Okay. Good afternoon, everybody. Welcome to the Physics Colloquium.
So I'm delighted today to be able to introduce a friend of mine from, uh,
from Garching in Munich, where the European Southern Observatory is Bob Fosbury.
He's been there for the vast majority of his career where he spent most of the time working with Hubble Space Telescope.
But, um, I have interest particularly on active galactic nuclei in star forming galaxies so far universe material.
But he has recently extended his wide range of interests to in particular the interpretation of colour images.
And so he has a, a very interesting website that you might want to look at for some interesting spectroscopy of various objects.
And today he's going to tell us about colours from Earth.
Oh, thank you, Roger. Yes.
Roger and I have known one another for a very long time, but I won't go into that.
It's very nice of you to invite me here. I'm very, very happy to be here.
As Roger implied, I spent a long time working on what I called Edge of the Universe stuff.
And when I retired from the European Space Agency and the Hubble job a few years ago,
I asked myself the question Should I continue with some very interesting collaborations
I had about the point or whether I should do something quite different?
And I decided to do something quite different. So in a sense, this is a bit of an experiment.
It's a subject towards a range of subjects that I'll talk about this afternoon
in which I'm I'm very interested in a very general sense about exoplanets,
the history of the Earth and so on, but with a very specific purpose of looking at the Earth as an example of an extra.
So looking at the Earth in ways that are very similar to the ways in which we would try and look at excellence in the future.
So it's the future is possibly the next generation of very large telescopes, both on the ground and in space.
And I think very many people are busy on thinking about this kind of problem.
But I think there are possibly ways in which we can hopefully make the ground based telescopes much more competitive with the space telescopes.
And if I'm going to tell you anything that's perhaps of interest for any experts in this area this afternoon,
it'll be an idea about using large ground based telescopes for looking at atmospheric properties of
terrestrial planets and avoiding some of the problems that we normally have from the from the ground.
So really on moving from distant universe to looking back at the Earth as an exoplanet.
And the obvious place to start, of course, is with Carl Sagan's Pale Blue Dot,
which I think was a masterstroke of of PR back in 1990 to point the Voyager cameras back towards the solar system and take a portrait of the planets.
It was actually quite difficult because from a distance of 6.1 billion kilometres, the planets are pretty close to the sun.
Certainly the terrestrial planets are very close to the sun.
And so the famous streaks in this image are sunbeams within the voyager camera, which is very hard to get rid of.
Nonetheless, one concealer, one can see a pale blue dot there.
And one of the things that's struck me many, many years ago is pale blue.
Why? Why is the earth pale blue? I find it very difficult to understand exactly why that was true.
And I hope I can say something this afternoon about that. So that was a masterstroke of imaging.
And then I'll just go through a series of these images of Earth from well away from the the Earth itself.
I'm sure you're all very familiar with these, but I think this is a lovely one from behind Saturn, using Saturn as a as a solar occult.
And to look at the pale blue dot, which if you enlarge a bit, you know, you can convince yourself you can see the moon as well.
This was done with the Cassini wide angle camera quite a long time ago in 2006.
And you may remember in 2013, I think it was July,
Nassr decided to take another image with the Cassini spacecraft using the the high resolution camera,
and they tried to get everyone in the earth to wave at the time that they took the picture.
What they also did actually, was to as well as to image the earth with from Cassini with Saturn in the foreground.
They also had a spacecraft, the Messenger spacecraft to Mercury.
And they looked back from the inside of the solar system to look at the earth.
At more or less the same time. So there are two images of the earth. So I took the Cassini data and lodged it to individual pixels.
And indeed, you can see the clear difference in colour between the earth,
which has a lot of blue pixels around it, and the moon, which is rather brown by comparison.
A nice picture I found from the the Mars Global Surveyor spacecraft from about 140 million kilometres of the earth in the moon.
And you can begin to resolve the surface structures on the earth from that distance and of course, the very famous Earthrise picture.
So Nassr have taken the trouble to build a couple of images of the earth called the blue marble.
I don't know if any of you have found this on the Web. I love looking at them.
They are composites of many individual images,
but they processed in such a way that they're supposed to represent a fairly true colour image of the of the earth.
So you can see which things contribute, which colours.
Well, I did. I took that image and I just separated the red and the blue channels in the in the image.
And this is the blue channel and this is the Red Channel.
And you can see something interesting about the ocean.
If you look at the the Gulf of Mexico here and Florida, you can see that Florida is brighter than the the Gulf of Mexico Ocean in the Blue Channel.
But in the harder to see. But you can see it can here it looks it looks it looks darker than the Gulf of Mexico in in in in the Blue Channel.
And that brought me to a little bit of history. I'm I'm a great admirer of C.V. Raman as a as a physicist.
And, in fact, I knew Roman Sun Radhakrishnan, the radio astronomer, quite well.
And I've read a lot of the stories about Raman doing experiments of the kind that I like doing with his pocket spectroscope and so on.
And there's a very nice paper in the Proceedings of the Royal Society, written in 1921, published in 1922.
Roman visited. He was already feted as a as as a famous scientist in India.
And he visited Britain for about a year, but less than a year.
I think where he went around to various places became a fellow of the Royal Society and so on and on his journey back to India by boat.
He became fascinated by the colour of the sea, the colour of the ocean.
And he spent the whole time, I mean,
I can imagine Roman running around with his nickel prism and his spectroscope peering at the period the ocean under all different conditions.
And he wrote a very nice paper entitled On the Molecular Scattering of Light in Water and the Colour of the Sea,
in which he actually estimated the colour of the ocean and tried to understand what was producing the colour.
