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Archive for September, 2011

Big splash

An artist's concept of a large impact hitting Earth during the period of heavy bombardment

Last week I wrote about the leading theories for how our moon formed. This week, I’d like to write about what’s happened to the Moon since then and what lunar rocks and element isotopes tell us about the Moon’s evolution.

You would expect that once most of the material in Earth orbit was swept up into the new moon (a process that took only about 10-100 years), the debris that remained would have gradually continued to collide, adding to the Moon’s mass, but slowly tapering off. The leftover planetesimals in the solar system would have occasionally collided, but that should taper off as well to a point almost, but not quite, equal to zero today. However, the rocks brought back by Apollo tell a different story.

Apollo 8 photo

The Earth rising over the Moon as seen from Apollo 8

The original surface of the Moon was crystallized out of a magma ocean (the formation of the Moon within less than 100 years would have created sufficient heat to melt the crust). We know this from the pieces of anorthosite brought back, especially the famous Genesis rock found by Dave Scott and Jim Irwin on Apollo 15. These rocks date back to 4.5 billion years. Yet by far the most common type of rock brought back from the six landing sites and the several Luna sample return missions and by lunar meteorites found on Earth are lunar brecchias: small, angular pebbles and regolith (lunar soil) fused together from the heat of lunar impacts. And they’re all the same age; a narrow window between 3.85 and 3.95 billion years ago.

Potassium-40 is fairly common in lunar rocks (in the form of feldspar) and once it breaks down to Argon-40, the argon atoms are too big to escape the rock if it has crystallized, so determining the amount of Argon-40 in a rock gives a very accurate age of crystallization. We hardly find any rocks on the Moon (at least we haven’t found many yet) that date to the time between 3.9 and 4.5 billion years. It’s as if some event occurred that reset the isotope clocks at 3.9 billion years in most of the lunar rocks.

When we look at the lunar highlands, which are the oldest surfaces on the Moon, we see only craters. It’s as if the surface of the Moon has been pounded and pounded repeatedly, so that no area is without craters. Craters lie on top of craters, from the very large basins all the way down to the microscopic level. The pounding has thrown up pulverized rock and fragments that has formed a powdery layer the consistency of flour called regolith that is very deep in some places (it can’t properly be called soil because it wasn’t formed by erosion). The large basins themselves are from big impacts that occurred around 3.9 billion years as well, with the Imbrium basin among the most recent (it overlaps the others).

Moon cross section

Cross sectional diagram of the Moon

There are other oddities as well. The lunar maria (what ancient people thought were seas) are large areas of basaltic lava that have filled in the huge impact basins, such as Mare Serenitatis and Mare Nectaris. These lava flows, accompanied by rivers of lava, volcanic domes, lava tubes, and other features, occurred between 3.8 and 3.2 billion years ago. Of the 50 some odd basins, by far the majority are on the near side of the Moon (maria basalts cover about 37% of the near side and only 2% of the far side). Data from the Apollo seismic monitors show that the far side of the Moon has a thicker crust and therefore fewer maria; lava had further to go to reach the surface. How could this be?

At the same time period (3.9 billion years ago), Mars and Mercury also show evidence of heavy bombardment. This is called the Noachis Period on Mars. Until recently, we had only seen 1/3 of the surface of Mercury in detail. Now, with the Messenger probe orbiting Mercury, we see craters on top of craters as well. The solar system at that time was a violent, dangerous place as large planetesimals roamed through the inner solar system and pummeled the planets. Earth would have been hit as well, maybe ten times as often as the Moon. It would have been difficult for any life that developed prior to that point to survive, except a few extremophilic bacteria similar to those living in hot springs today. Interestingly, life on Earth seems to date from about 3.8 billion years, just as soon as this heavy bombardment settled down. Perhaps it was already here but all evidence before that was blasted away. Or maybe life gets going quickly where conditions are favorable.

The heavy bombardment could not have been just a regular trail-off of impacts left over from the formation of the solar system. Something extraordinary happened that dramatically increased the numbers of planetesimals reaching the inner solar system. There are several theories for this increase. One is that a large asteroid or small planet was broken up by Jupiter’s gravity. Contrary to what we might like to think, the solar system hasn’t always been a fixed and stable configuration of planets in nice, regular orbits. At present, 4.5 billion years later, it mostly is, but not back then. The regular pattern of planetary orbits first noticed by Kepler (who thought he’d found the music of the spheres) isn’t an accident or coincidence. The masses and orbits of the planets created resonances that pulled and pushed the planets and other objects around as the solar system settled down. These resonances could have broken up a planet trying to form where the asteroid belt is now and sent pieces flying around to smash into the young inner planets.

