Cold asteroids may have a soft heart

Apr 08, 2011 by David L. Chandler
One of the thousands of fragments recovered from the Allende meteorite, which fell in Mexico in 1969. The black area is a fusion crust, produced from the heat of slamming into Earth's atmosphere. New studies of one such fragment provided evidence that the object the meteorite originated from had a magnetic field.

A new analysis of one of the most well-known meteorites on Earth provides strong evidence that the prevailing view of many asteroids is wrong. Rather than randomly mixed blobs of rock and dust stuck together, it appears that the asteroid that was the source of the Allende meteorite was large enough to have had a molten core, even though its surface remained cold and solid. The new view also suggests that astronomers’ view of how planets like the Earth formed may need revision.

The Allende fell in Mexico in 1969, shattering into thousands of fragments as it slammed into the Earth's atmosphere and strewing them across dozens of miles of desert. More than two tons of scattered pieces have been found, and it has become perhaps the best-studied meteorite ever.

When the solar system formed, planets built up through the slow accumulation of smaller objects that collided and stuck together. When these growing collections of rubble reached a certain size, radioactive elements within them heated up enough so that the rock melted, and heavier elements tended to sink toward their cores. This separating process (known as differentiation) ended up producing concentric layers of different composition, structured like the layers of an onion. In the metallic cores at the centers of these bodies, swirling eddies of molten metal would produce a magnetic field. Planetary scientists have long thought that asteroids that formed cores must have completely differentiated and melted throughout their interiors. Now, new findings by planetary scientists at MIT and other institutions suggest that may not be the case: that many asteroids with cores might be only partially differentiated, with their outer regions largely unmelted.

“It’s a new paradigm for how people imagine the parent bodies of meteorites,” says Benjamin Weiss, associate professor of planetary sciences and paleomagnetism in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). The shift in thinking comes from a combination of laboratory work and theoretical modeling. The lab studies, led by former MIT postdoctoral scholar Laurent Carporzen, found evidence for magnetization, apparently built up over a period of millions of years, in a piece of the Allende meteorite. A separate theoretical analysis, led by Linda Elkins-Tanton, the Mitsui Career Development Associate Professor of Geology in EAPS, showed exactly how such magnetization could have occurred — and why that changes not just our view of asteroids, but also of how all the planets formed and where the water that fills Earth’s oceans came from.

The two lines of evidence were published this month in a two related papers, one appearing in the journal Proceedings of the National Academy of Sciences, the other in Earth and Planetary Science Letters. Weiss is a co-author of both papers.

The Allende meteorite is a type called a carbonaceous chondrite. Chondrites are conglomerates of tiny pieces (called chondrules and inclusions) stuck together, and the individual pieces are thought to be remnants of the primordial cloud of material that originally collapsed to form the solar system. “Many of these are the oldest solar system solids we know of,” Weiss says.

The new analysis shows that while newly formed asteroids melted from the inside out because of their radioactive elements, their surfaces, exposed to the cold of space and continuing to accumulate layers of new, cold fragments, remained cold. Computer modeling of the cooling process by Elkins-Tanton clearly shows this disparity of a molten interior and cold, unmelted crust, she says.

The decisive new evidence came from studies of the way mineral grains within the meteorite are magnetized: the magnetic orientations of all the grains line up, showing that they became magnetized after the material had all become stuck together, rather than being a remnant of earlier magnetic fields in the swirling cloud of dust from which the object formed. In addition, using a form of radiometric dating involving isotopes of xenon, they could determine that the magnetization took place over a period of millions of years. That rules out an alternative theory that the grains could have become magnetized as a result of a brief pulse of magnetism in the cloud of dust itself.

The finding has implications far beyond the specific that was the source of this meteorite: “It says there’s a whole spectrum of planetary bodies, from fully melted to unmelted,” Weiss says.

Erik Asphaug, professor of earth and planetary sciences at the University of California at Santa Cruz and a specialist in asteroids and comets, finds the case compelling. “The magnetic data is difficult to argue with — that the Allende meteorite acquired magnetization late, and apparently from a stable field. I am convinced about that,” he says. Weiss and Elkins-Tanton, he says, “have made a firm association, for the first time, between differentiated parent bodies and chondrule-rich objects.”

