Explained: The origins of New Zealand’s 10th meteorite

Monday, 25 March 2024

How did New Zealand’s 10th meteorite go from floating in space near the sun to crash-landing near Lake Takapō/Tekapo? Yashas Srinivasa finds out.

A rock discovered in Mackenzie Country last week is believed to be the 10th meteorite found in New Zealand.

A group of enthusiasts had gone out looking for it with Fireballs Aotearoa after a fireball was seen racing across the sky over North Otago and South Canterbury on the evening of March 13. Wellington man Jack Weterings noticed the unusual rock before even arriving at his designated search area.

The group is more than 90% certain the rock was a meteorite.

University of Otago Professor of Geology James Scott, a key figure with Fireballs Aotearoa, offers his expertise on the meteorite’s 149,000,000km journey.

Where did the meteorite come from?

The origin of meteorites is “tricky to gauge”, Scott says.

However, looking at the trajectory of the meteor as it arrived, a modelled orbit suggests that it came from closer to the Sun than Earth.

“Normally these rocks come from the Asteroid Belt (further from the Sun than Earth) and to a much lesser degree Mars, and also the Moon.

“So, this meteorite has an unusual orbit and origin. It doesn’t mean that it isn’t originally from the Asteroid Belt, but that its orbital projection is highly unusual. This makes it interesting to study.”

How old is the meteorite?

Most, but not all, meteorites are close to 4.56 billion years old.

This is because they formed part of small planetesimal bodies (small planets) early on in the solar system history and were never incorporated into either the Sun (99.9% of the mass of the solar system), or planets or moons.

“Most meteorites therefore come from small bodies that have been pulverised in the solar system, or are knocked from asteroids during collisions at a later time,” Scott says.

“Although we cannot yet be certain, our first impression from the colour of the interior seen in the chipped surface is that this is a chondritic [unmodified] rock, and therefore that this rock is ancient.

“We will attempt to date the rock by isotopic methods, and once we know what it specifically compositionally is [by looking at the chemistry of the minerals] we will be able to say a lot more about its origin and history.”

How and why did the meteorite survive the Earth's atmosphere?

The meteorite was large enough when it first entered the atmosphere that it did not completely burn up.

“We have all seen shooting stars – these are small, probably pea-sized rocks that burn up high in the atmosphere [70 to 90km high],” Scott says.

“When a fireball gets in low [15-25km high], it must have been larger in order to survive the difficult passage through the atmosphere.

“It is hard for small rocks to get in low because when they traverse the atmosphere, initially at speeds of greater than 15km per second, they compress air in front of the meteorite, causing it to heat and melt the rock.

“That is what gives rise to the fireball trail.”

If the mass is small, the rock is simply consumed in this stage. Larger rocks can survive this, but they will still lose a lot of mass.

How did the network of Fireballs Aotearoa cameras help?

The network of Fireballs Aotearoa cameras are configured for the low light conditions of the night sky.

The lenses are very good, the sensors are very sensitive, and the software, developed by the Global Meteor Network, is very good at identifying components of the sky.

Each camera captures a 90 degree by 45 degree field of view that extends for hundreds of kilometres.

“With wide distribution of cameras, these views then overlap and enable us to triangulate meteors that occur between different camera,” Scott says.

“The cameras are inexpensive and most are housed at schools, observatories or with the public.”

How do you map where a meteorite lands?

Scott says the position of the fireball can be triangulated from multiple meteor camera views that together reveal the luminescent path of the rock.

“However, when the rock slows down, because it is ramming into compressed air in front of it, the luminescent part [the fireball] ends, and the rock then falls towards the ground.

“This is the dark flight stage. To work out where it fell during dark flight, we use the last visible portion, coupled with estimates of the size [determined from the camera data], guesses for the density of typical meteorites, and the local wind directions and speeds, to calculate what is known as a strewnfield.

“This gives us a small solution of several square kilometres, which can then be searched.”

Why is this only New Zealand's 10th meteorite found?

“There should’ve been much more,” Scott says.

“New Zealand is sparsely populated and the land is commonly rough and vegetated. This means that if one does fall, it will be difficult to find.

“Until now, the chance of discovering one has been mostly dependent on luck: a farmer finding something in the paddock, or a rock falling through a roof.”

However, an estimated three or four meteorites with a mass greater than 100g should fall around the country each year.

New Zealand’s largest known meteorite was found in Dunganville, on the West Coast, in 1976. It was 50kg, and was found by a prospector by chance, in a stream.

Scott says people can report fireballs to Fireballs Aotearoa, which can attempt to recover the meteorites.

“With the nationwide presence, we have vastly increased the chance of recovering one because we can now track them coming in and calculate where they have fallen. Many people have seen fireballs, but now they know where to report them.”

What tests must be done to confirm it is a meteorite?

First, the mineral components that make up the meteorite need to be identified, which is best done through an electron microscope.

“We can also measure the chemical components of those minerals. The mineral compositions vary widely and are dependent on the composition and history of a rock.

“There are good databases of what meteorite mineralogies look like, and so we will compare our results with those.”

What will happen to the meteorite after the tests are done?

It will be donated to a museum where it will be on display to the public along with the story of its discovery and its composition.

How are meteorites named?

The story goes that once upon a time, meteorites were named after the nearest town with a post box. “But that is not the case any more,” Scott says.

It’s usually the scientist who classifies the meteorite that chooses its name, and the name usually represents the place the meteorite was found.