A Potpourri of Lunar Results

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'?"

The Moon's hemispheres
The near side (left) and far side of the Moon, as recorded by the Lunar Reconnaissance Orbiter Camera. An oval indicates the location of South- Pole-Aitken basin. Click on the image for a larger version.
Kelly Beatty
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.

Magnetic anomalies on the Moon's far side
The Moon's largest grouping of magnetic anomalies (left side) lies near the northern rim of South Pole-Aitken, the largest
impact basin on the lunar surface. Some 2,500 km across and 13 km deep, it formed during an oblique impact from the south to north more than 4 billion years ago.
NASA / Science / AAAS
Magnetic Mysteries

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.

Three key lunar basins
The dark maria on the Moon's near side are actually broad volcanic plains that fill deep basins created during huge collisions some 4 billion years ago. The three labeled basins, along with the landing site for Apollo 17, are proving key to deciphering the Moon's early history.
NASA / LROC
Confusing Chronologies

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.

15 thoughts on “A Potpourri of Lunar Results

  1. Rod

    Interesting summary here. Concerning tidal locked Moon, it looks like this is within past 1E+9 years in the orbital integration so does not apply to the 4.5 billion years orbital history. I did not see any info concerning initial lunar spin rate or if the Moon was within the Roche limit of 3 earth radii for this tidal lock model to work extrapolated over 4.5 billion years. Reports of magnetic fields and magnetic anomalies, magnetic fields decay without a dynamo to sustain them over billions of years and there have been past reports of young surface features like Ina Caldera or the graben systems too. Only the radiometric ages of the Apollo samples give really old ages for the Moon.

  2. Russ

    Apollo 17 astronauts brought back nearly 245 pounds of rock & soil samples. That one load was more that Apollo 11, 12 & 14 returned.. combined.

    It is pathetic, knowing that we should have had at least 2 lunar bases.. as early as the late 1970s. One on the far side, for radio astronomy.

    It is even worse knowing that somehow, we would just decide to chuck it all.. trash our hardware as well as cast the highly trained personnel – to the street. Trashed our superb science. No other nation on the planet would do this. Why..? Look at America today. Dumbed down & weak.

  3. Lin4dGary Linford

    Although this interesting article purports to be presenting recent results, it gets off to a bad start by referring to the Moon as the Earth’s “satellite,” a statement that has been out of date for several billion years now. An object orbiting less than 260,000 km of the Earth’s center lies within this planet’s “gravitational dominance.” Beyond that range, Mr. Sun dominates. Based upon conservation of angular momentum, at one time, the Moon orbited the Earth-Moon system’s center of mass at a distance of around 17,000 km, but as the Moon—using its own tidal torques—subsequently spiraled out to its present distance of almost 400,000 km, it obviously “escaped” from the Earth long ago and became a “satellite” of the Sun AKA “a planet” while ending up with 80% of the Earth-Moon system’s angular momentum. The author neglects to mention—also from the Apollo data—that the Moon possesses a liquid iron core, which helps explain why there is a remnant global lunar magnetic field. If we can just avoid destroying ourselves in the thermonuclear war predicted by Carl Sagan, some of us may live to see humanity entering into a new era of peace and prosperity by colonizing “our nearest neighbor” in space using inertial confinement fusion power. From that point on, the Universe may be our oyster!

  4. Frank ReedFrank Reed

    Gary Linford, your idiosyncratic theory that the Moon is not a satellite of the Earth is founded on a well-known, incorrect calculation. The distance you quoted, 260,000 km, emerges when one equates the STATIC gravitational acceleration of the Earth with the STATIC gravitational acceleration of the Sun (mathematically, it’s equating GM/r^2 for the Earth and Sun). If the Earth were pinned down by some non-gravitational force, motionless in space relative to the Sun, and if the Moon were dropped between those static objects at somewhat less than 260,000 km, the Moon would fall towards the Earth. If it were dropped in between them at slight6ly greater than 260,000 km it would fall away from the Earth towards the Sun. But this is nonsense. The Earth is NOT static in space. It and the Moon are in freefall orbit around the Sun. It is, quite simply, POOR physics. The limiting distance for the freefall case is found by calculating the location of the Earth-Sun Lagrangian point, L1, which happens to be a bit more than four times further away than the Moon’s present distance (mathematically found by equating the Sun’s TIDAL acceleration with the Earth’s static gravitation). The Moon is therefore –obviously– a bound satellite of the Earth. I should add that there are, in fact, ambiguous cases for objects at much greater distances. Temporary "moons" which are not permanently bound but apparently orbit the Earth for long periods can exist.

  5. Bruce Mayfield

    To second Frank Reed’s above post I add that the mass of the moon is only about 1.23% of the earth. The semimajor axis of the moon’s orbit (around the earth!) is about 384,400 km. Therefore the baricenter of the "earth-moon system" is only 1.23% of that distance, or about 4738 km from the center of the earth in the moon’s direction. Both bodies orbit around this point in about 28.5 days. This point, (the earth moon baricenter) orbits the sun every 365.25 days. Gary, I share your hopeful optimism toward mankind and the potential usefullness of our nearest neighbor, but dude, if you hold onto the notion that the moon is not a satellite of the earth people might think that your, well, loony.

    Also, there was an article on temporary

  6. Bruce Mayfield

    Also, there was an article on temporary natural satellites of the eart (near earth asteroids) that orbit outside the moon’s orbit ocationally a few months ago on this website.