Asphaug adds “I think their conclusion has very significant implications, in that many differentiated asteroids can be 'dressed' in chondrule clothing.”

The new research also provides important information about the whole process of planet formation and how long it took, says Elkins-Tanton. The analysis shows that the parent body must have formed within just 1.5 million years, she says. “The question is, what fraction of planetesimals formed in that period of time? It turns out to be a lot.”

Her calculations show that the planetesimals that stuck together to form the early Earth, even though the heating process would have made them drier than previously thought, would still have retained enough water within their unmelted outer regions to produce the oceans. That contradicts a widely held view of planet formation in which the vast majority of the water and other volatile materials on Earth arrived later, delivered by impacting comets and asteroids.

It also implies that this process must have been commonplace in planet formation, and greatly improves the odds that most of the planets around other stars will also have abundant water, she says, which is considered an essential prerequisite for life as we know it. As we study distant planets around other stars, “This increases the probability of finding life in a form that we would recognize it,” she says.


This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

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Yellowdart
1.6 / 5 (7) Apr 08, 2011
If you keep looking to the low pressure of outer space for the heating mechanism, you will never find it.

You can generate enough heat if the chondrules were highly compressed and under high pressure, as the olivine will compress into spinel releasing an exothermic reaction. Ejecting that out of the high pressure would then cause rapid cooling.

Why continue with the poor assumption that it has to do with planetary formation?
omatumr
1 / 5 (6) Apr 08, 2011
The magnetic field was probably from a nearby pulsar.

Allende and other carbonaceous meteorites formed from poorly mixed supernova debris, as shown in these illustrations of experimental observations:

Nature 240 (1972) 99-101
http://www.omatum...Data.htm

Etc.

With kind regards,
Oliver K. Manuel
PinkElephant
5 / 5 (5) Apr 08, 2011
Why continue with the poor assumption that it has to do with planetary formation?
Because anything that experienced those sufficiently high pressures you mentioned, would have been blended together into one smooth piece of rock, rather than consisting of granules like the carbonaceous chondrites do.
The magnetic field was probably from a nearby pulsar.
So both the granules and the chondrite into which they accumulated orbited this pulsar without any precession or indeed tumbling motion, so that all of their individual magnetic fields ended up perfectly lined up together in aggregate, following which they neatly stacked themselves together into the chondrite while preserving their orientation. By the way, did I mention that I'm an actual pink elephant in real life? And so are you, by the way, don't deny it, you just aren't aware of my brilliant theory that explains it all via Neutron Repulsion!
ACW
not rated yet Apr 10, 2011
In addition, using a form of radiometric dating involving isotopes of xenon, they could determine that the magnetization took place over a period of millions of years.
How does that test work?
Yellowdart
1 / 5 (1) Apr 11, 2011
would have been blended together into one smooth piece of rock,


Most chondrules are spherical. Chondrites are just conglomerations of chondrules.

So it seems your condition is still satisfied.

PinkElephant
not rated yet Apr 11, 2011
Chondrites are just conglomerations of chondrules.

So it seems your condition is still satisfied.
Huh? Conglomerations of chondrules is the same thing as a metamorphosed, monolithic slab of stone?
Yellowdart
1 / 5 (1) May 23, 2011
Chondrites are just conglomerations of chondrules.

So it seems your condition is still satisfied.
Huh? Conglomerations of chondrules is the same thing as a metamorphosed, monolithic slab of stone?


I pulled the phrase from the article. Where are you confused?
omatumr
1 / 5 (3) May 25, 2011
The magnetic field recorded in the Allende meteorite, the separated d- and l-amino acids in you and me, and the association of all primordial Helium with r-products in isotopes of tellurium, xenon and krypton in the Allende meteorite, . . .

All these experimental observations recorded the catastrophic birth of the Solar System from the death throes of an evolved star [1,2]:

1. "Strange xenon, extinct super-heavy elements, and the solar neutrino puzzle", Science 195, 208-209 (1977):

http://www.omatum...enon.pdf

2. "Isotopes of tellurium, xenon and krypton in the Allende meteorite retain record of nucleosynthesis", Nature 277, 615-620 (1979):

www.nature.com/na...5a0.html