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This week, our skies will see a rare daytime astronomical event. | 0:00:02 | 0:00:06 | |
On Monday, if the weather's better than this, | 0:00:08 | 0:00:10 | |
we'll be able to watch a transit of Mercury | 0:00:10 | 0:00:13 | |
when the solar system's smallest planet passes in front of the sun. | 0:00:13 | 0:00:16 | |
Centuries of work allow us to predict precisely | 0:00:17 | 0:00:20 | |
when this event will occur. | 0:00:20 | 0:00:22 | |
But although we know exactly where Mercury is, | 0:00:23 | 0:00:26 | |
it's a planet that we know surprisingly little about. | 0:00:26 | 0:00:29 | |
I like to think of it as the solar system's problem child | 0:00:31 | 0:00:34 | |
cos it so often confounds our expectations. | 0:00:34 | 0:00:37 | |
A planet this close to the sun should be baked dry | 0:00:38 | 0:00:42 | |
and yet there's ice on its surface. | 0:00:42 | 0:00:45 | |
You'd such a tiny world to have solidified into | 0:00:47 | 0:00:51 | |
an inactive ball of rock... | 0:00:51 | 0:00:52 | |
..but Mercury is geologically alive. | 0:00:53 | 0:00:56 | |
It even appears to be shrinking. | 0:00:58 | 0:01:00 | |
On this month's programme, | 0:01:03 | 0:01:06 | |
we'll be investigating Mercury's curious behaviour. | 0:01:06 | 0:01:09 | |
And Pete Lawrence will be here to explain how you can watch | 0:01:11 | 0:01:14 | |
the transit in safety. | 0:01:14 | 0:01:15 | |
So, tonight, in all its glory, we give you Mercury - | 0:01:16 | 0:01:20 | |
the most puzzling planet in the solar system. | 0:01:20 | 0:01:23 | |
Welcome to The Sky At Night. | 0:01:23 | 0:01:25 | |
At first glance, the thing that strikes you about Mercury | 0:01:57 | 0:02:00 | |
is that it looks a lot like our moon - | 0:02:00 | 0:02:02 | |
a bare, pale rock covered in craters. | 0:02:02 | 0:02:05 | |
It's even a similar size. | 0:02:06 | 0:02:08 | |
But in reality, Mercury is not at all like the moon. | 0:02:10 | 0:02:13 | |
In fact, it's not like anywhere else in the solar system. | 0:02:13 | 0:02:17 | |
Mercury has long been seen as an enigmatic planet, | 0:02:19 | 0:02:23 | |
but it's difficult to observe because it's orbit is so close to the sun. | 0:02:23 | 0:02:26 | |
So, whenever we look at it, it's lost in the sun's glare. | 0:02:26 | 0:02:29 | |
Even so, with observations from Earth, | 0:02:31 | 0:02:34 | |
we can map out Mercury's unusual orbit. | 0:02:34 | 0:02:37 | |
Unsurprisingly, Mercury, being the closest planet to the sun, | 0:02:38 | 0:02:42 | |
has the shortest orbit. | 0:02:42 | 0:02:44 | |
It takes just 88 days to go all the way round. | 0:02:44 | 0:02:46 | |
It also has the most elliptical orbit | 0:02:46 | 0:02:48 | |
of all the planets in the solar system. | 0:02:48 | 0:02:51 | |
At its closest approach, it's just 46 million kilometres | 0:02:52 | 0:02:55 | |
away from the sun, and, at its furthest, 70 million kilometres. | 0:02:55 | 0:02:59 | |
Now, that's unusual, but things get really weird | 0:02:59 | 0:03:01 | |
when you take into account Mercury's rotation. | 0:03:01 | 0:03:04 | |
Mercury rotates very slowly. | 0:03:08 | 0:03:10 | |
It actually takes 59 Earth days for it to complete one revolution. | 0:03:10 | 0:03:13 | |
But as it rotates, Mercury is also moving around its orbit. | 0:03:14 | 0:03:18 | |
And the combination of rotation and orbit | 0:03:19 | 0:03:22 | |
causes the sun to move very slowly across the sky. | 0:03:22 | 0:03:25 | |
So slowly in fact that from the planet's surface, a day | 0:03:26 | 0:03:30 | |
from sunrise to sunrise actually lasts two complete orbits. | 0:03:30 | 0:03:34 | |
So, on Mercury, a day is twice as long as its year. | 0:03:34 | 0:03:38 | |
That means that, on the surface, daylight lasts the equivalent of | 0:03:38 | 0:03:42 | |
three Earth months and temperatures rise to around 450 Celsius. | 0:03:42 | 0:03:47 | |
That's followed by three months of night-time, | 0:03:47 | 0:03:49 | |
where the temperatures plummet to minus 180. | 0:03:49 | 0:03:52 | |
This temperature difference of over 600 degrees | 0:03:54 | 0:03:57 | |
is the highest experienced anywhere in the solar system. | 0:03:57 | 0:04:00 | |
That made it all the more surprising when astronomers at the Arecibo | 0:04:03 | 0:04:07 | |
radio telescope bounced radar pulses off the planet. | 0:04:07 | 0:04:10 | |
The signals that came back showed bright spots near the poles | 0:04:14 | 0:04:17 | |
that bore the unmistakable signature of water ice. | 0:04:17 | 0:04:21 | |
The shape of the ice deposits clearly showed that they were | 0:04:22 | 0:04:25 | |
concealed in the depths of polar craters - | 0:04:25 | 0:04:28 | |
the only areas on the planet that never receive any direct sunlight. | 0:04:28 | 0:04:32 | |
That means that, on this planet, | 0:04:34 | 0:04:36 | |
three times closer to the sun than Earth, | 0:04:36 | 0:04:38 | |
