> Earth was inhospitable until 3.8Gya

There is much uncertainty about this. The early Earth was a volatile place with a lot of raw materials and energy available, and a lot of chemistry happening. I wouldn't rule out that some part of Earth may have had the right conditions for life right from the beginning. Certainly if we expect life to be able to survive a ride on a piece of ejecta from Mars, it's plausible to imagine it surviving in some boundary niche amidst the heat and fury of the Hadean Earth.

> wouldn't rule out that some part of Earth may have had the right conditions for life right from the beginning

Even if it was, it wouldn't have survived the impact that created the Moon.

Bear in mind that hypothesized impact is estimated to have occurred around the 4.5Gya mark. That’s still a lot further back than 3.8Gya. And I still wouldn’t categorically rule out microscopic life/life-precursors surviving the event somehow.

Geological evidence shows that the earth’s crust was molten until 3.8Gya. This corresponds with the ending of the late heavy bombardment on the moon, which Apollo samples dated. It’s hard to imagine life persisting in or around a magma ocean.

What if Mars collided with Earth and that's what killed Mars and transferred life to Earth and also made the moon?

I think generally speaking people refer to life arriving and then staying as the start of life on Earth. You're probably right that it could have been and likely was spontaneously popping up and then disappearing on Earth prior to the pretty massive window as when we believe life to have "began" on Earth.

At this point, given how much Mars had been roved, why hasn’t some fossil record been discovered yet?

At the very least, I would expect something akin to chalk left behind by proto-diatoms that had calcium carbonate skeletons.

Other posts answered, but only if you already know the answer. The rovers we've used on Mars have done an extremely superficial job of scouring the planet. The most recent rover Perseverance has, to my knowledge, the most capable drill with a max depth of 2.4 inches. [1] And the drill is used extremely sparingly because it tends to break quickly, as it did on Curiosity. And of course it can only drill, not excavate/cleave. The first humans on Mars will likely provide more information in a week than decades of probes and rovers have.

[1] - https://attheu.utah.edu/facultystaff/qa-perseverance-rovers-...

I am imagining an astronaut digging with a shovel and 2.5 inches below the surface are just dinosaur skeletons and stuff. Or something even more fantastical.

Exactly. There's absolutely no reason to think that advanced life did not exist on Mars except for the lack of evidence. That sounds funny, but the absence of evidence is not evidence of absence, especially in a case like this where the planet has been probably sterilized on a time frame of potentially billions of years.

The Great Pyramid of Giza is a massive and extremely hardy structure that will outlast all modern buildings. Yet it's substantially deteriorating after less than 5,000 years, and it will eventually just completely disintegrate, absent the already ongoing preservation/restoration efforts. It might take 20,000 years, maybe a million. A quick search on the internet suggests nobody really knows, other than that it will happen. And now we're talking about time frames of billions of years. So what would a sterilized, perpetually radiated, atmosphere free Earth over billions of years look like, paired with never-ending massive dust storms (as on Mars)? Well, probably not especially unlike Mars does today. But oh what treasures would await you far below the surface!

I don't necessarily think anything does await us, but who knows? It's a tantalizing possibility that we won't be able to dismiss until we have humans on Mars. It's vaguely analogous to the early observations of 'canals' on Mars [1] whose explanation ranged from running water to active geoengineering from another advanced species on the planet. Their exact meaning was debated all the way into the mid 20th century, until we finally were able to get probes that could take 'high res' photos of the surface that showed they were definitely just optical illusions. A disappointing discovery, but nonetheless emphasizing that the importance of actually testing things instead of just relying on indirect instrumentation/data of this era or that.

[1] - https://en.wikipedia.org/wiki/Martian_canals

Kurzgesagt did a video on this.

Though they discuss how advanced civilizations could have existed on Earth and we might not even know it because there wouldn’t be much left and what was left might be so unusual that we might chalk it up to natural phenomena.

https://youtu.be/KRvv0QdruMQ

There's been less than 150 miles of Mars roamed by mostly small rovers that often could barely do more than take pictures and do some basic soil chemical analysis. Do you expect to see obvious dinosaur remains with an RC car with a small shovel driving around in Kansas for 100 miles, especially if you don't even know what a dino even looks like?

I think there have been less than 100 drill holes by curiosity and perseverance the deepest being ~3” and none of those samples have returned to earth. Not sure how deep any potential fossil layers might be but that doesn’t sound like a lot.

How would we know? It’s not like like we have a geologist over there with a rock hammer breaking open rocks to look for fossils.

Yet.

I think it’s actually pretty darn stupid that we aren’t running one way missions.

Feels like a conspiracy when you run into these super simple, realistic use cases of generic but progressive ideas.

Things kids can think up, that we can most certainly do that would put us insanely ahead but ... we just don't?

Like, why do we visit the moon but not have people stay there at all? Why are there no structures there?

Why do we ignore the Earth's mysteries a lot and go directly to Mars?

I, and I bet other people bored on Earth with nothing to look forward to, would gladly sign up for a one-way trip to Mars.

The people who would be valuable in that role, namely highly trained scientists, mostly aren't interested in what's effectively a suicide mission. They have better options and things they're tied down to on Earth.

> At this point, given how much Mars had been roved

I mean maybe its a lot compared to 20 years ago but in absolute terms? It’s practically nothing, isnt it?

They don't carry petrographic microscopes, nor dig deep boreholes.