And he did this in terms his unit of brightness was actually the air equivalent
number of kilometres you needed to really scatter that amount of light.
And so he had a rather odd units and his spectrum of the sea looked like this using scattering data as it was known at the time.
But I've corrected that for the radio, scattering from the sky and produced the green curve here,
using his data which represents the colour of the ocean as well as he had estimated it at the time.
And you can see on this blue image that the image is very bright around the edge, and that's because of the increased optical depth of the atmosphere.
So a little really, really scattering, going on around the edge, much less going on in the middle.
But the combination of the of the scattering and the colour of the sea gives you the rather peculiar colour of the of the earth,
which is a very unsaturated greenish blue.
This is due to vibrational band absorption, the vibrational overtones, absorbing the red light very effectively.
But here you have what I call the green sky phenomenon.
So you have a really scattering source function, lambda the minus four, but you also have right really scattering same function.
You have the absorption as well. So you combine those two together.
It's the same effect that gives you the the actual green twilight sky somewhere between the orange and the blue.
You see, as you go upwards from the horizon, you go through red, orange, apple green and then eventually blue.
The apple green is exactly this effect of the combination between the the radio
scattering source function and the each of the minus one over land as the fourth.
Also, each of the minus lambda is the fourth from the scattering anyway.
So I think that gives us an understanding of the two contributors.
So rather balanced contributors to the pale blue dot the rest.
The pale bit coming from all the clouds. So this brings me to address the ways in which we will look at terrestrial exoplanets.
It's a rather simple argument. I mean, we either look at reflected light coming from the parent star bouncing off the planets,
and that's extremely difficult for very obvious reasons.
The stars are extremely bright, 1 to 10 billion times for solar type stars and earth like planets.
Somewhat better, but still daunting in the thermal infrared. So that's going to be a very difficult problem.
And, you know,
a lot of thought has been given to the design of the European extremely large telescope and its instruments to address that particular problem.
But it is going to be extraordinarily difficult.
The other way, of course, is transmission through the atmosphere as the planet transits transits in front of the parent star.
We were talking just before Coffey about the transit of Venus and you have this
image of a rather large sun and a rather tiny Venus transient transiting across it.
So this is a problem as well because during the transit you're actually intercepting a rather small amount of light.
And if you want to learn about the atmosphere, you have to look at their light that has actually transmitted this very,
very thin skin of atmosphere around the earth.
So that's that's a rather daunting problem as well. But, you know, we're getting big telescopes.
We're making very stable, precise instruments.
So sometime it'll happen. What do we expect to see from something like a terrestrial planet, given that it could be at any stage of its evolution?
Well, we'll see reflection and transmission of the atmosphere, the effect of clouds and so on.
We'll see oceans by reflection, although oceans are actually very dark, there's very little reflection from the oceans.
There are lovely blue colour, but they don't reflect very much blue light. And the oceans of course have a profound effect on the atmosphere.
They provide water for clouds and so on. We will see continents by reflection.
And also, I'm sure we have the possibility of seeing the effects of volcanic activity on up on potential earth like planets life, of course,
very lengthy discussions about biomarkers and what they are and how many of them you need to convince yourself that there's life on a planet.
But certainly the kind of biomolecules that we can see, the 203 methane, nitrogen dioxide, etc., some of those we I think will be able to see.
And O2 may be difficult, but I think ozone will be probably the easiest to see.
Still very difficult, but the easiest to see.
They'll be bioproducts, for instance, you know, the oxygen oxidisation, state of rocks and so on, but also chlorophyll.
Chlorophyll is a very interesting biomarker because it has such a strong spectral signature in the red
at about 700 nanometres where the reflectivity of of plant material chains changes very abruptly.
So that is an interesting biomarker. And of course the extension of that is to look for non-equilibrium signatures in general, whatever they might be.
Civilisation, I, I put that down, but I think, you know, 20, 30 years ago people thought, oh, well,
the obvious way to look for life on planets throughout the galaxy is to look for civilisation or look for broadcast.
But, you know, you look at the earth today. I mean, how much energy are we putting into broadcasting, into outer space?
Not very much. And probably in a few years time, nothing at all. So I'm not sure that civilisation is going to be done in this kind of way.
So I want to talk in the middle part of my talk.
I want to talk about the looking at the earth as an exoplanet in those two ways, first of all, by reflection, and then secondly, by transmission.
I think the more interesting part for me anyway, is the transmission, but so I'll allow a little bit more time for that.
But of course, experiments like this have been done and the most valuable asset we have in performing these experiments is actually the moon,
because it acts as a very good reflector nearby and we can do a lot of experiments by just bouncing light back off the moon.
And that's been very valuable for us. You can also do it by flying spacecraft a long way away from the earth.
And looking back at the earth, that has been done on quite a number of occasions.
But they tend to be very short measurements.
You can't do terribly much with them.
So if you want to look at the integrated spectrum of the illuminated face of the earth, then Leonardo's your man.
I mean, Leonardo da Vinci figured out how to do this by looking at earth shine.
The light reflected off the dark part of the new moon.
There's a beautiful example, actually, in Munich where I live.
There was a wonderful example at the beginning of this currently nation where we saw a wonderful moon and a earthshine.
It's also been done with other planets. So I found this Cassini image which showed Jupiter as being illuminated by Saturn by reflection.
So this can be done. And this is a page from one of Leonardo's books which shows that he did indeed understand
the geometry of the of the process with the sun and the and the earth and the moon.
And this is an image, actually, I hate to admit it, but I think it's a synthetic image by Martin Kornmesser.
