Hm, why would chirality need to be a consequence of the panspermia hypothesis? I thought the mission defined "necessary ingredients for life" as any bio markers that might have seeded a primordial soup on Earth.
I don't understand this. Aren't cells just spherical structures that would form naturally from hydrophobic molecules suspended in water/tide pools? That seems likely to me, but I have no background in chemistry/biology.
Nick Lane argues in "The Vital Question" [0] that simple cell membranes are not enough for complex life and complex membranes may have evolved in matrixes around hydrothermal vents.
AFAIK there isn't even a known way to start with "normal chemicals" and produce highly-chiral reactions reliably in the lab. We get all our chiral molecules by extracting them from biology.
If we found say, a high pressure synthesis method which was heavily biased chirally, then that would be good evidence but none has been discovered.
The fact that the components on the asteroid is racemic meant or heavily suggested that they were formed using non-biogenic means. And if so, it also means that Earth could have had the same thing happened a long time ago, leading to the seeds of life.
tl,dr: this discovery weaken the panspermia hypothesis.
Nonzero-weighted sum of something and nothing is something.
Actually finding chirality in space would be extremely cool in that it would mean one of two things:
- the panspermia hypothisis is correct, or
- some non-biological process creates chiral molecules.
Physics itself has plenty of chiral processes, but they only show up in the weak interaction. As the name implies, the weak interaction is really really weak and essentially doesn't exist on the energy scales of molecular interactions. So chirality would be a bit of a smoking gun for panspermia.
On the other hand, not finding chirality just means we don't have a smoking gun. There might be another asteroid flying around that is 100% chiral, or maybe 50% of them and we were just unlucky.
The briefing joins the point about chirality with evidence against panspermia, but really that might miss the point. Chiral or not, abundant amino acids in space means that one of the many steps to create life is relatively simple. If we could show that every subsequent step is simple that would be a big blow against panspermia. But in that case ruling out panspermia would be pretty cool, since it would suggest that life exists everywhere.
The more asteroids we look and not find any asymmetry, the more evidence we have that life never existed on any of them.
What I don't understand is why would chirality and panspermia be so tightly linked.
The data right now still leaves every option on the table just because having any ratio of chiral molecules doesn't have to define how life evolves. It can't answer whether those molecules formed on Earth or hitched a ride on an asteroid, or life itself formed here or was brought here.
We can assume that in a soup with balanced proportions of each chirality, the left handed molecules created a self replicating mechanism (some definition of "life") first or faster than right handed molecules, either accidentally or because some yet undiscovered advantage. Whether this happened on Earth, or was brought to Earth by one or more of the millions of asteroids is hard to prove.
If we find one asteroid with chirality that doesn't match earth, it's good evidence that self replication just happens spontaneously rather than being seeded.
If we find only one asteroid with one chiral molicule that does match earth, that supports panspermia very weakly: it still might just be random chance. But multiple matching molecules, on multiple asteroids, starts to seem like evidence for a common source.
On the other hand: If we find multiple molecules with matching chirality, on multiple asteroids, and none of them match with earth... well, we should probably start preparing for the invasion.
In chemical synthesis, most (but not all) processes tend to preserve chirality of molecules: replacing a bunch of atoms in a molecule with another set will tend to not cause the molecule to flip to a mirror image. If you start from an achiral molecule (one where its mirror image can be rotated to the original), almost all processes tend to end up with a 50-50 mix of the two racemates of the product.
In biochemistry, you can derive all of the amino acids and sugars from a single chiral molecule: glyceral. It turns out that nearly all amino acids end up in the form derived from L-glyceral and nearly all sugars come from D-glyceral. The question of why this is the case is the question of homochirality.
There's as yet no full answer to the question of homochirality. We do know that a slight excess in one racemate tends to amplify into a situation where only that racemate occurs. But we don't know if the breakdown into L-amino acids and D-sugars (as opposed to D-amino acids and L-sugars) happened by pure chance or if there is some specific reason that L-amino acids/D-sugars is preferred.
– enantiomer refers to each mirror image of the molecule. https://en.wikipedia.org/wiki/Enantiomer
– racemate refers to a 50-50 mix of the two enantiomers. https://en.wikipedia.org/wiki/Racemic_mixture
I guess the reason it makes things more likely is because homochirality happens so aggressively that finding right handed molecules at all drastically reduces the odds of left.
Do any of these affect the fields that would have selected for molecules on Earth? The Sun's rotation, Earth's rotation, the direction of revolution in our by now almost coplanar solar system, Galactic rotation
Since they are a mixture of right and left-handed they definitely come from space.
Chirality of life is not a consequence of panspermia, just a consequence of incompatibility so one had to win eventually.