Why the two sides of the Moon look so different

One of the biggest questions in planetary science is why the side of our Moon that faces us has been extensively re-surfaced with lunar lava producing the so-called “maria” that give it a relatively smooth appearance; the far side of the moon on the other hand - also known misleadingly as “the dark side of the moon” (nothing to do with Pink Floyd) - is absolutely peppered with impact craters. Something must account for why the near side is much more geologically active than the opposite hemisphere. Well now scientists think they know. Measurements made from space have determined that the Moon’s mantle - which is the majority of the material inside - is warmer on our side than the dark side, possibly owing to more radioactive thorium being there. And that might be sufficient to make geological processes - like volcanism - occur more frequently on our side than the far side. In fact, as Caltech’s Alexander Berne explains, had the dinosaurs had telescopes, they might even have been able to witness recent lunar eruptions…

Alexander - One outstanding question with the lunar interior has been the very dramatic difference in the amount of volcanism which has occurred on the near side relative to the far side. We've flown past the far side several times. The first time was in 1959. One of the most striking things was the lack of the black mare basalts on the surface on the far side. And we've never really understood to what extent that asymmetry and structure extends to depth, how it evolved and whether it is associated with other important features of the lunar interior.

Chris - Because it is a striking aspect of the Moon, isn't it? The side that we see, which always faces towards us, is covered in a smoother surface, which we say is because of lava. The far side is pockmarked like it's been belted relentlessly with all kinds of things. And people are saying, well, why should it have two different faces like that? So what are the current theories as to why it looks different on one side to the other? Why might it look like that?

Alexander - Well, I should quickly preface my answer by saying that we don't know. There are two hypotheses for how the asymmetry formed. The idea that you had a large impact which redistributed material early in the Moon's history. The second hypothesis is that there was asymmetric mantle convection. So the mantle basically circulated a lot more on the near side than it did on the far side spontaneously in its early history.

Chris - How have you sought then, in this study, to try to get underneath what is going on on the Moon and some of the reasons why it might look the way it does?

Alexander - We've taken data from a mission that flew around the Moon about 10 years ago called the GRAIL mission. This was a NASA mission. It was actually two spacecraft that basically tracked their relative positions. So by taking that data of the relative positions of these two spacecraft as they both orbited the Moon, we're able to determine variations in the lunar gravity field. There are two components of that field. What's called the static component. This is the part that doesn't change in time. And then there are what are called temporal variations. And these are parts of the gravity field that actually fluctuate with time. And temporal variations in the gravity field occurred almost entirely due to tides. The Earth exerts a gravitational field on the Moon and that field deforms the Moon, basically changes its shape. And that change in shape moves mass around which then creates a time variation in the gravity field. So if you have a weak layer in the interior, for example, a warm nearside mantle, then that will create an anomalous signature in the lunar gravity field. So what we were able to do, we found an anomalous temporal signature in the lunar gravity field. We found that we're able to explain our anomalous GRAIL data if the nearside half of the mantle is about 100 to 200 degrees Celsius warmer than the far side.

Chris - Is it effectively, it's a bit like if I had a material like some plastic and I warmed it up, it would be more deformable when it's warm and less deformable when it's cold. So if you've got this temperature difference, it makes the Moon bend a bit more on one side compared to the other. And that's why you're seeing this gravity fluctuation because one side's bending and shifting more under the influence of gravity.

Alexander - That is almost exactly correct, yes.

Chris - Oh, you almost. So I almost nailed it. But why would there be that asymmetry?

Alexander - The origin of the asymmetry is still unknown. We don't know how material got redistributed on the surface. But what we can say is there's much more radioactive thorium on the nearside versus the far side. Once that thorium is in place, it's very easy to warm up the nearside mantle so much that it results in melting and convection of that side that doesn't occur on the far side.

Chris - Is this segregating like that and differentiating across towards the Earth-siding face because the Moon is what we call tidally locked? It always presents one face to us and then the opposite side out towards space. And is that why? Because it's in that one position, things can get drawn to one side and not the other.

Alexander - Yeah, that's correct. So we don't necessarily need for the redistribution to have happened in the present configuration. What could have happened is material was redistributed, it heated up the mantle, that heating up drove a mass, basically a mass anomaly, and the Moon reoriented in response to that mass anomaly. So the present day nearside may not have been the nearside when the mare or thermal anomaly in the mantle originally formed. It may have reoriented in the process of tidally locking.

Chris - But at the same time, what we've ended up with is a soft side and a harder side. And the soft side potentially might be more amenable to lava outflows which have resurfaced and smoothed out the side we see, but not the opposite side that's harder.

Alexander - That is correct.

Chris - And that's a hypothesis, obviously. Your data suggests that might be what's happened. But how can we test that? How can we find out if that is the reason that will explain this age-old mystery of why the Moon looks so weird?

Alexander - Just to be very clear, what we're finding is that in the present day, there is a nearside-farside variation in mantle temperature of about 100 to 200 degrees Celsius. So in our minds, that's very striking and suggestive that whatever process formed the nearside mare three to four billion years ago is still active today, still present in the deep interior. And the Moon may even be sporadically active volcanically. So there was a recent study that found that volcanism may have occurred as recently as 120 million years ago. So the short answer is by detecting this thermal anomaly, we have potentially tied together surface processes with deeper interior processes. And then the next step would be to actually identify the origin of this asymmetry way back at the beginning, whether it's due to an impact or spontaneous asymmetric differences in the amount of convection in the mantle.