The Moon: Earth's Sneezeguard

Mandi Walls writes "SF Gate is running an article about looking on the moon for pieces of Earth that may have been knocked into space by collisions with asteroids, etc. The article claims the guys responsible for the idea came up with it while stuck in traffic. They were probably digging for change for a toll in the seats."

Comments

Isn't the moon itself a piece of the earth?

[T.Hobbes]

I had heard in SciAm awhile ago that the moon was created when a mars-sized asteroid hit a young (~4bn yrs), molten earth. This sent a v. large mass of rock into orbit which coalesced into what is now the moon.

In fact, after a little searching I found this at NASA:

How did the Moon come to be? The leading theory is that a Mars-sized body once hit Earth and the resulting debris (from both Earth and the impacting body) accumulated to form the Moon. Scientists believe that the Moon was formed approximately 4.5 billion years ago (the age of the oldest collected lunar rocks). When the Moon formed, its outer layers melted under very high temperatures, forming the lunar crust, probably from a global "magma ocean."

A few more links: Perty image and more detailed explanation; a google search on the topic.

Re:Isn't the moon itself a piece of the earth?

[T.Hobbes]

Do you have any evidence to back up your claims? Were they true I would suggest writing a paper in Science post-haste. That being said, a couple things: RM dating may not be perfect; no scientific tool is. The question is not its perfection, but rather its precision and accuracy. Second, so far as I know the moon is currently falling towards the earth at the rate of ~1cm per year (I could be wrong on that figure).

Re:Isn't the moon itself a piece of the earth?

[T.Hobbes]

FYI - a link on 'the uses and limitations of radiometric dating'. Hard to find amongst the chaff of creationist debunking attempts.

Re:Isn't the moon itself a piece of the earth?

[T.Hobbes]

My mistake. That'll teach me not to use sketchy memories of grade 12 physics as evidence :)

Re:Isn't the moon itself a piece of the earth?

[T.Hobbes]

Lab error? This paper, and the site that it's on, give a good rebuttal; specifically,

"As magmas move through the crust towards the surface or as lava flows over the Earth's surface, they may pickup chunks of surrounding rocks. Sometimes, the magmas and lavas are not hot enough to melt the captured rocks. Once the molten rock solidifies, the plucked rocks remain trapped in the igneous matrix. These trapped rocks are called xenoliths. If the trapped materials are individual minerals, they are called xenocrysts.

Xenoliths, zoned phenocrysts, and xenocrysts (like metamorphic and weathering features) are often easily identified under the microscope and sometimes even in the field. In some cases, a geologist may be interested in dating xenoliths, zoned phenocrysts, or the xenocrysts. However, obviously, if the geologist is interested in dating the younger matrix, he/she will look for and avoid any xenoliths, zoned phenocrysts or xenocrysts. While mainstream geologists know how to avoid xenoliths, zoned phenocrysts, and xenocrysts when dating igneous rocks, creationist Steve Austin apparently was not careful to avoid them when he conducted his "research" at Mt. St. Helens or the Grand Canyon, see " A Criticism of the ICR's Grand Canyon Dating Project by Chris Stassen and compare with Excess Argon within Mineral Concentrates from the New Dacite Lava Dome at Mt. St. Helens Volcano, by Steven Austin. In his Mt. St. Helen's study, Austin collected what he thought was a freshly solidified dacite. He removed the gabbro xenoliths, but there's no mention if he found and removed any lighter colored, less obvious xenoliths, such as andesites or quartz diorites. Austin states that xenoliths of gabbro, quartz diorite, basalt, and andesite are common at the Mt. St. Helen site."

The study by Austin the author, a Dr. Henke, is referring to is one which claims impossibly old dates for fresh dacite.

We could go on for a long time, but I would simply say that science is self-correcting: it is a field which encourages criticism and revision, so long as the criticism is valid and demonstrable. No good scientist would ever claim to any perfection in his or her work. A scientific theory is only valid so long as it has been demonstrated in experiment and has not been disproven by other experiments. The reason why I have such a hard time accepting what you're saying is because, so far as I can tell, it has not stood up to peer review.

