21 December 2012

Fire or Ice? Options for the Apocalypse

Posted by Ryan

Image Credit: Unknown (Google Image Search)

Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.

-Robert Frost

Well folks, this is it. As of tomorrow, December 21, 2012, we will reach the end of the current b’aktun of the Mayan Long Count calendar. And then, well, you know what will happen.

That’s right, we’ll start the next b’aktun! And also, the world will not end. That said, I thought it might be instructive to take a look at our neighboring planets, Venus and Mars, which have actually met their apocalyptic fates. One ended in fire, the other ice.

Let’s start with Mars, which ended in ice. By now, with all of the various orbiters, landers and rovers that have been sent to the red planet, we know that once upon a time, Mars was a place where water flowed on the surface, and pooled to form lakes or perhaps oceans. The problem is, that couldn’t happen on Mars today. The average surface temperature on Mars these days is around -81 degrees Fahrenheit (-61 C), and on a hot day near the equator the temperature might barely creep above freezing. Not only that, but Mars has lost its atmosphere. The current pressure is about 1% the atmospheric pressure here on Earth. With such frigid temperatures and low pressures, any liquid water on the surface would exist only briefly, boiling away even as it froze solid. The only water left on Mars today is in the form of ice. Some is stored in the polar ice caps, some is stored in glaciers, and some is buried under the surface. What happened to make Mars so cold and inhospitable, if it once had lakes and rivers? Well, first I should clarify: nobody ever said Mars was a nice place to live. It’s entirely possible that it has always been quite colds and dry, with only intermittent periods of liquid water, lakes, and all the rest. Still, it’s hard to argue with the geomorphology: there used to be liquid water, at least occasionally,and now there isn’t.

That’s because Mars lost its magnetic field. It’s a smaller planet than the Earth, so it cools more quickly, and a planet’s magnetic field depends on convection from the hot core to the cool surface. If the core gets too cold, convection stops. Without a magnetic field, Mars has been exposed to the full force of the solar wind, which strips away atoms in the upper atmosphere, and over billions of years has reduced the atmospheric pressure to a fraction of what it once was. Without an atmosphere, not only is water unstable, but the surface is bathed in UV radiation from the sun, making it even less hospitable for life. Any life that got its start on Mars likely had to flee underground, following the remaining liquid water water and residual warmth of the dying planet.

Staying warm is hardly a problem on Venus though! Our sister planet is just slightly smaller than the Earth, and is closer to the sun, so you might expect it to be warmer. You would be very, very right. The average surface temperature on Venus is about 860 degrees Fahrenheit (460 C), thanks to monstrous carbon dioxide atmosphere with a surface pressure 93 times that on Earth. All that CO2 causes a runaway greenhouse effect that traps the sun’s energy and gives Venus the highest average surface temperature in the solar system.

Venus wasn’t always so hellishly hot. Based on measurements from Venus Express, we know that Venus once had water. Venus Express has detected an excess of heavy hydrogen atoms called deuterium in the upper atmosphere. Since all the planets formed out of the same original star stuff, they should all start with the same fraction of heavy and normal hydrogen isotopes. The excess heavy hydrogen on Venus means that some process has selectively been removing lighter atoms from the atmosphere. Enter, the solar wind. Venus also lacks a protective magnetic field, and just like Mars, the solar wind has worked diligently at stripping away its atmosphere. But Venus is a bigger planet than Mars, so it can hang onto atoms like carbon and oxygen, keeping it swaddled in its thick cozy carbon dioxide blanket. Light atoms like hydrogen can get stripped away by the solar wind though, and since regular hydrogen is twice as light as heavy hydrogen, it gets stripped away much faster.

Without hydrogen, you can’t have water. Once, long ago, Venus might have had liquid water and temperatures that were pleasantly warm. But as the early Sun got brighter, any liquid water evaporated and joined the CO2 in the atmosphere to act as a greenhouse gas. The warmer the planet got, the more water evaporated, leading to more warming and a runaway greenhouse effect. To top it all off, after Venus reached a certain temperature, it because too hot for carbonate rocks to form, preventing any CO2 in the atmosphere from being trapped as rocks and removed from the system. So Venus ended up as a planetary oven, its scorching, volcanic surface hidden from view by beautiful sulfuric acid clouds.

Any life that arose in the early lakes or seas on Venus would have had to adapt to live in those caustic clouds to find a pleasant temperature and pressure, and the loss of almost all of the water from the planet makes even the clouds an unlikely haven for venusian microbes.

Robert Frost might have been amused to learn that our neighboring planets ended in fire and ice, respectively. As for the end to our own planet? Well, it certainly has nothing to do with the Mayan calendar. Our planet hit the cosmic jackpot and exists in a sweet spot that allows it to have large oceans. These oceans buffer the climate and help to keep it pleasant for life, although our climate has been known to change significantly as continental drift changes the ocean currents. Our planet is large enough that the core hasn’t cooled, and so our magnetic field protects us from the ravages of the solar wind. These factors make Earth a decidedly nicer place to live than our neighbors. The biggest immediate question about how our world will end is what we humans choose to do with the finely-tuned planet that we have been given. We are already seeing the climate effects of human-made greenhouse emissions, but whether these effects last for just a few hundred years, or whether they trigger larger changes in the planet’s climate is yet to be seen.

As disastrous as climate change could be for humans and many other species, it will take more than that to wipe out all life on the planet. Barring a massive impact, or a nearby supernova, it is likely that some form of life will survive on the Earth for billions of years, until the sun swells into a red giant and consumes the planet. Ultimately, the world will end in fire. But that’s a long way off. Maybe before then, some form of life from Earth will find a way to become a multi-planet species, so that the loss of a single planet, even one as nice as the Earth, won’t mean the end of all life.

At the very least, I expect those future spacefarers won’t get so worked up when they need to get a new calendar.