A planet spanning civilization on Mars is less likely. However, there are still ways it could be found, depending on how long ago it was. I list three ways belo

A planet spanning civilization on Mars is less likely. However, there are still ways it could be found, depending on how long ago it was. I list three ways below, and suspect there are more. First, there is the possibility of a civilization dying and evidence being erased by tectonic plate recycling. Mars is approximately 4.6 billion years old. We know it has volcanic activity, and most consider it to have inactive tectonics now. Some scientists believe it could have had significant activity in the past, based on the 3 distinct regions of Mars. Earth's tectonic plates vary in their rates of recycling: the oldest non-oceanic tectonic plate is 3.6 billion years old, and shows no sign of recycling. Oceanic plates recycle on the average of every 180 million years though. Second, there is the possibility of technology that didn't leave a visible trace. This is not supported by any evidence and is mere conjecture. If the society tore down non-organic structures and converted to using all biodegradable/organic materials then evidence of the society would be scarce, if existent at all, especially if bodies were cremated as many do today. Chemical analysis could detect this I suspect, or more importantly prove it wrong. Last, if for some reason civilization evolved underground and stayed underground, then there wouldn't be any visible evidence above ground. This is also pure conjecture. Micro-robots that explore Mars' caves might find something, as might ground penetrating radar.