38 things you might not know about the Moon

(Unless, of course, you are extremely keen on the Moon, in which case you probably do. Anyway . . .)

1) Our Moon is the largest moon relative to the size of its planet in our Solar System. Some moons are larger - Titan and Ganymede, for instance - but are hundreds of times smaller relative to Jupiter and Saturn. This of course leaves out Pluto and Charon, which besides no longer belonging to the "planet" class are more of a several-body system. On that topic, our Moon is more than five times the mass of Pluto!

2) It's as bright as a sunspot and as dark as coal. A sunspot is a darker point on the Sun's surface. If you could isolate it, it would shine as bright as a full Moon - which looks extremely bright in the sky, especially when full. But its rocks are about the colour of coal. Its apparent whiteness is because it's nearby, and we see it contrasted against a darker space. Its albedo is roughly 0.1, which means it only reflects 10% of the light that hits it. (Ice is about 0.9, meaning it reflects 90% of the light, and charcoal about 0.04, meaning it reflects about 4%.) The Earth's albedo is about 0.3, though of course it varies from place to place. Mercury's is similar to the Moon's.

3) In pictures, the Moon is almost always drawn much bigger than it really is - because it is so bright and captures our imaginations. Its angular size in the sky is between 29.43 to 33.5 arcminutes, which is actually very small. The Andromeda galaxy appears six times larger than the Moon from Earth - but then, of course, we can't usually see that.

4) The size of the Moon appears to vary because of its elliptical orbit - and not because of where it is relative to the horizon. The famous "Moon illusion", the fact that the Moon appears huge when it's on the horizon, has been documented at least since Aristotle. Various theories have been put forward to explain it: one from Ancient Greece was that the Earth's own atmosphere had a magnifying effect. In fact, it doesn't - and you can check this for yourself by holding an object of fixed size next to the moon, at a fixed distance from your eyes. The cause is probably from the way we see the sky: we imagine it as fairly flat, or at most, a gently curving dome, making objects near the horizon seem further away than objects immediately overhead. There are also more landscape features like trees and buildings to compare the moon to on the horizon.

5) Actually, it's not quite simple. The Earth's atmosphere does have a lensing effect on the Moon which can turn it into funny shapes - but you need to be lucky to see it! It can be due to layers of air with different temperatures, such as here:

From the International Space Station, astronauts have seen a "squishy" Moon as a result of the Earth's atmosphere diffracting sunlight!

7) The Moon is 1/81th of the mass of the Earth, but its gravity is 1/6th. Why? Because of Newton's laws of gravity. (To be pedantic, the inverse square law specifically, which was not Newton's alone.) Gravity gets stronger the closer you are to the centre of something. If you have two bodies of exactly the same mass, but one is smaller than the other, the smaller one will have a greater gravity, though over a smaller area. This is why, in a binary star system where one star has died, the white dwarf, neutron star or black hole will often start accumulating matter from the star. As the Moon has a much smaller radius than the Earth, an astronaut standing on the Moon is much closer to the centre of the Moon than she would be to the Earth's standing on the surface of the Earth. Surprisingly, this effect is more significant than the mass of the body - double the mass and you double the gravity, but halve the radius and you multiply the gravity by four.

8) There is no atmosphere or liquid water on the Moon, meaning there is no weather. The escape velocity of the Moon is 2.38 kilometres per second (so it's much easier to fire rockets off the Moon than off the Earth). At Earthly and Moonly temperatures, this is easily low enough for all gases to escape immediately. (At a very, very cold temperature, such as Pluto and Charon, gases move far slower, so it's easier to hold onto them.) This has many implications . . .

9) . . . for example: Moon dust is dangerous to astronauts and their spacesuits! It's extremely abrasive - because no water has rubbed it and rounded it, the way rivers make pebbles smooth on Earth. It's very hard, because meteorite impacts give it a melted, glassy coating. This hardness and abrasiveness means it pierces spacesuits easily. It's very fine, and having no air or water to drive this fine dust around or turn it into soil, it stays there - static and clingy, getting into the Apollo craft and into the astronauts' lungs. The static comes from UV light which knocks electrons off the atoms and molecules. All this will need to be taken into account if we want to live on the Moon.

10) It has often been thought that there is water on the Moon - the Tintin characters, going there in the 1950s, see stalactites and stalagmites - and that hypothesis was correct: there is. It's ice, of course, and trapped within the rock - no rivers or lakes exist, and any water that seeped to the surface would immediately boil or be rapidly photodissociated. But absorption spectra by NASA's Moon Minerology Mapper show the presence of a tiny amount of water within the rock. This has wannabe lunar colonisers very excited indeed. Recently, researchers at the University of Michigan did a study on moon rocks which suggested that the water seems to have been there from the time the Moon formed. This is odd, because the Moon is thought to have been very hot when it formed, which would have boiled off any water. (The same, I suppose, goes for the Earth. Our water may have been brought via comets from the Late Heavy Bombardment.)

11) A "blue moon" is actually not a blue coloured moon at all, but simply the second full moon of any given month. Since the Moon's orbit is 27 days 7 hours 43 minutes, and your average month is 30 days and 10 hours, this doesn't happen often - every two or three years. Occasionally some Facebook page will tell you that a blue moon means something incredibly significant and spiritual. It doesn't: it's simply an inevitable lining up of human generated unequal series of numbers. Calendar months are entirely human choices. Sorry!

