A History Of Violence: Iron Found in Fossils Suggests Supernova Role In Mass Dying

Space and events that transpire there directly affect our lives and those of our remote ancestors. Credit: Bob King
Space and events that transpire there directly affect our lives and those of our remote ancestors including early humans who walked the planet two million years ago. Credit: Bob King

Outer space touches us in so many ways. Meteors from ancient asteroid collisions and dust spalled from comets slam into our atmosphere every day, most of it unseen. Cosmic rays ionize the atoms in our upper air, while the solar wind finds crafty ways to invade the planetary magnetosphere and set the sky afire with aurora. We can’t even walk outside on a sunny summer day without concern for the Sun’s ultraviolet light burning out skin.

So perhaps you wouldn’t be surprised that over the course of Earth’s history, our planet has also been affected by one of the most cataclysmic events the universe has to offer: the explosion of a supergiant star in a Type II supernova event. After the collapse of the star’s core, the outgoing shock wave blows the star to pieces, both releasing and creating a host of elements. One of those is iron-60. While most of the iron in the universe is iron-56, a stable atom made up of 26 protons and 30 neutrons, iron-60 has four additional neutrons that make it an unstable radioactive isotope.

Crab Nebula from NASA's Hubble Space Telescope
The Crab Nebula, shown here in this image from NASA’s Hubble Space Telescope, is the expanding cloud of gas and dust left after a massive star exploded as a supernova in 1054. Supernovae propel a star’s innards back into space while creating new radioactive isotopes such as iron-60. Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)

If a supernova occurs sufficiently close to our Solar System, it’s possible for some of the ejecta to make its way all the way to Earth. How might we detect these stellar shards? One way would be to look for traces of unique isotopes that could only have been produced by the explosion. A team of German scientists did just that. In a paper published earlier this month in the Proceedings of the National Academy of Sciences, they report the detection of iron-60 in biologically produced nanocrystals of magnetite in two sediment cores drilled from the Pacific Ocean.

Magnetite is an iron-rich mineral naturally attracted to a magnet just as a compass needle responds to Earth’s magnetic field. Magnetotactic bacteria, a group of bacteria that orient themselves along Earth’s magnetic field lines, contain specialized structures called magnetosomes, where they store tiny magnetic crystals – primarily as magnetite (or greigite, an iron sulfide) in long chains. It’s thought nature went to all this trouble to help the creatures find water with the optimal oxygen concentration for their survival and reproduction. Even after they’re dead, the bacteria continue to align like microscopic compass needles as they settle to the bottom of the ocean.

These are transmission electron microscope images showing tiny magnetofossils left by bacteria about 2.5 million years ago.
These are transmission electron microscope images showing tiny magnetofossils containing iron-60, a form of iron produced during the violent explosion and death of a massive star in a supernova. They were deposited by bacteria in sediments found on the floor of the Pacific Ocean. Click for more details. Credit: courtesy Marianne Hanzlik, Chemie Department, FG Elektronenmikroskopie, Technische Universität München

After the bacteria die, they decay and dissolve away, but the crystals are sturdy enough to be preserved as chains of magnetofossils that resemble beaded garlands on the family Christmas tree. Using a mass spectrometer, which teases one molecule from another with killer accuracy, the team detected “live” iron-60 atoms in the fossilized chains of magnetite crystals produced by the bacteria. Live meaning still fresh. Since the half-life of iron-60 is only 2.6 million years, any primordial iron-60 that seeded the Earth in its formation has long since disappeared. If you go digging around now and find iron-60, you’re likely looking at at a supernova as the smoking gun.

Co-authors Peter Ludwig and Shawn Bishop, along with the team, found that the supernova material arrived at Earth about 2.7 million years ago near the boundary of the Pleistocene and Pliocene epochs and rained down for all of 800,000 years before coming to an end around 1.7 million years ago. If ever a hard rain fell.

Reconstruction of Homo habilis at the Westfälisches Museum für Archäologie. Credit: Lillyundfreya / Wikipedia
Reconstruction of Homo habilis at the Westfälisches Museum für Archäologie. Credit: Lillyundfreya / Wikipedia

The peak concentration occurred about 2.2 million years ago, the same time our early human ancestors, Homo habilis, were chipping tools from stone. Did they witness the appearance of a spectacularly bright “new star” in the night sky? Assuming the supernova wasn’t obscured by cosmic dust, the sight must have brought our bipedal relations to their knees.

