“…Yet across the gulf of space, intellects vast and cool and unsympathetic, regarded our planet with envious eyes, and slowly and surely drew their plans against us….”

Yes, I’m listening to Jeff Wayne’s version on ‘War of the Worlds’.

Wells’s opening narration has to be one of the most wonderfully chilling things I think I’ve ever read. On the face of it, it breaks every narrative rule in the book (third person, passive voice, no dialogue, pure exposition) and yet, I find it impossible to read without shivering.

Interestingly, for its day, ‘The War of the Worlds’ was also scientifically-plausible. The Victorian Mars was believed to be a living world, if likely rather a dry and desolate one. Indeed there was even some evidence of supposed activity on the surface – the infamous ‘cannali’. (Actually ‘channels’ – the word was mistranslated into English as ‘canals’, which are a rather different entity from that which Schiapperelli meant to imply!)

Of course, Science Has Marched On (as it’s rather prone to). Our Mars is a nearly-airless, lifeless world. If anything does live on it – and there can’t be very much, else we would long since have known of it – then it couldn’t be more than a few microbes, perhaps huddling in some warm, wet reservoir beneath the surface. Our Mars almost certainly did have liquid water, warmth and air, but that phase was billions of years ago.

Our Mars is just too small.

Its core cooled and froze out aeons ago, stilling the planet’s internal dynamo. With no global magnetic field, there was nothing to shield it from the solar wind, and the storms of the Sun slowly stripped away its air. In addition, without plate tectonics, the atmospheric cycle slowly ground to a holt. Mar’s air was gradually lost to sediments deposited at the beds of the Noachian oceans. Then those oceans themselves gradually evaporated, their vapour rising into the stratosphere, where solar ultraviolet broke apart the heavy water molecules. The light hydrogen atoms would simply have escaped into space, Mars’s low gravity being too weak to hold them. The oxygen would have been lost into compounds by reaction with other materials in the Marian crust and air – oxygen is a notably reactive gas! (And as well it is – if it were any less reactive, our entire human metabolic system would be quite impossible.)

Over the millennia, these processes ran their course. Mars was left dry, desolate and lifeless. If there ever was anything living there, it does not seem to have been able to survive the transition from Blue Mars to Red Mars. Today, Mars averages something like -60 Celsius and it has a surface air pressure of a mere 7.6 millibars, or less than 1% that of the Earth. The only remaining water are some suspected permafrosts and the two small, bright caps of ice at either pole. There is also a breath or so of water vapour in the thin atmosphere, but not enough to form more than a single pond. Our Mars is a cold, cratered orb, staring blindly into the heavens.

Still, one has to wonder.

“…The chances of anything coming from Mars, he said, were a million to one…”

Image credit: NASA, via Wikimedia Commons

I think Kepler-22b is an exciting and significant result, and I’m enjoying watching the Kepler candidates followed up. However, I’m depressed to see that the press coverage here has been even more woeful than usual.

Here’s the TL;DR version; the relevant paper is right here. It’s a planet, it exists, it’s ‘Earthlike’ in the sense that it’s probably-but-not-certainly a terrestrial, and it orbits inside a star’s habitable zone. Other than that, we don’t know that much about it.

Longer version … there’s a lot of crap floating around about this object. Allow me to cut through the haze by listing the details that aren’t speculative:

• The star (Kepler 22) is a type G5, so somewhat solar-like, but also somewhat cooler and fainter (K-22 has an effective temperature of 5518 K, against 5578 K for the Sun).
• The planet has a radius of 2.38 times that of the Earth, with an error range of 0.13. Or to put it another way, it’s somewhere between 28,700 Km and 32,000 Km in diameter. (I’m rounding to the nearest ~100 Km there, incidentally.)
• Based on the (lack of) Doppler shifting in the star’s light, the planet must weigh less than 124 Earth masses. By contrast, Jupiter weighs 317.7 Earth masses, so K-22b is definitely a planet and not a star or brown dwarf.
• The orbital period is 290 days.
• The blackbody temperature for 22b – assuming a terrestrial reflectivity! – is 262 K, or -11 degrees Celsius.

And, umm, that’s it.

The temperature figures that are getting a lot of attention are somewhat inferential. First off, the orbital period has been used to infer an orbital radius; this isn’t a problem, incidentally. (Although note that it tells us little about the temperature on K-22b today, as the number you’ll get out from this won’t tell you anything about eccentricity, inclination and so on, and these could all impact on surface temperature.)