12) The Moon is red during a lunar eclipse because the light that reaches it is filtered through sunrises and sunsets. To visualise what is happening, look at this beautiful picture of Saturn:

Saturn's air itself is carrying the Sun's light around the planet (as the air does on a cloudy day on Earth). Earth's atmosphere does the same thing: the atmosphere around the edge of the planet carries the light on and diffuses it into Earth's own shadow. It's red light, for the same reason as sunsets are red: blue light is scattered and all goes off at an angle, leaving red light's path comparatively clear. There are also plenty of particles in the Earth's atmosphere, and particles tend to turn light redder. Light filtering through a sunrise and sunset has the most atmosphere to travel though. That's why people say there's "no protection" when you get sunburnt at noon: the Sun's light goes straight through the thinnest layer of air.

I made a silly little diagram to illustrate this for my March Galactic Orchids talk:

and incidentally, exactly the same thing happens when looking at spiral galaxies face-on versus edge-on:

(This lovely pair are NGC 4126 and NGC 3814. From the Sloan Digital Sky Survey telescope.)

13) We see perfect solar eclipses because of a wonderful cosmic coincidence: the Sun and the Moon appear exactly the same size. The Sun is 400 times larger than the Moon, but also 400 times further away. Actually, this varies - if the Moon is near apogee (furthest point away) due to its elliptical orbit during a solar eclipse, it won't quite block out the Sun, and we get the "ring of fire".

The shadow of the Moon on the Earth is actually surprisingly small. From the Planetary Habitability Laboratory:

(Eclipses are very emotional events. If you want to cheer yourself up, I strongly recommend watching this Sky at Night episode about Chris Lintott's trip to Turkey to see one!)

There is almost certainly no other planet on the Solar System where we could see such perfect solar eclipses - and this is in time, too, as well as space, because . . .

14) . . . the Moon used to be closer to Earth than it is now - and is moving away from us at the rate of 3.8 cm per year. We check this constantly by firing laser beams at retroreflectors placed on the Moon by the Apollo astronauts. As we know the speed of light, we can time how long the reflection takes to get back to us and get the Moon's distance to an accuracy of millimetres. In my lifetime, so far, it's moved away about 1.16 metres.

On average, that is. Its elliptical orbit varies by a huge amount more than that. But there's a slow progression. And the unavoidable conclusion is that the Moon used to be a lot closer to Earth than it is now. Tides would have been more dramatic; the Earth's own crust would have been under more strain, as would the Moon's. The friction this caused is why the Moon's face is always pointed towards us - and why we are heading the same way . . .

15) Earth's own orbit is slowing down, so one day we will always be showing the same face to the Moon, too. Our faraway descendants will never see a moonrise . . . But that will not be for a very long time.  At the moment, we only need to add a "leap second" less than once a year.

As Phil Plait put it: "I hope you liked 2008. Because you're going to get an extra 0.0000031689% of it today." (2008 was possibly the worst year of my life so I was not pleased, but the extra 0.0000031689% passed quickly!) As he explains, the Moon isn't the only influence - there's also the Sun, the fact that the Earth's structure is part solid and part liquid and generally uneven, earthquakes and tsunamis, and even the weather. So it's slow. But it seems that one day, a lunar orbit and an Earth day will be the same length - 47 of our present Earth days.

We know from fossil records and even rocks that the Earth's day was once 21 hours when life was very young, and 23 hours in the time of the dinosaurs. Coral is particularly good at showing this - they grow leaving marks like tree rings, marking days and years (or rather, periods of light and dark, and seasons, respectively). The older the records, the more days there seem to be in a year - indicating that days were shorter.

16) The Moon is speeding up! This is why it is receding from us, and how it will eventually only see one side of the Earth, as we do it. Essentially, it's taking energy from the Earth's rotation around its axis and putting it into its own orbit - like grabbing the hand of a spinning ice-skater. However, even though the Moon speeds up, it takes longer to complete its orbit, since it's further out.

17) The Moon may be responsible for the seasons. Some planets are very tilted (some ridiculously so, like Uranus); some are very sensibly aligned, with their equators on the same plane as the Solar System. Earth's is pretty tilted, and it keeps the tilt consistent as it goes round the Sun - hence, of course, the seasons. If the Moon was created in a giant impact (see later), this would have knocked us over; Uranus is thought to be tilted for the same reason. However, Earth also has a "wobble" (a very steady one; it's not going to fall over like a spinning top) which is shown in Milankovitch cycles. However, all these are steady and fairly minor, unlike Mars, which wobbles all over the place as its two titchy moons fail to exert any stability against the massive objects such as Jupiter pushing it around in the Solar System. The Moon has been shown to have a stabilising effect on the Earth's orbit - though this, much like the eclipses, will cease as it gets further away from us.

18) It may seem obvious, but art does not always capture it: the appearance of the Moon indicates where the Sun is, like an arrow. You will never see a crescent Moon looking like an open parachute: if the crescent is on its side, the "horns" will point upwards, indicating that the Sun is"below" the Earth's horizon. A full Moon indicates that the Sun is "behind" the Earth. My favourite ever scientist, Cecilia Payne-Gaposchkin, wrote in a 1954 astronomy textbook, agreed at the time to be the best in the world: "It is an amusing pastime to note the 'impossible moons' portrayed by some artists: a new moon high in the northern sky, for instance; a full moon near sunset in the west; or a crescent with horns pointed downward." (That is probably the American version of this book - I wonder if I have bought the only 1954 copy on the market?)

