Iron Isotope Provides New Clues to Solar System’s Past

Scientists at the University of Chicago’s Department of the Geophysical Sciences and Enrico Fermi Institute have found the radioactive isotope iron-60 – the tell-tale signature of an exploding star – low in abundance and well mixed in Solar system material. These findings challenge the theory that the force of an exploding star forced the formation of the Solar system.

Artist’s impression of planet-forming disc (ESO)

The new study looks at remnants of stellar explosions in meteorites to help determine the conditions under which the Solar system formed.

Some remnants are radioactive isotopes: unstable, energetic atoms that decay over time. Scientists in the past decade have found high amounts of the radioactive isotope iron-60 in early Solar system materials.

“If you have iron-60 in high abundance in the Solar system, that’s a ‘smoking gun’ evidence for the presence of a supernova,” said Prof Nicolas Dauphas, who co-authored the study published in the journal Earth and Planetary Science Letters (arXiv.org version).

Iron-60 can only originate from a supernova, so scientists have tried to explain this apparent abundance by suggesting that a supernova occurred nearby, spreading the isotope through the explosion.

But the new results were different from previous work. The team discovered that levels of iron-60 were uniform and low in early Solar system material. They arrived at these conclusions by testing meteorite samples. To measure iron-60′s abundance, the scientists looked at the same materials that previous researchers had worked on, but used a different, more precise approach that yielded evidence of very low iron-60.

Previous methods kept the meteorite samples intact and did not remove impurities completely, which may have led to greater errors in measurement. New approach, however, required that the scientists ‘digest’ their meteorite samples into solution before measurement, which allowed them to thoroughly remove the impurities. This process ultimately produced results with much smaller errors.

To address whether iron-60 was widely distributed, the team looked at another isotope of iron, iron-58. Supernovae produce both isotopes by the same processes, so they were able to trace the distribution of iron-60 by measuring the distribution of iron-58.

“The two isotopes act like inseparable twins: Once we knew where iron-58 was, we knew iron-60 couldn’t be very far away,” Prof Dauphas said.

They found little variation of iron-58 in their measurements of various meteorite samples, which confirmed their conclusion that iron-60 was uniformly distributed. To account for their unprecedented findings, the authors suggest that the low levels of iron-60 probably came from the long-term accumulation of iron-60 in the interstellar medium from the ashes of countless stars past, instead of a nearby cataclysmic event like a supernova.

“If this is true, there is then no need to invoke any nearby star to make iron 60. However, it is more difficult to account for the high abundance of aluminum-26, which implies the presence of a nearby star,” Prof Dauphas said.

Instead of explaining this abundance by supernova, the team propose that a massive star – perhaps more than 20 times the mass of the Sun – sheds its gaseous outer layers through winds, spreading aluminum-26 and contaminating the material that would eventually form the Solar system, while iron-60 remained locked inside the massive star’s interior. If the Solar system formed from this material, this alternate scenario would account for the abundances of both isotopes.

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Bibliographic information: Haolan Tang, Nicolas Dauphas. 2012. Abundance, distribution, and origin of 60Fe in the solar protoplanetary disk. Earth and Planetary Science Letters, vol. 359–360, pp. 248–263; doi: 10.1016/j.epsl.2012.10.011