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<blockquote data-quote="Umbran" data-source="post: 6851017" data-attributes="member: 177">I don't find it credible that we'd find the first evidence of a past Earth civilization on *Mars*.&nbsp; A civilization advanced enough to cross interplanetary distances (so, major energy use, resource development sufficient to build spaceships, and so on) would have left evidence on Earth we would have found by now.The risk of what?Contagion?&nbsp; No, the risk *reduces* as the biology becomes more alien.&nbsp; The greatest contagion risk comes from things that are similar to what we already have (so it can take advantage of our biology easily), but to which our current immune systems are not responsive.&nbsp; Truly alien contagion would need to *just happen* to want an environment like our bodies, and *just happen* to be based on the same mechanisms as our systems, and so on.&nbsp; You are at more risk from a variant E. coli than from Marspox.The third case (alien visitation) does require the use of advanced technology for the site to be there, but does not imply that anything that we can actually study is present at the site. In sci-fi, The Ancients always seem to build technology that will work for millions or billions of years, but in reality?&nbsp; Entropy eats everything.Prior Earth civilization?&nbsp; Amazingly unlikely, as noted before - it would have needed to use many resources, but left no sign of that resource use, or anything else.&nbsp; If they didn't leave sign here, they didn't leave it there, either. <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f61b.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=":p" title="Stick out tongue&nbsp; &nbsp; :p"&nbsp; data-smilie="7"data-shortname=":p" />Mars Civilization?&nbsp; Mars probably hasn't had persistent surface water for a billion years.&nbsp; What artificial structure would last that long?&nbsp; Or, if they managed to become subterranean, why did they build this one thing on the surface... in a cave, and *nowhere* on the surface?&nbsp; This doesn't make much sense.</blockquote>

[QUOTE="Umbran, post: 6851017, member: 177"] I don't find it credible that we'd find the first evidence of a past Earth civilization on *Mars*. A civilization advanced enough to cross interplanetary distances (so, major energy use, resource development sufficient to build spaceships, and so on) would have left evidence on Earth we would have found by now. The risk of what? Contagion? No, the risk *reduces* as the biology becomes more alien. The greatest contagion risk comes from things that are similar to what we already have (so it can take advantage of our biology easily), but to which our current immune systems are not responsive. Truly alien contagion would need to *just happen* to want an environment like our bodies, and *just happen* to be based on the same mechanisms as our systems, and so on. You are at more risk from a variant E. coli than from Marspox. The third case (alien visitation) does require the use of advanced technology for the site to be there, but does not imply that anything that we can actually study is present at the site. In sci-fi, The Ancients always seem to build technology that will work for millions or billions of years, but in reality? Entropy eats everything. Prior Earth civilization? Amazingly unlikely, as noted before - it would have needed to use many resources, but left no sign of that resource use, or anything else. If they didn't leave sign here, they didn't leave it there, either. :p Mars Civilization? Mars probably hasn't had persistent surface water for a billion years. What artificial structure would last that long? Or, if they managed to become subterranean, why did they build this one thing on the surface... in a cave, and *nowhere* on the surface? This doesn't make much sense. [/QUOTE]

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