February 28, 2009
There is a brief but vociferous debate about the value of human spaceflight over at Space Politics, under a discussion of the new NASA proposed budget. An often expressed opinion is that in general, humans contribute little to the scientific exploration of space. Indeed, my scientific colleagues often make this argument, largely in the hope that some cancelled human space program cash will magically work its way into their programs.
I’m sometimes asked what we learned by going to the Moon with Apollo (1969-1972). People vaguely remember that we brought back some rocks and maybe some remember that experiments were laid out on the surface of the Moon. But they don’t remember any great discovery on the Moon, like mountains of solid platinum or the uncovering of some new “unobtainium” element with anti-gravity properties.
Indeed, such was not found on the Moon. The Moon is rather ordinary in composition. It’s made of the same elements found in Earth rocks, although there are some interesting differences, such as the lunar samples are depleted in the so-called “volatile” elements, i.e., those with very low boiling temperatures. So the conventional wisdom is that we found nothing of value on the Moon.
In truth, we found true scientific gold. The rocks brought back from the Moon told us the story of the Solar System’s early history, details both surprising and astonishing. It was a time when planets collided and giant asteroids blew holes in planetary crusts hundreds to thousands of kilometers across. The outer part of the Moon completely melted, forming a global ocean of liquid rock. Our ideas about planetary formation and evolution had to be re-written from scratch after Apollo.
What does this have to do with human exploration? Because people went to the Moon, we now have a completely different view of how life has evolved on Earth. That’s a bold assertion, but I believe it to be true.
The lunar rocks are shaped by the process of hypervelocity impact and shock wave mechanics. These events leave tell-tale physical and chemical signs, learned through extensive laboratory experiments, field studies of terrestrial impact sites (like Meteor Crater, Arizona), and complex numerical modeling. Because we had done this work before Apollo, when the lunar rocks were returned, we knew how to read and interpret these signatures.
Now fast-forward to the early 1980’s. Geologist Walter Alvarez studies sediments in Italy, trying to figure out the rates at which limestone accumulates. Luis Alvarez, his father and Nobel-prize winning physicist, suggests to his son that he should use the concentration of the element iridium, which is rare in the Earth’s crust but common in meteorites, as a sedimentation “clock.” Because meteorites constantly rain down on Earth’s surface (including oceans) at a known rate, this meteoritic “sedimentation” is used to measure how fast the limestone accumulated.
When they applied this technique to the rocks, the Alvarezes got a big surprise. A huge spike in iridium concentration was found at the boundary (formed 65 million years ago), between the older Cretaceous rocks and the overlying Tertiary rocks, a boundary coincident with the extinction of more than half of all fossil species, including most famously, the dinosaurs.
To everyone’s astonishment, the iridium spike at this boundary is found worldwide. But the story gets even better: other iridium spikes are found elsewhere around the world at other rock sequence boundaries and many, if not all, are associated with mass extinctions, as evidenced in the fossil record. In fact, impact mass extinctions are a driving mechanism of evolution, as the disappearance of species opens an ecological niche for their replacement by new creatures, such as the rise of the mammals after the Cretaceous-Tertiary extinction of the dinosaurs.
What does all this mean? The lunar rocks melted by impact show enhanced amounts of certain iron-loving elements, including iridium. These elements are added to the rock during high velocity impact. It took years of painstaking study by geologists, chemists, and physicists to understand these distinctive features and diagnostic properties of the impact process. This knowledge was first applied to the Apollo lunar samples, which led to deciphering the impact history of the Moon. Later, this experience allowed us to uncover a wholly new and unexpected page in Earth’s geological history.
By going to the Moon, we gained a new perspective on the history of life on Earth and new insight into how the process of evolution actually works. Not a bad scientific return from a program whose real motivations were geopolitical, not scientific.
Why couldn’t this have been done with robot spacecraft? There are lots of answers to this question, but fundamentally, it boils down to the fact that we would never have gone to the Moon strictly for science. However, because we went to the Moon for other socially compelling reasons, exploration enabled science. Only human explorers, trained in the methods of terrestrial field science, were capable of finding and selecting geologically controlled samples, rocks for which we could reconstruct a context that made their stories understandable.
I’m not done with this topic. Next time, I want to look at the value of human spaceflight from a philosophical perspective.
The web editors have closed comments for this blog.