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"theoretically, life could have started on Mars just as easily as it did here" Yes, leaving aside the 20% lower luminosity of the sun[1] a few billion years ag

"theoretically, life could have started on Mars just as easily as it did here" Yes, leaving aside the 20% lower luminosity of the sun[1] a few billion years ago (when Mars had a reasonable atmosphere) putting Mars outside the habitable zone, and Mars's lack of plate tectonics and a protective magnetic field: - Life certainly could have evolved on Mars. I expect that one day prion-like or viroid-ish molecules may well be found. But so what? It's one or two billion years from there to a bacterium, let alone a lost civilisation. And Mars did not have the time. [1] https://en.wikipedia.org/wiki/Faint_young_Sun_paradox https://en.wikipedia.org/wiki/Faint_young_Sun_paradox

davedx 6y ago
Your linked article actually says there is evidence Mars was warmer and wetter in the past. How then was it "outside the habitable zone"? And what proof do you have that life 4B years ago needed to be in what we define now as a habitable zone, needed a magnetic field, and needed active plate tectonics to arise? These are exactly the kinds of assumptions the article is challenging.
- tuatoru 6y ago
  Life didn't need those things to arise; that's just a matter of the right chemistry and physics being available for a relatively short period of time. But it would seem to need them for sustained evolution to be possible. So as I said, I'd expect lifeforms at the complexity level of viroids, but not at the level of eukaryotes. Evolution needs time, lots of time in which conditions don't change too much. (Chemistry that works in dilute solution at 300K doesn't work well at 200K or 500K.) It also needs populations to be able to spread to new areas. Without a magnetic field, Mars's atmosphere got stripped off. No more liquid water, no more areographical distribution, so no more evolution. Plate tectonics is needed to ensure that nutrient chemicals don't get locked up permanently in sedimentary rock. As for the habitable zone, life as we know it needs liquid water on the surface. Mars may have briefly been habitable, but not for long enough. To be a challenge, the article needs to provide plausible alternative evolutionary pathways that get us eukaryotes without needing these things (time, liquid water, chemical recirculation, limited variability in physcal conditions). It doesn't do that. It's just spitballing. If early Mars was OK for life to begin, then so was early Earth. We don't need any interplanetary travel.
  - davedx 6y ago
    Thanks for the detailed explanation, great points. I really wonder how the volume of liquid water on Mars has changed over the aeons. Considering there is still liquid water today, I would still be cautious about claiming "no more evolution on Mars". https://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars https://www.nasa.gov/press-release/nasa-confirms-evidence-th...

davedx 6y ago

Your linked article actually says there is evidence Mars was warmer and wetter in the past. How then was it "outside the habitable zone"? And what proof do you have that life 4B years ago needed to be in what we define now as a habitable zone, needed a magnetic field, and needed active plate tectonics to arise? These are exactly the kinds of assumptions the article is challenging.

tuatoru 6y ago
Life didn't need those things to arise; that's just a matter of the right chemistry and physics being available for a relatively short period of time. But it would seem to need them for sustained evolution to be possible. So as I said, I'd expect lifeforms at the complexity level of viroids, but not at the level of eukaryotes. Evolution needs time, lots of time in which conditions don't change too much. (Chemistry that works in dilute solution at 300K doesn't work well at 200K or 500K.) It also needs populations to be able to spread to new areas. Without a magnetic field, Mars's atmosphere got stripped off. No more liquid water, no more areographical distribution, so no more evolution. Plate tectonics is needed to ensure that nutrient chemicals don't get locked up permanently in sedimentary rock. As for the habitable zone, life as we know it needs liquid water on the surface. Mars may have briefly been habitable, but not for long enough. To be a challenge, the article needs to provide plausible alternative evolutionary pathways that get us eukaryotes without needing these things (time, liquid water, chemical recirculation, limited variability in physcal conditions). It doesn't do that. It's just spitballing. If early Mars was OK for life to begin, then so was early Earth. We don't need any interplanetary travel.
- davedx 6y ago
  Thanks for the detailed explanation, great points. I really wonder how the volume of liquid water on Mars has changed over the aeons. Considering there is still liquid water today, I would still be cautious about claiming "no more evolution on Mars". https://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars https://www.nasa.gov/press-release/nasa-confirms-evidence-th...