19) The Moon has a molten core and magnetic field. Both are very small - but it's enough to perturb the solar wind from the Sun. The molten core was found with the help of seismometers left by the Apollo astronauts. This is pretty interesting, since not all planets have a molten core or magnetic field - Mars's, for instance, has pretty much shrivelled up, and Martians have no protection from the solar wind.

20) Tides are caused by gravity. Nothing else. Contrary to what you may have heard, the Moon does not preferentially pull on water. It does not affect us "because we are mostly water". True, the Moon does have some cultural effects, hence the werewolf, the word "lunatic", and the Sussex police claiming that there's a rise in crime around the full moon. I've heard people tell me in all seriousness that they "feel different when it's a full moon", that they "felt really angry when the Moon was red during an eclipse", but the Moon isn't picking some of the molecules in your body and dragging them around whilst ignoring the others. I'm not going to pretend I know why these mood alterations seem to happen, but I suspect a lot of it's simply that we expect them. Moonlight is a wonderful, shivery thing, after all - I will never forget taking a walk in a moonlit wooded area around the lake on my university campus. The water was black and silver and the moonlight reflected astonishingly off the silver birch trees. The thrill was slightly marred by the fact that it was so muddy that one of my companions had wrapped his shoes in plastic bags . . .

The Moon's gravity - not to mention the Sun's - pulls on everything, including the Earth's crust. CERN had to take this into account building the Large Hadron Collider, if I recall correctly what was said on my visit there.  There are tides because water moves around easily. As for the Sussex police, I also recall Chris Lintott's comment on a podcast called "Living Space": "Anything to do with the Moon in Sussex has got to be Patrick's fault."

21) The much-missed Sir Patrick Moore wrote his first paper about the Moon when he was only 14. He was invited by a local astronomer, W.S. Franks, to come and use the Brockhurst Observatory which was very near where the young Patrick lived. Mr Franks was suddenly killed by a car knocking him on his bicycle, and Patrick was asked to take over the observatory. He presented a paper to the British Astronomical Association named "Small Craters in the Mare Crisium". You'll find the Mare Crisium on the far right here. "I sent it in, and was notified by the Association's Council that it had been accepted, but I felt bound to explain that I was not exactly elderly. I still have the reply, signed by the then secretary, F.J. Sellers: 'I note that you are only fourteen. I don't see that this is relevant'." (This, by the way, is exactly what you should be saying to young people interested in science.) You can read more in Sir Patrick's autobiography. (You can also leave a tribute for him here - and yes, he did play the xylophone.)

22) The darker areas of the Moon, "the man in the Moon", are called maria, meaning seas. Tell that to someone you know named Maria, if she'd be interested? The maria are of course not seas - they are in fact old lava flows, quite possibly made as the result of impact craters that filled with lava. There still seems to be debate whether the impacts caused an upwelling of lava or whether it was volcanism. They are dark because they are more iron-rich than the rest of the Moon's surface, indicating that they come from closer to the core. (Just like the Earth, the Moon has more iron towards its centre, because iron is heaviest, and it started off molten so heavy things sank to the bottom.) In this picture by Alan Friedman you can see a large lava basin which then received a later impact:

23) There isn't a "dark side of the Moon". There is a "far side" of the Moon that never points our way. But, as you can see for yourself, every side of the Moon experiences day and night with its rotation just as the Earth does. We can see the far side of the Moon, however, using spacecraft such as the Lunar Reconnaissance Orbiter:

24) The Moon does not, strictly speaking, orbit the Earth. Rather, the Earth and the Moon orbit a common centre of mass. This centre of mass, or barycentre, is just under 2000km below the surface of the Earth. That means that Earth goes round in a little circle, and the Moon goes round in a big one. (If they were the same size and mass, the centre of mass would be exactly halfway between them.)

The Earth and Moon feel equal and opposite forces: gravity balances a "centrifugal force" (I say that in inverted commas, because there's actually no such thing as centrifugal force, and the idle use of the term drives some physicists crazy. It's actually the law of inertia: the urge of the body to keep going in a straight line. If either body started going in a straight line, it would be going away from the other one. It doesn't, of course, because of gravity. When you whizz lettuce in a salad whizzer, it is flung against the wall of the bowl because it "wants" to go in a straight line, but the container prevents it from doing so). That's why we have two tides, not one. Water nearer the Moon feels the Moon's gravity more, and wells towards it. Water on the far side of the Moon feels the "centrifugal force" more, and moves towards outer space.

25) There are more maria and larger craters on the "near" side of the Moon that points our way, and the core is about 2km closer to Earth than the actual centre. It is tempting to conclude that all this is because the Earth's gravity pulled the heavy parts of the Moon towards us. But the Moon feels exactly the same effect as in the item above this: it has an equal and opposite "centrifugal force" pulling it away from the Earth as towards. It could be due to a massive impact (or several) coming from roughly Earth's direction; but we don't know. Perhaps one day we'll find out?