The real issue is that this number doesn’t include any atmospheric effects – the number of 22 degrees was arrived at by assuming an exactly Earthlike reflectivity and an exactly Earthlike atmosphere. (The blackbody temperature for Earth is -19 degrees Celsius; the atmosphere adds another 30 or so degrees in greenhouse heating.) Neither of these is likely at all for K-22b. The combination is particularly unlikely.

The paper-writers, I want to note, make no bones about the limitations of this calculation:

• …Using Equation 2, and assuming a planet with a surface and an atmosphere with thermal properties similar to that of the Earth (which is unlikely) and a Bond albedo of 0.29, the surface temperature of Kepler-22b would be approximately 295 K. [295 K = 22 Celsius. Emboldening added by me]

Needless to say, the press are conveniently ignoring that bit. If they even noticed that it was there in the first place, that is.

Depressingly, the press coverage about this object seems to be telling us a lot more about the media than it is about exoplanets. It’s almost a classic case of ‘churnalism‘, where lazy and/or time-pressed journalists simply regurgitate press releases, with neither fact-checking nor criticism. It’s also one of the reasons why more and more people are abandoning the mainstream media – if all you get is a mix of rented-mouth political propaganda and stale churnalism, one has to wonder what the point of the media actually is?

Ahem. Yes, as it happens, I do feel strongly about factual accuracy in science-related articles (note that I’m not linking to any of the offending articles – if you must find them, Google is your friend). Anyway, ranting about press releases and dodgy newspapers aside, I think the point I’m trying to make is that Kepler-22b and their ilk are interesting and exciting objects, and as such they deserve to be considered on their own, factual merits, not on the basis of vague, ill-informed, emotive guff.

Also, there is a further problem with this. Declaring each new planet ‘habitable’ could have the effect of raising the public’s expectations – only to dash them down again. First off, it’s not a fair way to treat people, secondly it’s bad for public understanding of science and thirdly, it could backfire on the world of astronomy. Do we want the taxpaying public to be conditioned into a cynical expectation of disappointment? I think not!

Anyway, I would write more, but I don’t wish to add any more idle speculation to an already ill-informed debate.

And the much bigger dA version…

Although it’s a long way from perfect, I’ve had more success with this piece than I usually seem to with digital art.

There are various things that could be tweaked, but I decided it would probably do here! I’m not 100% sure about the texturing attempt on the rifle, for instance, but I think it’ll do as it is. I’ve also fixed the eyes – they were well off before – and I’ve tweaked a few bits of the shadows here and there. (That said, the shadowing/lighting could be a lot better than it is.

The painting itself is essentially a bit of playing with ideas for a current writing project of mine. I say ‘current’ but I actually haven’t got anything done since August on it (gah! need to be less busy! gah!)…

I am, of course, very quiet at the moment. That’s because I’m well into the thesis-writing season. If that wasn’t enough, I’m also drafting a second paper – and unlike the last one, this one is on a pretty tight deadline. I need to get it submitted by Christmas; given that the last one took 18 months, this could prove interesting, shall we say.

Anyway, when writing for this thing I try to aim for interesting and high-quality articles. Unfortunately, that comes with a price, in the form of research time, drafting-time and proof-reading time. And time is rather in short supply at the moment! I could tap out lots of poorly-researched, badly-written, typo-filled pieces of article-padding, but I’m not going to do that. I have no desire to patronise the collective intelligence of the readership!

So I guess this is an extended way of saying that it’s likely to be a bit quiet around here for the next few months. If anything truly exceptional/unexpected happens, I may break my silence, but that’s a possibility rather than a promise.

Anyway, in the meantime, there’s a presentation I need to go and work on…

(A much larger version is available on my dA page, as always.)

UKIDSS T dwarfs and the Galactic halo.

It’s a boringly low-tech HTML site, and I need to sit down and properly proof-read it … but it exists now, at least.

It’s time this stuff ended. Please. Before any of this gets any worse.

I hope that everyone reading this is safe, and I hope that things remain that way.

(Regarding my own situation, I’m very fortunate that where I am seems to be calm at the moment, and there’s no sign of that changing. Thank goodness.)

Image via: Wikipedia and the Galileo orbiter.

Ganymede is hard to miss. It’s sufficiently prominent that it was originally discovered by Galileo on the 7th of January, 1610 – in fact, it may well have been the first object ever discovered via a telescope. (We just don’t know precisely which of the Galilean Moons Galileo saw first; we do know that he saw all four of them on the same night!) Ganymede is actually fairly bright, for an outer-system object. At magnitude 4.5ish, it would actually be a naked-eye object, except for the glare from Jupiter.