But it's very realistic of the relative brightness of the, uh, the earthshine and the bright moon.
And of course,
the advantage of this kind of experiment is that you have the bright moon to calibrate the earth shine data so you can take the ratio between the two,
get rid of all the solar nonsense, and just look at the effect of the Earth's atmosphere and reflectivity of the planet.
This is an example of doing it from outer space.
It's the epoxy spacecraft looking at the earth with three colour filters in the visible, and you can see the very various continents going past.
You can see Saudi Arabia and you see the moon going past.
See how dark the moon is. It's quite remarkable how dark the moon is.
So that gives you a very direct visual impression of what the integrated earth looks like when you see it from a long way away.
I'll repeat that movie in a little bit, replacing the red filter by a near-infrared filter.
So you go along beyond the chlorophyll change and you can see the effect on the visibility of vegetation and forests.
So this is a visible spectrum of Earthshine taken with the Palomar five metre, in fact, by some very good friends and colleagues of mine.
It was done over two or 3 hours and it's an average of observations.
The black line is an average of observations taken at that time. I mean, you can look at this and think, yeah, okay.
It's not actually very exciting and it doesn't tell you very much. And the signatures are not tremendously strong.
And I think that's that's the problem with looking at reflected light.
You're looking down through the atmosphere. You're looking at light getting reflected back from the surface.
You're also looking at a lot of light reflected from the upper parts of clouds.
And you are and you look at the total path length through the atmosphere.
And it's not very much I mean, it's a bit more than an earth mass on average, but an air mass on average, but not very much more.
So all you see in the visible and a near infrared is a rise there.
Doing due to really scattering is somewhat affected by the ozone.
SHAPLEY Balance here you see rather weak oxygen, molecular oxygen absorptions and really very weak water.
So, yeah, it's it's an interesting experiment to do, but it's hard to learn very much about the Earth from it.
Well, I did do I had a room in when I was in Durham a year and a half ago.
I had a room which overlooks the Botanical Gardens,
and I used to get up and put my spectrometer out of the window and I took a spectrum of the botanical gardens and normalised that with the nearby sky.
So this is a relative colour of the botanical gardens in Durham.
Durham is well known to be fairly wet and you see this huge feature here.
This is water absorption in the far right and this is the reflection of, of chlorophyll.
Uh, so it has a very strong signature. And you may think it's surprising that while perhaps you do see this signature extremely weakly in the,
the Palomar data of the Earthshine, it is extremely weak.
And I think there's an interesting reason for that. It's because first tend to attract clouds.
So you have a lot of nice little forests on the planet at any one time, but there's a high probability that those forests will be covered by cloud,
and that's indeed a cloud that taken at about the same time from satellite data.
And you can see the Amazon, uh, you can see the African equatorial belt, you can see a lot of the way that the northern regions are covered by cloud.
So it's very little grassland or forest peeking through that.
So that's a that's a tricky part of trying to observe chlorophyll during, uh, during daytime.
This is going back to the, the epoxy image.
This is the same set of data except for the exchange of filters.
And I think you can see very clearly even Australia has a lot of vegetation at
this particular time and you can see the northern uh the northern forests,
you see the forests in Africa and of course the uh South America and North America shine up quite brightly.
So the chlorophyll signal is, it is extremely strong.
In fact, if you can see it, it's an extremely strong biomarker. So let's move on to the transmission efforts.
And here we can really exploit the utility of of lunar eclipses.
And we've done this. In fact, I so I'm an amateur in this.
We only started this a couple of years ago, but we had the advantage of getting a young Chinese student on an exchange program with,
uh, with Beijing, who spent two years with us and engaging.
And I must say he was absolutely brilliant.
And he did a number of lunar eclipse experiments, which I found extremely interesting.
And I learned a tremendous amount from this.
I mean, I including one thing we learned which may have a very profound effect on the use of large ground based telescopes.
So let me go through this and I'll, you know, some of it will be, uh, some will be interesting in a technical sense what you have to look out for.
But the last bit, I think, is, is more interesting in a in a philosophical and a utilitarian sense.
So I'll just take you through this. I hope you'll learn something from this.
I mean, there may be many experts in the audience, but I hope you'll learn something.
I have to show one. So movie here of what transits look like gives us all a bit of a break, but, uh, actually, it does show two things.
It shows, one, planets completely eclipsing the sun, which is what we're doing with lunar eclipses.
And then it shows a more conventional view of a here we get the complete eclipse happening.
And then a bit later we've got a small planet, planet going across the disk of the star,
which is more typical of the, of the kind of transit, uh, we want to observe with spectrometers.
It's still quite a big planet. I mean, it's much bigger than Venus in the Sun, but it gives you an idea of what they look like.
So this is taken from the International Space Station.
It's more or less what the moon sees in the umbra of a lunar eclipse.
So when you have the earth cutting across the sun, it completely eclipses the sun,
except that the Earth's atmosphere refracts a certain image into the Umbra.
So if you're sitting in the umbra of the if you're sitting on the moon within the umbra,
you actually see bright sunlight coming through the shell of the atmosphere at some point.
And you can calculate how big that point is and so on, and what the effect of altitude is.
But you also get the integral of the blue sky light from around the whole earth shining onto the umbra.
So both of those sources affect the spectrum that you get.
That's picture by so subtly. He is the guy who does in Baltimore who does very, very many of the Hubble images.
He's the image maker. We also do it in Cushing, but salt is a very good friend of ours.
This for this particular eclipse, this is the path of the moon in within the umbral within the umbral shadow.
And what we do, actually, as we look at the Tycho crater, the Tycho is quite highly reflective.