(It is not shameful, by the way, to answer a scientific question with "we do not yet know". It doesn't mean "scientists are stupid or lazy", but that the Universe is too big for us to have explored the whole thing yet. Otherwise we wouldn't need any scientists! Indeed, it's often when we think we're close to tidying up and claiming to understand the Universe that the greatest surprises of all occur - relativity and quantum theory, for example, or the accelerating expansion of the Universe. OK, rant over.)

26) The Moon is the second densest moon in the Solar System. The densest is Jupiter's volcanic moon Io.

27) Because of the Moon's elliptical orbit, we do get to see occasional corners of "the far side". This is called libration. Here, have a pic and get nice and dizzy.

There are other reasons for the libration: the moon's own axis is not quite at right angles to the plane of its orbit, so we see a "nodding" movement; and the fact that we're over 6000km from the Earth's centre, so we ourselves see the Moon from a very slightly different angle at the beginning and end of a night.

28) Phil Plait has a huge list of debunkations for conspiracy theorists who claim that the Moon landings were faked. You can read it here. I'll just go through one: the idea that upon launching, the Moon's dust "should have been blown around more".

Do an experiment. Next time you get out of the shower and the bathroom's full of steam, watch that steam for a while. Then blow on some steam quite far away. Or a balloon, if you desire; or a boiling pot of water or the kettle or a candle half a room away - whatever takes your fancy. You will notice that there is a pause, and then it will start swirling about. What did that?

Air, of course. The air from your lungs? Well, partly. But also all the air that's in between you and the steam. The air from your lungs knocked into air molecules, which knocked into more air molecules, which knocked into . . . well, on the Moon, this doesn't happen. Blown air, or blown anything, meets a vaccuum. Only that which is right next to the blowing gets touched. On the Moon, air will quickly dissipate into space.

For more lighthearted stuff, the Clangers are always willing to help.

29) For Moon observers, the best time to look at details is not at full moon, but when it's a crescent so there are lots of shadows. (This is only something I've been told. I'm a rubbish observer; I've used my telescope twice and then broken it!) Shadows show detail. Thank you Graham Bowes for this amazing image:

30) We've known for many centuries that there can't be air on the Moon, because its edges are sharp. Look at the Earth's own horizon, and it'll be misty, blue, and blurred. That's air getting in the way. You don't see that on the Moon. William Herschel, however, speculated that there might just be air in the craters - at the lowest points on the Moon's surface - and that aliens might live in these. He pointed out that at craters are on average 50% lit and may be lit from any angle, so alien buildings capturing the warmth of the Sun would probably be circular. Incidentally, Herschel has craters named after him on our Moon and on Saturn's moon Mimas.

31) Very occasionally, we see abrupt changes on the Moon, such as a spot turning brighter or darker or changing colour. These are transient lunar phenomena, thought to be caused by impacts, outgassing etc. You can hear a lot about the first ever recorded instance, in 1178, in Carl Sagan's "Cosmos". Some monks at a monastery in Canterbury saw "a splitting" of the very new crescent moon, and reported fire, smoke, darkening, and that the Moon "writhed" and "throbbed", "like a wounded snake". This was probably a large impact, and has been suggested to be the formation of the Giordano Bruno crater. Other odd phenomena include that static, UV-light-blasted moon dust, from number 9: the Apollo astronauts saw "twilight rays" towards the horizon, which was probably dust in a continually rising-and-falling state from the surface.

32) Going to the Moon made us much more aware of our own Earth. If you've ever spent a long time in a country other than your own, you'll know that you learn a huge amount about your own country, too. The first ever complete photograph of the Earth from space, "The Blue Marble", taken from Apollo 17 in 1972, and "Earthrise", taken by Bill Anders as Apollo 8 orbited the Moon in 1968, had a great effect on people's environmental awareness.

33) We're now pretty sure the Moon was formed by a giant impact on Earth, probably by an object the size of Mars. We can tell this from the fact that the Moon is made of fairly similar materials to the crust of the Earth, but not the core (it's less dense). Other theories of formation, historically, have included that it was captured, that the Earth and Moon formed together, or that the Earth was spinning so fast a piece of it bulged out and broke off!

It would take extreme luck to capture a passing body that was going at precisely the right speed to start orbiting the Earth, rather than colliding or simply escaping. If they had formed together, the Moon's iron core would probably be larger. This is also the case with the spinning theory. The giant impact hypothesis is supported by the fact that the Moon's composition is fairly similar to the Earth's crust. A more recent hypothesis is that there was a three-body collision, out of which the Earth and Moon formed together.

34) The temperature variation on the Moon is huge. At "night" - and a night lasts 2 weeks, since the Moon rotates at the same rate as it orbits the Earth - the temperature falls to -173 ºC, while by daytime at the equator, it rises to 127ºC.

35) There is a place near the Moon's south pole which is the coldest known place in the Solar System! There are very deep craters where sunlight never reaches the bottom. These have been measured to be -240ºC, 33ºC above "absolute zero" and 10ºC colder than Pluto. Of course, there might be similar permanently shadowed and even colder places in the Solar System just waiting for us to find them . . .

36) The Apollo astronauts brought back 2,415 separate samples of lunar rock, weighing a total of 380kg. You can go and see a piece of it at the Science Museum in London.

37) Nobody owns any land on the Moon. This has been decided in a treaty which specifically prohibits any country's sovereignity or its use for military purposes. So if someone tries to sell you a plot of land on the Moon (or to name a star after your loved one, or what have you), they are defrauding you - no matter how fancy the certificate you receive.