you can still find water ice. | 0:04:38 | 0:04:40 | |
That's pretty amazing. | 0:04:40 | 0:04:42 | |
But there's only so much we can tell by observing from Earth. | 0:04:45 | 0:04:48 | |
To learn more about Mercury, we need spacecraft data. | 0:04:50 | 0:04:53 | |
Only two missions have ever visited the planet - | 0:04:55 | 0:04:58 | |
the first was Mariner 10 in 1974. | 0:04:58 | 0:05:01 | |
Its camera only produced grainy pictures, | 0:05:03 | 0:05:06 | |
but the other instruments it carried began to reveal major | 0:05:06 | 0:05:09 | |
differences between Mercury and the other rocky planets. | 0:05:09 | 0:05:13 | |
I've come to the Open University in Milton Keynes to talk to | 0:05:14 | 0:05:18 | |
Mercury expert David Rothery. | 0:05:18 | 0:05:20 | |
When they see images of Mercury, like this one, | 0:05:21 | 0:05:23 | |
you can't fail to be intrigued by it. | 0:05:23 | 0:05:25 | |
It's got a wonderful landscape. | 0:05:25 | 0:05:27 | |
It's a beautiful image as well, but we've only sent two spacecraft | 0:05:27 | 0:05:31 | |
two Mercury, compared to many that have been to Venus and Mars. | 0:05:31 | 0:05:34 | |
Why is that? Why is Mercury being neglected? | 0:05:34 | 0:05:36 | |
Well, Mars and Venus are the low-hanging fruit - | 0:05:36 | 0:05:39 | |
they are closer to the Earth. | 0:05:39 | 0:05:41 | |
And at first sight, certainly they are more interesting. | 0:05:41 | 0:05:43 | |
Mercury, when we first sent a probe by it, it's airless, | 0:05:43 | 0:05:46 | |
it's heavily cratered, it's a little bit dull. | 0:05:46 | 0:05:48 | |
Now we realise it is not dull at all. | 0:05:48 | 0:05:51 | |
There's all kinds of things going on there. | 0:05:51 | 0:05:53 | |
Even that first mission, the Mariner mission, | 0:05:53 | 0:05:55 | |
-that flew past made some remarkable discoveries. -It did. | 0:05:55 | 0:05:59 | |
It was equipped with a magnetometer | 0:05:59 | 0:06:01 | |
to look at the interaction between the solar wind, | 0:06:01 | 0:06:03 | |
the charged particles from the Sun and the planet's surface, | 0:06:03 | 0:06:05 | |
but it found that the planet's generating its own magnetic field. | 0:06:05 | 0:06:09 | |
It's like a scaled-down version of the Earth's magnetic field. | 0:06:09 | 0:06:12 | |
And that's surprising, for such a small planet | 0:06:12 | 0:06:14 | |
to have a magnetic field. | 0:06:14 | 0:06:16 | |
-It's really, really unusual. -Absolutely. | 0:06:16 | 0:06:18 | |
Mars, the Moon and Venus don't generate a magnetic field, | 0:06:18 | 0:06:21 | |
but Mercury does. | 0:06:21 | 0:06:22 | |
And, you know, it caught everybody by surprise. | 0:06:22 | 0:06:25 | |
And thank goodness Mariner 10 carried its magnetometer. | 0:06:25 | 0:06:27 | |
But what does it tell us? | 0:06:27 | 0:06:28 | |
What do we need to be able to generate this magnetic field? | 0:06:28 | 0:06:31 | |
OK, well, some people say, "Well, iron in the core. | 0:06:31 | 0:06:34 | |
"You've got a core made of iron - it will be a magnet." | 0:06:34 | 0:06:36 | |
But that doesn't work. It has to be fluid. | 0:06:36 | 0:06:38 | |
You have to have an electrically conducting fluid churning around, | 0:06:39 | 0:06:42 | |
so we think the outer part of Mercury's interior core | 0:06:42 | 0:06:46 | |
is made of molten iron, and that's the explanation that holds for the Earth as well. | 0:06:46 | 0:06:50 | |
Well, that would make sense, apart from the fact that Mercury's such a small world. | 0:06:50 | 0:06:54 | |
On Mars, for example, we think that there's no magnetic field | 0:06:54 | 0:06:57 | |
because it's cooled down and there's no longer any fluid core. | 0:06:57 | 0:07:00 | |
Yeah, well, Mercury should be cooling down as well | 0:07:00 | 0:07:03 | |
cos it's got a large surface compared to its volume. | 0:07:03 | 0:07:05 | |
It was probably quite hot to begin with, but there is also clearly some | 0:07:05 | 0:07:09 | |
way of generating heat in the core to stop it having frozen, | 0:07:09 | 0:07:12 | |
and also something to reduce the melting | 0:07:12 | 0:07:15 | |
temperature of the outer part of the core - | 0:07:15 | 0:07:17 | |
we think that is probably sulphur. | 0:07:17 | 0:07:19 | |
-Mixed in with the iron? -Mixed in with the iron. | 0:07:19 | 0:07:21 | |
There has to be enough iron to make an electrical conductor. | 0:07:21 | 0:07:23 | |
So what we think is happening is iron is still today sinking | 0:07:23 | 0:07:28 | |
inwards to join the frozen inner core of solid iron | 0:07:28 | 0:07:33 | |
and, as the iron sinks inwards, | 0:07:33 | 0:07:35 | |
it's turning gravitational energy into heat | 0:07:35 | 0:07:38 | |
and that's heating the outer part of the core, which is gradually | 0:07:38 | 0:07:41 | |
becoming richer in sulphur, reducing its melting temperature | 0:07:41 | 0:07:44 | |
