A 1.3kg, grapefruit-sized meteorite made headlines around the world when, on a June morning in 2004, it tore through Phil and Brenda Archer’s roof and bounced off their leather couch. Photo / Richard Robinson
It crashed through the roof of an Ellerslie home at a speed of hundreds of metres per second.
Now, two decades later, scientists think they have a better idea how the famous Auckland meteorite got here.
The 1.3-kilo grapefruit-sized space rock made headlines around the world when, on a June morning in 2004, it tore through Phil and Brenda Archer’s roof and bounced off their leather couch.
At the time, scientists believed the later-named Auckland meteorite was some 4.5 billion years old and may have travelled as far as 700 million kilometres, from an asteroid belt between Mars and Jupiter.
Since then, it’s been on display at the Auckland Museum, where hundreds of thousands of people have viewed it - yet its origin had never been fully documented.
For Otago University geologist Professor James Scott, the extra-terrestrial intruder posed plenty of burning questions overdue for answers.
“What kind of meteorite was it? What was its history? Where did it come from?” Scott said.
“I’d been working on Martian meteorites and had been wondering what ‘Auckland’ was. Could it be a rare meteorite?”
Of just nine meteorites ever identified here in 160 years, Auckland was only New Zealand’s second “fall” - or those seen to have plummeted to Earth - with the others, called “finds”, all discovered by accident.
It was also the only one known here to have hit a roof - something the Archers likened to the sound of an explosion.
But even globally, it was a special case.
While there have been instances of meteorites hitting roofs, cars and driveways - as just happened in New Jersey, US - most are recovered in Antarctica or the deserts of northern Africa, where they stand out on the ground.
“It was pretty fortuitous really - the chance of one hitting a house in New Zealand is pretty unlikely given the landscape,” Scott said.
“Falls are important because they don’t have time to be affected by Earth processes - that is, they freshly preserve their extra-terrestrial histories.”
Most meteorites happened to have remarkably long histories, stretching back billions of years, to slightly after the time our solar system was formed.
“Apart from breaking up, little else has happened to them in the intervening 4.5 billion years - so 20 years in a museum isn’t long in its history,” Scott said.
“It’s also exciting to hold something that has travelled through space and is older than the Earth.”
Still, he said, just a few of our nine confirmed meteorites have been suitably studied to explain their origin - among them, the 1908 fall known as Mokoia, and which proved one of the rarest types known.
“So, when a casual inquiry showed that the Auckland Museum was prepared to provide us with some for analysis, we jumped at the chance.”
Scott’s team took tiny fragments, measuring a few millimetres in diameter, then used an electron microscope to reveal their minerals and textures.
“With this information, we can compare the rock against other meteorites and see what it is most similar too, because the mineral compositions vary between different meteorite classes.”
Meanwhile, Dr Kevin Faure of GNS Science and Derek Knaack and Matthew Leybourne of Canada’s Queens University carefully measured the fragments’ oxygen isotope compositions.
“When the rocks formed early in the solar system, there was a lot of variation in the oxygen isotope compositions with distance from the Sun,” Scott explained.
“So, these measurements helped to precisely show what the ‘parent body’ of the meteorite was.”
The analysis - described in a newly-published study - ultimately told them several new things about the meteorite and its past: namely that it was a chondrite of the “class H” type.
“The composition means that this rock represents material that never was incorporated into a planet or large asteroid, because those larger bodies get internally hot enough to form an iron core - which is where iron meteorites come from,” Scott said.
“So, it appears that Auckland comes from a small asteroid.”
Having compared it with others, the meteorite also most likely belonged to an asteroid that broke up over 400 million years ago - probably after it collided with another.
“That event subjected the ‘parent’ to enormous pressures and was catastrophic, spreading debris that today orbits the Sun,” Scott said.
The team also spent a long time assessing the meteorite’s surface, to help reconstruct what happened as the rock - perhaps at that point the size of a beachball - crossed Earth’s atmosphere.
“We found that the outside was mostly a thin film of glass that had formed in the seconds that it traversed the atmosphere, but that melting extended several millimetres into the rock.”
The oxygen isotopes also showed it took up Earth’s oxygen from the atmosphere as it melted - yielding further insights into the rapid processes that occur when a meteorite hurtles toward us.
Scott figured it would have been initially travelling at a pace of 15 to 20km per second.
“This inhibits air getting out of the way in front of it, which compresses it and raises the temperature to over 1000 degrees Celsius, which in turn causes the rock to melt.”
This melted and vapourised material created the bright trails that we associate with fireballs - but also quickly consumed much of the meteorite.
“They lose over 90 per cent of their mass as they traverse Earth’s atmosphere, before slowing down, the fireball winking out, and the rock falls to Earth,” he said.
“So, the small fragment of Auckland, just prior to entry, was probably part of a larger, perhaps half a metre in diameter, meteorite.”
While documented meteorites are rare, Scott suspected New Zealand would annually receive as many as three or four with masses greater than 100 grams.
The largest ever found here was the 50kg “Dunganville”, discovered in a Westland creek bed in 1976.
“So, there are certainly more in New Zealand waiting to be found, and there will be more coming each year.”
Scott’s group Fireballs Aotearoa was set up to help co-ordinate meteorite reports, while at Otago University, he and students have built 50 meteor-tracking cameras set up across the country.
Each recording a small portion of the night sky, these cameras feed data into an international network documenting meteor showers and meteors around the globe.
“New Zealand now has one the most rapidly growing globally coverages, as well as one of densest in the Southern Hemisphere.”
In March, cameras set up in Otago and south Canterbury captured two fireballs on different nights, later calculated to have come from the Ort Cloud, at the margin of the Solar System.
“These were travelling at 50 to 70 km per second, burnt up high above the surface, and were almost certainly comets,” Scott said.
“However, we’ve also had at least four meteorites land in New Zealand since early 2022.”
The largest, weighing as much as 10kg, was spotted over Otago last year, and smaller ones had been seen flying into Northland, the Waikanae Ranges and South Canterbury.
To date, none of them have been recovered.
“While we can pinpoint to a few square kilometres where these should’ve landed, there are variables that mean there’s some error in where the meteorite could be,” he said.
“So, our goal is to maximise the chance of recovering New Zealand’s 10th meteorite.”
To science, meteorites remained hugely important.
Many were debris which never got incorporated into a planet, so represented the building blocks of the planets, moons and asteroids, while others bore traces of the solar system’s very earliest solids.
“There are also Martian meteorites and meteorites from the Moon, but it’s much cheaper when they come to us, than us going there,” Scott said.
“So, if we want to better understand our place in the solar system, how Earth formed and what it is built from, meteorites provide critical pieces of information.”
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