38) You can help explore the far side of the Moon and classify its craters with citizen science. Go to www.moonzoo.org.uk.

Thank you Graham Bowes for this ghostly galleon too.

1081 Apostrophes

That is, astrophotos, but a friend made a treasurable misread - often those (like lyrics you think you hear in songs) are much more enjoyable and thought-provoking than the original thing.

On New Year's Day 2011, Jules set the Zooites a challenge: that every day for a year somebody must post an astrophoto. And we did it. Here's the result.

(Click for larger version.)

My camera is pretty hopeless in the dark; the best it'll do of the Moon for example is a bright speck in complete blurry blue or black - or, even better, a wobbly streak where my hand moved. I took some of the pictures of the sun though.

As she says, there were 28 contributors - most of whom were far more assiduous and successful than me! - and a great many types of photo were taken of different types of astronomical phenomena. Which, of course, does include the Sun. Every so often, if the Sun is low on the horizon and possible to look at through mist or clouds (health warning: actually, you are not supposed to do even this - and never look directly at the Sun when it's bright; it can damage your eyes permanently), it strikes me very deeply that here is a star, a star like any one of those glittering lights in the skies above at night. And this Earth is what a Sun like that can support.

When every one of us was experiencing a cloudy night, we'd photograph our astronomy books, magazines, equipment, gadgets, Astrofest tickets (yes, we get very excited about things like that) and so on. That made up just over 10% of our total. Two of my favourites - and quite possibly some more once I've gone through the thread in enough detail - are going to be part of an astronomy slideshow I'm going to be showing prior to any Skeptics in the Pub talk I give. Here they are: Orion through the trees by Bill Keel and a gorgeous shot of the Moon by Infinity. Many thanks both for their permission and for e-mailing me their best sharpened versions of their artwork.

Happy Christmas Blogosphere!

I hope everybody reading this has a lovely Christmas and New Year, whether you celebrate such things or not!

This is my nutpuss Izzy last year "helping" us with the present wrapping, and the border is globular clusters taken from Jules's Starcluster Index on the Galaxy Zoo Forum.

Thank you very much to all of you who've read and especially left me comments and feedback - I promise I'll try and write more in the New Year. Special thanks to Dean without whom I could never have started Wales's first Skeptics in the Pub, and to the zookeepers for endless enjoyable citizen science and a very funny Advent Calendar!

Have a great one!

Celebrating Ice

I see that, just like last year, a bit of water in its solid form on the ground has, well, ground things to a halt again - apparently it's making even more news than the coming Royal Wedding. It's not stopping the students demonstrating in the open air, though! Oh and I hear we're out of grit? Come and sweep some off Haverfordwest's streets, there's so much I mistook a huge puddle of it for the results of an over-enthusiastic night out . . .

All right, sarcasm over (head over here for more from me if you like that sort of thing).

I have to say a personal thank you to the ice that froze on the inside of my car's windscreen, because it finally cleaned it. I'd dried off some condensation with a cloth bag weeks before, and it had been mucky ever since - despite umpteen cleans with tissue paper, towel, ice scraper, glass cleaner, and you name it - which made driving in the sunset a particular eye-watering nightmare. But every bit of that dirt fell off with the ice!

But look what came out of our bird bath a few days ago . . .



The leaves are actually less obvious to the naked eye than to my mobile's not-very-good camera. I think the ice actually expanded away from them, but retained their veiny patterns, so that it looked like a laser cutting into a piece of glass. Sadly, it's melting now . . .


I noticed that when I arrived home in the rain . . .

Off the topic of ice for a moment, I had one of those headache-inducing-ly annoying days at work today. It was supposed to be my day off, but of course certain people (who don't usually work in my office) decided to summon in me, and someone else who's disabled and doesn't come in on a Friday, all the same for our delightful monthly meeting. What happens in these is that we provide a lot of tea and biscuits, shove all our stuff off our information table for our guests, get yelled at for not doing it fast enough and generally criticised because our desks aren't empty (doh . . . believe it or not, we do do paperwork in my office), put the table somewhere stupid, arrange chairs for those who demand to be waited on hand and foot, apologise when the phone rings because somebody actually needs us, and take minutes while the same two people (neither of whom work at our office) go on and on and on about how sleepless they are about our future and how many idiotic irrelevant things they demand we must do instead of look after the people we're supposed to be serving. You get the picture. Pretty typical office meeting, I should imagine. To be fair, it isn't half as dishonest, unprincipled, bullying, or generally stressful and soul-destroying as the meetings I sat in on when I was teaching . . .

Anyway, today the person who generally takes the minutes and provides the records in my department hadn't turned up. So this task fell to me, as did taking the prolonged public kicking for not knowing the things this person knows, who has been here 15 years longer than me, and because somebody didn't know something that I thought they knew and that wasn't part of my plans so not my job to tell them . . . Again, you get the picture. I'll shut up now.

I then had an icy walk to the car and shopping to do, and my head was pounding. On the roads leading away from the town I work in are signs to one of our local beaches. They point right, while my home lies left (roughly speaking).

So today I thought . . . I will follow those signs.

