[pyrois]

The linked paper does not suggest that Martians built nuclear reactors on Mars, which then melted down. The paper suggests that there were "large fireballs in the atmosphere such as Tunguska-like events , with mid-air explosions, but of much greater energy release than Tunguska".

[jerf]

It is worth pointing out that at least the "natural reactor" theory is not crazy: http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor Though it is unclear to me how such a process could go critical, in much the same way that one of the biggest challenges with building a real bomb fission bomb is not so much getting it to explode, but keeping it from exploding gently.

Whether we have the data to parse out something to this level of detail on Mars when we're still arguing about what killed the dinosaurs on this planet I'm substantially more "meh" about. But it is not, intrinsically, impossible or stupid.

The same goes for the alien hypothesis... but even taking the possibility seriously, I'd submit that A: we don't have enough evidence to eliminate the possibility that Mars could have once had an intelligent civilization but B: even moreso, let me underline that, even moreso, we have no evidence to suggest that it ever did. Such speculations would be pure science fiction right now. Right now we still know very little, full stop.

And I'd observe that the stories about these civilizations, viewed through modern technological eyes have some really weird aspects to them, such as, why would a civilization with technology capable of destroying planets (and, in this case, really, really destroying them, Death Star-style, not merely sterilizing the planet which is literally ten+ orders of magnitude easier [1]) only settle on Earth after the disaster? We're not planning on waiting for Earth to go bad before heading to Mars... we're pretty much only blocked on the requisite tech and on cosmological scales the instant we have it we'll be there. There's little reason to believe that a technological civilization of that scale would actually be destroyed even by its home planet going up in smoke. I'd submit the most likely hypothesis is that they were indeed written by humans thousands of years ago, who ultimately had no idea what technology was going to look like. (Heck, even we futurists are still only grasping at smoke in terms of what we'll have 50-100 years from today, to say nothing of trying to guess thousands of years ago....)

[1]: http://qntm.org/destroy

[NoMoreNicksLeft]

The alien genocider theory doesn't pass the smell test.

Why have they not bothered to come get us yet? We have our own nukes. They waited a little too long, or so it would seem.

Now, if they wiped out Mars with a designer black hole, or something of that nature, I'd say that we're still not too big for our britches for them to come do the same... but if they're just lobbing atom bombs, then they're overdue.

It's almost certainly some sort of natural, random event. If not another Oklo, then something not so different from it.

[jerf]

"Why have they not bothered to come get us yet? We have our own nukes. They waited a little too long, or so it would seem."

Oh, any alien genocider that may or may not exist would still be thoroughly unimpressed by our ability to fight back. Anything that could cross the stars in any period of time since civilization started need simply ram Earth to wipe humanity out as we know it. Call it a 10000 kilogram craft travelling at one-thousandth the speed of light from Alpha Centauri, setting sail 4000 years ago or so; if that simply rammed Earth it would be ~150,000 Hiroshima bombs [1]. That's pretty conservative for a genocider's capabilities, really, too. Obviously they're not right here, so anything that can travel here in time to get us is also a weapon that can wipe us out.

(This is to say nothing of the extreme opposite end of the scale and what a mature nanotechnology ought to be able to, even without Drexlerian extremes. I sometimes ponder "Our entire civilization was uploaded in its sleep ~3000BC and the real Solar System has long since been converted to computronium, and the simulated universe is lifeless to keep the processing simple." Pick your date of upload to suit your taste.)

To be clear, while I remain open-minded my current "top-probability" pick for resolution to the Fermi Paradox is "life is far more rare than science currently guesses". But it's still fun to discuss the alternatives and it's not like I could put that even remotely near 100%... it's just my best guess in a field where we have virtually no data, and YMWV.

[1]: https://www.google.com/search?q=10000kg++*+%2830000km%2Fs%29... - 63TJ is the Hiroshima explosion size.

[wongarsu]

I don't think it says anything that no aliens destroyed us yet. If such aliens would exist and such an event would have taken place in our solar system, that would still have been a long time ago. I don't think it's fair to assume that that alien race would just go around and kill all planet with live like a child stamping on ants.

It would be far more likely that they would be at least as sophisticated as humans, with emotional and strategic reasons, complex governments that change every now and then etc. By now they would probably be entirely different people from back then, just as our ethics evolve and and human empires rise and fall over a few centuries.

I don't think it's a very likely theory, but I don't see a general problem with it.

[roywiggins]

They're too busy trying to keep warm on Planet X, which has an extremely eccentric orbit that keeps them well out into the far reaches of the Solar System most of the time. They're kept warm by engineering a blanket of aeresolized gold particles in their atmosphere. They came to Earth the first time and enslaved us to mine for more gold. After the rebellion, they decided it was too hard to keep us under control and withdrew to their planet, which has since retreated into the cold depths. Soon, it will return and they will need more gold...

[This is all based off the nonsense Planet X theories that were going around. There was a good serialized YA novel that mined the ideas for plot, along with other Ancient Aliens nonsense, to quite good effect)