Almost every sci-fi story begins (and sometimes ends) with the terraforming of Mars to turn it into a more hospitable world.
But with its frigid temperatures, remoteness from the sun and general dustiness, changing Mars to be more Earth-like is more challenging than it seems (and it already seems pretty tough).
A dead world
The thing is, Mars used to be cool. And by cool, I mean warm. Billions of years ago, Mars had a thick, carbon-rich atmosphere, lakes and oceans of liquid water, and probably even white fluffy clouds. And this was at a time when our sun was smaller and weaker, but occasionally much more violent than it is today — in other words, our solar system is a much more favorable place for life now than it was 3 billion years ago, and yet Mars is red and dead.
Sadly, Mars was doomed from the start. It's smaller than Earth, which means it cooled off much faster. The core of our planet is still molten, and that spinning blob of iron-rich goo in the center of Earth powers our strong magnetic field. The magnetic field is a literal force field, capable of stopping and deflecting the solar wind, which is a never-ending stream of high-energy particles blasting out of the sun.
When Mars cooled off, its core solidified and its magnetic force field shut off, exposing its atmosphere to the ravages of the solar wind. Over the course of 100 million years or so, the solar wind stripped away the Martian atmosphere. When the air pressure dropped to near-vacuum, the oceans on the surface boiled away and the planet dried up.
It's so tantalizing: Mars was once Earth-like, and so is there any way to bring it back to its former glory?
Thankfully (or unfortunately, depending on your point of view), we humans have plenty of experience in warming up planets. Inadvertently, through our centuries of carbon emissions, we've raised the surface temperature of Earth through a simple greenhouse mechanism. We pump out a lot of carbon dioxide, which is really good at letting sunlight in and preventing thermal radiation from escaping, so it acts like a giant invisible blanket over Earth.
The increased heat encourages moisture to leave the oceans and play around as a vapor in the atmosphere, which adds its own blanketing layer, adding to the increase in temperature, which evaporates more water, which warms the planet more, and before you know if prime beachfront property is now better suited as an underwater submarine base.
But if it works on Earth, maybe it could work on Mars. We can't access the OG Martian atmosphere, because it's completely lost to space, but Mars does have enormous deposits of water ice and frozen carbon dioxide in its polar caps, and some more laced just underneath the surface across the planet.
If we could somehow warm the caps, that might release enough carbon into the atmosphere to kick-start a greenhouse warming trend. All we would need to do is kick back, watch and wait for a few centuries for physics to do its thing and turn Mars into a much less nasty place.
Unfortunately, that simple idea probably isn't going to work.
The first issue is developing the technology to warm the caps. Proposals have ranged from sprinkling dust all across the poles (to make them reflect less light and warm them up) to building a giant space mirror to put some high-beam action on the poles. But any ideas require radical leaps in technology, and a manufacturing presence in space far beyond what we are currently capable of (in the case of the space mirror, we would need to mine about 200,000 tons of aluminum in space, whereas we are currently capable of mining … well, zero tons of aluminum in space).
And then there's the unfortunate realization that there isn't nearly enough CO2 locked up in Mars to trigger a decent warming trend. Currently Mars has less than 1% of the air pressure on Earth at sea level. If you could evaporate every molecule of CO2 and H2O on Mars and get it into the atmosphere, the Red Planet would have … 2% of the air pressure on Earth. You would need twice as much atmosphere to prevent the sweat and oils on your skin from boiling, and 10 times that much to not need a pressure suit.
Let's not even talk about the lack of oxygen.
To counter this lack of easily accessible greenhouse gases, there are some radical proposals. Maybe we could have factories devoted to pumping out chlorofluorocarbons, which are a really nasty greenhouse gas. Or maybe we could shove in some ammonia-rich comets from the outer solar system. Ammonia itself is a great greenhouse blanket, and it eventually dissociates into harmless nitrogen, which makes up the bulk of our own atmosphere.
Assuming we could overcome the technological challenges associated with those proposals, there's still one major hurdle: the lack of a magnetic field. Unless we protect Mars, every molecule that we pump (or crash) into the atmosphere is vulnerable to getting blasted away by the solar wind. Like trying to build a pyramid from desert sand, it's not going to be easy.
Creative solutions abound. Maybe we could build a giant electromagnet in space to deflect away the solar wind. Maybe we could girdle Mars with a superconductor, giving it an artificial magnetosphere.
Naturally, we don't have nearly the sophistication to realize either of those solutions. Could we ever, possibly, terraform Mars and make it more hospitable? Sure, it's possible — there's no fundamental law of physics getting in our way.
But don't hold your breath.
Learn more by listening to the episode "Could we really terraform Mars?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.
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Paul M. Sutter is a research professor in astrophysics at SUNY Stony Brook University and the Flatiron Institute in New York City. He regularly appears on TV and podcasts, including "Ask a Spaceman." He is the author of two books, "Your Place in the Universe" and "How to Die in Space," and is a regular contributor to Space.com, Live Science, and more. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy.