PDA

View Full Version : How much could spectroscopy tell us about an exoplanet's atmosphere?



Somes J
2010-Jul-07, 06:54 PM
Here's something I'm wondering about. Exactly how much could we learn about the atmosphere of a possible habitable exoplanet from spectroscopy instruments like the Darwin Mission or Terrestrial Planet Finder would carry?

I mean, would we be able to know in detail the composition of the atmosphere, or would we just be able to tell, for example, whether or not there's some oxygen in it?

According to the TPF site (http://planetquest.jpl.nasa.gov/TPF-I/signsOfLife.cfm):


The oxygen and ozone absorption features in the visible and thermal infrared respectively could indicate the presence of photosynthetic biological activity on Earth anytime during the past 50% of the age of the solar system. In the Earth's atmosphere, the 9.6 Ám ozone band is a poor quantitative indicator of the oxygen amount, but an excellent qualitative indicator for the existence of even traces of oxygen. The ozone 9.6 Ám band is a very nonlinear indicator of oxygen for two reasons. First, for the present atmosphere, low resolution spectra of this band show little change with the ozone abundance because it is strongly saturated. Second, the apparent depth of this band remains nearly constant as oxygen increases from 0.01 times the present atmosphere level of oxygen (PAL) to 1 PAL.
Does this represent about the limit of presently plausible technology or could we get more detailed information on things like the amount of oxygen in an exoplanet's atmosphere?

Nereid
2010-Jul-07, 07:40 PM
Here's something I'm wondering about. Exactly how much could we learn about the atmosphere of a possible habitable exoplanet from spectroscopy instruments like the Darwin Mission or Terrestrial Planet Finder would carry?

I mean, would we be able to know in detail the composition of the atmosphere, or would we just be able to tell, for example, whether or not there's some oxygen in it?

According to the TPF site (http://planetquest.jpl.nasa.gov/TPF-I/signsOfLife.cfm):

The oxygen and ozone absorption features in the visible and thermal infrared respectively could indicate the presence of photosynthetic biological activity on Earth anytime during the past 50% of the age of the solar system. In the Earth's atmosphere, the 9.6 Ám ozone band is a poor quantitative indicator of the oxygen amount, but an excellent qualitative indicator for the existence of even traces of oxygen. The ozone 9.6 Ám band is a very nonlinear indicator of oxygen for two reasons. First, for the present atmosphere, low resolution spectra of this band show little change with the ozone abundance because it is strongly saturated. Second, the apparent depth of this band remains nearly constant as oxygen increases from 0.01 times the present atmosphere level of oxygen (PAL) to 1 PAL.

Does this represent about the limit of presently plausible technology or could we get more detailed information on things like the amount of oxygen in an exoplanet's atmosphere?
I don't know if it's mentioned on that site, or other related ones, but what you can tell from a spectrum - on what atomic or molecular species are present, and at what relative abundance - depends heavily on your ability to estimate, reasonably accurately, quite a few things about that atmosphere. Perhaps the most important is its optical depth, at the wavelengths covered by your spectrum. So, for example, if all the oxygen were in a layer below that at which the atmosphere becomes opaque (for your spectrum), you'd never know - directly - there was oxygen in it.

Another thing, already pointed to: the choice of waveband, for your spectrum, limits your ability to tell what atomic or molecular species are present; if there are no lines, or bands, of the species you are interested in in your chosen waveband, you would have only indirect indicators, at best, that they were present (or not).

Somes J
2010-Jul-07, 09:47 PM
Doing some internet searching, the infra-red component of TLP and Darwin would look for ozone as a proxy for oxygen, and wouldn't be able to tell whether more than .1% present oxygen levels were present.

http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA484941
http://adsabs.harvard.edu/full/2003ASPC..294..621F

I'm going to see if I can find equivalent information on the visual light component of TPF.

Another thing I'm wondering: what's the smallest object TPF or Darwin could detect in another solar system? Could it tell whether a planet had a large moon like Earth does?

John Jaksich
2010-Jul-10, 01:00 AM
Dear Somes J,


I don't know if it's mentioned on that site, or other related ones, but what you can tell from a spectrum - on what atomic or molecular species are present, and at what relative abundance - depends heavily on your ability to estimate, reasonably accurately, quite a few things about that atmosphere. Perhaps the most important is its optical depth, at the wavelengths covered by your spectrum. So, for example, if all the oxygen were in a layer below that at which the atmosphere becomes opaque (for your spectrum), you'd never know - directly - there was oxygen in it.

