February 4, 2011
Journey to the Center of the Moon
New interpretation of the lunar interior (from Weber et al., 2011, Science 331, 309-312)
A recently published science paper presented results of a re-analysis of seismic (moonquake) data sent to the Earth from a network emplaced by the Apollo astronauts 40 years ago. The scientists processing the old data found that the Moon may have more than a simple core – it may have a layered, partly liquid metallic core.
Why is this important? Scientists have known for many years that the Earth has a layered interior structure. The outermost layer, called the crust, is the only part of the Earth directly accessible to us for study. The crust varies in thickness, ranging from a few kilometers in the ocean basins to over 20 km in continental areas. The next zone down is called the mantle. The mantle is very thick – almost 3000 km. It is made up of a dense, iron- and magnesium-rich rock type called peridotite. Partial melting in the mantle is the source of basaltic magma that erupts to make up the floors of ocean basins worldwide. The innermost part of the Earth is the core, comprised mostly of metallic iron and nickel, and over 3000 km in radius. The outer layer of the core is liquid, but the enormous pressure that contains the inner core keeps it solid.
The Earth’s core is electrically conducting as the rotation of the Earth induces currents within it. It is thought that these electrical currents are responsible for the dynamo that generates the magnetic field of the Earth. Because most of the Earth’s iron is contained in the core, we know that in bulk composition, the Earth is made from chondrites, the same stony material found as primitive meteorites in space. Thus, understanding the core is relevant to the origin of its magnetic field and the internal structure and bulk composition of the Earth.
For these reasons, we are interested in the possibility of a core within the Moon. Even before we went to the Moon, we understood that an internal structure similar to Earth was not likely. A property called moment of inertia told us in broad terms that, unlike the layered structure of Earth, the Moon was more or less homogeneous inside. The moment of inertia indicated that any core inside the Moon must be smaller than a couple of hundred kilometers at most (the Moon’s radius is 1740 km).
Seismometers, deployed on the Moon as part of a surface network during the Apollo missions, operated for over seven years collecting data on tremors within the Moon. Because certain rocks have known physical properties (e.g., density), we use the velocity of seismic waves in an indirect way to infer the presence of these rock types and physical structure. From our initial analyses of these data, we determined that the Moon had a fairly thick crust (from 50-80 km, more than twice the thickness of Earth’s crust) and a very thick mantle, almost the remainder of the lunar radius.
The question of the existence of a lunar core remained uncertain. One moonquake resulting from a fairly large impact on the far side of the Moon a couple of years after the Apollo missions had ended produced a signal that suggested the presence of a small core (less than 400 km radius). Moreover, because seismic waves come in two varieties – P-waves, or compression (or sound) waves and S-waves (shear waves, which cannot propagate through liquids) – the partial suppression of S-waves through the center of the Moon during this event suggested that the lunar core might be partly liquid.
But this result was so uncertain that few lunar scientists actually believed it. They proceeded to try and constrain the dimensions and composition of a lunar core through other means. A core may be important in the generation of an early global magnetic field that some of the lunar samples seems to indicate (the current Moon has no global field). By carefully measuring the ways in which the magnetic field of the Sun and Earth is modified when the Moon passes through it (as it does during its orbit around the Earth), it was thought that it might be possible to “sense” the presence of a lunar core by measuring these deviations. Results indicated that the core of the Moon had to be small (less than 400 km in radius) and probably made of iron sulfide (FeS).
After seven years of operation, the Apollo seismic net was turned off to save money. Up until it was turned off, we had received a large amount of data but processing it was extremely difficult. The Apollo instruments, although sensitive, were very noisy and not well coupled to bedrock as are seismometers on Earth. Fortunately, faster and more capable computers, along with new techniques to process and analyze noisy data, were developed. And a new generation of scientists came forward to re-examine the old seismic data to see if anything could be discerned from it.
The new results are surprisingly detailed. Not only do these researchers think they have detected a core inside the Moon, but a core with three separate layers – an inner solid core and outer core, very similar in structure to that of the Earth, but with the added wrinkle of a partly molten outermost layer. The entire core is almost 500 km in radius, slightly larger than the diameter inferred from deep magnetic sounding.
The presence of currently molten core inside the Moon is rather startling; even the earlier idea about a partly molten zone was viewed askance by most lunar students. But this new idea has revived concepts about a magnetic core dynamo inside the Moon, generating a global field early in lunar history. Such a dynamo might explain a lot about the remnant magnetic fields measured in some of the returned lunar rocks. But there is no obvious reason why such a field would suddenly stop being generated.