Ganymede is so relatively-bright because it’s big. In fact, it’s the biggest moon in the solar system. At 5,268 Km in diameter, it’s actually wider than the planet Mercury, and it’s only about 1500 Km smaller than Mars. The total mass of Ganymede is slightly more than twice that of our Moon. This means that it’s also first in terms of mass as well as radius. (Second place goes to Titan, in both lists.)

Ganymede is believed to have a fully-differentiated structure. Here’s another picture, via Wikipedia, to illustrate that:

In this sense Ganymede is much more similar to the inner, terrestrial planets than to the more icy moons of Saturn, Uranus and Neptune. It has a solid, rocky core, surrounded by a silicate mantle. Over this is believed to be a water mantle. The crust-material at the moon’s surface appears to be predominantly water ice.

Ganymede’s surface terrain has two distinct portions. The darker, crater-saturated regions are believed to be the oldest, with an estimated age of around four billion years. The lighter regions are thought to be younger – partly indicated by their comparatively-low crater numbers – and it is thought these were originally tectonic features. Whether these processes are still active today is unknown, as far as I’m aware, but it generally seems to be believed that they aren’t.

Ganymede has an additional weird aspect; it’s the only known moon with a reasonably-strong magnetic field. This is thought to be due to the presence of that silicate-iron core, deep inside the moon. In fact, it’s magentic field is about three times stronger than that of Mercury (Ganymede does seem to like spiking Mercury’s guns, doesn’t it?).

Ganymede’s large mass allows it to affect other bodies significantly, even at some distance. The timing sequences of its orbit and that of Europa combine to ‘pump’ the orbit of Io, helping it maintain its eccentricity. This eccentricity is why Io is such a volcanically-active object (I have a previous post on the subject of Io, in fact).

Ganymede also has a connection to another inner moon; like Europa, it’s believed to possess a subsurface ocean. This does raise the possibility that there may even be something living down there, although I suspect it’s unlikely that we’ll ever know for a fact!

One more thing is worth noting; when Ganymede first formed, it would presumably have had surface water. This would be due to Jupiter; Jupiter’s formation would have released a lot of heat and for at least a few million years, Jupiter would have behaved much more like a very small brown dwarf than like the planet we’re used to. (It still emits more heat than it receives from the Sun, even to this day – see this post of mine if you’d like to see how Jupiter’s IR spectrum compares to that of a brown dwarf.) Whether this would have been long enough for any biological activity to occur, of course, is a question I can’t answer. But who knows?

Whatever else, Ganymede certainly remains an interesting (and rather large) object.

Credit: Cassini/NASA, via Wikipedia

Dione was first observed in 1684 by Giovanni Cassini, who also discovered three others of Saturn’s satellites[0]. Dione wasn’t actually formally named until 1847; Cassini called his discoveries the ‘Sidera Lodoicea’ (‘Stars of Louis’) after his patron, the French king Louis XIV. (This sort of fawning was entirely normal at that point in history … luckily, it’s one practise we seem to have left behind!)

At 1,122 Km in diameter, Dione is reasonably large; it ranks as the 15th biggest moon in the Solar System. It’s also more massive than all of the smaller ones combined – although in fairness, that’s less impressive than it sounds, given that many of the smaller ones are barely more than pebbles. Dione’s average density appears to be somewhat more than that of ice, suggesting that it probably does have some kind of rocky core deep inside it. Some estimates say it might contain about 46% rocky material to 54% ice. It’s also very, very shiny indeed, with a surface that reflects about 98% of all incoming light. (Again, this is consistent with a primarily icy outer surface.)

The moon’s surface is a mixture of heavily- and lightly-cratered plains, along with tectonic features such as scarps and fractures. However, it’s biggest claim to oddness is probably not its surface, but the other moons it exists in an orbital relationship with.

You see, Dione doesn’t orbit Saturn alone. Rather, it shares its orbit with two other objects, called Helene and Polydeuces. These two bodies are located respectively at the L4 and L5 points of Dione. The L- or ‘Lagrangian’ points are a series of five locations, associated with any two-body orbit, where a much less massive third body could, essentially, ‘sit still’ relative to the other two. In any system, there will be five L-points, numbered from 1-5. Numbers 1-3 are literally just points, and as such are of limited use – any body that is even minutely off-centre will quickly ‘slide away’ from these points. L4 and L5, however, function as small ‘regions’. As such, it’s possible for things to ‘pool’ inside them.[1]

This is exactly what’s happened with Polydeuces and Helene. The two moons drift along, 60 degrees behind and 60 degrees ahead of Dione, respectively, forever shadowing it as it orbits Saturn. Of the two, Polydeuces is the smallest, being about 2 miles across. Helene is somewhat more respectable, at roughly 40 Km in average diameter.[2]

The Dione-Saturn system is not the only place where this remarkable arrangement is seen. Tethys, another Saturnian moon, acts as the ‘parent’ to a pair of rocks called Telesto and Calypso. The planet Jupiter has accumulated for itself two enormous pools of asteroids on its L4 and L5 points; these are known as the Trojans. Mars too has accumulated a handful of Trojan rocks as well.

