It's been nearly 40 years since astronauts returned the last Apollo samples from the Moon (and 35 since Luna 24 brought back 170 grams from Mare Crisium). Since then several orbiting spacecraft have mapped the lunar surface from top to bottom, repeatedly. So a casual observer might conclude that we've learned everything there is to know about the Moon.
Ha! If anything, questions about how Earth's satellite formed and evolved are more numerous than ever. As evidence, I submit the following summaries of research that's been published in the past few weeks. Let's start by asking, as a trio of researchers have recently posed, "Why do we see the 'Man in the Moon'?"
A Curious Countenance
It's our luck, good or bad, that we see the same lunar hemisphere all the time. The Moon circles Earth in what's called synchronous rotation, making one rotation during each orbit. The reasons for coincidence are now clear. The Moon's globe is slightly oblong, shaped like a football, and long ago the incessant tug of Earth's gravity forced the long axis to point perennially inward. (Technically, we can see about 60% of the lunar sphere due to slight nodding motions called libration, but that's another story.)
Yet planetary dynamicists believe that long ago the Moon must have been spinning much more rapidly and been much closer to us. Earth's gravity must have created tidal bulges in the lunar interior, causing it to heave and sag as it spun around. This tidal energy sapped the Moon's rotational energy, slowing it down until it became locked in the same-face-inward configuration we see today. But why, Oded Aharonson, Peter Goldreich, and Re'em Sari ask, is the half dominated by dark maria facing inward — as opposed to the crater-dominated far side?
As they describe in a forthcoming issue of Icarus and at a conference next week, it's all about how quickly the Moon's rotation slowed down. If the despinning had been relatively rapid, then the odds of the "Man in the Moon" facing us would have been about 50:50 — basically a coin flip. But because the slowdown was very gradual, the odds favored what we ended up with by about 2:1. (When they first tackled this problem a couple years ago, the trio thought it might have been as high as 3:1.) "The coin was loaded," Aharonson quips in a Caltech press release.
But don't consider this a "case closed." For one thing, geologists aren't sure which came first — the tidal spin-down or the formation of the maria. And other factors might have influenced the hemispherical coin flip, such as a big collisional whack relatively late in the game or even a wholesale redistribution of the lunar crust.
When Apollo astronauts returned from the Moon, surprised geologists found that some lunar stones were strongly magnetized. A rock needs two things to get that property. First, a strong ambient magnetic field must exist. That's no longer the case on the Moon, though conceivably one existed billions of years ago. And, second, the rock must contain considerable iron or some other magnetically susceptible mineral. That's problematic for lunar samples, which have little iron in them.
Yet the Moon rocks are magnetic, and how they got that way has puzzled researchers for decades. To duplicate the strong remanent magnetism found in the lunar samples would take some wildly implausible crustal geology: either a solid layer of highland rock at least 60 miles (100 km) thick, or giant slabs of mare basalt far thicker than the ones now present.
New clues have emerged thanks to a global magnetic map acquired by Lunar Prospector more than a decade ago. In the March 9th issue of Science, researchers Marc Wieczcorek, Benjamin Weiss, and Sarah Stewart explain that some of the strongest magnetic anomalies lie along the northern rim of a giant far-side basin named South Pole-Aitken. Some 1,500 miles (2,500 km) across, "SPA" is the biggest, deepest, and oldest impact feature on the Moon and arguably in the entire solar system. Notably, this giant hole is oval in shape, not circular, suggesting that the impactor struck the Moon obliquely and sprayed debris predominantly northward.
Wieczcorek and his collaborators believe that material thrown out during the basin's formation was highly magnetized, which would explain the magnetic deposits on SPA's north rim and elsewhere. But what's its composition? The lunar crust contains too little iron, so the three researchers postulate that the impactor itself was the source. If SPA formed roughly a half billion years after the Moon's formation, while a magnetic dynamo still churned in the deep interior, then the impact could have splashed the entire lunar surface with iron-rich debris that locked in the magnetic field as it cooled.