and enabling it to churn round and generate a magnetic field. | 0:07:44 | 0:07:47 | |
So we know about one process that is happening inside Mercury - | 0:07:47 | 0:07:50 | |
what do we know about the rest of its structure? | 0:07:50 | 0:07:52 | |
We do know that the core must be very, | 0:07:52 | 0:07:55 | |
-very large compared to the size of the planet. -Wow! | 0:07:55 | 0:07:59 | |
Beyond the iron-rich inner core and outer core, you've got the rock. | 0:07:59 | 0:08:03 | |
Most of the rock is what we call the mantle, | 0:08:03 | 0:08:05 | |
there's a crust on the outside that is slightly chemically different. | 0:08:05 | 0:08:08 | |
This rocky outer part is much, | 0:08:08 | 0:08:10 | |
much thinner on Mercury than it is on any of the other rocky planets. | 0:08:10 | 0:08:13 | |
It's the opposite of the earth, where we have quite a thick mantle and a thin core. | 0:08:13 | 0:08:16 | |
So, to me, this is one of the great mysteries of Mercury. | 0:08:16 | 0:08:19 | |
Why do we have this large core surrounded by a thin mantle? | 0:08:19 | 0:08:22 | |
Absolutely. | 0:08:22 | 0:08:23 | |
Mariner had started to reveal Mercury's inner secrets, | 0:08:24 | 0:08:28 | |
but we had to wait another 35 years for a really good | 0:08:28 | 0:08:31 | |
look at Mercury's surface. | 0:08:31 | 0:08:33 | |
That finally came in 2011, when NASA's MESSENGER probe | 0:08:35 | 0:08:40 | |
started returning high-definition images like these. | 0:08:40 | 0:08:43 | |
The team here in Milton Keynes are using these images to create | 0:08:45 | 0:08:49 | |
detailed geological maps of Mercury, | 0:08:49 | 0:08:52 | |
but they're also finding plenty more evidence of Mercury's strangeness. | 0:08:52 | 0:08:56 | |
So, Jack, if we can interrupt, what are you working on? | 0:08:58 | 0:09:01 | |
I'm making a geological map of a region on Mercury, this area | 0:09:01 | 0:09:04 | |
you see here. I'm using data from NASA's MESSENGER satellite, | 0:09:04 | 0:09:08 | |
planetary images mosaiced together, | 0:09:08 | 0:09:10 | |
and I'm interpreting the geological units I see at the surface. | 0:09:10 | 0:09:13 | |
And so what I see when I look at this globe of Mercury is | 0:09:13 | 0:09:16 | |
craters and I think that's what people think of. | 0:09:16 | 0:09:18 | |
Like the moon, it's a grey body with a cratered surface. | 0:09:18 | 0:09:21 | |
You're absolutely right. | 0:09:21 | 0:09:22 | |
It is a heavily cratered surface in places. | 0:09:22 | 0:09:25 | |
However, there is more to Mercury. | 0:09:25 | 0:09:26 | |
For example, these lobate scarps that you see here, | 0:09:26 | 0:09:30 | |
this one is called Carnegie Rupes. | 0:09:30 | 0:09:31 | |
So that's this line running from top left to bottom, | 0:09:31 | 0:09:34 | |
-right across the image. -Absolutely. | 0:09:34 | 0:09:37 | |
This is an escarpment in the landscape caused by faulting | 0:09:37 | 0:09:40 | |
within the crust of Mercury. | 0:09:40 | 0:09:41 | |
So we see this kind of thing on Earth, | 0:09:41 | 0:09:44 | |
these places where you have this sort of raised up area. | 0:09:44 | 0:09:46 | |
Yes, you see these things on Earth. | 0:09:46 | 0:09:48 | |
On Earth we have plate tectonics | 0:09:48 | 0:09:50 | |
and, because of the collisions of plates, we find them building | 0:09:50 | 0:09:53 | |
mountains and making escarpments in the landscape such as this one. | 0:09:53 | 0:09:56 | |
However, there's no evidence to suggest Mercury has multiple | 0:09:56 | 0:10:00 | |
tectonics plates that move around and collide with each other, | 0:10:00 | 0:10:03 | |
so instead this is internal deformation within one plate | 0:10:03 | 0:10:06 | |
that's being drawn in from within. | 0:10:06 | 0:10:08 | |
So it's almost as if the whole planet is shrinking. | 0:10:08 | 0:10:11 | |
Yes, the planet is in a state of global contraction. | 0:10:11 | 0:10:13 | |
And people have looked at the distribution of these lobate | 0:10:13 | 0:10:16 | |
scarps over the planet and added up all their effects, | 0:10:16 | 0:10:18 | |
and they calculate that perhaps the planet has | 0:10:18 | 0:10:20 | |
-lost as much as 7km of its planetary radius. -That's enormous. | 0:10:20 | 0:10:24 | |
-That's Everest-ish... -It's hard to imagine a planet shrinking, | 0:10:24 | 0:10:29 | |
but Mercury demonstrates that it has. | 0:10:29 | 0:10:31 | |
So what's causing this? Why is there this shrinking? | 0:10:31 | 0:10:35 | |
As the planet loses heat, this causes a volume reduction, | 0:10:35 | 0:10:38 | |
particularly because as the liquid part of Mercury's iron core freezes, | 0:10:38 | 0:10:41 | |
that causes a volume reduction, which pulls the crust in everywhere. | 0:10:41 | 0:10:44 | |
So it's quite a simple process but, Dave, | 0:10:44 | 0:10:46 | |
this isn't the only thing that's happening on the surface. | 0:10:46 | 0:10:49 | |
