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Comment by mach1ne | Hacker News Reader

Mars lost all of its (surface) water presumably due to solar wind, and thus never had time to evolve complex life. Earth would have faced the same fate were it not for the fact that the Moon collided with us at some distant point in time, surrendering its iron core to ours. Now, our Rare Earth has an unnaturally large active iron core, resulting in an unnaturally efficient magnetic field which is able to protect our waters from being blown away by Sun.

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Just wondering, what would have stopped the Earth from having enough iron on its own in the first place? If the Moon could have that much iron, the Earth could as well have had it without needing to mix with the Moon, they're made from the same original matter after all.

The OP mispoke. It wasn't the moon that collided with earth, but another planet around mar's size. As a result of the collision the the core of that planet combined with the earth and the crust and parts of the mantle were ejected and coalesced into the moon.

https://en.wikipedia.org/wiki/Giant-impact_hypothesis

I think the point still stands though, right? The moon is only 1% the mass of the Earth so if we had all that extra crust it wouldn't make our crust to core ratio so off that we couldn't have an atmosphere.

Read TSiege's correction again, it wasn't a (bad numbers) 1000T planet being impacted by a 10T object, it was a 500T object (earth) being impacted by a 500T object (now also earth) and ejecting a 10T object (now moon) from the debris.

Without the event we'd have a planet even smaller than mars!

I guess the question is rather than having 2 planets of 500T each form then collide, couldn't a single 1000T planet have formed from the same material?

I think the answer is yes, but the only way you get the dynamo of our core(thus strong magnetic field) is that collision, but that's just my fuzzy understanding from occasional reading over the years.

According to the article below, the impact may have started the magnetic field by mixing the core. It doesn't mention anything about "adding more iron" being in itself more significant. I wonder if the OP is half-remembering a distorted version of this theory.

https://phys.org/news/2017-12-doesnt-venus-magnetosphere.htm...

One might be inclined to attribute the amount of good luck to devine intervention, but to this I would say: If the conditions on our planet would not have allowed for higher life, nobody would have been there to ask themselves why the conditions were slightly off.

If you haven't heard of it, you've just stated the weak anthropomorphic principle.

*anthropic

Huh. Didn't realize I had those two words mixed up. Thanks!

Thanks, I did not know there was a term for this idea.

It's not that the water "blew away" in the solar wind, it's that the atmosphere did, taking the water vapor in the atmosphere with it. This is a very gradual process, which would happen on Earth too, if not for the magnetic field.

I don't know that the collision with the moon is the secret here or not. I think it's complex.

Neither Venus nor Mercury have substantial magnetic fields presently (compared to Earth).

I don’t know enough to know if that supports the GP’s conclusion or not, but it may.

These are pretty well researched, observed, and understood subjects. https://en.wikipedia.org/wiki/Atmosphere_of_Venus explains it quite well, particular https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Induced_ma... which explains how Venus is currently losing its atmosphere in the way described above. And https://en.wikipedia.org/wiki/Atmosphere_of_Mercury: "The Mercurian exosphere consists of a variety of species originating either from the Solar wind or from the planetary crust [...] This corresponds to a surface pressure of less than 10^-14 bar (1 nPa)" which is about 1% of what is considered "ultra-high vacuum" (https://en.wikipedia.org/wiki/Orders_of_magnitude_(pressure))

Mercury has no magnetic field, and no meaningful atmosphere like we'd understand it. Venus has a minimal magnetic field, and is covered in hot dense CO2 layer because the solar wind blew away all the hydrogen and continues to blow away the atmosphere. The Earth has a much stronger magnetic field that has protected our oxidized hydrogen, and good evidence points to the giant impact hypothesis.

I have very little reason to believe life evolved in Earth or even in Mars for that matter. The more likely scenario is that there’s rocks floating around everywhere that have spores of life throughout the galaxy ready to seed a suitable planet in geologically instantaneous timelines. That will explain the extremely rapid emergence of life on earth initially while it took a few billion years to evolve any further. In which case if Mars had flowing water at some point it likely also harbored at least single cell life.

Source: biologist but not specializing in evolution.

But you only introduce an unnecessary extra step in the same theory. Life still had to evolve somewhere, just that now it had to embark onto interstellar trip.

Not entirely unnecessary.