There’s even a possibility that an increase in cosmic rays from the event affected our atmosphere and climate and possibly led to a minor die-off at the time. Africa’s climate dried out and repeated cycles of glaciation became common as global temperatures continued their cooling trend from the Pliocene into the Pleistocene.

Cosmic rays strike our atmosphere all the time, but their energy is spent creating showers of secondary, less energetic particles. Credit: Simon Swordy, University of Chicago, NASA
Cosmic rays strike our atmosphere all the time, but their energy is spent striking atoms to create showers of secondary, less energetic particles, a few of which sometimes make it to the ground. Credit: Simon Swordy, University of Chicago, NASA

Cosmic rays, which are extremely fast-moving, high-energy protons and atomic nucleic, rip up molecules in the atmosphere and can even penetrate down to the surface during a nearby supernova explosion, within about 50 light years of the Sun. The high dose of radiation would put life at risk, while at the same time providing a surge in the number of mutations, one of the creative forces driving the diversity of life over the history of our planet. Life — always a story of taking the good with the bad.

The discovery of iron-60 further cements our connection to the universe at large. Indeed, bacteria munching on supernova ash adds a literal twist to the late Carl Sagan’s famous words: “The cosmos is within us. We are made of star-stuff.” Big or small, we owe our lives to the synthesis of elements within the bellies of stars.

Cosmic C.S.I.: Searching for the Origins of the Solar System in Two Grains of Sand

Composite Spitzer, Hubble, and Chandra image of supernova remnant Cassiopeia A. A new study shows that a supernova as far away as 50 light years could have devastating effects on life on Earth. (NASA/JPL-Caltech/STScI/CXC/SAO)

“The total number of stars in the Universe is larger than all the grains of sand on all the beaches of the planet Earth,” Carl Sagan famously said in his iconic TV series Cosmos. But when two of those grains are made of a silicon-and-oxygen compound called silica, and they were found hiding deep inside ancient meteorites recovered from Antarctica, they very well may be from a star… possibly even the one whose explosive collapse sparked the formation of the Solar System itself.

Researchers from Washington University in St. Louis with support from the McDonnell Center for the Space Sciences have announced the discovery of two microscopic grains of silica in primitive meteorites originating from two different sources. This discovery is surprising because silica — one of the main components of sand on Earth today — is not one of the minerals thought to have formed within the Sun’s early circumstellar disk of material.

Instead, it’s thought that the two silica grains were created by a single supernova that seeded the early solar system with its cast-off material and helped set into motion the eventual formation of the planets.

According to a news release by Washington University, “it’s a bit like learning the secrets of the family that lived in your house in the 1800s by examining dust particles they left behind in cracks in the floorboards.”

A 3.5-cm chondrite meteorite found in Antarctica in Nov. 1998. Dark meteorites show up well against the icy terrain of Antarctica. (Carnegie Mellon University)
A 3.5-cm chondrite meteorite found in Antarctica in Nov. 1998. Dark meteorites show up well against the icy terrain of Antarctica. (Carnegie Mellon University)

Until the 1960s most scientists believed the early Solar System got so hot that presolar material could not have survived. But in 1987 scientists at the University of Chicago discovered miniscule diamonds in a primitive meteorite (ones that had not been heated and reworked). Since then they’ve found grains of more than ten other minerals in primitive meteorites.

The scientists can tell these grains came from ancient stars because they have highly unusual isotopic signatures, and different stars produce different proportions of isotopes.

But the material from which our Solar System was fashioned was mixed and homogenized before the planets formed. So all of the planets and the Sun have the pretty much the same “solar” isotopic composition.

Meteorites, most of which are pieces of asteroids, have the solar composition as well, but trapped deep within the primitive ones are pure samples of stars, and the isotopic compositions of these presolar grains can provide clues to their complex nuclear and convective processes.

The layered structure of a star about to go supernova; different layers contain different elements (Wikimedia)
The layered structure of a star about to go supernova; different layers contain different elements (Wikimedia)

Some models of stellar evolution predict that silica could condense in the cooler outer atmospheres of stars, but others say silicon would be completely consumed by the formation of magnesium- or iron-rich silicates, leaving none to form silica.

“We didn’t know which model was right and which was not, because the models had so many parameters,” said Pierre Haenecour, a graduate student in Earth and Planetary Sciences at Washington University and the first author on a paper to be published in the May 1 issue of Astrophysical Journal Letters.