Another theory is that Jupiter migrated around in an unstable orbit as it grew larger; Saturn also wobbled around, and when these two planets reached a 2:1 resonance, the combined gravity of Saturn and Jupiter sent Uranus and Neptune spiraling outward, which in turn scattered the large number of planestimals and Kuiper Belt Objects. Many objects were spun outward and escaped the solar system. Some were tossed inward. Computer models show this possibility and agree that about 500 million years after the formation of the solar system would have been a likely time for such a resonance to occur. It was like a cosmic shooting gallery. These icy bodies could have provided much of Earth’s water supply, and caused the blasting of medium and large craters seen on the Moon, Mars, and Mercury.

Facts about the moon

Cut away diagram of the Moon, with known facts

Since the maria basalts stopped erupting about 3.1 billion years ago, the Moon settled down into a basically steady state. Occasional moonquakes occur deep in the mantle near the boundary with the Moon’s asthenosphere. These are weak and long lasting (several minutes) and help reveal the Moon’s interior. Now and then meteorites still hit the Moon, splashing bright rays over the dark maria (such as those of Tycho, Copernicus, and Kepler craters). But that’s about all.

There’s a great activity done in many Earth Science classrooms to demonstrate the sequence of events that shaped the Moon’s surface. Start with a cake pan about ½ full of flour and sit it on a tarp or drop cloth. Take a number of small and medium  sized rocks and drop them from various heights and angles into the flour, carefully removing the rocks each time so as not to disturb the craters made in the flour. After a while, the craters start overlapping, with younger craters showing sharp and clean and older craters getting obliterated. This is the lunar highlands. Then drop in larger rocks to make deep basins. Take cocoa powder and sprinkle it carefully in a thin layer in the deepest holes. This is the maria basalts. Then take small rocks and drop them into the maria. The white flour underneath will splash out over the top of the dark maria, making rayed craters. When you’re done, you have a very convincing model of the Moon’s surface.

Moon crater activity

Moon crater simulation activity

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Earth-Moon

Earth and Moon today

We’re back in session at Walden School of Liberal Arts and this year I’m teaching courses in astrobiology, forensic science, multimedia, 3D animation, and computer literacy. We alternate chemistry every other year, since we are a small school, and therefore I’m able to teach some unusual science classes. Because my focus is on astrobiology this semester, the blog posts for this Elements Unearthed website will have a decidedly planetary science flavor for the next few months. As much as possible, I’ll try to weave the stories of the elements into our quest for life elsewhere in the universe.

I’ve spent much of the summer trying to arrange authentic learning experiences using real data for my students and for the students in the physics classes. I still get e-mails from NASA programs that I’ve participated in, and these often contain some wonderful student opportunities. I’ve been pretty successful finding some fun and meaningful projects.

Moon

Our Moon today

Our first is to create a realistic animation of how the moon formed for the Center for Lunar Origin and Evolution (CLOE) in Boulder, Colorado. Here is a link to their website: CLOE Homepage. They are part of the NASA Lunar Science Institute and study the evidence brought back by the Apollo astronauts, trying to determine how our moon first formed and how it evolved over time. This has important implications for astrobiology because it gives us clues to the early solar system and how planets form in general and how some planets (such as Earth) develop life and others (such as the Moon) don’t.

A successful theory of the Moon’s formation has to explain some strange, anomalous facts about the Moon and its rocks. First, the Earth-Moon system has too much angular momentum compared to other planetary-moon systems. No other planet (unless you count Pluto) has a moon so massive compared to the planet, and adding up the total mass and rotational and orbital speeds gives too much energy for a stable system. In fact, the Moon is slowly spiraling away from the Earth.

Second, the rocks brought back show that at one point about 4.5 billion years ago the entire surface of the Moon was molten, a magma ocean, at a time when the Earth already had a solid crust. If the moon formed slowly, by accretion, then there wouldn’t have been enough energy to totally melt the surface. The moon must have formed quite quickly (in a period of only a few years) for there to have been enough heat. Also, since the Moon is smaller than the Earth, one would expect it to have cooled down sooner, not later.