As soon as I'd parked the car and got out to hear those waves between the still, silent cliffs, I felt better. The wind was painful around my un-scarfed face, it got in under my coat, and the clouds were dark; but the sea was a surprisingly bright blue-green.


This is Broad Haven according to the holiday websites . . .


This is it earlier today . . .


. . . yep . . . lots of water had frozen right there on the stones and sand. I walked on ice puddles which made wonderful noises but did not snap, merely created bubbles and pushed the sand around! (I'm actually really cheesed off - you see that blurry bit on the bottom right? That was an amazing ice puddle in which stones were nestled, but I somehow managed to delete that photo when I got home).

The ice had fascinating effects on the sand. It was wonderful to see where there was both solid and liquid water, and the alien landscapes it was managing to produce . . .





If someone had told you this was a satellite photo of Mars, would you have believed them? I'm afraid I might! But it's just ice and sand. Look at the effects . . .


. . . and just next to the image above:


It's a funny thing, ice. Most materials - as far as I know - contract when they cool. But the water molecule is a very special thing. It's sort of Mickey Mouse shaped, two hydrogen atoms and one oxygen atom. The oxygen atom is much greedier for electrons than the hydrogen atoms are, so they spend, on average, more time with the oxygen - or to put it in another way, their "probability cloud" or where they will be is somewhat skewed towards the oxygen. This means that the oxygen has a negative dipole, the hydrogen a positive one - so, like a magnet, they will be attracted to each other (this wonderful article mentions an old description of two hydrogen atoms desperately in unrequited love with an oxygen atom). But this affects other water molecules too: the oxygen will also be attracted to hydrogen atoms of other water molecules, and vice versa, which makes the general structure stick together very nicely. Most molecules so small, and made of such small atoms as hydrogen and oxygen, would be gas (think of nitrogen, methane, carbon dioxide . . .). But water's a liquid, because of this dipole. And although they slip under and over and around each other a lot, their strong bonding also causes capillary action - just watch rain falling on the window and notice how a new drop will leap into the track of an old one, rather than make a new trail.

But once water goes solid, the molecules form hexagonal lines. Russell Stannard, in his book "Ask Uncle Albert", describes ice molecules (water molecules below freezing point, if you will) as being like long, long lines of people all sitting on each other's laps. This rigidity leaves plenty of gaps between those lines, which is why it expands. That's why, when water trickles into rocks and soil and then expands, we get weathering. I believe that's probably what's happened in the last picture above.

It was also very satisfying scientifically to notice that the water that remained liquid seemed to be coming from under the stones or sand - since ice expands, it's lighter than liquid water, and so it rises. Again, this isn't true of most liquids. But it's made pondlife and icebergs possible. Without this characteristic of water, I bet a lot of Earthly life would be hugely different.

And meanwhile, something's a bit wrong with the cliff . . .


But after only a short time of tramping across the beach I was starving. So I went to the cafe by the road. It was closed, but another was open. I bought scampi and chips and a hot chocolate with whipped cream and marshmallows. While I waited, I chatted with a family already scoffing chips there. They had two utterly adorable and very well-behaved dogs, who let me stroke them like cats, one hand on each. I honestly cannot recommend a better remedy for awful meetings!

Anyway, yes, I took my scampi and chips and hot chocolate and two sachets of tomato ketchup straight out there to the beach again. I went closer to that cliff . . .


And mostly stayed away from the water; can you imagine how slippery it was there? I plonked myself down on a rock, found a little well-like bit to put my hot chocolate in, and gobbled scampi for a while. It was raining slightly but I decided I didn't care. Surprisingly it really wasn't that cold.

Nevertheless, although the huge icicles hanging from the cliff were shedding water and indeed a few chunks of ice onto the sand, clearly the ice wasn't going to melt very fast!


What a sight!






While a chap nearby did a crazy dance, I photographed and photographed, my hand over my mobile to shield it from the rain - also to put it in shadow, since the picture went especially dark whenever I tried to include a bit of sky. I wondered how long it had taken those icicles to form. I remembered a phrase I'd heard in my first year of university, describing Antarctica, which really tickled me: "thermal inertia". I think the gist was that an increased amount of water vapour in the atmosphere would increase the size of Antarctica to some extent, since the water molecules would be likely to stick to it. If the size of Antarctica increases, that will increase its albedo - in other words, it will reflect away more sunlight. Of course, it will also raise the average temperature of Antarctica, too, just the same as if you pour warm water into a bucket of ice.

Another beautiful characteristic of water is its high specific heat capacity or enthalpy of melting. Heat is basically molecules wriggling about frantically due to having plenty of energy. (Absolute zero is of course when they stop moving altogether.) Now, because of the strong attractions between water molecules, it takes quite a lot to make them separate, or wriggle away from each other. That means that you have to put an awful lot of heat in before it'll warm up. The converse is true, too: water will take a long time to freeze, because it's got so much spare energy you had to put in to warm it up in the first place.

And that is why the Earth's surface is (contrary to how it often feels) remarkably similar in temperature all over. OK, OK, so hot countries seem incredibly hot and cold ones seem incredibly cold, but that's because we're adapted to a narrow range of temperatures, and what cosmologically speaking is only a slight variation seems extreme to a biological entity. When water evaporates from the equatorial regions, it carries plenty of heat to the poles. And those cold deep currents that start at the poles and head towards the equator remain cold, which again is useful, because more oxygen can dissolve in cold water - so cold water upwellings are particularly useful for marine and coastal life in the equatorial regions.