So it gives it just gives us more signal to noise than we would get by looking at a random part of the moon.
So we just point the telescope at Tycho and let the eclipse proceed and then work out the geometry of the of the light path.
And it turns out that for this particular eclipse,
the turning point of the light from the sun to the moon crossed the Antarctic coast south of Australia.
Interesting. I mean, what is the atmosphere like in and over the Antarctic coast?
And Mount Travis is marked in there for a reason which will become apparent
in a moment and it's raised an issue that we haven't completely solved yet.
But this is a not to scale representation of the particular geometry.
So you've got the sunlight coming in refracted in the Earth's atmosphere, which is very much exaggerated in depth here.
Bouncing off the moon and going back to the telescope actually was not in Beijing itself, but not very far away from Beijing.
It was actually a very nice house.
The student from from China came to us with high resolution spectral data of this eclipse, and we were extremely impressed.
It was far better than anything we'd seen before, and he was able to do very interesting things with it.
You look at both the bright moon and the eclipsed moon through more or less the same vertical extinction.
And so when you take the ratios that cancels out, in fact,
you can often design the observation time so that the masses of the two observations are more or less identical.
So in principle, even though Beijing is a great source of pollution, we shouldn't have any pollution showing due to Beijing.
We should perhaps have pollution showing from Antarctica.
This, again, is an exaggerated scale calculation of the image that is seen by the part of the moon we were looking at.
So you get a distorted image of the sun, which varies slightly during the process of the eclipse,
and it has an effect of altitude in the in the atmosphere around about 15.
Actually, the mean the mean altitude is around 12 and a half kilometres.
So it's it's almost like looking through a pinhole. So you get a clean path through the atmosphere, through a very small pinhole.
And you can model on that particular basis.
And this is what you see. We're very familiar with this picture now, but this is what you see.
This is the ratio of the eclipsed the umbral eclipsed moon and the bright moon.
So it's the ratio of the light that's passed through the Earth's atmosphere to the sunlight that's illuminating the same part of the moon.
And, well, just a I mean, a crude description.
You see the decreasing transmission as you go into the blue.
And a lot of this is due to both really scattering, scattering and aerosol scattering.
Interestingly, you see this really very broad feature here with these lumps and bumps.
And this is due to ozone, the sharply bands of ozone,
which are actually the strongest two lowering feature that we will see in the visible for an earth like planet,
much stronger than the oxygen and even the water. The water is somewhat problematic.
And it's interesting here you see some noise here where you should see water, but you don't see any water.
This is a surprise because we had looked at other lunar eclipses and seen a fair amount of water,
not as much as we see from the ground, but quite a lot.
But in this case, we see very little water.
So I showed this to the Alma guys at ESO and they said, yes, we know, don't tell us there's no water in Antarctica.
So the water is presumably frozen out at that altitude.
And we don't we really don't see any water at all or very little of it.
The other features, I mean, the oxygen, molecular oxygen of yes, we also see these used to be called dinosaurs, the oxygen to oxygen to combination.
So I think the modern terminology, they're called collision induced absorptions.
They turn out to be quite important. They turn out to be quite strong under some circumstances and they're very useful because they depend.
The strength of those bands depends quite drastically on the pressure.
And so you can use them as indicators of whether you're actually seeing through the low atmosphere or not.
Because many times when you look at a lunar eclipse, the low atmosphere will be obscured by aerosols and clouds and so on.
And if you don't see any water, you may think, well, the water is just obscured by cloud.
We don't see it at all. But if you see that, if you see these dimmer features, then you know, you're seeing a low into the atmosphere.
So that was an interesting thing we learnt on the way.
I did mention that you got a little bit of skylights as well as the pinhole image of the sun and in
fact here the model without the skylight is this little faint line underneath here in the blue.
So this, the skylight from around the earth contributes a small amount in the blue, but not very much.
And these are the contributors. I mean, this is the this is the menu of molecules that you have to use to produce this kind of spectrum.
Many of them are obvious, like the O2, the aerosol scattering, the molecular scattering, the Rayleigh scattering.
The interesting things are, well, the very weak H2O.
These are the dimer features you can see, which are reasonably strong,
hard to see for an exo earth, but you know, they're interesting to know that they're there.
The thing that surprised us was to have very strong nitrogen dioxide.
We didn't expect this because nitrogen dioxide is normally considered to be a pollution product.
You see lots of it over Los Angeles and I'm sure over Beijing as well.
But we didn't really expect to see it over Antarctica. So I actually got in touch with Clive Oppenheimer,
who's volcanologist at Cambridge and discussed this with him because he's actually the guy who runs the non service volcanic observatory.
In fact, he spends a month of the year December camped at 3000 metres on the slopes of Mount Everest.
I get the most amazing emails from him. At least I have the most amazing email origins.
I got one from me the other day from Pyongyang.
He was invited by the North Vietnamese to to look at a volcano on the Chinese border because they were very worried about it.
Anyway, it turns out.
Of course, I didn't know this, but it turns out that Mount Travis is the strongest point source of nitrogen dioxide in Antarctica.
And it's not a volcanic gas. It actually gets fixed above the hot lava like there's a permanent lava lake in Mount Everest.
And that the temperature, the high temperature above the lava lake fixes the nitrogen and produces a significant amount of nitrogen dioxide.
So we thought for a while that nitrogen dioxide might have been coming from Mount Everest.
We're a little sceptical about that now.
It's, I think, more likely that the nitrogen dioxide is a is a decay product of nitrous oxide, which is produced by bacteria.
And in the spring snow melt,
the bacteria produce quite a lot of nitrous oxide and dump it into the atmosphere where it can get oxidised into nitrogen dioxide.
