(Image: NASA--These are mountains at the northern ridge of the South Pole-Aitken basin)
The Moon might be our closest neighbor in what we think of as space, but we are constantly coming across things that let us know how little we really know about it--and how much solving these mysteries can help us understand our own planet Earth.
Recently, scientists used some data from spacecrafts used for NASA's Gravity Recovery and Interior Laboratory mission, otherwise known as GRAIL, to measure small changes in gravity around the Moon. They then used lunar topography (measuring the lay of the land, like peaks and valleys, etc) data recorded by the Lunar Reconnaissance Orbiter to see if there was a correlation between gravity and the topographical features. Lo and behold, they did find something.
Put in its most basic form with no math involved, you can think of the force of gravity as the result of two masses interacting with each other across a distance (that's a super simplification, but it works for our purposes here). Two bodies of mass have some value of gravitational pull on each other--it's just very small for smaller masses, and becomes weaker the further those masses are from each other. So the equipment NASA and the scientists doing this study measured the pull of the Moon's gravity on the equipment. As they surveyed the moon's surface, nothing big stood out, until they got to the area of the South Pole-Aitken basin.
(Image: NASA/University of Arizona--This is a false-color image of the South Pole-Aitken basin. The blue colors are places that are low (like valleys, or in this case, craters), and the red places are the highest (mountains, etc). The purples are the lowest spots. The circled area is where the anomaly is.
DISTRACTED MEL MOMENT: Wait, what's that? You don't know what the South Pole-Aitken basin is? Well, we have to fix that, because, honestly, it's a REALLY cool feature on our Moon. The South Pole-Aitken basin is a crater--a huge crater--on the moon made about 4 billion years ago. It's not the biggest crater in the solar system. But it is considered to be one of the best preserved craters in the solar system. What this means is that other repeated impacts and/or erosion haven't happened here, and so the surface of the crater is much the same as when it was formed so long ago. This makes it an interesting place to study and learn about all kinds of things, including planet formation, impact effects, etc. China landed a spacecraft here at the beginning of 2019. Peter B. James, PhD., assistant professor of planetary geophysics at Baylor University says this basin is "one of the best natural laboratories for studying catastrophic impact events, an ancient process that shaped all the rocky planets and moons we see today." We can't see this basin from Earth, sadly, because it's on the far side of the Moon, the part that always faces away from us. But we've seen it via spacecraft. Why has it been so well preserved? Because it's toward the "bottom" of the moon, and most impacts happen in the orbital plane of the solar system versus the tops and bottoms of planets and moons.
(Image: NASA. The dark area at the bottom is the South Pole-Aitken basin. It is about 2,000 km at its widest point, the distance between Washington DC and Waco, TX. It's a darker color than the surrounding area because it's a smoother surface--not as many newer impacts there as on the rest of the moon. The same can be seen in the darker spots facing the Earth--those are places where impacts haven't happened after the big basin/crater was formed)
OK, where was I? Oh, yes. So they measured the gravitational pull near the South Pole-Aitken basin, and found it was stronger than on other places on the Moon. Instead of this (what they expected, same gravitational pull around the whole Moon):
(fairly constant gravitational pull)
Scientists discovered THIS:
What does this mean? According to the gravity formula Newton came up with, F = [(Gm1m2)/r^2], this effect suggests that there is a big body of mass hundreds of miles underneath the Moon's surface in this spot. How big?
"Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground. That's roughly how much unexpected mass we detected," says Peter B. James, PhD, the lead author of the report in Geophysical Research Letters.
What could this mass be? Did we finally find the lunar monolith from Arthur C. Clarke's 2001: A Space Odyssey? (cue music)
Well, probably not. One likely explanation for this mass is that the metal from the asteroid that hit the Moon and created the basin in the first place is still embedded/dispersed in the Moon's mantle, instead of sinking into the Moon's core and being melted and distributed among the matter there. Scientists performed computer simulations to test this theory, and found that under the right conditions, this could indeed be possible.
Another theory is that this large mass is the remnant of dense oxides (types of heavy metal rust, basically) that was formed when the last of the liquid magma ocean beneath the surface of the Moon cooled and solidified. This theory isn't as exciting, maybe, as the first, but it's still something to work with.
OK, so what? Why is this important and how does it impact your life as a tax-paying citizen? Why should you want to help fund research into this? Well, there are a ton of reasons, depending on where your interests lie. Researching this will shed light on how planets, including our own, were formed. This in turn will lead to new knowledge about how conditions formed that made life possible. It can also help us understand the effects of giant impact events on planets, so we can be better prepared for one when it happens on Earth. It can help us understand more about the cooling process of planets--Earth is still in the process of cooling, and will some day, long after humans as we know them are probably extinct, be a cold dead planet like the Moon or Mercury. We can learn about the effects of cooling on us today, on how our food grows, on how our environment is changing. We might be able to predict the weather differently, learn how animals migrate, etc, all based on how matter moves through the Earth. There are definitely applications for the knowledge we can glean from this discovery.
So, what do you think? Any questions? Concerns? Ask away!
NOTE: This content is free right now here and on my Patreon because I really feel strongly that the world needs access to science. We need a better educated populace, we need people to know about the world they live in. But if you had $2 a month to throw at me, I could put that toward my own education so that I can help spread the most accurate science I can. I would love your help to make the world a better, more educated, place.
Melanie R. Meadors is an author of fantasy, a role playing game designer/editor, and a blogger with The Once and Future Podcast and GeekMom. She also loves science, sewing, music, comics, DMing for a couple of gaming groups…In other words, she’s a professional nerd. You can learn more about her at her website. And if you like what she’s doing here, please support her Patreon!