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The news today was the discovery of phosphine in the
atmosphere
of Venus. It might indicate the existence of life on Venus.
It is known that life tends to favour making chiral (organic)
compounds disproportionately of one chirality.
Chiral compounds transmit light with circularly polarisation
of
one type (left circularly polarised light) more than the other
(right circularly polarised light).
So, a point telescope at a planet (or exoplanet) and get a
spectroscopic signature for each of left and right circularly
polarised light.
If there is any spectroscopic signal that is relatively stronger in
one polarisation than another, then this indicates life might
exist
on the planet.
Homochirality
https://en.wikipedi.../wiki/Homochirality
[xaviergisz, Sep 16 2020]
Will the Webb telescope be able to detect life signs at nearby exoplanets?
https://earthsky.or...appist-1-exoplanets
[xaviergisz, Sep 17 2020]
the truth is out there
https://arstechnica...e-at-least-not-yet/
[theircompetitor, Jan 18 2024]
[link]
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It's an interesting thesis - if it is the case that observations of life (on earth) favour chiral compounds of
one kind, is it also the case that non-living processes generate non-chiral (or statistically neutrally
chirally biased)organic compounds? |
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As soon as you have a process complex enough to produce organics, even if it has a tiny bias, I'd imagine it'd
tend to generate an unstable dynamic state that'd want to resolve itself one way or another. I suppose this
last depends on how the organics react with one another post creation - which perhaps is another way of
describing what "life" does, so the idea certainly could have legs. i.e. that life forces chirality to polarise
in favour of one direction or the other. |
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Although there are non-biological processes that can form
homochiral compounds (or at least compounds that are
significantly skewed to one chirality), I think this is fairly rare. |
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Good idea, but astronomy, like the microscopy i do, is
operating right on the edges of what's possible with tiny
amounts of light. At the moment, back-illuminated electron
mutiplier CCDs are the sensitivity standard at ~95% quantum
efficiency. They're being replaced with the newer CMOS
detectors, mainly because of cost and massive speed increase,
although they're lucky if they get 85% QE. The problem is that
detecting circulary polarized light. All the detectors are junk
with no efficiency. So it could kind of be done, you'd just need
a 10x larger telescope. |
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The typical human behaviour pattern is: |
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2. Find new and interesting life-forms |
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3. Hunt them to extinction. |
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It's only a matter of time ... |
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//So, a point telescope at a planet (or exoplanet) and get a spectroscopic signature for each of
left and right circularly polarised light.// |
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I don't think this will work as described. (Disclaimer: my understanding of some of the following is
fairly weak; some of it might be wrong.) |
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As I understand it, the detection of phosphine gas was though absorption at a particular
wavelength. Similar to how the composition of the sun and other stars is
determined.
This has the advantage that one doesn't need to illuminate the sample - which is a significant
advantage when the sample is another planet or something else far away. |
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Chiral compounds do have some interesting interactions with polarised light, and apparently can
absorb left- and right-handed polarised light differently.
(I found this is called "circular dichroism".)
So that would be fine, except that to measure this one does need the sample to be irradiated
with circularly polarised light. In both rotations. Sunlight
isn't initially polarized at all, so further processes would be required to genrate that. Linearly
polarised light can by created through scattering, so I imagine one could conceivably
measure, say, very specific areas at the edge of a planetary 'disc' to see that - but if you want to
collect data remotely you'd also need another physical
process to generate circularly polarised light from that - which I'm not sure is available. |
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Furthermore, chiral compounds are generally more complex than simple inert byproducts. That
means they're rarer, and hence harder to detect. For example,
if you were trying to evaluate the Earth from a distance, I imagine that it would be reasonable to
measure oxygen or carbon dioxide in the atmosphere (by
wavelength absorption, say). But amino acids, or sugars etc would be harder. |
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Finally, the idea that "life makes compounds disproportionately of one chirality" is true, but it's
specific for each molecule type. For example,
amino acids in proteins are predominantly L-stereoisomers. Conversely, most monosaccharides
are the D-stereoisomer. In any case, have some
recollection that the description of the chiral centre doesn't necessarily relate to the direction in
which it will reorient polarised light.