But that's not really been resolved yet. I thought this plot end because it just shows you what you can do by looking at the moon with a spectrometer.
This is the computed spectrum of nitrogen dioxide absorption, given the the basic cross-section data, and this is the residual of our observations.
So I was surprised when I saw how how well we could do with such a complex, weak spectrum.
Coming back to water for a moment. This is the top spectrum is a lunar is an earlier lunar eclipse observation.
In fact, I think it was the first one that was published. It was published by.
All right, Pele from the Canaries in Nature in 90 something 2009.
Beg your pardon? And we look we looked at this, and you can see here, the water's quite weak, but there is some water.
This is an eclipse above the Arctic rather than the Antarctic.
But it actually happened when there was a volcanic eruption in in in the Arctic region.
And so there was probably quite a lot of aerosol extinction.
And this is what the spectrum of the sunset looks like.
So your ground level, this is actually a sea level from Norfolk.
And you see the water bands are extremely strong.
You can also see these these oh four features, the diamond features are also quite strong in these data.
This, incidentally, is taken with our one of our little portable spectroscope.
So we have a number of portable electronic spectrometers, which we use for atmospheric and plant studies and so on,
for generating reference spectra for a lot of these things.
Very nice devices. I just wanted to put a little rest into the into the talk by talking a bit more about ozone because I realise,
I realise that very few people realise what a profound effect ozone has on what we see around us, especially during Twilight.
I came across this by accident quite a number of years ago when I was trying to understand the the colour of the zenith sky at twilight.
When, when you, when you have a clear sky and the sun is setting and you look directly upwards, you see a remarkably deep, steely blue colour.
And I try I measured this with a little three colour photographer,
a light metre with three filters and tried to model it using Rayleigh scattering
and I could get nowhere near modelling the colour blue from the zenith sky.
And I eventually looking in Allen's astrophysical quantities,
the little used to be in my day a green book where they have the coefficients of really scattering and aerosol scattering and so on.
They also have a little table of the SHAPLEY band absorption coefficients in this spectral range.
So I put that into my little model. And lo and behold, it's ozone that makes a blue colour.
And I'll show you in a moment. If you take the ozone away, it's totally different.
So this is a this is an observation, again, taken with one of our little portable spectrometers of about an hour before.
Uh, yes. Well, it's a it's an hour and a quarter before sunset.
And these are spectra normalised to the spectrum, taken an hour before sunset.
So we just take the subsequent spectra and divide them by the, uh, the first spectrum.
And you can see the development of this structure in the orange as the twilight proceeds and the,
the blue lines are the ones before sunset and the red lines are the ones after sunset.
And you can see this is an enormously strong absorption feature.
This is a zero real zero level here. Um, and this has a very, very profound effect on the colour.
So when you within about half an hour of, of sunset, the blue colour of the sky has almost nothing to do with right here with Rayleigh scattering.
It's all due to ozone and it makes a tremendous effect.
And you don't notice this so much because your eyes adapt to the blue.
And it's only when you take a picture with a digital camera set.
So sunlight, white balance that you realise you must have done something wrong because the pictures are so incredibly blue,
but the colour temperature will show you and the colour temperature gets extremely high in twilight.
So this not only tells you that you're dealing with something which has a strong spectroscopic signature,
which is of interest for exoplanets, but it also tells you something about the earth we live on.
And, uh, not many people realise that this is such an important effect.
What I've done, I, I took what I think is the first observation of the SHAPLEY band absorption,
which is reported in a little note in Astrophysical Journal in 1934 by Wolff, Moore and Melvin.
And it's actually Absorbance they're measuring. So I turned it upside down.
For the sake of argument. And you can see that this is done with a pocket spectroscope in a box camera.
Uh, but it's a pretty good, pretty good.
And this is what I was saying about the the colour temperature, the brightness.
Of course, as you approach sunset goes down dramatically.
This is the colour temperature measured with a a colour temperature metre.
So it's just measuring in the visible spectrum. It doesn't go in far into the red.
But you say it goes up to almost 15,000 kelvins.
And this has profound effects on on animals, as I'll come to in a moment.
It's it's quite remarkable. The eye level.
This side, the blue hour this is known as the blue are by artists and poets.
I don't know if any of you are that way inclined, but this is a phrase which is used quite a lot in artistic and poetic literature.
The blue are they realise that this is a time when the the landscape around them was much slower than they might have expected.
This is a neat little set of data we had.
We were we were observing the moon from the CFT for another reason.
But this gives us a picture of the spectrum as a function of the height of the tangent path through the Earth's atmosphere.
So by looking at different times, at the reflected light from the moon, we could change the height.
So this goes from about five kilometres altitude, up to about 100 kilometres, and you can see a low altitudes.
You lose more or less everything in the visible.
But there's a there's a region later on, just as you're coming out of Eclipse where you can again, you get this very blue spectrum due to the ozone.
And I found there's a German lady photographer who is not actually a scientist,
but she's very keen on photographing things of scientific interest and she's done a whole
series of photographs of the Eclipse moon and this is one of them actually she gave to me.
The intensity range here has been very much compressed, of course,
but this shows you that as you get close to the edge of the umbral eclipse, you get a blue moon.
And this is a blue moon for a reason, which is quite different from the normal accepted blue moon ni scattering explanation.
You can't see this by eye if you look carefully at the right time, but it's I think it's quite remarkable.
Another thing, which is I'm sure the observers amongst you have seen is the the Earth's Shadow or the dark segment.
This is a picture taken of Paranal as the sun is setting.