Third, the elemental composition of the Moon’s crust very closely matches Earth, especially Earth’s mantle, right down to the precise isotopes of elements. Fourth, the Moon has a smaller iron core than it should have for an object its size (Earth, for example, has an iron/nickel core that takes up 1/3 of its mass, the Moon’s iron core is less than 5%). If it formed on its own either in Earth obit as a twin planet or was captured later, it should have different isotopes and a larger iron core than it does.

Fifth, the Earth’s axis is tilted about 23.5° from the plane of the solar system (the ecliptic) and the Moon’s orbital path is closer to Earth’s tilt than it is to the ecliptical plane (only about 5° off). Otherwise, we would have lunar and solar eclipses each month.

Sixth, the moon’s elements are different than one would expect for an object that size compared to other similar objects in the solar system. It has more titanium and aluminum but much fewer volatiles – that is, chemicals like water, methane, and ammonia that evaporate easily. Even the rocks are extremely dry, without any hydrates to speak of except small deposits in shadowed craters near the lunar poles. So the Moon is both very similar and somewhat different than the Earth.

How do you account for these facts? The theories of lunar formation prior to Apollo were: 1.) The Moon formed at the same time as the Earth, both accreting from the same cloud of planetesimals, with the Moon already in orbit around the Earth as it formed. 2.) The Earth started out rotating very fast and spun the Moon off. 3.) The Moon formed elsewhere and was captured into Earth’s orbit.

A careful look at each theory shows facts that contradict it. Theory 1 (co-formation) is negated by the high angular momentum of the Earth-Moon system. Theory 2 (spin-off) is contradicted by the magma ocean early in the Moon’s history and by the fact that the Earth couldn’t have ever spun fast enough for this to happen. Theory 3 was the leading contender for years, despite the Moon’s large size, but the identical isotopes show that the Moon must have come from the Earth, not elsewhere. None of these theories can account for the unusually small iron core and lack of volatiles.

Giant impact

Giant impact of a Mars-sized object 4.5 billion years ago

Gradually, during the 1970s and 1980s, a new theory emerged and gained acceptance in planetary science circles. It is called the Giant Impactor theory. If true, then about 50-100 million years after the Earth formed (4.5 billion years ago), a large object about the size of Mars collided with Earth. It wasn’t a head-on collision, more of a glancing blow, and it knocked off a goodly amount of the Earth’s mantle into space and knocked Earth partially on its side while leaving Earth’s core intact. This planetesimal, called Theia, would have formed nearly in the same orbit as Earth and the closing speed was slow – only about 5 km per second. The impactor was demolished after the first collision – most of its iron core spiraled in and joined with Earth, the rest of it joined the splashed mantle material to form a ring around the Earth. The lighter volatile materials escaped from Earth’s gravity entirely. Within a fairly short time (maybe only ten years or so) most of the ring coalesced into the Moon. The heat of this rapid formation caused the Moon’s surface to melt and crystallize. The final lunar surface was therefore a mix of the Earth’s mantle (similar isotopes of oxygen and other elements) and impactor material (different aluminum and titanium). Here is a poster from CLOE that summarizes this theory: Moon formation poster

I find it really fascinating from both a chemical and a planetary science standpoint that by analyzing a few hundred kilograms of moon rocks brought back by the Apollo astronauts, we can tell so much about events 4.5 billion years ago and answer age-old questions. This is why we must eventually send humans to many locations on Mars – even with sample return robotic missions, the chances of answering the riddles of the Red Planet are small unless we have people with trained minds and eyes on the surface to put it all in context and find the right specimens to study back on Earth.

Our job will be to take this theory and turn it into a believable animation. My astrobiology students will research the details and evidence, create the storyboards, and ensure the accuracy of the animation while my 3D modeling students create the actual objects, textures, scenes, and animations. It will be challenging, involving particle effects, physics, and some very sophisticated compositing techniques, but I think we’re up to it. I look forward to the challenge! Meanwhile, I continue to apply to programs that we can participate in where our unique capabilities will be put to good use.

If you’d like a more detailed description of the giant impactor theory, check out this link: http://www.psrd.hawaii.edu/Dec98/OriginEarthMoon.html

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