Since when? Or rather: Still? Its not even clear wether there was at one time life on Mars, and even if there was it was not beyond monocellular level and is extinct.
Several points, Susano:
1: It is fairly clear that the early Martian environment was similar to the early Earth environment, especially in certain areas useful to the formation of life (eg., warm(-ish), wet). Given the seeming rapidity with which life formed on Earth after the end of the LHB, there are good, if indirect, grounds for believing it likely, at least, (which is all he says) that there was some form of life on Mars at some point, even if it perhaps never advanced past naked molecules to actual cells.
2: No one is disputing that it is very unlikely that there was multicellular or even eukaryotic life on Mars. The habitable phase that would allow for that kind of complex development probably didn't last long enough.
3: It is not certain (albeit extremely likely) that Martian life is extinct, as extremophile life forms on Earth have shown that life can exist in surprising and difficult-to-study habitats, such as
deep underground. Looking at what lithoautotrophs (the type of bacteria just referenced) need to survive, and where they sometimes live, it seems fairly clear that they could easily survive on Mars, not even necessarily aware of the vast changes on the surface since they evolved billions of years ago. Obviously, our current probes have not, and cannot, probe kilometers into the bedrock to detect whether or not there are small communities of bacteria living there.
The recent discovery of anomalously high methane levels on Mars further supports the idea that there may be remnant habitats suitable for and inhabited by ancient bacterial lifeforms, as that gas is mainly released by volcanic and biological activity. Mars, then, must either be more volcanically active (and thus more habitable) or more inhabited than we had previously thought.
Antipater said:
Quite a response for my first thread!
Anyway, there seems to be a lot of disagreement the right amount to spend on space exploration; everyone agrees there will be scientific benefits, but cost seems to be the issue. So at what point would this (theoretically) no longer be an issue? Will it ever be feasible for space missions to pay for themselves, and therefore be viewed as an investment?
You would need much lower costs. Launch costs are frequently bandied about, eg. the $10,000/kg cost the Space Shuttle supposedly incurs (although the question of actually calculating costs is rather complicated), but the real showstopper is the cost of payloads--
Cassini, for instance, has cost about $3.3 billion, but the launch vehicle "only" cost about $420 million, less than 1/6th of total costs. (I expect that less specialist payloads such as GPS satellites, remote observation satellites, or communications satellites would be cheaper in relation to the launch vehicle, but it's much harder to find information, especially for the commercial ones). For another perspective, per-launch costs have clearly gone down since the late '60s or early '70s due to inflation, but there hasn't been a massive explosion in the use of space like some predicted, or as looked imminent in the mid-'90s before the massive fiber build-out that seriously damaged the communications satellite market.
Anyways, it's proven difficult to get lower costs due to the harsh environment and the high cost of replacement if the satellite should fail, meaning that people spend a lot of money to build something that can work reliably in that environment.
And finally, obviously some space missions
do pay for themselves, since otherwise some businesses couldn't exist. It's just not the
crewed missions that do that, since there you have to pay for all the life-support. I think you might be waiting on actual AI before you can get something akin to humans on Mars, at least in terms of intelligent "life" there that talks to us. But otherwise...well, monkeys in a can is not necessarily the best way to do things, is all, and very expensive. Plus, some of the best long-term investments are a long, long way from being useful right now (eg., asteroid mining requires a lot of R&D for the microgravity vacuum processing and production facilities; many, many, many things that work on Earth just wouldn't in space).
Antipater said:
Space tourism seems like the most obvious way for this to happen, but, that can't be sustainable in the long run. That leaves mining for valuable minerals.
So, what will it take for space exploration to pay for itself? Severe natural resource depletion at home, or just advances in technology?
Erm, not necessarily. You forgot to mention things like research or manufacturing (of products that require ultra-super-mega high vacuum or microgravity conditions. As I said, launch and payload costs will be key. So, for profitable (crewed) space missions, you would need high prices on Earth for whatever is going on in space, lower costs for actually going off and doing whatever, and an inability to automate it (completely). Which is possible--AI is tricky, especially the brute-force "simulate the brain!" is certainly decades or more away*--but isn't likely for a while, say several decades at least.
* I spent this summer doing computer simulations of (tiny segments of) two proteins binding to each other. Now, obviously any decent whole-brain simulation is likely to need to take into account protein behavior to function properly. It took about 3 months of work to produce about 20
nanoseconds of simulation. Of two tiny (less than 100 residues, that is amino acids) protein hunks interacting, with nothing else around. Scale that up to a whole brain operating for hours or days...