And you get this very dark band here, which has an interesting colour.
It's, it's, it's a very steely grey blue.
And in fact, this is, I think, the result of a double scattering. This is a set of data I got from a guy called Ray Lee.
He's a very well known atmospheric physicist, works at the US Naval Academy and he had actually observed the spectrum of the dark segment,
the to the orange and the green curves here in the visible.
I think he had a trouble with background subtraction over here but and then this orange line
is my model of the solar spectrum with two scatter rings and a and a deep ozone layer.
So it's the earth's shadow is a shadow of the ozone layer.
It's not actually a shadow of the solid earth. It's a shadow of the ozone layer.
So this is a this is a sunset sky model.
I took the spectral fist of the data and modelled the colour of the blue sky somewhere up in the air above you.
Not necessarily the zenith, but to give you this I don't think the colour calibration is very good.
But this is the the colour of the sunset sky, including ozone.
And if you take the ozone away it becomes a sort of grey, strong yellow.
So it has a very, very profound effect on the sky.
So you learned something. Okay. And this picture taken by my son actually in Scotland, which I grabbed immediately.
He's very careful, careful about his colour balance. This is a picture taken at dawn or just before slightly before dawn in Dumfries and Galloway.
You can see the mountains on the snow is the snow on the mountains is fairly nice and white.
But the rest of the picture is incredibly blue. And this is the ozone blue in the way it actually looks like.
But if you were standing there looking at it, you would really want to see that because your eye would flat field it out or colour field it out.
Now, this is the kind of subject that I now find very interesting, but this has some remarkable effects on the adaptation of animals behaviour.
And there are many examples of this too.
I'll point out to you one I heard about only fairly recently.
Another one I worked on myself some some years ago. The people have looked at the eyes of reindeer in the in the Arctic regions and they
find that during the winter the eyes adapt to see blue light very effectively.
And they do this because the the top into the lining of the eye behind the retina is reflective so that they
can maximise the the light coming in because there's a double pass processed through the through the retina.
And in the summer, the temperature reflects orange light.
And if you shine a torch into a reindeer eye in the summer, you see an orange reflection.
But if you do the same thing in the winter, you see this very dark blue ultraviolet reflection.
And so they adapt their eye to allow them to live in this incredibly blue environment.
And that adaptation allows them to see food much more easily.
They can see the lichen and they can see things on the ground.
They can see that the predators more easily.
And there's a thesis from the University of Tromso, which I found which shows two pictures of the top part of the orange,
a turquoise one, a summer eye, and the the blue one have a winter eye.
There's actually a lovely YouTube video and there's a group at UCL who have studied this, and they made a video about a year ago about reindeer eyes.
If you look up reindeer eyes or something on YouTube, you'll find it very easily and it's a very nice description of what they did,
but I don't think they realised it was something they were dealing with.
I really must write to these guys. Mention Ozone anyway.
So that's the reason. The other thing I did get involved in was this little guy.
He's a madagascan lemur called The Eye.
I'm sure you've heard of the eye. He's the lemur that has the extended finger that it's used for picking out bugs from the leaf litter and so on.
They forage in the rain in the rainforest at twilight where they're shadowed, generally shadowed from direct sunlight and moonlight.
So they're just looking at skylight and they have retained their their blue, obscene blue visual pigment to be very blue sensitive.
And I by chance, via what I post on Flickr,
I got in touch with a a primatologist in the US who was actually working on this problem and he was puzzled.
He didn't know what the evolutionary driver was for this item and maintain this capability.
And we were thinking that it was that the eye feeds on the not on the seats of the travellers tree itself.
There's this big tree that grows in Madagascar called the traveller's tree, which is the same family as the banana.
But it's a huge thing with big fan shaped leaves and it has big flower clusters about this big,
hard things and it produces seed pods which when they open up into this little
tripod of but it contains this artificially blue arrow fluff around the seed.
So black seeds and I have this, this incredibly blue arrow, which I bought one of these seeds went somewhere,
but I was convinced that this was dyed artificially to make the seeds more attractive.
And it was only years later that I realised this was a real blue arrow.
It's an extremely unusual blue in in animals or plants, and the protein has not yet been identified.
As far as I know, people in University of Florida were looking at it. But the interesting thing was it was a by by-products of this investigation,
this tree was known to produce a biological molecule, the molecule that you.
I get with jaundice. I've forgotten the name, but it's something somebody remember.
It's the. No, no. It doesn't matter.
But this was apparently the first biological model molecule.
It's the first animal protein that had been found directly in a in a plant.
And there was a great deal of excitement about it. But anyway, I think that turned out to be a red herring.
I think the the real reason for the adaptation, again, is the ozone colour,
which gives you a very blue visual environment where they still can maintain some colour vision, even at very low light levels.
Now. What have I got? I want to do another discussion of the excellent transit measurements.
And this is the I mean, the normal way of doing transit measurements is you look at the you look at the photometry of the star being transited.
And by looking at the depth of the eclipse signal as a function of wavelength,
you can work out the, the, the absorption of the, uh, of the molecules in the planetary atmosphere.
So it's a floating metrically based experiment. You have to measure it very precisely.
This is why Hubble has been so successful in doing this.
Not in earth like planets, but in somewhat bigger parts, because it can do such stable photometry.
But it's extremely, extremely difficult to do this on the ground. And so we've been investigating two effects that you get during a transit,
which are a result of the fact that the transiting planet is masking regions of the stellar surface.
So the star is not just a uniformly amazing surface.
It has variations and it has variations which are symmetric, like, uh, um, centre limb variations in brightness and line depth and so on.