In addition, a recent discovery has been 2010 TK7, which turns out to be in a Trojan relationship with our own planet (it’s sat on the Earth’s L4 point). Sadly for us, 2010 TK7 is even less impressive than Polydeuces, being a mere 300 metres in diameter. (If it hit us – which it can’t – it would struggle to take out even a tiny banana republic. Which is utterly pathetic!) This minute diameter is mainly why TK7 was only spotted very recently.

One interesting possibility for the future is that of Trojan exoplanets. It is possible that a relatively-close Jupiter-analogue might have smaller, terrestrial-mass planets associated with it in a Lagrangian relationship. It’s also possible that a particularly-big Jovian-type might also have Uranus/Neptune-mass gas giants Trojaning with it. In this case, the Dione system could end up being a scaled-down prototype for a category of much bigger systems.

Time will tell…

___________________
[0] In addition, in 1672, Cassini and a colleague called Jean Richer were also the first people to measure the distance from Earth to Mars, via a rather cunning bit of trigonometry.
[1] The reason for the existence of Lagrangian points involves maths … as best I understand it, the L-points come from the play-off between gravitational and centripetal accelerations. I shan’t dig deeper into that here, though. There’s a discussion on Wikipedia, however.
[2] One awkward issue of L-points is that bodies occupying them have to be ‘small’ relative to the masses of the other two, otherwise the third body has enough gravity to complicate things.

(Image via Wikipedia, from Cassini’s September ’05 flyby.)

Hyperion is weird.

No, really, it’s weird.

First of all it has a massively eccentric orbit around Saturn. Secondly, despite being fairly large, the moon itself is appreciably egg-shaped (as you can clearly see in the above photo!). Its longest axis is 360 Km, its shortest is 205. Egg-shaped orbit, egg-shaped moon, apparently!

But there’s more. In terms of composition, Hyperion also appears to be a freak. Its mass is tiny. Despite being around 2/3rds the size of Mimas, it has only 15% of that other Saturnian moon’s mass. Probably this is partly why it’s so un-spherical; there’s just not enough surface gravity to flatten things out. Hyperion’s density is also rather low. In fact, some estimates claim that its insides are as much as 40% empty space.

Yup, that’s right, it appears to be partly-hollow. Presumably its interior must contain some absolutely massive caves.

Of the bits of it that actually are solid, current thinking holds that they’re mainly composed of water ice. This at least is fairly normal for Saturn’s moons. At Saturn’s distance from the Sun, things are cold enough for water to remain frozen pretty much indefinitely; it’s only once you get inside the asteroid belt that solar radiation would have been intense enough to evaporate off the early store of water. (That’s one of the reasons why there’s such a sharp compositional change between the rocky inner planets and the big, icy moons of the gas giants; basically we live inside the so-called ‘snow line’.)

However, in addition to its icy surface, Hyperion also appears to be covered in some sort of dark material. Its reflectivity is too low for pure ice; Hyperion reflects about 20-30% of incoming light. By comparison, snow reflects about 70-80%. What exactly this material is isn’t known, although there is a suggestion that its source might be another moon called Phoebe.

Another bit of compositional weirdness is the dark material that fills the bottom of Hyperion’s craters. This dark reddish substance appears to be some sort of hydrocarbon, perhaps similar to the stuff that gives Iapetus its dark hemisphere. But again, this material has never been directly-sampled, so its exact nature is uncertain.

There is one final point of high weirdness. Hyperion’s rotation is unstable. Its rotational axis flops around pretty much at random, almost as if the moon were severely drunk. This appears to be down to the conflicting tides it experiences from Saturn and Titan, and also due to the dramatic eccentricity of its orbit. (The tidal force falls off with the cube of the distance, not the square, and so is very sensitive even to small changes in orbital distance.)

Although possibly the ugliest moon in the solar system, Hyperion is an interesting object, and one which poses some interesting questions for models of planetary formation and structure. It also seems to be one of these things where we know just enough for it to be really tantalising, but not quite enough yet for any really concrete answers.