This hypothesis might seem a bit far-fetched, especially because geophysicists think it's a stretch that the lunar dynamo lasted so long, but it's far more reasonable than a 100-mile-high stack of highland rock. Because it likely dredged up material from the Moon's upper mantle, the SPA basin is a strong candidate for a future sample-return mission. Careful analysis of rocks plucked from the basin's floor could resolve this magnetic mystery — not to mention the earliest chapters of lunar history — once and for all.
The truth is that planetary scientists really don't know much about the Moon's first half billion years of history. It was erased by a savage bombardment, as countless chunks of rocky leftovers from the planets' assembly pummeled the lunar surface. Many of the samples gathered by Apollo astronauts date back to 3.9 to 4.0 billion years ago. So either the incessant rain of primordial impactors came to a fairly rapid end, or the Moon endured a cataclysmic pulse of collisions (dubbed the "late heavy bombardment") at that time.
Lunar specialists have debated about these two scenarios for decades. However, thanks to the exquisite imagery provided by NASA's Lunar Reconnaissance Orbiter Camera (LROC), research teams can place better constraints on what happened and when.
For example, Simone Marchi, a postdoc at NASA's Lunar Science Institute, led a team that carefully examined craters found on or near the Nectaris basin, a near-side feature that's 520 miles (860 km) across. Marchi's team found that the Nectaris-era craters average 30% to 40% larger than those found in older comparable populations. The most logical explanation is that when Nectaris formed, roughly 3.9 billion years ago, the incoming projectiles were hitting twice as fast as those that left craters on more ancient terrains.
As the team reports in April's Earth and Planetary Science Letters, "This dramatic velocity increase is consistent with the existence of a lunar cataclysm." As noted in a NASA press release, the upturn in impactor velocities seems to have occurred after the creation of the South Pole-Aitken basin but before the appearance of most of the other big near-side basins.
Where all those LHB impactors came from isn't clear, but higher velocities imply a source in the asteroid belt or farther out — a cascade perhaps triggered by an abrupt reconfiguration of the giant planets' orbits.
Meanwhile, another research team has focused on the geology in and around the Apollo 17 landing site. Many of the samples returned by astronauts Gene Cernan and Harrison "Jack" Schmitt were basaltic rocks that had been molten when they were blasted out of some huge, distant impact before plopping down onto the lunar surface and solidifying. Because the landing site was right next to Mare Serenitatis, 420 miles (675 km) across, most researchers have believed pointed to the Serenitatis impact as the samples' source.
But it's not that simple, say lunar specialists Paul Spudis, Don Wilhelms, and Mark Robinson. As the trio detail in a recent issue of Journal of Geophysical Research, there's conflicting evidence as to whether Serenitatis is old or young, as lunar basins go. Their analysis of LROC images shows that the Sculptured Hills, knobby outcrops, extend far beyond the spot where Apollo 17 touched down. More likely, they conclude, they are a consequence of the formation of the Imbrium basin.
Now, here's the conundrum. Imbrium is thought to be 3.84 billion years old, whereas the Apollo 17 samples largely date to 3.89 billion years. If the Apollo 17 site is really awash with rocks ejected by the Serenitatis blast, then these two giant basins, along with up to 25 others thought to be between them in age, must have formed in a very narrow, 50-million-year-long window. As they note, this would have been "truly an impact 'cataclysm' in its most extreme form."
Conversely, if the Apollo 17 rocks came from the Imbrium basin, their composition should match the composition of material elsewhere on the Moon thrown out during that event. But they don't, in which case (as the Spudis, Wilhelms, and Robinson pointedly admit), "We possess little systematic understanding of the effects of large-body impacts." In other words, it'll be back to the drawing board.
There you have it: a potpourri of recent research results that demonstrate how much — and how little — we really know about our nearest neighbor in space.