But it also has variations which are asymmetric, and that is the rotation of the, of the star.
And the rotation of the star is called the Russell Mclauchlan Effect.
And it's been used for very many years now in spectroscopic binaries and, and,
and even in exoplanets in recent years to figure out whether the planets are rotating in the same direction as they as the star.
So it's a well developed technique, but in fact it can be adapted to look at the effective radius of the planets because the part
of the star that you actually mask or eclipse by the planet varies with the effect of radius,
and that variation carries an intrinsic signal about the atmospheric properties.
Now this is actually suggested back in 2004 by Chanel and in Leiden, but nobody had ever actually done it.
Uh, so we decided to do it with a lunar eclipse and see if we could do it.
It's a very easy case because you eclipse the entire sun.
So it's a it's a strong signal in that sense. But to show you the way it works is that we have here the centres limb variation of the sun.
This is the Kurt Sutter Atlas data. We just divide the the solar limb spectrum by the central spectrum and of course the
lines don't completely go away because the lines are different deaths that the oh I mean,
all the solar people know this very well. The lines are different deaths.
So in the subtraction, you say this goes up to 20, 25%, especially in the blue.
You get the effect of the central and variations of the line profiles and line deaths.
Similarly, where, where you have a penumbral spectrum of the, uh, of the sun or penumbral spectrum of the,
of the Earth's shadow, you're cutting off part of the sun by the, by the Earth.
And so you're when you do your division, you're something different parts of the stellar surface.
So this is a a set of data during a lunar eclipse where we've done the penumbra divided by the bright moon, we see a very similar pattern.
Now, this is very important to understand in detail, because if you're looking for faint absorption lines due to the atmosphere of the planet,
you have to understand what this central and variation effect can actually do to destroy your signal.
And it's quite can be quite profound under some circumstances.
That's just a simple diagram of a penumbral eclipse at different stages.
And this is the peak solar atlas looking at a spectral line, one of the calcium triplet lines in the infrared.
You can see the profiles are quite different. When you divide one by the other, you get this characteristic emission absorption feature.
You can degrade the resolution a bit for the kind of observation you might make.
And this is the example of this line with the CFT data.
So you can see the kind of feature you get.
And when you're trying to look for a in Earth's atmosphere absorption line in the ratio spectrum, you have to understand this.
And in fact, it's quite daunting. I think this is the sodium one of the sodium d lines looking again at real data of the day.
The day line has a rather weak centre limb variation, but this is the absorption line that we know exists because of the laser guide star work.
We know the optical depth of the sodium in the Earth's atmosphere, and this is close to the maximum that we see in doing these experiments.
And you can see the relative size of that compared to the the the variation effects.
So that's and I don't think people realise that at that level that it's such a daunting problem.
So if we can measure planetary atmospheric transmission from purely radial velocity data of the study taken during transit using this Rossiter
in effect it since it doesn't require very high precision photometry it's a it's a real potential for ground based ELTs for earth like planets.
And I, I suspect that this will become a serious scientific case for the E-ELT.
We've already discussed it with the VLT people, and so we did this experiment using a lunar eclipse, realising that it was an easy experiment to do,
but interested nonetheless in seeing what lessons we had to learn and how strong the signal actually was.
So the amplitude of the signal, as I said, depends on the effect of radius, which is wavelength dependent because of the atmospheric transmission.
And it's a kind of radial velocity tomography of the surface of the stellar surface that we do in this way.
It will be dependent on surface activity, sunspots and so on.
We're well aware of that. But we use the Harps telescope and I'm sure you all know Harps.
It's the the great planet finder. It's a resolving power of over 100,000 beautiful data.
It's wonderful to look at the data, I must say, as a spectroscopy. So I just look at the data, gosh, these are the shallow orders you can see here.
That's a bit of a gap in the middle. There are 72 spectral orders going across that wavelength range.
That's what a spectrum of the bright moon looks like. If we just look at a bit round the action k lines.
The Beautiful, isn't it. I love that.
So okay what we do is we measure for each harps spectral order and I think those 69 useful orders that we can use,
we measure the radial velocity, okay? And we measure that at various points during the eclipse.
And this is the, the basically the raw data before eclipse, the bright moon coming along, radial velocity, the first penumbra going down.
And then there's the umbra. I won't talk about the Umbra.
You can ask about it if you want.
I think this is we're seeing a number of different effects here, but then we pick up the pin number again on the other side.
So we got a good sample of the bright moon spectrum.
Now, this is the computed radial velocity that we would expect after because of the the relative motions of the sun, the moon and the earth.
And this is taken from the standard JPL horizon EPHEMERIS There's an offset here,
which I think is due to the way the template is used in the in the HARPS pipeline.
It doesn't matter because we, we just flatten everything by dividing by this curve.
And so we get a zero based measurement of the velocity of the, of the reflected light from the moon.
And obviously what you're doing your your first of all obscuring part of the sun that's redshifted.
And then the earth moves across and then you're observing part of the sun, which is blue shifted.
So it gives you the signature of the of the stellar rotation.
And what we do here, we actually model the stellar rotation. We have a great of points, 100th of a solar radius grid of the.
And so we assign velocities, the differential rotation and the actual rotational rotation to those points and then model the rotation.
And this is just an extract of that signal before where you see the pre the first
penumbra with this turn up because of the asymmetries and the second penumbra.
So what we actually do, we take the maximum signal we have here of the of the velocity on one side and the other and
plot those and those are the red and the green lines and the artificially normalise to zero.
It doesn't matter. We don't, we don't worry about the normalisation.
But I think the first thing you spot here, there's a correlated noise between those two signals.
