Quora: Can We Terraform the Moon? It's Complicated

The Moon
A passenger aircraft, with the full "Harvest Moon" seen behind, makes its final approach to landing at Heathrow Airport in west London, September 19, 2013. Toby Melville/Reuters

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Answer from Nicolas Nelson, space enthusiast:

Can the Moon be terraformed for humans? First of all, you have to understand the difference between Hollywood terraforming (or “space opera terraforming”) and pragmatic terraforming.

Hollywood/Space Opera terraforming involves a magical transformation of a moon or planet to look just like some idealized facet of Earth’s biosphere, usually a garden planet of bucolic sylvan loveliness (in which lurks certain death, or which is threatened with certain death, or there’d be no story worth telling). This is pure fantasy. Until humanity evolves godlike powers to remake the solar system and shape entire planets to our liking, that ain't happening, and the Moon would be a real challenge even for those future worldmakers.

So it's confusing when reputable scientists use the same word to describe something completely different and far less satisfying to the average person: real scientists are talking about a more pragmatic approach to terraforming that involves relatively slight and gradual changes to an existing planet or large moon that will make it “more Earthlike” only to a degree that makes it less inimical to human presence and activity, and sometimes even hospitable… but never actually like Earth.

So for example, no amount of pragmatic terraforming will increase the gravity of the Moon, or give it a breathable shirt-sleeve atmosphere. No amount of pragmatic terraforming will give Venus a 24-hour day. No amount of pragmatic terraforming will plant deciduous forests on Io’s unprotected surface.

But with a pragmatic approach to terraforming, you could initiate some relatively small changes that would make a big difference in some planetary habitats. Let's look at two examples, Mars and Triton.

Mars is an excellent candidate for pragmatic terraforming because it already has so many similarities to Earth: a 25-hour day, an atmosphere, plenty of water in the permafrost and geothermal brine, lots of interesting geologic activity that has produced a wide variety of useful minerals, and plenty of easily-accessible carbon and oxygen. It has slightly more than a third the gravity of Earth, perhaps not ideal, and the atmosphere is super thin, making flight difficult (but making weather very tame, which is a decent trade-off).

Most interesting, there are huge amounts of carbon-dioxide ice and frost locked in both the soil and the south polar cap. Mars is cold but not bitterly so, and a global temperature rise of only a few degrees can trigger the runaway sublimation of that CO2 ice into gaseous form, thickening the atmosphere enough to allow water to remain solid or liquid on the Martian surface, which would restore a surface hydrological cycle to the planet.

A Martian water cycle, a Martian “global warming”, remember. Still nothing like Earth. Still just 38% Earth gravity, still a very thin atmosphere, still way colder mean temperatures, a weak and arbitrary-seeming magnetic field… but at least the thicker atmosphere would block radiation much better, and the climate would be a bit less harsh even though storm winds would have a bit more force, and having liquid surface water during the long northern summer could have profoundly positive effects long-term, for human settlement and for possible Martian native habitat development (or renewal!) Plus it would allow for truly amazing scientific research opportunities with implications for both Mars and Earth climatology.

That's what pragmatic terraforming sounds like.

Triton is a very different case. The mean temperatures on Triton are so much colder than they are on Mars, our whole frame of reference has to shift downwards one order of magnitude, regarding the phase changes of substances.

  • Things that we think of as “normally liquid” we have to think of almost as minerals on Triton: it never gets hot enough to melt water, there, unless volcanic activity were involved, in which case liquid water would honestly act like lava on Triton.
  • Things we think of as “normally gaseous” on Earth we have to think of almost like water, on Triton: methane and ethane vapor forms clouds and condenses into snow, dew, and rain— rivers of methane flow into lakes and oceans. The kind of erosion that water does on Earth, hydrocarbon “gases” do that in liquid form on Triton. That means the mineralogy will be bizarre indeed, if liquid hydrocarbons, not water, are interacting with the elements in Triton’s crust the way water does on Earth and on Mars!

The idea of warming the surface or atmosphere of Triton to human norms is pretty much off the table as a terraforming approach. The temperature shift is just too enormous to attempt, and would catastrophically change the entire moon. Instead, human outposts or settlements would take advantage of the natural cryocooling for outdoor rocket propellant storage. The challenge would no longer be keeping it chilled to liquid form, but keeping it from being contaminated by other cryoliquids seeping into our storage tanks and ponds.

So, what “small change yielding great improvement” might we try, then?

One possible approach is to drill down to Triton’s liquid-water oceans (if indeed they are down there under the thick icy crust) and warm a deep inner thermocline there, as if we were establishing an ocean-floor colony on Earth. Oddly, this would be analogous to aliens from a super-hot homeworld arriving on Earth and drilling down to establish a peaceful colony within our molten mantle! Crazy, yes, but it's an idea we can test right here on Earth under the North polar cap. Convenient to be able to do basic proof-of-concept testing in a decently analogous environment without leaving Earth at all. “Pragmatic terraforming” in this scenario would mean small changes to Triton’s “ocean floor” to make it similar enough to, say, our North Polar Seafloor, to become a habitat for terrestrial sea life.

I hope this gives you a very different, more pragmatic understanding of what real terraforming is. And even pragmatic terraforming is by no means easy: with current technology it is impossible, but with our current level of scientific understanding it is possible indeed, even plausible; that is to say, we don't have the tools to pragmatically terraform Mars or Triton now, but we do have the tools and knowledge to make those terraforming tools and infrastructure, if we found the funding for it.

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