Another thing, already pointed to: the choice of waveband, for your spectrum, limits your ability to tell what atomic or molecular species are present; if there are no lines, or bands, of the species you are interested in in your chosen waveband, you would have only indirect indicators, at best, that they were present (or not).


Currently, there are approximately 600 or so detected exo-planets (according to info- from a seminar which I viewed recently)---and there is much interest in how to detect and ultimately model the atmospheres to degree where it is readily simple to understand their dynamics.

My point is --- dynamically, what is known about these current exo-planets' atmospheres is modeled and based on our own Solar System atmospheric dynamics---and the case in point is Jupiter and Saturn atmospheres. (Since most of the exo-planets seem to be large Jupiter-sized --and they are much warmer.)

You might want to view the following seminar ---if you are interested---

It gives a good survey (I believe) of some current understandings of "Planetary atmospheric-dynamics."


http://www.youtube.com/watch?v=Xmh_k4ClJ7Y



The presenter is Mark Marley from NASA Ames ---and here is a separate link:


http://www.youtube.com/watch?v=Xmh_k4ClJ7Y&playnext_from=TL&videos=5f_Slca6Wxw&feature=sub

Dale Botha
2010-Jul-15, 10:31 AM
Hi

Does the spectra from individual atoms vary depending on whether they are single atoms or in molecules?

How much of the spectra exists out of the visible fequencies?

Nereid
2010-Jul-15, 08:29 PM
Hi

Does the spectra from individual atoms vary depending on whether they are single atoms or in molecules?
Yes; the spectra of molecules made up of two, or more, atoms of the same element are totally different from the spectra of the atoms alone.

This is because lines are emitted when an electron 'jumps' from one allowed energy level to another. The allowed energy levels of a molecule of diatomic oxygen, say, are quite different from those of ozone, or oxygen atoms. In fact, molecular spectra are far richer than atomic spectra, because molecules can also vibrate (the bonds stretch and contract, for example) and (often) rotate; this gives rise to bands, which are many, many lines spaced very closely together.

I tried to find an example - oxygen atom, diatomic oxygen molecule, ozone - but failed.



How much of the spectra exists out of the visible fequencies?
For most species, of atoms, ions, and molecules, most of the spectrum is not in the visual waveband. Molecules, for examples, emit (or absorb) in the infrared or microwave regions, typically; highly ionised atoms, in the UV or even x-ray regions.

Nuclei also have allowed energy states, and a transition from a higher one to the ground state is usually accompanied by emission of a gamma ray. These 'nuclear lines' are useful in astronomy too.

John Jaksich
2010-Jul-18, 12:05 AM
Yes; the spectra of molecules made up of two, or more, atoms of the same element are totally different from the spectra of the atoms alone.

This is because lines are emitted when an electron 'jumps' from one allowed energy level to another. The allowed energy levels of a molecule of diatomic oxygen, say, are quite different from those of ozone, or oxygen atoms. In fact, molecular spectra are far richer than atomic spectra, because molecules can also vibrate (the bonds stretch and contract, for example) and (often) rotate; this gives rise to bands, which are many, many lines spaced very closely together.

I tried to find an example - oxygen atom, diatomic oxygen molecule, ozone - but failed.


For most species, of atoms, ions, and molecules, most of the spectrum is not in the visual waveband. Molecules, for examples, emit (or absorb) in the infrared or microwave regions, typically; highly ionised atoms, in the UV or even x-ray regions.

Nuclei also have allowed energy states, and a transition from a higher one to the ground state is usually accompanied by emission of a gamma ray. These 'nuclear lines' are useful in astronomy too.


Oxygen is normally a very stable species . . . and so please let me explain ------->

The oxygen molecule O2 may exist in either a so-called singlet or triplet species . . .


Triplet oxygen is the more stable of the two, and is therefore considered to be the "ground state." Because molecules tend to bond with electron pairs--- but the electronic configuration of oxygen has two pairs of "non-bonded electrons" that can force the molecule into the singlet species when excited.

So let me try to re-cap----the oxygen molecule normally exists where all of the electrons are paired---but when the non-bonded electrons become excited the molecule becomes re-active . . . or more to the point, the triplet species becomes a singlet species.


Because electrons like to be paired with opposing spins ---according to Pauli's Principle---the singlet species in molecular oxygen is a rare molecule to find and reactively toxic---consider ozone and rust?

More to come ------>

John Jaksich
2010-Jul-18, 12:13 AM
Oxygen as we know---does not have an associated color --so its spectrum --is (1) complicated by its aforementioned electronic properties (2) the blue of our "sky" is the result of so-called "Rayleigh scattering."