So we're not getting a uniform velocity as a function of wavelength and we can't use that directly.
And what we think is happening is that the sun suffers from what's called a convective blue shift.
The convective cells in the sun come up hot and go down cold.
And so they're more light emitted from the upwelling than the than the down welling material in the sun.
And it produces a blue shift that the surfaces understand this very well, but it depends on how deep the lines are and so on.
So each individual harps.
Order will contain a distribution of lines formed at the distribution of deaths, which will be different for each different order.
And so the actual velocity we measure for each order will be, will have this correlated noise associated with it.
And so, but the point is that the blue shift noise, the convective blue shift noise is symmetric around the centre of the sun,
whereas the signal we're looking for is anti symmetric around the sun.
So we just subtract the two eclipse measurements to get to the line underneath and then we just have to fiddle around a bit.
We have to press the button. The. We have to correct,
we have to take the continuum then darkening into effect because that has a a slight weighting effect on the on the velocity measurements.
And then we we correct for that and we get a signal.
So this is the variation of the radial velocity measured in metres per second against wavelength.
Okay.
And you've got this very characteristic signature and the red line here is a model of this derived from the components that I talked about in the,
in the, in the eclipse spectra. So it's the basically the really scattering and the ozone absorption.
So this bump here at 600 nanometres is the effect of the SHAPLEY ozone absorption.
And the rise up here is due to the radius scattering.
So we have simply from measurements of radial velocity done to a precision of well within the HARPS capabilities,
it's a precision of a few metres per second. We actually make a measurement of the.
Of the transmission of the atmosphere. So I think given that we're going to have extremely stable,
extremely high resolution spectrometers on the very large telescopes, which are the last to get very high signal to noise.
We're just looking at the light from the star. We're not looking at a planet at all, so we're just looking at the light from the star.
So actually, it's a very I think it's a very cool method of trying to get information about the atmosphere.
It's going to be hard. It's going to be in getting down to the centimetres per second.
It's going to mean coping with the the intrinsic noise of the stellar surface and so on.
But I think it's it's promising and we're trying to simulate some of these effects at the moment.
Now. I should finish now. Um, I just very quickly want to show you some quite different things that we've been doing
in trying to understand the earth and the effects it can have on spectrum and so on.
I will run through all the quickly, but I think it just to give you a flavour.
Um. We're interested in volcanoes. We're interested in volcanic gases.
We're interested in the effect that it may have on the spectra.
So we mounted an expedition to Mount Etna with the help of the Volcanic Observatory.
Again, through this, Clive Oppenheimer in Cambridge helped us organise this.
And we measured with them using our own spectrometers.
We measured Mount Etna and they were very fortunate in having a fairly significant eruption at the time.
So we were able to look at the lava as well as the volcanic plume and night time here of the of the eruption.
And that's infrared 1 to 2 and a half micron spectrum of the lava seen at two distances,
one in ten kilometres, and a rather crude estimate of the temperature of the lava.
1260 Kelvins pretty close to what I expect for a basaltic lava.
I think, um, the deviation of the, the red in there is due to the water.
The water's a lot of water absorption and this is right down in the ultraviolet.
We have a nice little ultraviolet spectrometer which actually was shown in that picture.
It's only about this size. It's the one all the volcanologists use, I think an ocean optics spectrometer which starts measuring it to 90 nanometres.
So we go from 290 to 510 nanometres and we can get the sulphur dioxide absorption.
So that's a the Wiggles. There are a data and a model of the sulphur dioxide.
We think we can measure column densities and so on. And I just want to finish with a rather nice experiment, which is, again, these are based.
If you go to the north of Paranal in Chile, the very northern part of the Atacama Desert, it's probably the driest place on the planet.
And there are big solar plains there which contain bacteria, which are cyanobacteria,
probably evolved very little over the last two and a half billion years.
And so we've been looking at the properties of this bacteria and testing the idea
of remote sensing it in some way long time before we do that on the next to Earth.
But we might be able to do it on Mars. So this is the region, the Big Salt Lake Uni.
I've actually been there a fantastic place, 220 kilometres across.
It's the it's the zero point for the chips model earth.
So flat is within a centimetre of being completely flat.
And then we go down here somewhere and it's within, it's not too close to the ocean.
There's a mountain range, a low mountain range here. So it never gets the the view coming in from the fog coming in from the sea.
So it's incredibly dry. I think the rainfall is about one millimetre every four years.
Uh, you can see and these are the nodules of salt, pure salts covered by a bit of dust.
And within the salt you get these bands of, uh, bacteria.
If you can pronounce the word, your better than me croak out to the opposites.
Um, it's a very well known bacterium. It's an extremophile which will survive almost anything in Antarctica and in the hot deserts.
And some people even think this came from Mars, but maybe, uh, a lot of interest in this particular bacterium.
But with our little spectrometers, we were looking at whether we could see either the transmission or the fluorescence of the chlorophyll.
And in fact, the fluorescence of the chlorophyll is quite significant.
And this red peak here is the effect of shining blue light on the on the rock and looking at the reflected light.
So it's potentially this is a technique that's used by Earth resource satellites to look at the health of plants on the ground,
and they measure the fluorescence by measuring the depths of Fraunhofer lines.
Because if you've got a fluorescent signal underlying the reflected light, you reduce the depths of the front of the lines and that can be used.
So I don't know whether it'll work.
Um, this is my finishing slide, which I won't talk to, but it's one of the fun things one can do with a little spectrometer.
It's the spectrum of a thundercloud, and it has all kinds of interesting features.
Anyway, that's enough. Thank you very much.