Prometheus (2012) Part II - "How Long's It Been Dead?"

Spoiler Level – Moderate

The Movie – Cave paintings from different times and cultures have a common message pointing scientists toward a distant planet where aliens might have engineered humanity.

While I think this movie has been overly bashed, there is certainly some terrible science here. What particularly bothers me is that I think fixing the science would fix more general problems with the movie and make it an all around better picture.

The Scene – While exploring an alien structure, the scientists discover a dead alien. One of the scientists takes a "carbon reader", probes the alien body, and concludes it's been dead for "2000 years, give or take".

First Law of Interstellar Relations: As soon as you encounter an alien species, you must immediately shove a probe into it. From Prometheus.

The Science – This is getting close to nitpicking, but I like having the opportunity to explain carbon dating. I think this is also a chance where fixing the bad science can help strengthen the movie as a whole.

Carbon dating is a common technique for figuring out the age of once living things. We are carbon-based, but there are multiple kinds of carbon. Carbon-12 is stable and common, while carbon-14 is rarer and radioactive. The latter decays at a known rate, where half of it is lost every 5,730 years. As long as something is alive and still bringing in new carbon through the food it eats, the ratio of carbon-12 to carbon-14 remains constant. But as soon as it dies, no new carbon is brought in. So if you know the ratio of carbon-12 to carbon-14 at the time it died and you can measure the same ratio in the dead sample, you can use the known rate of decay to calculate how long ago it died.

The tricky part here is figuring out what the starting ratio of carbon-12 to carbon-14 was. The ratio in a dead creature should be same as that in the atmosphere. That's because the carbon in animals come from plants (or animals that ate plants), and the plants get their carbon from the atmosphere (photosynthesis). But the carbon-12 to carbon-14 ratio in the atmosphere isn't constant. We can correct for this problem on Earth through various methods. We can look at the carbon ratios of samples with known ages, such as trees, which can be independently dated based on their rings. We can also find samples of ancient atmospheres trapped in ice cores.

The scientists in Prometheus have not done the analysis of this new planet necessary to have any idea what the carbon ratio was when the alien died. It's pretty clear that when they landed, there had not been any previous unmanned missions to collect basic data. Giving them the benefit of the doubt, the best they could have done was take a reading of the current atmospheric levels and assumed that the levels were constant. And this may not be terribly inaccurate as long as natural variations were the only things affecting the carbon ratio on this planet. However, if the aliens ever tested nuclear weaponry, then the variations could be massive.

Yeah, this will pretty seriously mess up your carbon dating calibration. Among other things... From Wikimedia Commons.

Nothing else is particularly wrong with the carbon dating shown here. Normally it takes some large and complicated equipment to make these measurements, but I'm willing to believe it can be done with a handheld device in the future. The alien is presumably carbon-based, so carbon dating is an appropriate technique to use. And 2,000 years is well within the time range that carbon-dating is useful for; it's actual effective for up to tens of thousands of years. I even sort of appreciate the use of the phrase "give or take" to acknowledge that these measurements always have errors. However, you should be wary of any movie when carbon dating is used to measure the age things that were never alive or things that are millions of years old. There are other ways of doing these things that use similar methods, but not with carbon.

Fixing the Scene – This is a pretty straightforward and something that would make logical sense for the movie as well. Before an expensive manned mission was launched, an unmanned probe that could collect basic information should have been sent. Supposing archaeologists discovered cave paintings from a variety of times and cultures that unambiguously pointed toward a specific star system, I would believe that a government or corporation would deem this enough evidence worth sending a probe to. And if the probe found a structure on that planet that was clearly made by an intelligent agent, then a manned mission would be important for further investigation. While the manned mission is on it's way, the probe could spend years collecting important data, such as samples necessary for calibrating carbon dating. This would solve a lot of problems for the movie. The manned mission makes more sense with this background. The hypothesis about alien engineers gains some evidence that would persuade other scientists that it's worth looking into. Once again, solving a science problem solves an overall logic problem with the plot.

Next Week – I'll finally get around to the biggest problem with this movie and discuss panspermia.