All in all, you're
hoping that there would be a significant difference in the absorbance of the two circularly
polarisation options through biological matter in aggregate, but
I'm not confident that that's true, and it's certainly not as good as you can see in a solution of a
single compound. |
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// The typical human behaviour... // |
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I say we nuke the planet from orbit. It's the only way to be
sure. |
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You wanna push the button? |
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//The typical human behaviour pattern// |
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You're one to talk, you brane-obsessed space zombie. |
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"Obsessed" is a trifle strong, don't you think ? |
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hmmm... off topic but, does chirality during gestation play a role in people being either right of left handed? |
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I'm no expert, but I think it's more at a molecular level -
you eat food, its constituent amino acids and peptides are
chiral, and your body is able to process them due to the
shape of their molecules a shape which is deeply
influenced by chirality. So if you met someone who came
from another planet and who was oppositely chiral, to
you, they'd not be able to eat your food, and vice versa -
maybe simple sugars, water and minerals, but probably
not meat or more complex foodstuffs anyway - thinking
about it, it might be a way to develop diet foods, they
might look the same as normal food, but due to chirality,
be able to pass through a person with next to zero
processable calories. Experts needed at this juncture to
puncture flagrant hypothesising in the face of any actual
knowledge or insight into molecular biology. |
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Basically, that's right. Enzymes will only operate on molecules of the "correct" handedness. Since even quite simple molecules, like lactic acid - which is only 3 carbons long, a propanoic acid - exhibit chirality, the chemical processes of life are fundamentally dependent on their "feedstocks" being correctly configured. Wrong enantiomer, and the enzymes won't work. |
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By "flipping" molecules, it's possible to create molecules that behave physically the same way (i.e. in cooking), but the body can't process. This has both advantages and disadvantages, for instance in creating "diet" foods, that pass through the system ... largely unmodified. |
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//creating "diet" foods, that pass through the system ...
largely unmodified.// |
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That was proposed, L-glucose was the candidate. It is a
silly idea for more than one reason. Firstly, we know what
happens when a human eats a sugar they can't process,
largely they groan, gurgle, complain & lament. Even then,
there's a good chance the calories still get absorbed, just
like with dietary fiber, there's plenty of gut microbes that
will use them, and then we absorb bits of them. |
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// amino acids in proteins are predominantly L-
stereoisomers.// |
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This made me stumble on a whole area of my own
ignorance. Surprisingly, there's plenty of activity in the D-
isomer amino acids. Humans not so much, but even we
have "racemases" with plenty of interesting little hints at
them acting as signalling molecules. |
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We even eat D-amino acids regularly, anything fermented
or subject to long term heating is a good candidate. |
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//As I understand it, the detection of phosphine gas was
though absorption at a particular wavelength. Similar to how
the composition of the sun and other stars is determined.
This has the advantage that one doesn't need to illuminate
the sample - which is a significant advantage when the
sample is another planet or something else far away.// |
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Venus was illuminated by the sun. The light reflected from
the atmosphere and surface showed an absorption
(attenuation) that indicates the presence of phosphine. My
idea would be essentially the same. If there are chiral
compounds with significantly greater concentration of one
chirality than the other, then there should be absorption of
one circularly polarised light than the other at a particular
frequency. |
|
| |
//Chiral compounds do have some interesting interactions
with polarised light, and apparently can absorb left- and
right-handed polarised light differently. (I found this is
called "circular dichroism".)
So that would be fine, except that to measure this one does
need the sample to be irradiated with circularly polarised
light. In both rotations. Sunlight isn't initially polarized at
all, so further processes would be required to genrate that.