If we look elsewhere, we can find evidence of other processes. | 0:10:49 | 0:10:52 | |
Far from it. There's all kinds of things that have gone on on Mercury. | 0:10:52 | 0:10:56 | |
One thing that comes to mind is something that we didn't know about | 0:10:56 | 0:10:59 | |
until MESSENGER got there, which is these areas called hollows. | 0:10:59 | 0:11:02 | |
There's a view here that's about 20-30km across | 0:11:02 | 0:11:05 | |
and it's showing an area of surface where the top 20m of material | 0:11:05 | 0:11:09 | |
is just gone, it's been stripped away to leave that irregular area. | 0:11:09 | 0:11:14 | |
There's some smaller hollows nearby. | 0:11:14 | 0:11:16 | |
It looks a bit like mould or Swiss cheese, or something like that. | 0:11:16 | 0:11:20 | |
Absolutely. How is it being removed? | 0:11:20 | 0:11:22 | |
It's not falling into caverns, | 0:11:22 | 0:11:24 | |
it's not being blowing away in the wind, there's no wind. | 0:11:24 | 0:11:27 | |
It's turning to vapour somehow and just being lost to space, | 0:11:27 | 0:11:30 | |
so something in the surface is volatile. | 0:11:30 | 0:11:33 | |
Volatile enough to turn to vapour and just go. | 0:11:33 | 0:11:36 | |
-So what could that be? -It's a big problem and we can't tell from this. | 0:11:36 | 0:11:40 | |
It could be sulphur, could be chlorine. | 0:11:40 | 0:11:42 | |
Is it driven by heat or charged particles breaking chemical bonds? | 0:11:42 | 0:11:45 | |
But there is evidence of volatile richness in the planet, | 0:11:45 | 0:11:48 | |
which was completely unexpected. | 0:11:48 | 0:11:50 | |
Something this close to the sun ought not to be rich in volatiles. | 0:11:50 | 0:11:54 | |
Why not? | 0:11:54 | 0:11:55 | |
Close to the sun you should be losing volatiles as you're | 0:11:55 | 0:11:57 | |
trying to grow a planet because it's hot and, because Mercury has | 0:11:57 | 0:12:01 | |
a large core, people thought it's had a violent birth. | 0:12:01 | 0:12:05 | |
How do you get such a large core and a thin, rocky area? | 0:12:05 | 0:12:07 | |
You blast the rock away, | 0:12:07 | 0:12:09 | |
but that should be stripping away the volatiles as well, | 0:12:09 | 0:12:12 | |
and yet Mercury has retained it's volatiles | 0:12:12 | 0:12:14 | |
and still got a large core. | 0:12:14 | 0:12:16 | |
-It doesn't fit. -And it's a world that's changing now. | 0:12:16 | 0:12:20 | |
Some of these processes are still happening. | 0:12:20 | 0:12:22 | |
The hollow-forming processes are still going on today. | 0:12:22 | 0:12:25 | |
When you look at fields of hollows, you don't find impact craters | 0:12:25 | 0:12:28 | |
superimposed. We think the hollows are still growing in some areas. | 0:12:28 | 0:12:32 | |
Who would have thought Mercury would be an active planet? | 0:12:32 | 0:12:35 | |
That adds up to Mercury being a very exciting place. Thank you very much. | 0:12:35 | 0:12:38 | |
Pleasure. | 0:12:38 | 0:12:40 | |
One of the reasons that we knew so little about Mercury for | 0:12:42 | 0:12:45 | |
so long was that it's difficult to observe from the Earth. | 0:12:45 | 0:12:49 | |
But this week's transit will give everyone the chance to see | 0:12:52 | 0:12:56 | |
this mysterious planet... | 0:12:56 | 0:12:57 | |
..and Peter's here to explain the best way to view the transit safely. | 0:12:59 | 0:13:03 | |
As planets go, Mercury isn't far away. | 0:13:07 | 0:13:09 | |
Only 48 million miles at closest approach. | 0:13:10 | 0:13:13 | |
But it's surprisingly hard to observe... | 0:13:16 | 0:13:19 | |
because being so close to the sun, | 0:13:19 | 0:13:21 | |
it is only ever visible for a short period, | 0:13:21 | 0:13:23 | |
just before sunrise or after sunset. | 0:13:23 | 0:13:26 | |
But this week's transit is a great opportunity to see | 0:13:28 | 0:13:31 | |
the planet during broad daylight. | 0:13:31 | 0:13:33 | |
We won't get another chance as good as this until 2049. | 0:13:34 | 0:13:38 | |
Eclipse glasses are a great way to look at the sun safely, | 0:13:40 | 0:13:44 | |
but unfortunately Mercury is going to be too small to be seen | 0:13:44 | 0:13:47 | |
with the naked eye. | 0:13:47 | 0:13:48 | |
It's about 1/155th the apparent size of the sun, | 0:13:48 | 0:13:52 | |
so to see it at all you're going to need something | 0:13:52 | 0:13:55 | |
with a bit more power - | 0:13:55 | 0:13:56 | |
say a telescope or a powerful pair of binoculars. | 0:13:56 | 0:14:00 | |
But to be safe, these must be fitted | 0:14:00 | 0:14:02 | |
with a certified solar safety filter. | 0:14:02 | 0:14:04 | |
There are various filters available, but one of the easiest ways | 0:14:04 | 0:14:08 | |
to achieve this is to get hold of an A4 sheet of solar safety film | 0:14:08 | 0:14:13 | |
and then make your own filter, | 0:14:13 | 0:14:15 | |
which slips on the front of the telescope - just like that. | 0:14:15 | 0:14:19 | |