According to Wikipedia----> Dr. R. Mulliken, who made the following discovery---->


"Direct detection of singlet oxygen is possible through its extremely weak phosphorescence at 1270 nm, which is not visible to the eye. However, at high singlet oxygen concentrations, the fluorescence of the so-called singlet oxygen dimol (simultaneous emission from two singlet oxygen molecules upon collision) can be observed as a red glow at 634 nm."

Interpretation of the atmospheric oxygen bands; electronic levels of the oxygen molecule R.S. Mulliken Nature Volume 122, Page 505 1928


More to come---->

John Jaksich
2010-Jul-18, 12:41 AM
The following spectrum was taken from Wiki: http://en.wikipedia.org/wiki/Diffuse_sky_radiation--The spectrum is


and according to wiki---->

"(Spectrum of blue sky somewhat near the horizon pointing east at around 3 or 4 pm on a clear day. Spectrum was taken using an Ocean Optics HR2000 spectrometer with a high-OH solarization-resistant fiber optic light guide. this)"

The obvious problem ---> (to me) would be to attempt the characterizing the bands of the spectrum ---and many times it can be trickier than how it was described---(at least in my attempts:exclaim:)


More to come------>

John Jaksich
2010-Jul-18, 12:56 AM
Again according to Wiki -----> http://en.wikipedia.org/wiki/Fraunhofer_lines

Oxygen bands supposedly occur at ----->


898.765 nm

822.696 nm

759.370 nm

686.719 nm

627.661 nm

But note ----->

again from the article:

"Note that there is disagreement in the literature for some line designations; e.g., the Fraunhofer d-line may refer to the cyan iron line at 466.814 nm, or alternatively to the yellow helium line (also labeled D3) at 587.5618 nm. Similarly, there is ambiguity with reference to the e-line, since it can refer to the spectral lines of both iron (Fe) and mercury (Hg). In order to resolve ambiguities that arise in usage, ambiguous Fraunhofer line designations are preceded by the element with which they are associated (e.g., Mercury e-line and Helium d-line).

Because of their well defined wavelengths, Fraunhofer lines are often used to characterize the refractive index and dispersion properties of optical materials."

Dale Botha
2010-Jul-19, 03:37 PM
Yes; the spectra of molecules made up of two, or more, atoms of the same element are totally different from the spectra of the atoms alone.

This is because lines are emitted when an electron 'jumps' from one allowed energy level to another. The allowed energy levels of a molecule of diatomic oxygen, say, are quite different from those of ozone, or oxygen atoms. In fact, molecular spectra are far richer than atomic spectra, because molecules can also vibrate (the bonds stretch and contract, for example) and (often) rotate; this gives rise to bands, which are many, many lines spaced very closely together.

I tried to find an example - oxygen atom, diatomic oxygen molecule, ozone - but failed.


For most species, of atoms, ions, and molecules, most of the spectrum is not in the visual waveband. Molecules, for examples, emit (or absorb) in the infrared or microwave regions, typically; highly ionised atoms, in the UV or even x-ray regions.

Nuclei also have allowed energy states, and a transition from a higher one to the ground state is usually accompanied by emission of a gamma ray. These 'nuclear lines' are useful in astronomy too.

Cool...that was informative! Thanks for the time Nereid!


Oxygen bands supposedly occur at ----->


898.765 nm

822.696 nm

759.370 nm

686.719 nm

627.661 nm

Thanks for your reply John! These frequencies seem to have been nailed down to a fine degree of accuracy! I understand that the specific molecules and atoms have a spectral signature that id unique...does that uniqueness apply to individual lines?

John Jaksich
2010-Jul-19, 05:20 PM
Given a "standardized spectrum" the bands would appear to be unique---but the "actual medium" that they occur in (numbers, types, and actual ppm of all species would alter how each individual spectrum would appear." So, in short--- a molecule's band might red-shifted or blue-shifted in the medium because individual molecular interactions.



Referring to spectrum --previously attached----oxygen molecule bands are extremely weak due to "a low extinction coefficient" ----> so in short ---they don't readily stand out.

More to come . . . ----->

John Jaksich
2010-Jul-19, 05:50 PM
The red-shifted and blue-shifted species mentioned in the previous post are normally cases where there are "extremely-weak molecular---Coulombic and Dispersive forces" affecting the resulting spectra.

In controlled laboratory spectral experiments----> (back in my laboratory days!) spectra were collected of individual molecules for purposes of identification and standardization. The concentrations of each measured moiety was controlled and measured against a blank -- or more to point--- the solvent that contained the molecule---- so what was actually measured was the spectrum of the molecule (infra red, visible, U.V., etc. . . .).

