Starship Asterisk*

Everyone who hangs out at Starship Asterisk* is probably aware that a monster asteroid slammed into the Earth some 65 million years ago and killed all the dinosaurs. Because of the sheer drama of this event, asteroids have received a lot of attention as potential killers, and there has been a lot of discussion and speculation and even moviemaking on how to best avoid these monster bullets from the sky.





But I recently saw a BBC documentary, which I can unfortunately no longer access, that claimed that the worst killers of terrestrial life in the history of the Earth may have come not from the sky, but from inside the Earth itself. According to this documentary, scientists have searched in vain for geological evidence for other killer asteroids than the one that ended the reign of the dinosaurs on the Earth. It could be that major asteroid impacts are really very rare, so rare that the Chicxulub impactor may have been the only one to hit the Earth with catastrophic results after complex life began flourishing on our planet.
The BBC documentary claimed that the worst recorded extinction event, the Permian-Triassic extinction event some 250 million years ago, may have been caused by tremendous volcanic eruptions. Check out this page and scroll down a bit until you come to four videos, Introduction, Permian extinction explained, Freeze and burn and Volcanic monster. The last video, which I can't play, is about evidence of the world's biggest ever eruption in the Siberian traps. According to the BBC documentary that I saw, it was this monster volcanic eruption (or series of eruptions) that caused the Permian-Triassic extinction. All right, I guess the jury is still out on that, but I found it fascinating to consider the the idea that volcanoes may cause more havoc and change planets more profoundly than asteroids. (Check out what John Lambshead wrote about the Permian-Triassic mass extinction.)

So what about Mars and Venus? The latest news about Mars says that early Mars was more likely cold and icy than warm and wet. I guess the jury is still out on that one, but it should be noted that all other things being equal, a planet that is farther away from its parent star will be colder than one being closer to the same star, because the more distant planet will receive less sunlight. So for the sake of the argument, let's assume that early Mars really was cold and icy. Then we don't have to explain why it is so cold today, because it was (almost) always cold. What we have to explain is how Mars ever could get warm enough to melt enough of its ice to produce all those dried out lake beds and river beds.
A definite possibility is asteroid impacts. But what about volcanoes? Mars has volcanoes, yes indeed. I believe Olympus Mons, seen here, might be the largest volcano in the solar system. It is extinct now, of course, but a long time ago it must have been active. Back then, it must have had titanic eruptions and released huge amounts of carbon dioxide into the Martian atmosphere, quite likely thickening it and also adding powerful greenhouse gases to the atmosphere. It may even have drawn water from the depths of Mars and released it into the atmosphere as water vapor. Couldn't such enormous volcanic eruptions have changed Mars enough to melt a lot of its ice and created rivers and lakes on the surface of the red, but temporarily semi-blue, planet?

And now consider Venus. Venus is anomalously hot, hotter than Mercury, which is closer than Venus to the Sun. One of the few things we know about the surface of Venus is that it has a lot of volcanoes. Could it be that the thick, carbon dioxide atmosphere and the hellish temperatures of our sister planet have been caused by all its volcanoes?
Ann

It's a logical fallacy to say it's either this or that. In reality it's most likely a combination of everything. It doesn't even necessarily have to be one giant event at any given point. What if you've got a relatively powerful volcanic eruption going on that stirs things up for a few hundred years and then during that time a relatively large impact also happens? Either alone might not tip the balance but the combination of both at once might. When a history becomes complicated with multiple factors it also makes it difficult to interpret whatever evidence gets left behind.

Absolutely, Geck. It just seems to me that we talk a lot about asteroid impacts and craters, but we talk a lot less about volcanoes. When it comes to Olympus Mons on Mars, we are told about its gigantic size, but I can't remember hearing a discussion on how the formation of a volcano that size could have affected the climate of Mars. And what about the giant canyon of Mars, Valles Marineris? Something made that canyon form, and it wasn't a crashing space boulder. Couldn't the formation of such a feature have released enough heat to melt some ice on an otherwise frozen Mars?

Ann

Several correlations have been made on Earth, between large igneous provinces (LIPs) and mass extinction events:
https://en.wikipedia.org/wiki/Large_ign ... ion_events

Mars is supposed to be a lot less tectonic in structure than Earth, which probably goes some way to explaining why Martian volcanoes are larger. Have a look at this excellent presentation of the granddaddy of all LIPs, the Tharsis region on Mars:

PIA02820 [Public domain], by NASA/JPL (http://photojournal.jpl.nasa.gov/catalog/PIA02820), from Wikimedia Commons

I have no evidence that the Tharsis region caused any mass extinction events on Mars, but just looking at the picture above, it is hard to imagine that the region didn't have a major role in the geological evolution of Mars.

Comet and asteroid impacts have clearly played a role in the evolution of the solar system, too. Indeed, in the mess of the early formation of the solar system, the line between impact/accretion and volcanism was probably less well defined.