In short, if the Earth's oceans were oil, the heat would be far less evenly distributed around the globe!

I love the water molecule. I actually fell in love with it during A level Chemistry, and kept up with it for my A level project and several of my university units. If you want more of what it can do, check out this site - I've linked to it before, but it has some desperately gorgeous close-ups of snowflakes!

I'll leave you with the most spectacular photos of that cliff . . . and by the way, it was very nice to get home!

Cascades of Cassini's wonders

Saturn at equinox, found on APOD, imaged by Cassini.

I first fell in love with Cassini back in early 2007 when Mark Leese, who works on the project, came to give us a talk at Sussex University. At the time, Huygens had comparatively recently dropped onto Titan's soil. I remember three things most clearly from the talk. One was the video of Huygens spinning down on its parachutes, one of its instruments going thud-thud-thud like my heartbeat. Another was when he asked if we wanted a break, and Tim Metham, our course tutor, replied: "No, this is riveting!" - he wanted to hear it all, right now! And finally, he took us on a tour through Saturn's rings . . . those little blocks of ice, once thought to be dust and rocks, but made of frozen water, so they gleam . . . many of which looked like little dots - but one was blue. Was it an anomalous blob? No - it's the Earth.

And it wasn't for another couple of years that I encountered the traditional Pale Blue Dot, which you can see and listen to here. But the lump in the throat was exactly the same.

Here it is again, an insignificant point of light, a tiny flicker against this backlit Saturn:
Do read its caption on APOD. Imaged by Cassini.

I'm a poor substitute for Carl, but nevertheless I tried to give a little of that sense of hugeness in my Tea with the Stars lecture the other night. I described to the audience how the rings had scattered the sunlight to brighten up Saturn from behind - and then I zoomed in on the Earth. I don't know if it came across. It's often too personal to say.

Back to more practical terms, I was thrilled to be asked to write a piece for Astronomy Now's yearbook on what Cassini will be up to next year, and Keith, the editor, is happy for me to blog about what I found out.

I e-mailed various Cassini scientists and was answered by two, Carl Murray and Joe Burns, both of whom agreed to my ringing them up and taking up lots of their time with asking occasionally silly questions. Although I really must invest in a dictaphone or something else to record what people are saying (at the time I just scribbled it down; they were very sympathetic about waiting!), it's definitely easier to get information out of people by talking to them than by e-mail. I could ask very general questions and let what they said lead up to specifics; often the specifics came by themselves, rolling on waves of enthusiasm. I love talking to people who are exhiliarated by what they're doing! I hope it goes without saying that neither they nor Keith are responsible for any errors I have made . . .

Meanwhile, Keith had kindly pointed me to Cassini's 2011 timetable. That took some dissecting - mostly drawing up tables of types of event. To summarise, it'll make 16 orbits, usually using Titan's gravity for the slingshot effect but making 30 course corrections. It'll look at the Sun and our pale blue dot 11 times. Cassini has an elliptical orbit, allowing it to view moons at different distances from Saturn, and also goes "through the ring plane", from north to south, 29 times! This isn't through one of the actual rings, obviously; it picks fairly empty areas. But even so, it'll need to "employ protective mesaures" half a dozen times or so. I asked Carl Murray what these were and he said mostly turning the instruments inward, except obviously the cosmic dust analyser which loves that sort of thing.

And what about Cassini's main job - the moons? Well, as you'll see, it heads past lots of those. Most passes are only distant ones, though these can be useful, Joe Burns explained to me, as they show you the whole moon rather than just a "patch" of it; this allows them to check general brightness, which in turn tells us about their atmospheres, temperatures and so on. But the important, nearby passes will be Rhea, Enceladus, and Titan. Those will be checking the moons in great detail.

Rhea and Janus from Cassini. NB I'm finding all these on APOD but (update, Feb 2011) have just been told off by the legendary Carolyn Porco for not making it clearer that Cassini took them. All the originals can be found at that link. I'm now updating the links wherever possible - it's not easy! I will however keep the APOD links alongside as they are friendly and informative.

Here's where Huygens landed:
Tethys behind Titan from Cassini.

(A friend remarked that whenever he talks about Cassini he feels he should just shut up and show the pictures. I know how he feels! You can easily waste half a day going to APOD Search and typing in "Cassini" . . .)

Titan is the only moon in the Solar System with a thick atmosphere. This atmosphere is actually denser than the Earth's, and is mostly methane. However, this methane is split apart by sunlight, and if Earth is anything to go by this generates free radicals, which would then react with other methane and anything else around such as nitrogen to generate quite large molecules. This makes the famous haze which we can't see through. Cassini can see through to some extent with radar and IR. Radar doesn't bounce off liquid, so these dark patches were lakes:

Found on APOD; taken by Cassini Radar Mapper.

Herewith some beauties that Huygens found when it detached itself from Cassini and dropped down into Titan's atmosphere . . .

The landing site (having trouble finding the original one here) . . .

From eight kilometres high (JPL/Cassini) . . .

About five kilometres high, a fisheye view . . .

Rivers and lakes, as predicted (Cassini) . . .

And rocks and sandiness, just like Earth, or Mars. (Cassini)

And an artist's impression of what it looked like there.