It's a big question as to why life started on Earth as soon as it did, pretty much as soon as it was even possible.

That could be because life is very easy to start if conditions are right. In which case it's quite likely to have started on early Mars too, and we should expect to find at least simple life all over the place.

The other possibility is that it didn't start on Earth, and was seeded from somewhere else.

Why not add more steps? The aliens¹ brought microbial life on an interstellar ship and seeded it. A lot better chance than riding out for a few million years on a piece of rock! See, it's approximately on the same trailing end of unlikely as panspermia, but I fancy this one more.

¹Let's call them Engineers

The big question is how local regions of low entropy that exhibit metabolism and replication (ie. life) came into existance out of inanimate matter. Where it happened is secondary.

Yes, that is the biggest question. Understanding where it happened may give us some clues of course.

But instead of having a single planet generating random molecular combinations to roll for proto-DNA, you have billions or more. Life would have to evolve extremely commonly for it to be more probable to evolve by itself on any specific planet than on literally any planet which then encouters an event causing panspermia.

But alas, that's an external observer viewpoint. It's important to note that it does not say that life is more probable to have evolved somewhere else than Earth (since we have an effective sample size of 1). It only says that the extra step introduced in the panspermia theory does not have a negative probabilistic impact, on the contrary.

If living organisms can survive on a meteorite, they can definitely survive on moon/mars, which contradicts our observations.

Nitpick: They'd survive in a meteorite.

They likely would not survive on a meteorite in the sense of active metabolism and replication. We'd be talking about very dormant spores.

That raises 2 problems for me.

1. How exactly are these spores of life getting into asteroids and being pinged around the Universe? The mechanism seems absurd to me. Earth is teeming with life, but outside of our explorations I believe none of it escapes our planet at all.

2. The life that would reach us would be already specialised for whatever environment it came from, and I would imagine would result a lower amout of variety, since it's already a successful species. But on our planet we have a massive amount of variety, which I believe points more to a bootstrapped start where competition was less intense, and odd things could develop, which were later outcompeted (see the Cambrian explosion).

Disclaimer: Not even a biologist, but I'm more willing to accept that life started and evolved here. Of course that might all be cast in doubt if we detect masses of life on asteriods or metoerites, but the first attempt at that seems to have come up with nothing so far.

Collisions? Large enough collisions eject large amounts of rocks into space. A good fraction of rocks ejected from earth would have some viable single cell life inside it, all you need are some cavities in them that don’t reach insane temperatures during this process, which is very possible for big enough rocks.

Remember that while you deal with insane numbers on the space side, viz. it’s emptiness, the number of planets etc., you’re meeting an insane adversary with biology as well. The number of cells in a single piece of sand in this planet is insane. The number of particles we might eject if there’s a catastrophic event from earth is insane. Anyone who worked on biology would know how hard it is to keep things truly sterile. I’m just extending it to outside space as well.

As for the conditions being too specific, while it’s true, I don’t see many outrageous combinations you can commonly observe in planets in the galaxy so I’m not sure if that’s a big deal breaker.

NASA proved in the 70s that some microorganisms survive atmospheric rentry. In fact, they could not possibly be killed by any means known to man, except for some enormous cosmic event with very powerful radiation / cosmic rays. This was done to question the idea of it being "safe" or not to travel in space or other planets, if it is possible to "scrub" our ships clean and avoid bringing germs with us. Apparently thats not 100% possible, some critters survive space and the harshness of re-entry just fine.

What makes you think panspermia is more likely than abiogenesis?

The only data point we have suggests the evolution of eukaryotic cells and multicellular life is simply less likely than abiogenesis. There are plenty of hypothesis to explain why that might be the case.

Seems like a bit of a stretch to say with certainty that Mars never evolved complex life 500+ million years ago.

Fair point, but even with rovers we have zero indication to the fact, and thus it's a very reasonable estimate.

Rovers are not the most effective tool for searching for fossils, especially microscopic fossils that can’t be seen from a distance on camera. They’re just the most effective tool we can send at the time we sent them. But a rover is only slightly more effective than just sending a dude with a buggy and a child’s beach kit.

You are making another assumption that a large meteor didn't destroy everything on the planet surface.

Considering we believe this happened to earth, it's not unreasonable to think the same happened to Mars at least once.

https://youtu.be/PENT_hnyO-o

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