> To learn as much as possible before you irreversibly change it.
That's valuable too. But Mars's main value to humanity will be as a second home, not as a geologic curiosity.
We'll be much more secure as a species if we have a second planet where we can walk outside and breathe the air, even if the temperature will be stuck at ~250K.
There is some value to this argument if Mars turns out to have life on it. If this is the cas we should definitely hold off on the terraforming, which is why I hope it isn't.
So... we ruined this planet by making it less habitable, and we'd ruin Mars by making it more habitable?
I take it we're supposed to leave the entirety of the universe exactly as it happened to be before we realized that we're capable of rearranging very tiny little bits of it?
I imagine we would lose the ability to learn about certain aspects of the planet (atmosphere, etc) if we changed it, but would the trade off be worth it given everything else that would be infinitely easier to study? I'm asking because I don't really know.
Mars is very cold right now with almost no atmosphere, and it has remained relatively unchanged for billions (yes, billions) of years.
Terraforming it would entail thickening the atmosphere and heating it up. That'd cause a lot of changes to its surface. Water frozen for eons might start flowing, gases currently frozen in the ground might start evaporating etc.
Eventually it might start raining, which would start massive planet-wide erosion. There's a reason you see craters on the Moon and not on Earth: It rains.
To learn as much as possible before you irreversibly change it.