So, what I am referring to in the beginning of this post is the effects that "medium" might have on an individual molecule--it might shift the spectrum within a one or more nano-meters one direction or the other--depending upon how and how many different species there are affecting the individual molecules (or atoms, for that matter) -- being measured.

And please note ---> I am sure that there many within this forum who do the spectral surveys within the galactic medium or ISM that could readily expand upon my "chemist's" notions of molecular and atomic spectroscopy.

Seek them out . . . :exclaim:

Nereid
2010-Jul-19, 06:47 PM
Cool...that was informative! Thanks for the time Nereid!
You're most welcome.



Thanks for your reply John! These frequencies seem to have been nailed down to a fine degree of accuracy! I understand that the specific molecules and atoms have a spectral signature that id unique...does that uniqueness apply to individual lines?
If all you have is just one line, in a spectrum, you're totally lost - you have no hope of identifying what electronic transition produced it ... except that if it's just a line, then it's not a molecule which produced it.

Why? Because a single line tells you nothing of the redshift of the object, so you could be seeing a line that is close to its lab-standard wavelength, or something from a quasar at a redshift of 6, or anything in between!

With some metadata - the shape of the continuum, the location of the object on the sky, whether it's an extended source or not, that sort of thing - you may be able to guess what the line might be.

With two lines you're much better placed ... astronomical objects have very distinctive spectra, and the ratio of the observed wavelengths, compared with the strongest observed lines in a wide range of typical objects, *may* give you a good idea of what the object is, and so the atomic transitions may be identifiable.

Molecules produce bands, and each band is quite distinctive; if your spectrum has molecular bands, you can nearly always identify the molecules.

In a spectrum with many lines, it is nearly always possible to identify the lines, because there will, nearly always, be several lines from each species (atom, ion), and their observed strengths must be consistent (e.g. temperature dependence). It gets tricky with faint lines and very minor components; in the solar spectrum, for example, there are many faint lines whose identity is not known for sure - too many possible choices, all of them more or less consistent. Line blends are another source of possible confusion.

John Jaksich
2010-Jul-19, 06:54 PM
Nereid,

Thanks for clarifying and putting a better answer to my post!

Cheers to you and Dale Botha--the original questioner

Dale Botha
2010-Jul-21, 01:25 PM
Cheers to you and Dale Botha--the original questioner

Cool cool!

Back at you - - Nereid and John Jaksich

1. Are the relative brightnesses of the individual lines for a specific molecules/atoms unique at specific temperatures as well?

2. If one has a spectrum with many lines is it safe to assume that the temperature determining the strength of those lines is constant for each molecule/atom causing those lines?

Sorry for continued questions...I often find it very difficult to nail down a specific piece of info on a topic where the only other alternative is pile through mounds of very technical literature!!

Thanks, Dale

Nereid
2010-Jul-21, 04:59 PM
Cool cool!

Back at you - - Nereid and John Jaksich

1. Are the relative brightnesses of the individual lines for a specific molecules/atoms unique at specific temperatures as well?
I'm not sure I understand the question, so what follows may not be a good answer.

Let's take a simple spectrum, with only hydrogen lines, of the Balmer series, in it. Let's also assume that the object emitting the spectrum is at a single temperature.

The strengths of each of the Balmer lines - as measured by something called 'equivalent width' - will be related to each other in a way that depends solely on the temperature (assuming that the source can vary in temperature and no other parameter).

In general, however, the strengths of the lines - of one atomic/ionic/molecular species - will depend on several factors in addition to temperature; for example, surface gravity (or pressure), composition, and how different layers in the source vary (the light we 'see' in a spectrum comes not from a zero-depth surface - except for white dwarfs! - but from several layers, and may be many tens of thousands of km thick!).



2. If one has a spectrum with many lines is it safe to assume that the temperature determining the strength of those lines is constant for each molecule/atom causing those lines?
In general, no ... because there may be many parts of the source that contribute to the line, and they may be at very different physical conditions. For example, the spectrum of a distant galaxy is the sum ('integrated') of the light from the billions of stars in the galaxy (and, if it's a spiral, the many nebulae too).



Sorry for continued questions...I often find it very difficult to nail down a specific piece of info on a topic where the only other alternative is pile through mounds of very technical literature!!

Thanks, Dale
Hey, that's what we're here for! :)

The more questions, the merrier.

Dale Botha
2010-Jul-22, 12:38 PM
I'm not sure I understand the question, so what follows may not be a good answer.

Let's take a simple spectrum, with only hydrogen lines, of the Balmer series, in it. Let's also assume that the object emitting the spectrum is at a single temperature.