You can point the telescope at the sun... | 0:14:19 | 0:14:21 | |
..and you're good to go. | 0:14:23 | 0:14:25 | |
With the filter in place, | 0:14:28 | 0:14:29 | |
you should get a view of the whole of the sun's disk, | 0:14:29 | 0:14:32 | |
on which it's possible to make out small sun spots. | 0:14:32 | 0:14:35 | |
This one is about the size that Mercury will | 0:14:37 | 0:14:39 | |
appear during the transit. | 0:14:39 | 0:14:41 | |
The transit will begin just after noon, with the sun high in the sky. | 0:14:44 | 0:14:48 | |
You will see Mercury make first contact with the eastern | 0:14:51 | 0:14:54 | |
edge of the sun and will then track southwest | 0:14:54 | 0:14:57 | |
until the transit finishes at 7:42 in the evening. | 0:14:57 | 0:15:01 | |
If you don't have a filter then there is another way to get | 0:15:04 | 0:15:07 | |
a decent view of the sun and that's to project it. | 0:15:07 | 0:15:11 | |
Now, projection is only really suitable for small refracting | 0:15:11 | 0:15:14 | |
or lens-based telescopes. But using a small refractor, | 0:15:14 | 0:15:17 | |
if you point this directly at the sun with | 0:15:17 | 0:15:20 | |
an eyepiece in the eyepiece holder, it's then possible to project | 0:15:20 | 0:15:23 | |
an image of the sun onto a piece of white card | 0:15:23 | 0:15:26 | |
and it actually gives you a really good view. | 0:15:26 | 0:15:29 | |
One final warning, if you're using this method, | 0:15:29 | 0:15:32 | |
is to not keep the telescope pointed at the sun for too long a period. | 0:15:32 | 0:15:36 | |
If you do, you run a risk of damaging the internal | 0:15:36 | 0:15:39 | |
components of the telescope. | 0:15:39 | 0:15:41 | |
If it's got plastic bits inside, for example, they may melt | 0:15:41 | 0:15:44 | |
and plastic eyepieces may melt, as well. | 0:15:44 | 0:15:47 | |
Also, be careful | 0:15:47 | 0:15:48 | |
because the temperature just behind the eyepiece is really hot. | 0:15:48 | 0:15:52 | |
I can demonstrate that with this little piece of black card. | 0:15:52 | 0:15:55 | |
Look at that. That didn't take very long at all. | 0:15:55 | 0:15:58 | |
Makes Bear Grylls look pretty pathetic, doesn't it? | 0:15:58 | 0:16:01 | |
HE LAUGHS | 0:16:01 | 0:16:02 | |
Hopefully, on the day of the transit, | 0:16:08 | 0:16:10 | |
the skies will be lovely and clear like they are today. | 0:16:10 | 0:16:13 | |
But if the clouds do come, | 0:16:13 | 0:16:15 | |
in the event is so long at 7.5 hours from beginning to end that we do | 0:16:15 | 0:16:20 | |
stand at least a decent possibility of some clear breaks | 0:16:20 | 0:16:23 | |
where we'll see something of it. | 0:16:23 | 0:16:25 | |
The end of the transit occurs with the sun just | 0:16:25 | 0:16:27 | |
nine degrees above the northwest horizon, | 0:16:27 | 0:16:30 | |
so if you do intend to watch the entire event then make sure | 0:16:30 | 0:16:34 | |
you've got a clear view in that particular direction. | 0:16:34 | 0:16:37 | |
So clear skies and good luck. | 0:16:37 | 0:16:39 | |
If you don't have the right equipment, | 0:16:42 | 0:16:44 | |
there will be lots of events happening all over the country, | 0:16:44 | 0:16:46 | |
like here at the Open University, where you can watch the transit | 0:16:46 | 0:16:49 | |
in the company of your local astronomers. | 0:16:49 | 0:16:51 | |
And if it does happen to be cloudy, like today, | 0:16:51 | 0:16:54 | |
then Isa are streaming the transit live from space using | 0:16:54 | 0:16:57 | |
satellites that will get a great view whatever the weather, | 0:16:57 | 0:17:00 | |
and we'll have details of that stream | 0:17:00 | 0:17:02 | |
and the list of events on our website. | 0:17:02 | 0:17:04 | |
But transits are more than just rare and remarkable events - | 0:17:04 | 0:17:07 | |
they also have significant scientific value. | 0:17:07 | 0:17:09 | |
We asked public astronomer Marek Kukula | 0:17:09 | 0:17:11 | |
from the Royal Observatory Greenwich to investigate. | 0:17:11 | 0:17:14 | |
To understand the importance of transits, | 0:17:18 | 0:17:21 | |
we need to geo back to the 17th century, | 0:17:21 | 0:17:23 | |
to the time just after the founding of the Royal Observatory. | 0:17:23 | 0:17:26 | |
This is what was known as the solar system in the second | 0:17:29 | 0:17:32 | |
half of the 1600s. The six inner planets all orbiting around the sun. | 0:17:32 | 0:17:36 | |
The outer planets, of course, hadn't been discovered yet. | 0:17:36 | 0:17:40 | |
It was a model we'd had since the time of Copernicus and Kepler. | 0:17:40 | 0:17:43 | |
We knew the order of the planets and we knew the shape of their orbits, | 0:17:43 | 0:17:47 | |
but there was one thing about the solar system we didn't know - | 0:17:47 | 0:17:50 | |
we didn't know how big it was. | 0:17:50 | 0:17:52 | |
Although the relative sizes of the orbits were understood, | 0:17:52 | 0:17:55 | |