Under the guidance of physics professor Dr. Christine Floss, who found some of the first silica grains in a meteorite in 2009, Haenecour investigated slices of a primitive meteorite brought back from Antarctica and located a single grain of silica out of 138 presolar grains. The grain he found was rich in oxygen-18, signifying its source as from a core-collapse supernova.

Finding that along with another oxygen-18-enriched silica grain identified within another meteorite by graduate student Xuchao Zhao, Haenecour and his team set about figuring out how such silica grains could form within the collapsing layers of a dying star. They found they could reproduce the oxygen-18 enrichment of the two grains through the mixing of small amounts of material from a star’s oxygen-rich inner zones and the oxygen-18-rich helium/carbon zone with large amounts of material from the outer hydrogen envelope of the supernova.

In fact, Haenecour said, the mixing that produced the composition of the two grains was so similar, the grains might well have come from the same supernova — possibly the very same one that sparked the collapse of the molecular cloud that formed our Solar System.

“It’s a bit like learning the secrets of the family that lived in your house in the 1800s by examining dust particles they left behind in cracks in the floorboards.”

Ancient meteorites, a few microscopic grains of stellar sand, and a lot of lab work… it’s an example of cosmic forensics at its best!

Source: Washington University in St. Louis

Isotopic Evidence of the Moon’s Violent Origins

Artist’s impression of an impact of two planet-sized worlds (NASA/JPL-Caltech)

Scientists have uncovered a history of violence hidden within lunar rocks, further evidence that our large, lovely Moon was born of a cataclysmic collision between worlds billions of years ago.

Using samples gathered during several Apollo missions as well as a lunar meteorite that had fallen to Earth (and using Martian meteorites as comparisons) researchers have observed a marked depletion in lunar rocks of lighter isotopes, including those of zinc — a telltale element that can be “a powerful tracer of the volatile histories of planets.”

The research utilized an advanced mass spectroscopy instrument to measure the ratios of specific isotopes present in the lunar samples. The spectrometer’s high level of precision allows for data not possible even five years ago.

Scientists have been looking for this kind of sorting by mass, called isotopic fractionation, since the Apollo missions first brought Moon rocks to Earth in the 1970s, and Frédéric Moynier, PhD, assistant professor of Earth and Planetary Sciences at Washington University in St. Louis — together with PhD student, Randal Paniello, and colleague James Day of the Scripps Institution of Oceanography — are the first to find it.

The team’s findings support a now-widely-accepted hypothesis — called the Giant Impact Theory, first suggested by PSI scientists William K. Hartmann and Donald Davis in 1975 — that the Moon was created from a collision between early Earth and a Mars-sized protoplanet about 4.5 billion years ago. The effects of the impact eventually formed the Moon and changed the evolution of our planet forever — possibly even proving crucial to the development of life on Earth.

(What would a catastrophic event like that have looked like? Probably something like this:)

Read more: What’s the Moon Made Of? Earth, Most Likely.

“This is compelling evidence of extreme volatile depletion of the moon,” said Scripps researcher James Day, a member of the team. “How do you remove all of the volatiles from a planet, or in this case a planetary body? You require some kind of wholesale melting event of the moon to provide the heat necessary to evaporate the zinc.”

In the team’s paper, published in the October 18 issue of Nature, the researchers suggest that the only way for such lunar volatiles to be absent on such a large scale would be evaporation resulting from a massive impact event.

“When a rock is melted and then evaporated, the light isotopes enter the vapor phase faster than the heavy isotopes, so you end up with a vapor enriched in the light isotopes and a solid residue enriched in the heavier isotopes. If you lose the vapor, the residue will be enriched in the heavy isotopes compared to the starting material,” explains Moynier.

The fact that similar isotopic fractionation has been found in lunar samples gathered from many different locations indicates a widespread global event, and not something limited to any specific regional effect.

The next step is finding out why Earth’s crust doesn’t show an absence of similar volatiles, an investigation that may lead to clues to where Earth’s surface water came from.

“Where did all the water on Earth come from?” asked Day. “This is a very important question because if we are looking for life on other planets we have to recognize that similar conditions are probably required. So understanding how planets obtain such conditions is critical for understanding how life ultimately occurs on a planet.”

“The work also has implications for the origin of the Earth,”  adds Moynier, “because the origin of the Moon was a big part of the origin of the Earth.”

Read more on the Washington University news release and at the UC San Diego news center.

Inset image: Cross-polarized transmitted-light image of a lunar rock. Photo by James Day, Scripps/UCSD