Thursday, December 06, 2007

Night at the Museum Pt. 2

Sometimes reality exceeds our dreams, and the Museum of Natural History is far larger than any movie set. In the opposite corner from Rose Center for Earth and Space is the Hall of Meteorites. This room has not been placed closer because it can't be moved. The Cape York meteorite fragment here weighs 34 tons! This rock from Space is so heavy that it is mounted on supports descending into bedrock.

Touching a big meteorite is a priceless experience. They are dense because they are made of nickel and iron forged by heat into metal. Sliced sections show the silver-grey texture of steel. Banging on a meteorite with the naked fist produces a ringing sound. Sometime in their past these rocks were exposed to intense heat, forged into a metal that survived the lesser heating of impact. For many centuries, meteorites were humanity's only supply of metals. The Cape York meteorite was mined by local Inuit for steel.

In this room are travellers from the asteroids, Mars and Beyond. the Zagami meteorite (lower photo, top) comes to us from Mars. The Camel Donga meteorite displayed below came from asteroid Vesta. How a tiny body like Vesta could have been so hot was a complete mystery. The lack of olivine in meteorites is another mystery.

The Brenham meteorite was found in a Kansas field. Farmers there in the 1880's often bumped into mysterious metallic rocks. A homesteader named Eliza Kimberly recalled a meteorite she had been shown as a schoolgirl. For five years she collected samples and wrote letters to scientists, despite teasing by her husband and neighbours. (Her work wasn't accepted by the arxiv, either.) Finally a scientist was convinced to examine her meteorites and the woman was proved right. The meteorites she found were billions of years old, dating from a time near Earth's formation.

Elsewhere in the hall is a model of Arizona's Barringer Crater. For many years this hole in the Earth was thought to be a product of volcanoes. A Princeton graduate and lawyer named Daniel Barringer became bored with the office and headed West to be a mining engineer. He tried to take geology at Harvard, but dropped the class when an instructor called his questions "childish." Gaining success in his chosen field, Barringer became obsessively interested in the crater. He spent years and most of his fortune making excavations and trying to convince the science community. Within Barringer's lifetime the world realised he was right too.

There are lessons in this room for all scientists. There have been many times (like today) when the textbooks' explanation for the Universe is lacking. Good ideas can come from outside the mainstream of science, even from a Kansas farm. These ideas may be ignored at first, even ridiculed. Determination and years of work can lead to the truth, even within a lifetime.

The books claim that Earth's nickel-iron core remains hot due to "radioactive decay." We can't get samples of the core, but meteorites like Eliza Kimberly's date from the time of Earth's formation. From their composition and what is known about Earth's density, scientists have concluded that Earth's core is also made of nickel-iron. These meteorites may be considered as samples similiar to the core.

The hypothesis of "radioactive decay" may also be tested here. Earth's core has temperatures exceeding thousands of degrees, hot enough to melt rock. The book claims that isotopes within Earth's core cause it to be hot. Since the Hall of Meteorites contains similiar samples, are any of them about to melt? If they contained even a tiny amount of radioactive isotopes, it would not be safe to go near this room. If they contained any isotopes, those would have decayed to nothing long ago. Today these rocks are cold as the New York Winter, yet Earth's core continues to produce heat.

If Earth's core formed around a singularity, that tiny object would generate heat indefinitely. Today it would have the mass of a small moon and the diameter of a grain of sand, far too tiny to suck us up. The tiny amount of Earth that it eats is far less than the mass that arrives each year via these meteorites. Presence of a singularity would also explain Earth's magnetic field and how Earth formed from dust grains in the first place. Like Eliza Kimberly and Daniel Barringer, a scientist should not be afraid of bold steps.

This week Robot Guy hosts the new Carnival of Space!

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Blogger nige said...

Thanks, Louise. This is extremely interesting and very informative! It's interesting that the dense meteorites, especially those composed of iron and nickel, tend to survive the ablation during their fall through the atmosphere, and hit the ground. Less dense stony objects of similar mass tend to heat up and then explode like an air burst nuclear bomb while still high in the atmosphere, as was the case of the Tunguska explosion of June 30, 1908 (an explosion equivalent to several megatons of TNT, see C. Chyba, P. Thomas, and K. Zahnle, "The 1908 Tunguska Explosion: Atmospheric Disruption of a Stony Asteroid", Nature, v361, 1993, p. 40-44).