for instance, we knew that the Earth was three times | 0:17:55 | 0:17:57 | |
further from the sun than Mercury, | 0:17:57 | 0:18:00 | |
the actual distances weren't known. | 0:18:00 | 0:18:02 | |
It was a solar system without scale. | 0:18:03 | 0:18:06 | |
And that's where this guy comes in. He's Edmund Halley. | 0:18:08 | 0:18:11 | |
He'd later become Astronomer Royal himself, but in 1677 he was | 0:18:11 | 0:18:15 | |
just an assistant astronomer, here at the observatory in Greenwich. | 0:18:15 | 0:18:19 | |
And he'd been sent to St Helena in the South Atlantic to observe | 0:18:19 | 0:18:22 | |
the southern skies. | 0:18:22 | 0:18:23 | |
While he was there, he watched a transitive Mercury, | 0:18:23 | 0:18:26 | |
just like the one that's due this week. | 0:18:26 | 0:18:29 | |
Watching Mercury crawl across the face of the sun, | 0:18:29 | 0:18:32 | |
Halley realised how a transit could be used to measure | 0:18:32 | 0:18:35 | |
the size of the solar system. | 0:18:35 | 0:18:37 | |
Halley's breakthrough was to understand that | 0:18:39 | 0:18:42 | |
if you viewed the transit from two widely spaced locations, | 0:18:42 | 0:18:46 | |
thousands of miles apart, then you'll see the transit differently. | 0:18:46 | 0:18:50 | |
Viewed from here, south of the equator, | 0:18:51 | 0:18:53 | |
the planet will appear here against the disk of the sun. | 0:18:53 | 0:18:56 | |
But viewed from north of the equator, | 0:18:56 | 0:18:58 | |
the planet will appear further down on the sun's disk. | 0:18:58 | 0:19:01 | |
What Halley realised was that | 0:19:02 | 0:19:04 | |
if you could measure the apparent separation between the points, | 0:19:04 | 0:19:07 | |
the parallax, then you could work out | 0:19:07 | 0:19:09 | |
the distance between the earth and the sun. | 0:19:09 | 0:19:11 | |
It was a calculation that required | 0:19:14 | 0:19:16 | |
some fiendishly complicated geometry. | 0:19:16 | 0:19:18 | |
But to produce an accurate figure, it also needed a number | 0:19:20 | 0:19:23 | |
of extremely precise measurements to be made during the transit... | 0:19:23 | 0:19:27 | |
..including, most crucially, | 0:19:28 | 0:19:29 | |
the exact time it takes for the planet to cross the sun. | 0:19:29 | 0:19:32 | |
But here Halley faced a problem. | 0:19:35 | 0:19:37 | |
Mercury was so small and travelled so fast that it would be almost | 0:19:38 | 0:19:42 | |
impossible to make the measurements required. | 0:19:42 | 0:19:45 | |
A more suitable target was Venus, | 0:19:47 | 0:19:49 | |
which appears both bigger and slower as it transits the sun. | 0:19:49 | 0:19:53 | |
But the next transit of Venus wasn't due for another 84 years. | 0:19:55 | 0:19:59 | |
Halley knew he'd be long dead by then, | 0:20:00 | 0:20:03 | |
so he laid down the gauntlet for future generations. | 0:20:03 | 0:20:06 | |
Almost a century later, the world's astronomers rose to that challenge. | 0:20:08 | 0:20:12 | |
There were transits of Venus in 1761 and 1769, | 0:20:14 | 0:20:17 | |
and expeditions were sent out all around the world to observe them. | 0:20:17 | 0:20:21 | |
These expeditions were among the first great international | 0:20:21 | 0:20:24 | |
scientific collaborations | 0:20:24 | 0:20:26 | |
and, in many ways, they were like the Large Hadron Collider | 0:20:26 | 0:20:29 | |
or International Space Station of their day. | 0:20:29 | 0:20:31 | |
The French, British and Austrians went to Siberia | 0:20:42 | 0:20:45 | |
and Northern Canada, | 0:20:45 | 0:20:46 | |
where they had to brave polar bears and hostile locals. | 0:20:46 | 0:20:49 | |
They went to the Indian and Pacific Oceans. | 0:20:49 | 0:20:51 | |
Captain James Cook was sent to Tahiti in 1769. | 0:20:51 | 0:20:55 | |
And these were major expeditions for the time, | 0:20:55 | 0:20:57 | |
involving perilous sea voyages sometimes lasting several years. | 0:20:57 | 0:21:01 | |
There was one French expedition to Mexico from which only one | 0:21:01 | 0:21:04 | |
person returned alive. | 0:21:04 | 0:21:05 | |
And to make matters worse, during some of this time, | 0:21:05 | 0:21:08 | |
France and Britain were at war | 0:21:08 | 0:21:09 | |
and special arrangements had to be made to give safe passage | 0:21:09 | 0:21:12 | |
to scientists from each side. | 0:21:12 | 0:21:14 | |
Once they'd arrived, each team had to spend weeks | 0:21:18 | 0:21:20 | |
calculating their latitude and longitude. | 0:21:20 | 0:21:23 | |
And this is dated from Cook's voyage in 1769. | 0:21:23 | 0:21:26 | |
You can see how detailed it is. | 0:21:26 | 0:21:28 | |
They're even using the moons of Jupiter to calculate their longitude. | 0:21:28 | 0:21:32 | |
And once they'd done that, they had to hope for clear | 0:21:32 | 0:21:34 | |
skies for the transit itself. | 0:21:34 | 0:21:37 | |
The crucial measurement was to time how long it took Venus to | 0:21:37 | 0:21:40 | |
