April 12, 2011

Partially molten asteroids may be abundant in space and some comets could have liquid water

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.

New findings by planetary scientists at MIT and other institutions suggest that many asteroids with cores might be only partially differentiated, with their outer regions largely unmelted.

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 asteroid 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.

Comet Wild 2, which NASA’s Stardust spacecraft flew by on Jan. 2, 2004. The picture on the left is the closest short exposure of the comet. The listed names on the right are those used by the Stardust team to identify features. “Basin” does not imply an impact origin. Credit: NASA/Stardust mission.

Centauri Dreams - Analysis of comet Kuiper BeltWild-2’s dust grains is changing our view of cometary interiors. A new study by Eve Berger and Dante Lauretta (University of Arizona) points to liquid water inside the comet at some point in its history, casting doubt on the idea that cometary ice is always in a terminal state of deep freeze. Berger and Lauretta found minerals in the dust grains, minerals that form in the presence of liquid water.

The paper is "Evidence for aqueous activity on comet 81P/Wild 2 from sulfide mineral assemblages in Stardust samples and CI chondrites".

University of Arizona press release - Frozen Comet Had a Watery Past

This artist’s impression shows the irregular surface of comet Wild-2 and jets spouting into space at speeds of several hundred kilometers per hour. A UA-led team of scientists now found evidence that Wild-2 harbored liquid water at some point in its history. (Image: NASA/JPL-Caltech)

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