On 18 January 2000, a fireball tore across the dawn sky above northwestern Canada. Fragments of the meteorite landed on the frozen surface of Tagish Lake in British Columbia. Days later a local resident collected some of the fragments and handed them to researchers.
Analysis of those fragments and others collected later revealed the Tagish Lake meteorite to be unusual—and not just because its prompt retrieval reduced exposure to terrestrial weathering. Resembling charcoal briquettes (see photo), the fragments contain amino acids, magnetite, carbonate minerals, and other substances whose creation required liquid water. The fragments are also old. They formed when our solar system itself was in its infancy.
The Tagish Lake meteorite’s parent body formed beyond the solar system’s snow line. How, when, and where did the parent become hot enough to melt water ice? To find answers, Yuki Kimura of Hokkaido University in Japan analyzed the magnetism of tiny particles extracted from the meteorite. From those measurements, he and his collaborators—Kazuo Yamamoto of the Japan Fine Ceramics Center in Nagoya and Shigeru Wakita of MIT—inferred a plausible history of the Tagish Lake meteorite and its parent body.
The particles that Kimura analyzed were made of magnetite and came from framboids, micron-sized spherical aggregates that look like raspberries. On Earth, framboids form in aqueous, sedimentary environments. Ranging in diameter from 100 nm to 250 nm, the particles are so small that Kimura resorted to electron holography, a variant of electron microscopy, to characterize their magnetism. Like its optical counterpart, electron holography uses interference and diffraction to create images. Unlike optical holography, it’s sensitive to magnetic fields.
In general, magnetite and other ferromagnetic materials acquire a magnetic field from their environment. Heating a magnet above its Curie temperature TC causes the source of the magnetism—the orientations of the domains—to become randomized and the magnetism to vanish. The so-called remanent field is restored when the magnet cools and the domains relax to their original orientations. The relaxation can be frustrated by lattice defects and internal stresses. If a magnet is heated above and beyond TC, the energy barriers to those sources of frustration are removed. If a magnet never becomes hot enough to remove the barriers, the frustration remains, and it is manifested in a slower-than-expected relaxation time. (See “Magnetic recording in rocks” by David Dunlop, Physics Today, June 2012, page 31.)
Magnetite has a TC of 585 °C. Kimura repeatedly heated the nanoparticles to a gradually increasing temperature and monitored how their magnetism changed as they cooled. He discovered that some of the particles had experienced a maximum temperature in their past of 250 °C, whereas others had experienced a temperature up to 150 °C.
That parts of the same meteorite were subjected to different temperatures is not surprising. Asteroids, the parent bodies of meteorites, and the cores of planets grow by the accretion of whatever dust, pebbles, and boulders are nearby. Those building blocks need not originate from the same material. Indeed, the brecciated composition of the Tagish Lake meteorite indicates that its parent body likely broke up and reassembled at least once.
The high ratio of carbon-13 to carbon-12 in the Tagish Lake meteorite implies that its carbon came from carbon dioxide ice. That inference, in turn, implies that the parent body originated in the frigid environment of the Kuiper belt 30–50 AU from the Sun. Liquid water can exist inside Kuiper belt objects, provided their internal pressure is high enough and provided they contain enough radioactive aluminum-26.
Most of the 26Al in the early solar system was produced by the supernovae of a previous generation of stars. Because the isotope’s half-life is a cosmically modest 0.72 million years, the time during which 26Al can serve as a strong internal heat source for Kuiper belt objects is limited. A second limit comes from the time it takes a Kuiper belt object to grow through accretion.
Calcium-aluminum-rich inclusions
The birth of the solar system from the gas and dust of solar nebulae took place 4.6 billion years ago. Subsequent stages, such as the formation of planets, lasted millions of years. But one stage, the formation of the solar system’s first minerals, began a relatively precise 4.567 billion years ago and lasted a comparatively brief 160 000 years. Known as calcium-aluminum-rich inclusions (CAIs), those first minerals are found in the oldest meteorites. Their brief formation serves as a temporal milestone for dating events in the solar system’s history. Using a numerical model of a Kuiper belt object, Kimura, Yamamoto, and Wakita calculated that for magnetite to form in an aqueous environment at 250 °C, the parent body of the Tagish Lake meteorite formed about 3 million years after the formation of CAIs.
What about the magnetite particles that experienced a temperature no higher than 150 °C? Conceivably, they could have formed after the parent body had cooled to that temperature, a process that Kimura and his collaborators calculated would take 4 million years. However, magnetite particles that formed so slowly would have a broad size distribution. The ones in the Tagish Lake meteorite are more or less the same size and shape.
The researchers speculate that an impactor struck the parent body. The sudden shock induced magnetites to form rapidly under the same conditions. Those conditions, the researchers calculated, could have been met if the parent body had a radius of 90 km, typical of Kuiper belt objects, and the impactor had a radius of 5 km and a speed of 5 km/s. They estimate the collision took place 4.3 million–5.3 million years after the formation of CAIs.
Collisions like that occur far more readily in the main asteroid belt between Mars and Jupiter than they do in the Kuiper belt, where the objects move more sluggishly and are farther apart. Jupiter’s outward migration from the inner solar system, where it formed, to its current position could have delivered the gravitational nudge that brought the parent body of the Tagish Lake meteorite to the asteroid belt.
The timing of Jupiter’s formation is uncertain. However, if its migration was responsible for bringing the parent body to the asteroid belt, the giant planet was formed at the early end of estimates, at least within 7 million years after the CAIs. (Y. Kimura, K. Yamamoto, S. Wakita, Astrophys. J. Lett. 917, L5, 2021.)