cross the disk of the sun to within a couple of seconds. | 0:21:40 | 0:21:44 | |
And these are drawings by Captain Cook himself and they perfectly | 0:21:44 | 0:21:48 | |
illustrate a really crucial problem that they discovered. | 0:21:48 | 0:21:52 | |
It's an optical illusion - the so-called "black drop effect". | 0:21:52 | 0:21:56 | |
And as Venus starts to cross the disk of the sun, | 0:21:56 | 0:21:59 | |
the disk of Venus appears to stretch out and blur, | 0:21:59 | 0:22:02 | |
and that makes it very difficult to measure | 0:22:02 | 0:22:04 | |
the precise time at which the transit begins. | 0:22:04 | 0:22:07 | |
The black drop effect made it impossible to record | 0:22:08 | 0:22:11 | |
the length of the transit with the desired accuracy... | 0:22:11 | 0:22:14 | |
..but the data they did collect was enough for astronomers | 0:22:17 | 0:22:20 | |
to start their calculations. | 0:22:20 | 0:22:21 | |
In 1771, the French astronomer Jerome Lalande calculated | 0:22:22 | 0:22:26 | |
a value for the astronomical unit, | 0:22:26 | 0:22:29 | |
the distance between the earth and the sun, of 153 million km, | 0:22:29 | 0:22:33 | |
which is impressively within 2.5% of the modern value. | 0:22:33 | 0:22:37 | |
Suddenly, the solar system had a scale. | 0:22:37 | 0:22:39 | |
That's why transits were important historically. | 0:22:42 | 0:22:44 | |
But today, transits are still important in helping us | 0:22:46 | 0:22:48 | |
understand our position in the universe | 0:22:48 | 0:22:51 | |
because of the role they play in showing how many other planets | 0:22:51 | 0:22:54 | |
there are outside the solar system. | 0:22:54 | 0:22:56 | |
Whenever a planet passes in front of the sun, | 0:22:59 | 0:23:02 | |
it blocks out a small but measurable amount of its light. | 0:23:02 | 0:23:05 | |
The same principle applies when we look at other stars - | 0:23:07 | 0:23:10 | |
it's very difficult to observe their planets directly - | 0:23:10 | 0:23:14 | |
but we can see the tiny drop in brightness | 0:23:14 | 0:23:16 | |
as the planet passes in front of the star. | 0:23:16 | 0:23:19 | |
It's exactly this technique that the Kepler space telescope uses. | 0:23:19 | 0:23:23 | |
It monitors 100,000 stars, looking for the telltale dip in luminance | 0:23:24 | 0:23:29 | |
that indicates a transiting planet. | 0:23:29 | 0:23:31 | |
By using this method, it has in the last seven years detected | 0:23:34 | 0:23:37 | |
nearly 6,000 possible exoplanets. | 0:23:37 | 0:23:40 | |
When you're watching a transit, like the one this week, | 0:23:43 | 0:23:45 | |
you should bear in mind a couple of things. | 0:23:45 | 0:23:47 | |
One is that what you are watching is a clear example | 0:23:47 | 0:23:50 | |
of the solar system in action - | 0:23:50 | 0:23:52 | |
planets actually moving along their orbits in real time. | 0:23:52 | 0:23:55 | |
But also what you're seeing isn't just a pleasing spectacle | 0:23:55 | 0:23:58 | |
because transits, perhaps more than any other phenomenon, | 0:23:58 | 0:24:01 | |
have helped us to understand the scale and scope of our universe. | 0:24:01 | 0:24:05 | |
Mercury is undoubtedly a strange world. | 0:24:09 | 0:24:12 | |
With its large iron core and its thin mantle, | 0:24:12 | 0:24:15 | |
it's not like any of the rest of the family of rocky planets. | 0:24:15 | 0:24:19 | |
So what happened in Mercury's formation to make it this way? | 0:24:19 | 0:24:22 | |
Maggie has been talking to planetary scientist Craig Agnor to | 0:24:23 | 0:24:27 | |
discuss the latest ideas. | 0:24:27 | 0:24:29 | |
Craig, can you describe to me | 0:24:31 | 0:24:32 | |
the old theory of the formation of Mercury? | 0:24:32 | 0:24:35 | |
One of the initial ideas was that Mercury's mantle was removed | 0:24:35 | 0:24:38 | |
through a giant impact. | 0:24:38 | 0:24:39 | |
And the initial modelling of this suggested a smaller object, | 0:24:39 | 0:24:43 | |
maybe a third the mass of Mercury, smashed in at very high velocity, | 0:24:43 | 0:24:48 | |
vaporised the mantle, blasted off into space | 0:24:48 | 0:24:50 | |
and this would have predicted a very hot but iron-rich planet. | 0:24:50 | 0:24:55 | |
OK, so an iron-rich planet, so a large core and a thin mantle, | 0:24:55 | 0:24:58 | |
so that does tie in with what we see of Mercury. | 0:24:58 | 0:25:00 | |
That's exactly right. | 0:25:00 | 0:25:02 | |
The problem is that recent spacecraft data has shown | 0:25:02 | 0:25:04 | |
that Mercury's mantle retains a significant | 0:25:04 | 0:25:06 | |
inventory of volatiles that wouldn't have survived the extreme | 0:25:06 | 0:25:09 | |
heating of this initial scenario. | 0:25:09 | 0:25:11 | |
They would have been blown off into space with the temperature. | 0:25:11 | 0:25:14 | |
-That's right. -OK, so there's a challenge there. -That's right. | 0:25:14 | 0:25:16 | |
