| I'm not a geologist much less an astro-geologist so I can only offer what I've picked up on this topic through curiosity along the way but here is my understanding: Tectonic movements are caused by temperature, density and other material differences, upwelling as you've mentioned and I've also heard that gravitational influences are important, effects of the moon, sun and earth gravity vs the density layout causing strain. My understanding of the Mars issue is that the planet cooled off much faster than the Earth did, as a result there isn't as much thermal induced stress and upwelling going on. As for why Mars cooled off faster, I don't think anyone knows definitively. Mars has very little atmosphere so it doesn't retain much heat. It is also much smaller than the Earth so it would be expected to cool off faster under equal conditions. On the other hand Venus is also smaller than the Earth but is one seriously hot place, and not completely due to its proximity to the Sun. So why doesn't Mars have an atmosphere? Well, it has little to no magnetic field so it is very poorly protected from solar winds, a massive solar ejection could have just ripped the atmosphere off. On the other hand maybe Mars used to have a magnetic field and it was only lost due to cooling effects that occur even with an atmosphere. Using Venus again as a counter point is also has nearly no magnetic field yet it maintains an extremely dense atmosphere so there is no 1:1 correlation with not having a magnetic field and not having an atmosphere. I don't know of any consensus on what it takes during planet formation to end up with an atmosphere to retain heat and a magnetic field to protect that atmosphere, I've heard lots of theories but its still an open question to be answered. |