Even though the old Apollo network data may still be mined for information, to fully understand lunar structure and history we must emplace a long-lived, global network of new instruments to fully characterize the interior of the Moon. Although studies are underway to determine how this might be accomplished, deployment of such a network is difficult to achieve by robotic spacecraft alone and long life on the Moon may require a nuclear power supply. Each and every time we start believing that we understand our Moon, a new discovery raises even more questions.
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Have you read Tom Powell’s speculation that tidal heating may have warmed the lunar interior? Does the existence of a molten core lend credence to Powell’s notion that planetary accretion and isotopic heating may not account for all the moon’s interior heat?
Comment by Hop David — February 4, 2011 @ 5:22 pm
[...] This post was mentioned on Twitter by Portal to Universe, Lights in the Dark. Lights in the Dark said: The Moon once had a magnetic field…but what happened to it? http://fb.me/SmN1z0Qx [...]
Pingback by Tweets that mention Journey to the Center of the Moon | The Once and Future Moon -- Topsy.com — February 4, 2011 @ 6:17 pm
Hop,
There’s no doubt that the early Moon was tidally heated; it was much closer to the Earth 4 billion years ago. But I think that most of the Moon’s internal heat was caused by two factors: the early rapid accretion of the Moon from small particles 4.6 billion years ago released a lot of heat (which melted the Moon globally) and then, by the decay of radioactive elements, which are present in high enough abundance to explain mare volcanism. Tidal heating is possible but not necessary.
Comment by Paul D. Spudis — February 5, 2011 @ 4:03 am
great news and your site offering nice space stuff
Comment by space zone — February 6, 2011 @ 12:24 pm
Thank you for the new blog. It lacks some of the punch some of your other articles have. But I suppose if you keep saying the same thing over and over, you will sound like a broken record.
I see the whole issue as the same problem. This world needs to be shaken out of its complacency. Being an American sure I am sad that the US is loosing its edge economically, intellectually, and in some ways militarily. However this US is not the US that the fore-fathers envisioned. I am certainly not anarchic, however change is sometimes for the better. I see a private, non governmental, group getting to space more appealing than a governmental one doing so.
By getting to space, I mean what government agencies should have been doing for the last 50 years. Not just making headlines and winning tax dollars, but making a way for humanity. Sure if you want to do it for only your country do so. But no one really has. So I am disenchanted by governmental groups accomplishing anything of lasting value.
In your paper “Mission and Implementation of an Affordable Lunar Return (December 2010)” with Lavoie, you still seem to cling to NASA as the provider. Why not put your weight behind any of the myriad other groups? However if you cannot convince the government to put forth funds to make a mission like this possible, perhaps you think that you cannot charm an angel investor to help an NGO to make your plan possible either.
I suppose I am trying to drum up more support for PERMANENT which I am a small part of. Because I am selfish and do want to be on the short list of people do go to Luna.
I am a welder fabricator by trade, and would love to ply my craft on Luna for a change of scenery.
With proper funds and recognition it would not have to take long to take humanity to the stars. I say fund because Mark Prado head of PERMANENT once said that volunteers rarely give 100%, however when someone is paid to do a job, they usually get it done.
I should not drink before posting, it leads to me saying exactly what I am thinking.
Comment by Rhyshaelkan — February 6, 2011 @ 11:10 pm
Rhyshaelkan,
Your post is in the wrong place, but as long as it’s come up here, I’ll discuss it.
I would not characterize my advocacy as “clinging to NASA.” I assume that, as we’ve had a federal civil space program for the last 50 years, we will continue to have one. That means we will spend billions of dollars per year on it. I think that we should get something in return for that expenditure. I supported the Vision for Space Exploration because it was a logical strategic direction that would have created new capability in space faring and it used the existing NASA budget to do so. I oppose the new direction because it discards our strategic direction and replaces it with (literally) nothing — but we still spend the money.
That said, I am most certainly not against private spaceflight and have myself been involved with several groups that were attempting to establish an alternate path to the Moon. If the private sector can establish a robotic resources capability on the Moon, more power to them. Let them raise the capital they need and do it. Stop telling us all the great things they are going to do — just do it. Likewise, if NASA is now completely irrelevant and a waste of money, it should be defunded and eliminated. But that’s not what many in the “New Space” business seem to want — they seem to want the money NASA was spending on Constellation for themselves. You can call that a lot of things, but free market capitalism isn’t one of them. I am not against NASA contracting for Earth to LEO launch services — but NASA should not give tax money to New Space corporations to fund their vehicle development. Let them offer a space service, if they can — payment on delivery.