The strengths of each of the Balmer lines - as measured by something called 'equivalent width' - will be related to each other in a way that depends solely on the temperature (assuming that the source can vary in temperature and no other parameter).
Equivalent Width....is that actually the width of the lines? I thought the strength of a line was a count of the number of photons at that specific frequency!? If the strength is measured by their width does that mean that there is a slight change in the individual frequencies of the photons? Or is it just an effect caused by the way in which the spectrum is 'recorded' (experimental error)?


In general, however, the strengths of the lines - of one atomic/ionic/molecular species - will depend on several factors in addition to temperature; for example, surface gravity (or pressure), composition, and how different layers in the source vary (the light we 'see' in a spectrum comes not from a zero-depth surface - except for white dwarfs! - but from several layers, and may be many tens of thousands of km thick!). Got it!


In general, no ... because there may be many parts of the source that contribute to the line, and they may be at very different physical conditions. For example, the spectrum of a distant galaxy is the sum ('integrated') of the light from the billions of stars in the galaxy (and, if it's a spiral, the many nebulae too). Cool. This can be tricky subject I see! Are you involved in spectroscopy?

Nereid
2010-Jul-22, 01:07 PM
I'm not sure I understand the question, so what follows may not be a good answer.

Let's take a simple spectrum, with only hydrogen lines, of the Balmer series, in it. Let's also assume that the object emitting the spectrum is at a single temperature.

The strengths of each of the Balmer lines - as measured by something called 'equivalent width' - will be related to each other in a way that depends solely on the temperature (assuming that the source can vary in temperature and no other parameter).Equivalent Width....is that actually the width of the lines? I thought the strength of a line was a count of the number of photons at that specific frequency!? If the strength is measured by their width does that mean that there is a slight change in the individual frequencies of the photons? Or is it just an effect caused by the way in which the spectrum is 'recorded' (experimental error)?

[...]
It's a rather tricky matter ... as I understand it, Equivalent Width (EW) is used far more in astronomy than in any branch of science.

A spectrum taken by a spectroscope, attached to a telescope, of a typical astronomical object (e.g. a star) is what it is.

The spectrum, when plotted as intensity (or flux) against frequency (or wavelength), often has 'lines' in it; if absorption lines, these are dips in the intensity, compared to the nearly spectrum (called the 'continuum'); if emission lines, these are peaks.

But a spectroscope has a certain 'resolution', which can be understood as the smallest difference in wavelength (or frequency) that can be distinguished, in a spectrum taken by the spectroscope; this is a little like pixels in a digital image.

Then the source atoms (say) are not sitting still; they have a certain temperature, which means that some are moving towards us, some away, and some neither towards or away - this makes a 'line' more like a Gaussian (do you know that this is?).

Then the spectrum of many astronomical sources is 'integrated' across a very large physical region - the spectrum of a star, for example, is all the light from the star (coming in our direction). If the star is rotating (and we do not see in 'pole-on'), this too will 'broaden' the line (do you know why?).

And so on.

So a line's intensity is not just how many photons are detected at the exact wavelength of the corresponding atomic transition; rather it is all the light that comes from atoms undergoing that transition, in the source (hopefully!). Mathematically, that means integrating across the line profile (which is like a blown-up, or zoomed-in, part of a spectrum); the result is expressed as EW.

A lot of astronomers spend a lot of their professional lives teasing out the various components in a line, from that introduced by the limited resolution of the spectroscope (and other 'errors'), to the temperature and pressure of the source atoms, to rotation effects, to electric and magnetic fields, to ...

I hope that explains it a bit more clearly.

Dale Botha
2010-Jul-23, 01:06 AM
Then the source atoms (say) are not sitting still; they have a certain temperature, which means that some are moving towards us, some away, and some neither towards or away - this makes a 'line' more like a Gaussian (do you know that this is?).
Yes

Then the spectrum of many astronomical sources is 'integrated' across a very large physical region - the spectrum of a star, for example, is all the light from the star (coming in our direction). If the star is rotating (and we do not see in 'pole-on'), this too will 'broaden' the line (do you know why?) yes


So a line's intensity is not just how many photons are detected at the exact wavelength of the corresponding atomic transition; rather it is all the light that comes from atoms undergoing that transition, in the source (hopefully!). Mathematically, that means integrating across the line profile (which is like a blown-up, or zoomed-in, part of a spectrum); the result is expressed as EW.

A lot of astronomers spend a lot of their professional lives teasing out the various components in a line, from that introduced by the limited resolution of the spectroscope (and other 'errors'), to the temperature and pressure of the source atoms, to rotation effects, to electric and magnetic fields, to ...

I hope that explains it a bit more clearly.
Yes...a lot! Thanks! Nice one!