So you have to look at the different types of giant impacts that | 0:25:16 | 0:25:19 | |
occur during planet formation. | 0:25:19 | 0:25:21 | |
One of the new ideas about this origin of Mercury is that | 0:25:21 | 0:25:23 | |
maybe Mercury hit a larger object at slower velocity | 0:25:23 | 0:25:27 | |
and this collision may be able to remove the mantle without | 0:25:27 | 0:25:31 | |
the extreme heating of the earlier scenario. | 0:25:31 | 0:25:33 | |
OK, so we keep the volatiles. | 0:25:33 | 0:25:35 | |
Wonderful. | 0:25:35 | 0:25:36 | |
So what we see in this animation here is kind of a proto-Mercury | 0:25:36 | 0:25:40 | |
and a proto-Venus on crossing orbits that will eventually | 0:25:40 | 0:25:42 | |
result in a giant impact. | 0:25:42 | 0:25:44 | |
-So the impact was actually between Venus and Mercury. -Right. | 0:25:44 | 0:25:47 | |
So what actually happens during impact? | 0:25:47 | 0:25:50 | |
The way we study this is through computer simulations. | 0:25:50 | 0:25:52 | |
You can model a planet in this | 0:25:52 | 0:25:54 | |
simulation from Arizona State by Erik Asphaug and Andreas Reufer. | 0:25:54 | 0:25:58 | |
An iron core shown in blue, rocky mantles are shown in red or orange. | 0:25:58 | 0:26:02 | |
This type of collision is called a hit and run collision, | 0:26:02 | 0:26:05 | |
where the two objects slam into each other, | 0:26:05 | 0:26:08 | |
they sheer off a portion of their mantles | 0:26:08 | 0:26:10 | |
and they leave the scene of the crime. | 0:26:10 | 0:26:12 | |
The impact happens at a modest velocity, | 0:26:12 | 0:26:15 | |
so this is quite a bit more gentle than the smaller, | 0:26:15 | 0:26:18 | |
high velocity impact collisions. | 0:26:18 | 0:26:21 | |
OK. So this could explain the core, the mantle, | 0:26:21 | 0:26:24 | |
but the volatiles as well. | 0:26:24 | 0:26:26 | |
-That's right. -So, in this scenario, what happens to Venus? | 0:26:26 | 0:26:28 | |
Part of the proto-Mercury's mantle may have been deposited onto Venus | 0:26:28 | 0:26:33 | |
and that may help to explain why Venus has a little more | 0:26:33 | 0:26:36 | |
mantle material relative to the size of its core than the Earth. | 0:26:36 | 0:26:40 | |
So this theory is looking pretty good now | 0:26:40 | 0:26:42 | |
because we've got this collision, | 0:26:42 | 0:26:43 | |
we've got Mercury left with a large iron core and a thin mantle, | 0:26:43 | 0:26:46 | |
we've got Venus with an extra mantle, | 0:26:46 | 0:26:48 | |
which is what we actually see in reality, | 0:26:48 | 0:26:50 | |
so it does seem to stand up. So what happens next? | 0:26:50 | 0:26:52 | |
It's not the end of the story | 0:26:52 | 0:26:54 | |
because its orbit continues to evolve | 0:26:54 | 0:26:56 | |
and, over the next five billion years, | 0:26:56 | 0:26:58 | |
there's about a 1% chance that its orbit can become so eccentric | 0:26:58 | 0:27:02 | |
that it again crosses the orbit of Venus. | 0:27:02 | 0:27:05 | |
It can suffer giant impacts with Venus or Earth, | 0:27:05 | 0:27:09 | |
or it may collide with the sun. | 0:27:09 | 0:27:11 | |
Gosh. 1% probability is quite high, really, | 0:27:11 | 0:27:14 | |
-that our solar system could change radically. -That's right. | 0:27:14 | 0:27:16 | |
So it's not as static as I take for granted. | 0:27:16 | 0:27:19 | |
-No, the solar system is an amazingly dynamic place. -Thanks very much. | 0:27:19 | 0:27:22 | |
Thank you. | 0:27:22 | 0:27:23 | |
We still don't understand everything about Mercury | 0:27:27 | 0:27:31 | |
or how it became the planet it is today. | 0:27:31 | 0:27:33 | |
It remains the problem child of the solar system. | 0:27:36 | 0:27:38 | |
But by studying Mercury's peculiarities | 0:27:40 | 0:27:42 | |
and troubled beginnings, we are producing new insights into | 0:27:42 | 0:27:46 | |
the processes that formed and shaped the whole of the inner solar system. | 0:27:46 | 0:27:50 | |
When we started working on this, I thought Mercury was the least | 0:27:57 | 0:28:00 | |
interesting of planets, | 0:28:00 | 0:28:02 | |
but the more you find out about it the more fascinating it is. | 0:28:02 | 0:28:04 | |
I was particularly interested in the fact that the formation | 0:28:04 | 0:28:07 | |
of Mercury tells us more about the dynamics of the early solar system. | 0:28:07 | 0:28:10 | |
Well, that's all we've got time for this month. | 0:28:10 | 0:28:12 | |
But if you're going to watch the transit tomorrow, good luck. | 0:28:12 | 0:28:15 | |
If you're watching us on repeat, I hope it was clear. | 0:28:15 | 0:28:17 | |
That's it for this month, | 0:28:17 | 0:28:19 | |
but do check out the website, where we've got Pete's star guide. | 0:28:19 | 0:28:22 | |
And in the meantime, get outside and get looking up... | 0:28:22 | 0:28:25 | |
-but never directly at the sun. -Goodnight. | 0:28:25 | 0:28:28 |