The International Space Station is growing mould, inside and outside

At the time, and for decades after, everyone assumed anything on the outside would dessicate and die immediately.

But, the universe has been getting more complex all the time - every stage of its history over billions of years has moved towards more things, more complex interactions, more abstract organizations - fundamental forces splitting off, plasmas, galaxies and stars, fusion and heavy elements, solids and liquids and planetary systems, chemistry tectonics and weather systems and day/light cycles, LIFE, multi-celled life, biological systems and their interaction with weather, communication, socialization, organization, co-operation, industrialization, it just gets more abstract and more involved.

Unless you think a God exists and put a barren radiated rock somewhere for a good reason, then you may as well conclude that the universe is trying to get more complex. Like you can say that humans moved grass over the ocean for our own benefit, you can also interpret it that grass got humans to move it over the ocean for grass's long term survival benefit. There is only one known life-o-sphere and if one-day it is going to outside Earth, it needs a highly intelligent species to appear and move all the rest of it. That could be us, it could be why it looks like we're screwing up the planet - because long term, the point is not for us to stay here as bucolic agriculturalists, but to be an ignition chamber for something (probably silicon) to burst out and go forth.

What makes it our job is that nobody else can. Whales cannot do it. Cows cannot do it. Oak trees and crickets and fungus and cockroaches and E.coli aren't going to. Only humans can escape Earth intentionally, so humans have the obligation to do that on behalf of all life.

Compare how many humans think life is important (a lot), with how many humans think it important that Mercury stay barren until it gets swallowed by the Sun at the end of its fuel supply (the number of people who have even considered the idea in all human history, rounds to zero).

What makes it important? WE make it important, because - in the absense of a deity giving orders to us - who else can give meaning to anything, except humans? If nothing else, a more populated universe would be more interesting for us. Eventually.

Earth is habitable now. There are plenty of plausible non-anthropogenic disasters that may make that so not so. Some typical examples include megavolcano eruption, coronal mass ejection, and asteroid impact. It would be good to not have all of our eggs in one basket here on Earth.

And it's for exactly the same reason of having redundancy that it would be a good thing for Earth to continue to remain habitable, i.e. we should stop ruining the planet through climate change.

It's not a moral hazard at all; the two outlooks are congruous.

No, it wasn't. Photosynthesis that released O2 evolved relatively quickly, and was more efficient than other forms of photosynthesis. Then it took another billion years for oxygen to accumulate in the atmosphere. Mind you, we had an entire planet with oceans teeming with photosynthetic life. That will be next to impossible to replicate on Mars.

There are two oxygen sinks on Earth that reacted with oxygen released from photosynthesis: the first was the ocean, the other was minerals on land. This is the reason that it took so long for atmospheric oxygen to accumulate. Mars will have a lot of oxidizable minerals and compounds that will react with oxygen before it can accumulate in its atmosphere. Even then, it will escape into space without a sufficient magnetosphere to trap an atmosphere.