  7. Rod

    Interesting comments here. Just where was the Moon 4.5 billion years ago in relation to distance in earth radii? Today its mean is some 60 earth radii. If folks think the Moon was about 3 earth radii away or within the Earth-Moon Roche limit (17,000 km or so) problems can arise. One problem is the geologic record. Where is the geologic evidence demonstrating that the Moon orbited with a period < = 2 days? The potpourri report indicates the Moon had the same side facing earth for no more than 1E+9 years. Magnetic field problems on the surface of the Moon, even this report and others don’t claim the Moon has a dynamo keeping it going for 4.5 billion years. Mercury dipole field is an example and this is hard to show a dynamo to explain the field. Other reports indicate a number of lunar areas show evidence of youth as I already mentioned in some craters. Conclusion – radiometric ages could be grossly inflated.

  8. Geoff Bell

    Bruce Mayfield’s argument made sense to me but I do have a query. The radius of the earth is in excess of 6350km which places his baricentre well within its surface. Does this not affect the calculation?

  9. Frank ReedFrank Reed

    Bruce, the mass of the Moon relative to the Earth actually doesn’t have much to do with it. Consider: what if the Moon were twice as big in diameter as it is and thus nearly eight times more massive? Note that the barycenter of the Earth-Moon system would then be well outside the radius of the Earth. This would change nothing in terms of the Moon’s status as a BOUND satellite of the Earth. It’s the mass of the Earth relative to the Sun that determines the distance of the L1 Lagrangian point. Another way to quantify this is the radius of the "Hill sphere" of the Earth: http://en.wikipedia.org/wiki/Hill_sphere. The Earth’s moon lies well inside the Hill sphere of the Earth. By similar reasoning, it is much closer to the Earth than the Earth-Sun L1 Lagrangian point. Therefore the Moon is a bound satellite of the Earth. Again, I should emphasize that there is some ambiguity for objects orbiting close to the limit (roughly four times further away than the Moon’s orbit).

    Geoff Bell, no it doesn’t matter that the Earth-Moon barycenter is within the volume of the Earth. It’s interesting to note that the barycenter of the entire Solar System is sometimes well within the body of the Sun and sometimes well outside of it, depending on the relative positions of the major planets. This has no real physical significance.

  10. Bruce Mayfield

    Geoff, to answer your question, yes, the baricenter of the earth-moon system is indeed well inside the earth, although it is not fixed. With relation to the earth’s center, it is always located on the line connecting the centers of both bodies, and it moves up and down each month as the moon moves from perigee to apogee. For gravitational calculations roughly spherical objects can be considered as point masses located at the centers. And to Rod, "Just where was the moon 4.5 billion years ago … ?" assumes that the moon even existed that far back. If, as I have no reason to doubt, the sun and the proto-solar system are 4.6 billion years old, when did the great impact of the early earth and a mars’ sized body that is thought to have formed the moon occur? Does anyone have info on this?

  11. Bruce Mayfield

    To Frank Reed, I was entering my above response to Geoff & Rod’s comments before seeing yours about mine. I was NOT trying to imply that the barycenter had to be beneath the earth’s surface for the moon to be a bound satellite. I was merely adding more info to the argument that the moon does indeed orbit the earth, therefore it is a bound satellite. I agree with all the points you’ve made and I’m quite amused that we’ve both had to expend so much time and thought in defense of what (at least to you and I) is so completely obvious.

  12. Rod

    Bruce, you raised some interesting points and asked when the giant impact took place forming the Moon. S&T has published on this topic along with others. One report was in August 2011 -"The Moon’s Uncertain Age" at http://www.skyandtelescope.com/news/128240223.html. Here are some thoughts from me:

    As the report indicated, there is an MIT model out that indicates the early Moon orbited very close to the early earth resulting in significant tidal heating and allowing the lunar magma ocean to remain liquid for 200 million years after its birth thus the difference in some of the lunar samples and their ages-radiometric dating.

    Bruce, yes the Newtonian tidal forces could be 100x or more than present. This raises all kinds of questions in my thinking like how did the giant impact avoid destroying the earth or form a moon like Ganymede, about 2.02 lunar masses. The wrong configuration here between Earth and Moon can leave the Earth similar to Io from tidal flexing. Enjoy.

  13. Rod

    For Bruce, I found another quick reference to the answer as to when. "ScienceDaily (June 7, 2010) — Astronomers have theorized that the planet Earth and the Moon were created as the result of a giant collision between two planets the size of Mars and Venus. Until now, the collision was thought to have happened when the solar system was 30 million years old, or approximately 4,537 million years ago. But new research shows that Earth and the Moon must have formed much later — perhaps up to 150 million years after the formation of the solar system." Title: Earth and Moon Formed Later Than Previously Thought, New Research Suggests

  14. Bruce Mayfield

    Rod, thanks for your references, and I did enjoy another fine Kelly Beatty article. So, the date of the big hit is about 4,597 +or- 60 mya, and as you correctly infer, it had to be just exactly, perfectly right in order to have produced our lovely, moonlit home. I can think of two scenarios; (1) The Great Celestial Mechanic made, in effect a perfect billard shot, or (2) Out of the ? x 10^?? universal solar systems He chose the one (or, one of the ones) that had randomly produced ours (or one of the ones like ours), having perfectly ordained the laws of physics so that such systems had a certain probability of occuring at least once.

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