Twists and turns

One of the things I love the most about science is how one thing leads to another.

Someone notices something anomalous, and thinks to ask, "why?"  The answer to that question leads to more "whys" and "hows," and before long it's led you somewhere you never dreamed of, and opened up new vistas for understanding the universe.

Take, for example, the strange phenomenon of lunar swirls.

Swirls near Firsov Crater [Image is in the Public Domain courtesy of NASA/JPL]

Lunar swirls are pretty much what they sound like; undulating curls of light-colored rock and dust, often overlying craters and other topographic features, but seeming not to follow any obvious contour lines.  This is odder than it may appear to be at first.  We see lots of looping, curly stuff on Earth -- cirrus clouds, the twist of hurricanes and tornadoes, the meanders of rivers -- but all of those occur because of some fluid flowing, be it air or water vapor or liquid water.  The Moon has no atmosphere, and never has had flowing water; so what's causing the sinuous shape?

The mystery deepened when lunar sampling missions found out that the light regions had somehow been magnetized.  This at least explained the color difference; the magnetized bits deflected the particles in the solar wind, causing them to hit nearby rocks instead.  This triggered a series of chemical reactions that darkened the rocks' surfaces, while the magnetized parts were spared and stayed light-colored.

But then the question was, how did the light-colored rocks get magnetized in the first place?

It happens easily enough on Earth; a lot of terrestrial rocks have particles of magnetite (iron II, III oxide), and while they're in the molten state the particles are free to move.  They respond like compass needles, aligning with the Earth's magnetic field, and when the lava cools the magnetite crystals are frozen in place, locking in a magnetic signature.  (You probably know that this property is how geologists found out that the Earth's magnetic field periodically flips -- something that was key to proving the plate tectonics model.)

The problem is twofold.  First, magnetite is rare in lunar rocks; and even more difficult to explain -- the Moon has no magnetic field.  So what are these magnetic crystals, and how are they aligning well enough to make the rocks magnetized?

A possible answer was the subject of a paper this week in the Journal of Geophysical Research, describing a study out of Washington University.  A rock called ilmenite, common on the Moon's surface, can form crystalline iron (which is highly magnetic).  As far as how the crystals got aligned, the research team found a process that could cause enough of a magnetic field anomaly to cause it -- if there was a flow of high-titanium magma underground.

"Our analog experiments showed that at lunar conditions, we could create the magnetizable material that we needed," said study co-author Michael Krawczynski. "So, it's plausible that these swirls are caused by subsurface magma...  If you're going to make magnetic anomalies by the methods that we describe, then the underground magma needs to have high titanium.  We have seen hints of this reaction creating iron metal in lunar meteorites and in lunar samples from Apollo.  But all of those samples are surface lava flows, and our study shows cooling underground should significantly enhance these metal-forming reactions."

So a formation on the lunar surface led to an inference about magnetism and the solar wind, and ultimately gave us information about the subsurface geology of the Moon.  I don't know about you, but I love this kind of stuff.  So many of us just look at things and shrug our shoulders, if we notice them at all.  And maybe that's what sets scientists apart; their capacity for seeing what the rest of us miss, and most importantly, wondering why things are the way they are.

It's pretty clear that science isn't just a list of vocabulary -- even though sadly, it's often taught that way.  Science is a verb.  As the brilliant polymath Jules Henri Poincaré put it, "Science is built up with facts as a house is with stones; but a collection of facts is no more a science than a heap of stones is a house."

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Moonstruck

A lot of times, it's the simple, easily-stated questions that are the hardest to answer.

Take, for example, the question of how the Moon formed.  Satellites around planets are common -- Jupiter has 79, for example -- but our own is a bit of an anomaly.  For example, if you make a list of moons in the Solar System in order of mass with respect to its host planet, the Earth's Moon is way out in front.  Its mass is 0.0123 of the Earth's.  Next in line would be Titan, which has a moon mass to planet mass ratio fifty times smaller (0.000237).