Two final points. First, I support NASA returning to the Moon because they get a lot of tax dollars and we should get something for that, not have it dissipated on aerospace corporate welfare. Second, the federal government has legitimate interests in establishing a system of space transportation that can access satellite assets in cislunar space. By doing that, we address national concerns (scientific, economic, and security) and create the capabilities needed to go beyond cislunar into interplanetary space.
Comment by Paul D. Spudis — February 7, 2011 @ 5:56 am
May I suggest a topic for a future post, based on one of your recent replies: the science that would be conducted at a lunar polar base. You could break it down (perhaps on a scale of 1 to 5) into categories such as research that can only be done at the poles; can best be done at the poles; finally, investigations that can be done anywhere on the Moon.
Comment by Dick Morris — February 10, 2011 @ 6:29 pm
Dick,
Science is only one of many motivations for the space program and in my opinion, not even the most decisive one. Still, I will give your suggestion some thought and perhaps will do a post on that topic sometime in the future.
Comment by Paul D. Spudis — February 11, 2011 @ 5:01 am
Just got your new book and am quite pleased with it. I’ve only read a few of the threads so far, starting with the “NASA Lost It’s Way” thread containing my posts from last spring, but will get to the rest as time permits.
Science is indeed only one of the motivations for returning to the Moon, but I think you will agree that it is an important one, and the more reasons we can present the more support we can generate. (Lack of support was a major factor which ruined the Shuttle program.)
And, contrary to how it may appear, I care a lot less about where our initial lunar base is located than about getting it started ASAP. If you can get a program started that is targeted to either, or both, poles I will salute, say “Yes Sir, All The Way Sir!”, and fully support it.
I tend to be less optimistic than you are about whether the polar ice is in an easily accessible form, but that remains TBD, and the only way to find out is to send one or more rovers up there to crawl around in those craters with the appropriate instrumentation. According to press reports, ESA is planning a lunar south polar lander, with a rover, so we may not have too long to wait to find out.
Comment by Dick Morris — February 12, 2011 @ 5:53 pm
We actually agree on most things. The biggest area of disagreement I see is that I think you are too pessimistic on the prospects for major reductions in the cost of Earth-to-orbit (ETO) transportation. That has been my major interest for the last few decades. Last night I was reading “The Moon: Creating Capability in Space and Getting Value for our Money” in your new book. You correctly identify many of the factors which lead to high cost space transportation, which, together, boil down to the “chicken or the egg” problem: In order to get low costs we need a large market – in order to get a large market we need low costs. I have been beating on that drum on the Internet for about a decade now, mainly on the sci.space.policy newsgroup and the Space.com and Mars Society websites.
In the comments following the article you state that:
“The point I am trying to make is that we’ve been trying to lower launch costs for the last 40 years and have essentially nothing to show for that effort. It’s time for a different approach…..I’ll believe it [$1000/kg to LEO] when I see it….[It] is not reachable in the near future.”
The “different approach”, ISRU, certainly can be a “game changer”, but so would low cost ETO transportation, and I disagree that we have actually been trying to lower launch costs for the last 40 years. NASA claimed that that was what they were trying to do (and some of them may actually have believed it), but the approach they adopted was the exact opposite of the proper approach. In all of their launch vehicle programs since the Saturn-V – Shuttle, NASP, X-33, SLI – their approach was to “Push The Technology” (the PTT paradigm) in order to “justify” a massive amount of technology development work for their research centers.
By the mid-60s, NASA fixated on the notion that “reusable launch vehicles must have wings”, and when the time came they were unable to develop their Vertical Takeoff Horizontal Landing (VTHL) design with flyback booster within the limit imposed by the OMB. The result was the overly complex, partly expendable design we now have. NASA then compounded the blunder during development with it’s obsession for absolute maximum performance with absolute minimum mass, which led to a very fragile and unreliable vehicle. It is the Shuttle’s fragility and unreliability, plus the lack of continuous abort modes, which leads to high pre-launch processing costs, not it’s high complexity.