Drilling a couple of inches into the surface a handful of times isn’t going to find any fossils.

That still doesn't answer the question. How did life start on Mars?

That's a different question though, and you're right that this wouldn't be answered in the scope of this discussion.

How does earth get seeded?

Look up ALH-84001. The debate about whether the structures inside this Martian meteorite are fossils is too long for a HN post. But in analyzing the rock it was shown that the during the entire trip from when it was blasted out of Mars to when it fell in Antarctica would have been survivable for rock boring microbes. Not just theoretically, but hard evidence. There are structures that would have been destroyed if the rock was heated enough to sterilize, but they survived the trip. Hundreds of tons of rock like this from Mars land every year.

Related talk: https://m.youtube.com/watch?v=ekJQUS7WP78&t=57s&pp=ygUPS2lyc...

> Hundreds of tons of rock like this from Mars land every year.

277 total Martian meteorites — with the largest weighing 14.5 kg — is not hundreds of tons yearly.

That's how many have been conclusively classified as Martian out of "the 72,000 meteorites [total] that have been classified". (https://en.wikipedia.org/wiki/Martian_meteorite)

That's obviously not the actual total that have ever made it here.

https://science.nasa.gov/solar-system/meteors-meteorites/fac...

> Scientists estimate that about 48.5 tons (44,000 kilograms) of meteoritic material falls on Earth each day.

That has been going on for billions of years. If the 277/72,000 proportion holds that's a lot of material.

While I recognize your logic, and even mostly agree, the point still stands that it cannot be conclusively and definitively said (as least as far as we've been alive/can tell) that hundreds of tons of Martian meteorites fall to Earth every year. Or even tons at all.

Tons of meteorites in general, sure — but not from Mars.

It is likely given these numbers at least some tons of Martian rock land here annually, which makes the seeding of life concept feasible. I'm on board with "not hundreds of tons", but it's a lot closer estimate than 277 ever.

(48.5 * 365) * (277 / 72,000) = 68 tons per year as an extremely speculative estimate here, ignoring entirely probable variances in what hits us (much of which is sand-grain sized) versus what we identify... and again, any estimate here we have to multiply by a few billion years.

> Scientists estimate that about 48.5 tons (44,000 kilograms) of meteoritic material falls on Earth each day.

Most of it gets destroyed on entry, right? No organic matter surviving?

No, most are smaller rocks and generally decelerate to terminal velocity in the upper atmosphere. It’s the big ones that have too much momentum and penetrate further at speed, causing them to go boom.

The underlying reason is that momentum scales with volume but air resistance scales with surface area.

If there were extra-terrestrial intervention, it would only have needed to be a one-time event.

And it wouldn't be technologically hard to do it such that no trace remains over those timescales.

So, yes, that's "crazy". On the other hand, it's unfalsifiable in the absence of other theories.

The burden of proof is on the person making the claim. It is not on someone else to disprove such a claim.

There are much saner theories for panspermia.

I'm not arguing against that.

I'm just saying unfalsifiability is a bitch, and it's hard to find hard evidence on geologic timescales.

Like? You're coming across as very dismissive for no reason.

There are meteorites on Earth which very likely originated from Mars. They're believed to have been launched into space by an impact, eventually finding their way onto Earth. If we're looking for the most realistic options for panspermia to have occurred, they'd be due to a natural event of this sort.

It’s very much like how islands in the sea are colonized by life. Some event causes a blob of living creatures to enter the sea on a raft and they drift until they make landfall somewhere else. Sometimes they survive long enough in the new place to reproduce.

Rats leaping from a sinking ship swimming to shore we were.

Volcanic eruptions throwing rocks into space. A large impact throwing a huge number of rocks into space.

Your numbers give Mars an extra ~800 million years. Nothing even close to 3/4 billion.

Besides, deciding what was inhospitable for early life is an exercise of noisy assumptions. I'd bet the error margin is larger than that difference.

3/4, as in the fraction 3 out of 4, 75%, not 3-4

To be fair, 3/4 billion is an unusual way to write that. Much more common is 750 million, or even 0.75 billion.

3/4 billion = 750 million, no?

Long billion vs short billion?

Didn’t the “long billion”, aka a trillion, stop being used sometime in the early 20th century on British English, if not earlier?

Some of our books in UK school in the 1980s listed 10^12 as a billion. That doesn't mean it wasn't officially changed in the early C20th though.

?????????

800 million is greater than 3/4 billion?

Maybe he misread it as 3-4 billion?

I misread it that way too. Fractions aren't commonly used for things like millions, billions, etc.

The longer time period would be partially offset by Mars having over three times lower surface area than Earth, though.

Complexity speed run to stagnation isn’t suspect at all, thats exactly what you’d expect when there is suddenly a new environment to exploit for life. Mutations are always happening. At some point a subpopulation will emerge that has a lucky few mutations that grant it an edge relative to others in fitness, then it will dominate the environment until the factors that confer success either change, or these mutants fall short of a more fit upstart mutant population. Its something you see play out even today, when an invasive species is introduced to a new environment where its traits serve it well, and quickly outcompetes native species.

Its also why we better be damn sure the mars sample return is absolutely sterile lest we contaminate our planet with extraterrestrial microbes.

Couldn't it just be that simple life isn't rare as long as the conditions exist for it to form. It took a very very very long time for even oxygen producing life to form after that.