Re:Isn't the moon itself a piece of the earth?

[pyr0]

Just to defend radiometric dating somewhat here, I have actually taken a course on the subject so I know a bit about it. Depending on what isotopic system you are working with (U-Th-Pb, Rb-Sr, Sm-Nd, etc.), you have to make several assumptions about the rock you are trying to date. When dating lunar material, many of these assumptions are completely valid, the main one being a closed system. There is no water to leach elements from these rocks, and no atmosphere to speak of on the moon, therefore the system has effectively been closed to the exchange of parent and daughter products since their time of crystallization. Even if there was weathering going on at the Moon's surface, all it takes is very careful sample selection as well as picking the proper isotopic system in order to get a very good date. This is why we can date rocks on the Earth at all.

The other interesting thing is that the same date is found +- a couple millions years or so using different isotopic systems, which is a great check for accuracy.

Re:Isn't the moon itself a piece of the earth?

[T.Hobbes]

First, the article referenced pointed out that Austin made no mention of certain types of xenoliths which were known to exist in the area. Re-read it for clarification (2nd paragraph). Second, the argument didn't base itself on the ad homonim attack. Furthermore, the attack itself put the word research in quotes and mention Austin's beliefs; while uncalled for, it is not what I would call a scathing attack. As to your last point: yes they could have. That's why you must base your trust of results on the relative skill, experience and record of the geologist submitting the sample and the technician conducting the measurement.

More generally, we could go on for days like this on any topic. Errors in data will always come out. You can never completely get rid of human error. If you want to convince me that radiometric dating is pseudoscience, you'll have to give some idea as to what, other than human error in measurement, would cause such skewed results. There are several areas with potential for inaccuracy: the methodology, incorrect assumptions about the properties of various radioactive elements used in measurement; errors in the underlying theory of radioactive decay. But simply giving specific examples where dating has been shown to be inplausable with no context for the scope of the studies nor the larger context of the field (with numbers for accurate/inaccurate results on objects with known time periods). There must be a reason for the inaccuracies; by themselves, as exceptions to the rule, they prove only that radiometric methods have been inaccurate in the past. They do not prove that radiometric methods have always been inaccurate, nor do they prove that radiometric methods will be inaccurate.

Archimedes, not Newton

[T.Hobbes]

You're thinking of Archemedies. From school-for-champions.com(link),

Story of Archimedes

The ancient Greek philosopher and scientist, Archimedes was asked by the king to determine if a gold statue he had was 100% gold. Since it was an odd shape, Archimedes could not simply measure the volume to determine the object's density and thus its composition.

Archimedes decided to take a hot bath, to help him think about this problem. When he got in the bath tub, he noticed the water rise. This clue led to the discovery that an object will displace its volume when immersed in a liquid. When Archimedes realized that objects displace their volume in water, he excitedly jumped out of the tub and ran down the streets shouting, "Eureka! Eureka!" which means, "I have found it!" Unfortunately, he didn't notice that he forgot to put his clothes on! When Archimedes put the statue in a container full of water, he measured the volume of the overflow to determine the volume of the statue. Then he measured the weight of the statue and compared its density with the known density of pure gold. He discovered that the statue was not made of pure gold, rather it contained some other metal, like lead.

Re:Archimedes, not Newton

[Amazing Quantum Man]

Nope, the OP was right. He used the principle by taking the crown, a block of gold, and a block of silver, all three of which had the same weight, and showing that the crown was not pure gold by measuring the volume displaced.

That's volume displacement for submerged objects.

Tidal drag.

[Christopher Thomas]

If the moon is receding, something is pulling it away; you can't "back calculate" that because it would be some kind of idiosyncratic effect. By default, the moon would simply spiral into the earth.

Well, if you consider that the Earth/Moon system is not the only gravity well in the solar system, you might come to the conclusion that the Sun and/or Jupiter and/or any other massive object might have a cumulative pulling effect. Because the orbits and masses of these objects is known, it would be relatively easy to calculate this effect.