It's easy to picture a planet the size of Jupiter or Saturn gravitationally capturing blobs of the coalescing matter during the Solar System's formation, but it's harder to see a small planet like the Earth having the gravitational oomph to snag something the size of the Moon.  Another oddity is that of the sixteen most massive moons in the Solar System, the Moon's orbit around the Earth is by far the most eccentric.  Eccentricity is a number between zero and one that indicates how elliptical an orbit is, with 0.000 eccentricity being a perfect circle.  The Moon's deviation from a circular orbit is twice the next contender (which is once again Titan; whether that's a coincidence or not isn't known).  But the elliptical nature of the Moon's orbit is why its apparent size from Earth fluctuates, and explains why when there's a solar eclipse, sometimes it's total (complete coverage of the Sun's disk) and sometimes it's annular (occurs when the Moon is farther away and has a smaller apparent size, so at totality there's a ring of the Sun's disk still visible).

A third peculiarity of the Moon only became apparent when scientists got their first views of the far-Earth side around 1960, and they discovered that the far side had few maria -- the darker regions that were named for the Latin word for sea because it was thought early on that they might be water-filled oceans.  The largest two, the Oceanus Procellarum (Ocean of Storms) and the Mare Imbrium (Sea of Showers) together cover about 10% of the near-side disk of the Moon, and given that they're dotted with impact craters they seem to be very old structures.  (The first Apollo manned landing, in 1969 in the Mare Tranquillitatis (Sea of Tranquility), showed that the darkness of the maria is due to their being made largely of the dark volcanic rock basalt.)

[Image is licensed under the Creative Commons Gregory H. Revera, FullMoon2010, CC BY-SA 3.0]

So something odd is going on here, but a research team headed by geophysicist Stephen Elardo of the University of Florida has come up with a compelling answer to at least one piece of it.  The best hypothesis for the formation of the Moon, the researchers say, is the head-on collision of two protoplanets, one about ten times larger than the other (the smaller is estimated to be about the size of Mars).

Wouldn't that have been something to see?  From a safe distance?

In any case, this colossal collision blew both planets to smithereens, creating a whirling cloud of white-hot rocks and dust.  When the debris cooled and re-coalesced, the heavier one (eventually the Earth) had a high enough gravity to sort out the mess and pull the denser elements, like nickel and iron, into the core.  The lighter one (eventually the Moon) didn't, so it was left asymmetrical, with one side enriched in uranium, thorium, and the elements collectively called KREEP (potassium [symbol K], the Rare Earth Elements [such as cerium, lanthanum, dysprosium, and yttrium], and phosphorus [symbol P]).  This combo is what created the maria.  Uranium and thorium are radioactive, and as they decay, they release heat.  One effect of rocks being enriched in KREEP elements is that it lowers their melting point.  This meant that the surface remained liquid much longer -- becoming the flat, dark basalt plains we now can see from Earth.  The other side, being much lower in uranium, thorium, and KREEP, froze solid very early, and the landscape largely lacks maria.

"Because of the relative lack of erosion processes, the Moon's surface records geological events from the Solar System's early history," said study co-author Matthieu Laneuville, geophysicist at the Tokyo Institute of Technology, in an interview with ScienceDaily.   "In particular, regions on the Moon's near side have concentrations of radioactive elements like uranium and thorium unlike anywhere else on the Moon.  Understanding the origin of these local uranium and thorium enrichments can help explain the early stages of the Moon's formation and, as a consequence, conditions on the early Earth."

So that's one piece of the puzzle.  It brings up other questions, though, such as whether the fact that all this happened on the near-Earth side is a coincidence or was driven by something about the collision that formed the Earth-Moon system.  But whatever the answer to that is, the whole topic is fascinating -- and the violence of our satellite's origin is something to remember the next time you're looking up on a clear, peaceful moonlit night.

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This week's Skeptophilia book recommendation of the week is for anyone who likes quick, incisive takes on scientific topics: When Einstein Walked with Gödel: Excursions to the Edge of Thought by the talented science writer Jim Holt.

When Einstein Walked with Gödel is a series of essays that explores some of the deepest and most perplexing topics humanity has ever investigated -- the nature of time, the implications of relativity, string theory, and quantum mechanics, the perception of beauty in mathematics, and the ultimate fate of the universe.  Holt's lucid style brings these difficult ideas to the layperson without blunting their scientific rigor, and you'll come away with a perspective on the bizarre and mind-boggling farthest reaches of science.  Along the way you'll meet some of the key players in this ongoing effort -- the brilliant, eccentric, and fascinating scientists themselves.

It's a wonderful read, and anyone who is an aficionado of the sciences shouldn't miss it.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]