What is the Habitable Zone for an Exomoon?

The search for life in the Galaxy will soon take on a new dimension.  We’ve been searching for exoplanets in the habitable zone of their parent star.  We’ve even begun figuring out how this habitable zone changes when more stars are involved.  Now, the promise of exomoons puts a new spin on the habitable zone concept.

What future exomoon hunters will find?
What future exomoon hunters will find?

The habitable zone for an Earthlike planet around a single star is conceptually simple.  Get too close to the star, and the planet gets too hot, losing its precious moisture and becoming a charred, lifeless rock.  Get too far away from the star, and even with a greenhouse effect turned up to maximum, the water on the surface freezes and animal life perishes.  Somewhere in between, the planet can boast liquid water on its surface, and provided the chemistry is right, a healthy atmosphere can be maintained.  This habitable zone is a shell around the star, which the planet needs to be inside (at least most of the time) for the planet to be “habitable”.

This habitable zone is determined entirely by the star’s radiation (and the planet’s response to that radiation).  Now if that planet is in fact a moon, then the situation gets more complicated.  There are extra sources of energy – the moon can be heated directly by the planet’s gravitational field, if the moon’s orbit is elliptical.  This is called tidal heating, and we see it in action in Jupiter’s moons.  Io is a volcanic nightmare thanks to this heating, but Europa seems to have a warm water ocean under its thick icy crust, that could support life (and it’s about time we visited).

How Tidal Heating makes Io the luxury resort destination it is
How tidal heating works.

There are also extra ways to lose energy.  As the moon orbits the planet, the planet will pass in front of the star, screening off all light on the moon for a short time.  If the moon orbits close to the planet, this will happen extremely often.

Exomoons are on the cusp of detection, as I’ve written about before, and their habitable zones could look very different to the exoplanet habitable zones.  There’s now a real drive for theoretical astrobiologists to describe these new habitable zones.  Some have taken an analytical approach – we took a numerical approach.  We took a climate model for a planetary system, and converted it into an planetary system with a moon like the Earth.  We looked at how the moon’s orbit affected its climate – just changing the direction of the orbit changes the average temperature of the moon, and causes large scale climate fluctuations (and that was without adding tidal heating).

Adding tidal heating often ruins the party.  Even making an orbit slightly elliptical will render an Earthlike moon a crispy Io…

The habitable zone without tidal heating.
The habitable zone without tidal heating…
And with tidal heating.  Blues are too cold, reds are too hot, greens are just right.  Purples are habitable but extremely variable.
And with tidal heating. Each dot is a simulation, run with a different planetary orbit (and the same moon orbit).  Blues are too cold, reds are too hot, greens are just right. Purples are just habitable but with extremely variable temperatures.

The habitable zone for an exomoon is not just about how close it is to the star, but how close it is to the planet.  And now that we are beginning to grasp this, we will be able to assess future detections of exomoons for habitability.  The number of Galactic holiday destinations just got a whole bigger…


The Good Things about JUICE

There’s been a lot of chatter in astronomy circles about the negative consequences of ESA’s latest L-class (i.e. large) space mission selection.  JUICE (The JUpiter Icy moon Explorer) was selected over two rival missions – the New Gravitational wave Observatory (NGO), and the Advanced Telescope for High ENergy Astrophysics (ATHENA).  In the current age of global austerity, one group’s win is several groups’ losses, and understandably the X-Ray and gravitational wave communities are upset at the choice.  Indeed, reading the comments section on astro blogs might make planetary scientists go a little pale. Not least the fact that ATHENA supporters have already delivered a 1450 signature petition demanding a rethink.  The fact that the decision making process has been somewhat cloudy doesn’t help matters.

It does indeed suck that this is a zero-sum game (in fact, probably negative-sum).   Sadly, from my view it looks like the ire surrounding JUICE would have been stoked and pointed at whoever won this contest.  After all, astronomers have begun crying foul so loudly out of a justified fear for their own fields.  Without ATHENA, X-Ray astronomy (a necessarily space-based pursuit) will struggle in the coming years.  NGO was also a great white hope for the gravitational wave community, it being the survivor of  LISA, a similar observatory which fizzled after NASA pulled out.

I’m not here to point the finger, or begin arguments.  I just want to mention that while JUICE might not be good for everyone, it is good for many other people.

One thing I’ve heard many times from detractors is that Juno, a NASA mission, is already on its way to Jupiter.  True, and admittedly there are overlaps in the missions, but its ultimate objectives are completely different to those proposed for JUICE.  Juno will map the magnetosphere of the planet, and assess its atmospheric water content – ultimately, it hopes to assess the nature of Jupiter’s core, a long standing question in planet formation theory. More than this, Juno will reach Jupiter in 2016 (after a quick swoop past Earth in 2013 to pick up some extra momentum we’re not needing).  JUICE won’t arrive until the 2030s, long after the Juno data is picked clean.

Artist’s impression of Juno as it arrives at Jupiter (Credit: NASA/JPL)

JUICE will look at Jupiter, its atmosphere, its magnetosphere, but its key science goal is to study the moons – Ganymede, Callisto and Europa in particular.  There are plenty of unanswered questions about the moons – questions thrown up from earlier missions such as Galileo.  We know these moons are icy, but how icy? Is there liquid water on these moons, and in what quantity?  The thickness of the crust of ice on Europa’s surface is currently a guess based on some incomplete data and extrapolated from models.  If we ever wanted to send a probe to the Europan surface, it would be good to know if we were landing on thin ice.

This leads us to a second comment that’s repeatedly levelled out missions of these type, from failed Mars missions to Cassini flybys.  The promise of liquid water is the promise of life on another world.  That’s all it is right now, a shaky promise built on (necessary) terrestrial chauvinism.  This is painted as a wild goose chase, assumption built on assumption, and JUICE’s mission goals are repainted into a straw-man argument that is easily demolished (interestingly, this argument might apply to NGO, which is also attempting to detect a theoretically defined but never seen phenomenon, but it is made far less often).

Understanding moon composition is not just so we can shake out the exo-bacteria – it also helps us to figure out how these objects formed in the first place.  If you thought planet formation was a tough nut to crack, then moon formation is a doozy.  We think they form out of discs that surround planets (after they are formed from discs that surround stars).  The circumplanetary discs are fed from the circumstellar disc, which then feed on to the planet or into coalescing moonetesimals (as you can see, the potential for jargon is hilarious).  Finding some slime mould would be an amazing bonus, but not finding mould would not be a mission failure.  The science haul from JUICE includes magnetospheric data, exospheric data, topographical surveys of the moons’ surfaces…all information that could help describe the early Solar System, not to mention the potential exomoon systems that exist out there.  Characterising these missions as merely chasing puddles does them an injustice, just as it would be unfair to characterise an X-Ray observatory as simply searching for large explosions.

There is one fair comment to be made against JUICE.  That it is an experiment, not an observatory.  When money is tight, space missions should be flexible, able to carry out science goals across all the fields of astronomy.  JUICE is limited to studying one moon system.  Of course, there are many observatories in existence (not many for X-Ray or gravitational waves, but there are some), and there are many experiments (the LHC is probably the most famous example).  But we need all sorts of approaches to make the scientific community work to the best of its ability.

If you asked me which mission astronomers needed most, I might well have said ATHENA, as it is the most flexible, and probably the quickest to deploy to a needy community.  But we have what we have – and remember that JUICE has a lot to offer us.  And ATHENA and NGO aren’t dead yet – a second round of selections are expected to be made by ESA, for launch in the 2030s.  Hardly the best, but certainly not the worst outcome.