So there we are. Titan is a solid world - but its surface is shaped like the Earth's, because of weather. That implies a cycle. Not a water cycle, for all water is frozen hard as rock there: it is, according to our best guesses, a methane cycle. It seems that those lakes and rivers are liquid methane, and methane rain falls from those hazy clouds. What we don't know is what drives the cycle: when does it evaporate or liquify? Is it the Sun - far less powerful out here, nearly ten times as far from Saturn as from the Earth (and those studying basic physics will know that means nearly 100 times less radiation, if I remember correctly) - or is it something else; volcanic activity perhaps? Can Titan hold onto its envelope of gas, or is it slowly losing it, as Mars probably lost most of anything it had lighter than carbon dioxide? The team may have found evidence of a changing coastline, but it's hard to tell, since these things take a long time to occur. There are still a few corners here and there of Titan unmapped (good old Huygens only lasted there an hour and a half), which is one of the tasks set for Cassini to do in its remaining estimated 7 years of life.

There is also some argument about wind on Titan. Dunes appear to point against the wind; this may be because only storms which occur rarely and go the opposite way from normal are strong enough to shift the sand. All in all, there are lots of delicious mysteries to go.

Besides its moons and rings, Saturn has a very complicated magnetosphere. Any planet with a molten core (that is, Earth plus the gas giants) has one of those. Do play around with it in this animation. Magnetic fields give off radio waves, which allows Cassini to study it, and it's got quite a few objectives on its (so to speak) hands. What's especially interesting about Saturn's magnetosphere is its interaction with its moons. Some moons have actually been found by local drops in the charged particles, which the moons take up - come to think of it I'm not sure if that's by gravity or by something else, sorry folks and anyone who can enlighten me and my readers, please do. (Good job this is a blog and not a news article. Of course, it may be that nobody yet knows why moons take up the charged particles . . .)

The effect of Saturn's aurora at its poles, from the VIMS probe, found at guess where. Sometimes these charged particle storms "punch through" Saturn's atmosphere, or indeed, drag it upwards. There are still a lot of mysteries and surprises - it was due to studying this magnetosphere that the moon Rhea has been hypothesised to have some kind of rings!

When I asked Carl Murray about this, he laughed wryly and said, "Well, that depends on who you believe!" He sent me a paper which describes their repeated efforts to go back to Rhea and find these rings again - which failed. Something is causing a local drop in charged particles; as yet, we know not what . . .

Rhea, the second-largest moon, whose surface is patchy and worn.

And, for me, the biggest surprise of all? Guess what's contributing not only to the very diffuse e-ring (the faint, outer one you see in that gorgeous backlit picture), but to the magnetosphere? The moon Enceladus.

Here it is, right in the middle.

These
These fountains are water. To be exact, they are salty water, indicating a rocky presence below, and they are ejected from the south pole by geyser activity. But why? Joe Burns remarked to me that something as small as Enceladus shouldn't still be hot; that it still is "calls into question our understanding of how things work" (always an exciting sort of sentence in science). It may be because it's in resonance with Dione, as Ganymede, Europa and Io are around Jupiter. This may be causing tidal shifts and heating.

Ultraviolet light from nearby stars is dimmed by these fountains, indicating that there's plenty of material there. Out in space, of course, with little or zero pressure but plenty of radiation flying around, much of it won't stay as water, but will break up into the charged particles that make up the magnetosphere.

Incidentally, the moons within the E-ring reflect more light than Saturn's other moons, indicating that they get blasted by these charged particles. I suppose to say that Enceladus "washes" them is going a little too far, but the thought made me smile. Actually, it's because such particles move pretty fast, and that melts their icy surfaces and keeps them smooth.

Another view of Enceladus's vents - if you click the first link, you'll see it's upside down! It makes for a terrific video too.

Enceladus looks, at first, like a pretty placid sort of world in comparison to all this trouble it's making - until you notice those tiger stripes. They indicate regions where the ice has melted. The stripes have a different temperature and composition to the rest of the moon, but as far as I know that's all we can say at the moment.

So, besides a very strange magnetosphere, unique rings, and the only moon in the Solar System with a dense atmosphere (Triton would have a similar one if it was warmer - on Triton, the methane is frozen, much as water is on Titan), Saturn has two moons where liquids are present. Could life exist in Titan's methane lakes, or under Enceladus's ice? Organic compounds and a liquid medium are present in both. I doubt it would be life like ours. I won't be personally disappointed if there isn't any. When people ask me if I believe in life on other worlds, I tell them that I accept my own ignorance on the matter and am simply waiting to see - which many people do not consider an acceptable answer! But life or no life, there's plenty for Cassini to do in its probably six remaining years.

Yes, it's due to last until about 2017. In fact, all flybys are planned until then. It may change, of course. But that's about how long the remaining rocket fuel should last. Once that time comes near, Cassini will head out a long way, 10,000km beyond the F-ring, to survey Saturn and all its moons from afar; then it will head in again - right into Saturn's atmosphere. While it's still transmitting, we might get our first glimpses of what it's like under that giant planet's visible surface. Further and further in it will head . . . until that's the end of the mission. And what an extraordinary mission it will have been.

Thanks to Keith at Astronomy Now, and Joe Burns and Carl Murray of the Cassini team for all their information, guidance and encouragement.

This was my last slide at my recent Cassini talk.