"Since the Hall of Meteorites contains similiar samples, are any of them about to melt? If they contained even a tiny amount of radioactive isotopes, it would not be safe to go near this room. If they contained any isotopes, those would have decaued to nothing long ago. Today these rocks are cold as the New York Winter, yet Earth's core continues to produce heat."

If a small rock was hot enough to measure the heat, the radiation would be lethal. A radiation dose of 10 Sieverts, which is equal to 10 Joules/kg for a quality factor of 1 (low LET radiations), is lethal within a few days. Since an average person is 70 kg, that means that 700 Joules of radiation is lethal. To make a rock hot and remain hot for long periods by the degradation of radioactive energy into heat, a larger amounts of radioactive energy are required, so the radiation from such a rock would be lethal.

The thing about the earth is that you have a lot of radioactivity distributed within it, and very little leakage of that energy. A few feet of earth or rock can keep the embers of a fire hot for a long time. If you take account of the thickness of the earth's crust, it traps heat very efficiently, so that a moderate amount of radioactivity keeps the core hot. (However, I'm skeptical about the details as I've not seen any convincing calculations from geologists so far.)

If you try testing those meteorites for radioactivity content, you will find there will be some content in them (probably little, but still a trace)! The earth does contain a lot of uranium:

"The convection in the core may be driven by the heat released during progressive solidification of the core (latent heat of crystallisation) and leads to the self-sustaining terrestrial dynamo which is the source of the Earth's magnetic field. Heat transfer from the core at the core/mantle boundary is also believed to trigger upwellings of relatively hot, and hence low density, plumes of material. These plumes then ascend, essentially without gaining or losing heat, and undergo decompression melting close to the Earth's surface at 'hot spots' like Hawaii, Reunion and Samoa.

"However, the primary source of energy driving the convection in the mantle is the radioactive decay of uranium, thorium and potassium. In the present Earth, most of the energy generated is from the decay of U-238 (c 10-4 watt/kg). At the time of the Earth's formation, however, decay of both U-235 and K-40 would have been subequal in importance and both would have exceeded the heat production of U-238. ...

"Measurements of heat have led to estimates that the Earth is generating between 30 and 44 terawatts of heat, much of it from radioactive decay. Measurements of antineutrinos have provisionally suggested that about 24 TW arises from radioactive decay. Professor Bob White provides the more recent figure of 17 TW from radioactive decay in the mantle. This compares with 42-44 TW heat loss at the Earth's surface from the deep Earth."

There's nothing in the universe that isn't radioactive. (Even clouds of hydrogen gas contain traces of tritium.)

Table 1 in that above-linked article shows that meteorites are 0.008 parts per billion uranium, the earth's mantle is 0.021 parts per billion uranium, and the continental crust is 1.4 parts per billion uranium. The concentration of uranium in the earth's core is not very well known (antineutrino measurements are available), but since uranium is relatively dense (denser than lead), there may be a considerable concentration of uranium in the earth's core, at least similar to that in the crust. Also thorium-232, etc.

The earth contains a tremendous amount of The earth's core is hot not because the radioactivity is capable of keeping isolated rocks hot, but because the rate of loss of heat is minimised due to the poor thermal conductivity of the outer layers, particularly the crust. This keeps most of the heat trapped.

The calculation to check the theory should be simple. Take the total radioactivity in the earth (in Becquerels, decays/second), multiply it by the average energy of the radiation emitted (0.3 MeV or so for a beta particle, 4 MeV or so for an alpha particle) and that gives you the total MeV/second, then convert that power energy from into Joules/second (watts). Then estimate the diffusion rate of the heat out of the earth.

10:53 PM  
Blogger L. Riofrio said...

Again nige's comments are very informative and point to new lines of study.

"However, I'm skeptical about the details as I've not seen any convincing calculations from geologists so far."

Concerning Uranium in the core, I can quote Andrew Alden from

"Uranium is an extremely heavy metal, but instead of sinking into the Earth's core it is concentrated on the surface. Uranium is found almost exclusively in the Earth's continental crust, because its atoms don't fit in the crystal structure of the minerals of the mantle. Geochemists consider uranium one of the incompatible elements, more specifically a member of the large-ion lithophile element or LILE group."

Heat flows toward cooler regions. Since the mantle is warmer and the crust very thin, most crustal heat dissipates into Space. Uranium in the mantle or core is an inferrence unsupported by data. For this reason most geologists hypothesise radioactive Potassium or Thorium. Since these elements have relatively short half-lives, none of them explains why the core is hot. The nthere is the magnetic field...

Detailed calculations are being made.

9:07 AM  

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