habitable planets.  There are other things that can make parts of the Galaxy uninhabitable, which we weren’t able to look at in this paper – if stars are too crowded together, they can knock habitable planets away from their host (which would make the galactic centre very bad); gamma ray bursts can explode much more powerfully than supernovae, with potentially more devastating results.

What we do know is that whatever the true Galactic Habitable Zone of the Milky Way is, it ain’t an annulus.