Linearly polarised light can by created through scattering, so
I imagine one could conceivably measure, say, very specific
areas at the edge of a planetary 'disc' to see that - but if you
want to collect data remotely you'd also need another
physical process to generate circularly polarised light from
that - which I'm not sure is available.// |
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Although the effect is more pronounced using polarised
light, chiral compounds still absorb more light of one type of
(circular) polarisation than another. This should be able to
be detected with polarised filter at the detector. |
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//Furthermore, chiral compounds are generally more
complex than simple inert byproducts. That means they're
rarer, and hence harder to detect. For example, if you were
trying to evaluate the Earth from a distance, I imagine that
it would be reasonable to measure oxygen or carbon dioxide
in the atmosphere (by wavelength absorption, say). But
amino acids, or sugars etc would be harder.// |
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Completely agree. This is going to be a very subtle effect. I
mentioned exoplanets, but that is unrealistic with present
technology. Much more realistic to use on the planets and
moons within our solar system. |
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//Finally, the idea that "life makes compounds
disproportionately of one chirality" is true, but it's specific
for each molecule type. For example, amino acids in
proteins are predominantly L-stereoisomers. Conversely,
most monosaccharides are the D-stereoisomer.// |
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Yep, so we'd be looking for any difference in the absorption
spectra at every single frequency. |
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//In any case, have some recollection that the description
of the chiral centre doesn't necessarily relate to the
direction in which it will reorient polarised light.// |
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Not sure about this. There may be some exceptions, but
generally the chirality of molecule will be detectable with
its effect on the polarisation of light. |
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//All in all, you're hoping that there would be a significant
difference in the absorbance of the two circularly
polarisation options through biological matter in aggregate,
but I'm not confident that that's true, and it's certainly not
as good as you can see in a solution of a single compound.// |
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Yep, it's going to be a barely detectable effect. We could
point a remote telescope at Earth to test out the idea. |
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// anything fermented or subject to long term heating is a good candidate. // |
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True, but those are at the very least de-natured, and more than likely pyrolysed (BCBs being a prime example foodstuff). |
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The problem comes when you need the molecule "undamaged", like vitamins. The autoclaving process employed in the preparation of vegetables for school dinners is a fine example of how dangerous contaminants can be made safe. Many survivors of the British educational system reached their md-20's before becoming aware that Brussels Sprouts and Cabbage were in fact solids rather than liquids. |
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The fermentation thing is interesting; sauerkraut contains lots of raffinose, which is an oligosaccharide that humans aren't equipped to digest, but the typical gut flora find (metaphorically) most toothsome, with spectacular consequences from a gas manufacturing perspective. |
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// So it could kind of be done, you'd just need a 10x larger
telescope. //
Sounds good to me. Tangentially related to this idea, I am
excited about the James Webb
Space Telescope. |
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The article I linked says that it will take between 10 and 30
'transits' of nearby exoplanets to determine the composition
of their atmospheres. So, optimistically, we might know in as
little as 10 years from JWST being operational if there is life
on nearby planets. |
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Of course this would be amazing, but this will also be sad in
that it is impossible (in our lifetimes) that we will ever be
able to see directly what that life looks like. |
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//Venus was illuminated by the sun. The light reflected from the atmosphere and surface showed an
absorption
(attenuation) that indicates the presence of phosphine. My idea would be essentially the same. If
there are chiral
compounds with significantly greater concentration of one chirality than the other, then there
should be
absorption of one circularly polarised light than the other at a particular frequency.// |
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Okay, but where are you getting the circularly polarised light from? Light doesn't come that way
from the sun. |
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As I said before, I'm not enormously confident in my understanding of light, but I don't think you can
derive the
measurements you want from incoherent (unpolarised) light.
You might be thinking "Oh, I can just split out the different sorts of light at the detector." But - as I
understand it -
quantum mechanics says no.
The photons... retcon themselves. |
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//Not sure about this. There may be some exceptions, but generally the chirality of molecule will
be detectable
with its effect on the polarisation of light. // |
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Also not sure about this, think you're right in what you say...
But I was just building up to the idea that a diverse mixture would tend to cancel out. However, I
think this is a
good point I hadn't considered: |
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//so we'd be looking for any difference in the absorption spectra at every single frequency.// |
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As you say, you'd only need a difference at one frequency. |
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