To address two different points in these messages:

- Why would the moon "spiral into the earth" by default? There's nothing moving it in either direction in an ideal system.

- The moon recedes from Earth because of "tidal drag". The moon and the earth each deform each others' surfaces (creating the tides that we know so well, among other things). The net effect of this is that if one or both of the bodies are spinning, you get angular momentum transferred. The earth's rotation slows down, and the angular momentum the earth loses goes into the moon's tangential motion about the earth, which pushes it into a higher orbit.

This is what caused the moon to be tidally locked to us in the first place (i.e. always showing the same face to us).

We can calculate the rate of momentum transfer, but I don't have the numbers for that off the top of my head.

Re:Would that make space a....

[Valdrax]

Ah, how it makes NASA long for the salad days of their youth.

Exactly

[ynotds]

If I had moderator points that would definitely have got an "informative" but I guess there are a couple of points I can add to justify a response.

Firstly, it is rather sad that such a basic yet interesting fact as this angular momentum transfer seems to be completely missing from the standard education curriculum.

Secondly, I've been trying to get to first base in compiling some data on the energy storage and rate of use in major planet-wide systems (down to, say, the gravitational potential of elevated water and ice stores) but am stuggling to find clear data.

Even in such an obvious area as total solar radiation the ratio between what the Sun is claimed to radiate (386 billion billion megawatts) and what the Earth is claimed to receive (4.4 x 1016 watts) seems to fly in the face of simple geometry which seems to me should have the earth intercepting one part in 1.1 billion of the Sun's radiation.

Digging for data on other energy systems, there is a total mess of approaches and even units used by different specialties that are going to make even a basic comparison table hard work to draw together, unless of course I am willing to become a "Creation Scientist".

Energy sources.

[Christopher Thomas]

Even in such an obvious area as total solar radiation the ratio between what the Sun is claimed to radiate (386 billion billion megawatts [seds.org]) and what the Earth is claimed to receive (4.4 x 1016 watts [nasa.gov]) seems to fly in the face of simple geometry which seems to me should have the earth intercepting one part in 1.1 billion of the Sun's radiation.

The number I keep hearing for solar energy flux is between 1 and 1.5 kW/m^2 at the Earth's distance from the sun, which gives between 1.5e17 and 2e17 watts for the whole earth. The amount that reaches the surface is less, and the amount that could be captured by any practical harnessing scheme would be much less, but I digress. This is roughly in line with the NASA numbers.

At this flux, total solar energy output is between 3e23 and 4e23 watts. SEDS claims 4e26, so they probably switched "million" and "billion" somewhere.

Digging for data on other energy systems, there is a total mess of approaches and even units used by different specialties that are going to make even a basic comparison table hard work to draw together

What gets me is conversion to/from ergs with old articles ;).

My suspicion is that solar power flux dwarfs most of the mechanical potential energy stored on Earth's surface features. Earth's angular momentum holds one heck of a lot of energy, though, as does that of the earth-moon system, so tidal will work for quite a while.

Wind is recycled solar. Ditto anything involving growing crops.

Energy stored in wind at a given time can be estimated by assuming an average velocity (maybe look up the speed of the trade winds). Mass of the atmosphere is easy to calculate (about 10 tonnes per square metre of the earth's surface; it's just the atmospheric pressure).

I have no idea how much energy is stored in hydrocarbon reserves. You can probably get an upper limit by looking at the elemental abundance of carbon in the earth's crust, and assuming that the upper km is accessible.

The other big source is geothermal, which is driven by radioactive decay in the earth's core. I'm afraid I don't have numbers for the energy flux offhand, but it should be straightforward to calculate. Anything that's still fissioning now will have a long enough half-life to last for billions of years more.

Fissionables on the earth's surface will be dwarfed by geothermal, which effectively makes all radioactives on the planet available for harvesting (albeit over quite a long time).

Good luck with your search, in any event :).