An international team of researchers has analyzed recent snow deposits in Antarctica and discovered the presence of a material that comes from outside our Solar System. They discovered samples of a particular isotope of iron, iron-60, in freshly fallen snow and they are confident that it could only have come from outside our planetary system.
They suspect this rare element arrived in Antarctica in the form of interstellar dust, and it arrived in the last 20 years.
The most abundant type of iron found is iron-56, which has 26 protons and 30 neutrons in its nucleus, and makes up almost 92 percent of all iron there is. It is one of the four stable isotopes of iron. Iron-60, which they found, has an extra four neutrons and is slightly radioactive, decaying with a half-life of 2.6 million years.
This radioactive element can be produced in certain nuclear processes and in supernovae. Astronomers have found it in interstellar space, but it has also been found on Earth at the bottom of the sea (dating back 2–2.5 million years) and on the Moon, suggesting that over the past few million years Earth has been showered with material from nearby supernovae, which should show up in geological formations.
In the study detailing their find, published in Physical Review Letters, the researchers were interested in seeing if this “shower” continued to this day. To do this they needed to analyze material from an uncontaminated site, so they collected 500 kilograms (1,100 pounds) of Antarctic snow from the last 20 years. They then melted it and analyzed the composition of the meltwater, which is where they discover the unexpected iron-60.
The team looked at the most likely scenario for the abundance of this rare isotope. Nuclear power plants and nuclear weapons can produce iron-60 but there was no global fallout to justify the excess of iron-60 seen in the sample from Antarctica. For this reason, the researchers point their fingers at an interstellar source, and suggest it rained down as dust.
What makes this research potentially very impactful is the insights we can glean into interstellar clouds and their enrichment from supernovae. The researchers think that the radioactive iron-60 from steller explosions should be caught up as dust particles in the Local Interstellar Cloud. It's thought that the Solar System crossed into the cloud roughly 40,000 to 50,000 years ago, and that we may have left it around 3,000 years ago – though we may still be skirting the edges. And in that time, the material has showered down on Earth as dust.
“By ruling out terrestrial and cosmogenic sources, we conclude that we have found, for the first time, recent iron-60 with interstellar origin in Antarctica,” the researchers conclude in the paper.
Investigating the ice cores from throughout this timeframe and comparing the abundance of iron-60 from when we first entered the cloud to the current sample might provide new information about the structure and even the origin of the interstellar dust clouds that streak across the Milky Way.
[H/T: Physics World]