The Earth travels around our Sun within our solar system, which travels through the Milky Way Galaxy. As we orbit, the environment around us changes and the solar system collects particles as it passes through clouds and dust which can end up on Earth. Samples of Iron-60 — an isotope that derives from massive-star supernova explosions — has been found in recent Antarctic snow as well as ice cores, tens of thousands of years old. The samples help to paint a picture of the trajectory our solar system has taken through the Local Interstellar cloud and when it will leave.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
We know that springtime in Missouri means rain of the water variety, but Earth has also been experiencing rain of the stardust variety.
Let's unpack that.
Our solar system, including Earth, is orbiting within our galaxy the Milky Way. It completes one orbit around every 250 million years. Our solar system, including us, is traveling at an amazing half a million miles per hour. You and I, and all 8.3 billion people on Earth, travel over a million miles every two hours.
Even after teaching that for 25 years, it still blows my mind.
OK, so we're traveling through space around our galaxy, and that means the environment on an extra solar system scale changes. Our galaxy is a spiral galaxy containing four spiral arms, which are over dense concentrations of gas, dust, and young stars, and between them are less dense regions where little new stars are being formed. Over time, the environment we're in changes as we orbit around, and the spiral arms also move with time.
That environment — the one outside our solar system — has been captured here on Earth as we pass through it, and that information has been published in a physics review letter by Dr. Dominic Cole and collaborators from the Helmholtz Zentrum Dresden-Rossendorf, or HZDR for short. They looked for a particular isotope of iron called Iron-60 in ice cores from Antarctica. Iron-60 is very special as it does not occur on Earth, and so every atom of Iron-60 we find came from somewhere else — and not just anywhere else. Iron-60 is only formed inside massive stars and then distributed by supernova. But with a half-life of only 2.6 million years — short on our galaxies, or even our solar system's time scale — it is only distributed somewhat locally.
And that's what makes this article interesting.
Now we know that our solar system formed with a little pressure help from a nearby supernova some 4.6 billion years ago. All of that iron-60 is long gone, but we use longer-lived supernova particles like Pyrilium-10 and Aluminum-26 for that determination.
From Moon rocks and deep sea sediments, we also know that our solar system had supernova deposits at least twice in the past: 7 million years ago, and then again around 2 to 3 million years ago — capturing telltale Iron-60. But surprisingly, Iron-60 has also been found in Antarctic snow less than 20 years old, and that led Dr. Cole's team on an investigation.
There hasn't been a nearby supernova to produce that Iron-60, so where did it come from?
To investigate, Dr. Cole's team were able to obtain an ice core sample from a European ice drilling project called EPICA. The core comes from 40,000 to 80,000 years ago. That sample, weighing over 660 pounds, was very carefully chemically processed, resulting in only a few hundred milligrams of dust. Dr. Cole's team then had to use the heavy ion accelerator facility at the Australian National University, which is the only facility in the world capable of detecting minute quantities of Iron-60. From the HZDR press release, Dr. Annabel Rolofs — a co-author from the University of Bond — says, it's like searching for a needle in 50,000 football stadiums filled to the roof with hay. This machine finds the needle in an hour."
Wow, that's pretty impressive.
It turns out our solar system is passing through what's called the local interstellar cloud, which is a region of very diffuse gas spread out over tens of light years, yet containing less than half the mass of our sun. And that cloud was seeded sometime in the past by Iron-60 from a supernova.
At this point, I need to reveal that it's not actually news that we are in the local interstellar cloud; that has actually been known for some time. But what Dr. Cole's work provides is a way to measure when we entered, when we'll be leaving, and different density bumps within that cloud.
Beyond their ice core and the recent Antarctic snow, there are also deep sea sediments from around 20,000 years ago with detected Iron-60 in it, and it has the most. So the ice sample from roughly 60,000 years ago has the least, the 20,000 year old sample has the most, and the recent snow is in between, but closer to the sea sample than the ice one. It's thought our solar system entered this cloud a little over 100,000 years ago, so the ice sample is very close to that, but not quite there, and amazingly, we'll be leaving this cloud in just a few thousand years. What a special time we live in.