All of NASA’s attempts to replace the Shuttle have been single-stage, horizontal-landing designs which were chosen for no other reason than that they were the most difficult ways to build a launch vehicle that they could imagine: the PTT paradigm. Indeed we do need a new approach.
Had NASA chosen a sensible design for the Shuttle we could have had low cost ETO transportation 30 years ago. The key is to develop a simple (Vertical Takeoff and Landing) fully-reusable design using proven technologies and generous safety margins (requires 2-stages) which emphasizes durability, reliability, and maintainability. That will minimize recurring costs and allow us to develop new markets, such as tourism, and expand existing markets, which will allow us to address the fixed cost problem.
Comment by Dick Morris — February 15, 2011 @ 5:31 pm
Interesting article Paul! The idea that the Moon still has a molten core is news to me.
As for why the formerly strong magnetic field might “suddenly” shut down, consider that the Coriolis effect generated by rotation of the Earth results in rotation of the outer core, thus inducing the magnetic field. Ergo, once the Moon became tidally locked to the Earth, kinetic energy provided by the rotation of the Moon would no longer be available to generate the Coriolis effect, and the Lunar dynamo would shut down.
Ordinarily, we think of the process of the Moon becoming tidally locked as a gradual process that must have taken millions of years. But what if the Moon’s own magnetic field interacting with the Earth’s magnetic field played a role in slowing down the Moon’s rotation? That is, maybe it’s possible that the Moon’s magnetic field acted as sort of an electromagnetic brake, causing the Moon’s rotation to slow down much faster than if gravity were the only braking force.
The prediction would be that older rocks would have a rather strong residual magnetism, and that there would be a rather sharp age transition to rocks with little to no residual magnetism. Indeed, this age transition could be used to date when the Moon became tidally locked.
Another prediction is that conservation of momentum as the Moon’s rotation was electromagnetically decelerated would cause the Moon to “spin out” from it’s earlier orbital radius to its present location much sooner than previously thought.
Also, I guess it would have the same effect on the Earth’s rotation, causing it to slow down faster than if tidal forces alone were acting. Don’t know if this effect would leave “fossil” traces on Earth rocks….
At any rate, the presence of a liquid core these days would seem to indicate that the interior of the Moon is hotter and more dynamic than previously believed. Maybe there’s also a few mantle plumes that could cause localized hot spots that might not be hot enough to produce volcanoes, yet might be hot enough to cause the zone of liquid water that I had speculated might exist in a comment to your “A Wetter Moon Impacts Understanding of Lunar Origin” article of June 19, 2010, to rise significantly and result in a sort of Lunar “Yellowstone Park” where there might be some intermittant fumarole and geyser activity; such geysers could serve as feeder mechanism allowing “primordial” water to escape and eventually find its way to the polar cold traps.
Consider the fact that the site of highest concentration of “observed” transient Lunar phenomena (TLPs) is Aristarchus Crater and is located at a fairly high northern latitude, and that that highest concentration of anomalous, high-CPR craters also occurs in the northern polar regions.
Coincidence? I think not!
Which reminds me: you still owe us Constant Readers an article on those pesky TLP’s….
Comment by Warren Platts — February 16, 2011 @ 9:32 am
Warren,
once the Moon became tidally locked to the Earth, kinetic energy provided by the rotation of the Moon would no longer be available to generate the Coriolis effect
The Moon didn’t stop rotating when it became tidally locked with Earth — it rotates once per month. One would think that such a slow rotation might not permit a field dynamo, but Mercury has a global field and it rotates once every 59 days, less than half the rotation rate of the Moon.
Aristarchus is not really at a high latitude (24 degrees north) and the problem with TLP is that, as singular observations, they cannot be confirmed. I know of at least one study that is attempting to document and record episodes of TLP, but it has not had success as of yet. If I hear about any new discoveries, I’ll let you know.
Comment by Paul D. Spudis — February 17, 2011 @ 5:22 am
The Apollo 12 and 14 Suprathermal Ion Detector Experiments detected a fairly substantial water vapor release over a period of about 14 hours on March 7, 1971, which the investigators concluded could only have come from within the Moon. They estimated the quantity of water released to be on the order of 10^3 or 10^4 kg.
While we are setting up the seismic network, it would be well to include something like the Apollo SIDE instruments to characterize any future releases as to composition and quantity, and triangulate the locations. We could then correlate them with any detected seismic events. If we can locate an active vent, it could be a useful source of raw materials.
Comment by Dick Morris — February 17, 2011 @ 7:36 pm