Ancient rocks tell us plenty about the history of the Earth, but they also may harbour hints of billion-year-old cosmic encounters with mysterious dark matter.
That’s the idea from a group of physicists, who suggest we might spot tiny telltale tracks left by dark matter particles as they jostled atoms in crystals out of their ordered arrangement.
In a paper published in Physical Review D, the team outlined their plan for a dark matter detector, and how to tease out those minute signals from background noise.
It sounds completely “out of the box”, said Lindsey Bignell, a physicist at the Australian National University who was not part of the group, but the concept takes a tried-and-tested technique and puts it to a new use.
“The technology that they’re using has actually been used in other particle detectors, albeit in a very different way.
“In principle, it’s a very clever idea.”
Why dark matter matters
Normal matter, the stuff we see and touch, only makes up around 15 per cent of matter in the universe. The lion’s share is dark matter.
And while we know it’s there — we can see light bending around dark matter and galaxies would fly apart if they didn’t contain it — we don’t know what it’s made of.
The problem is dark matter is invisible, hence its “dark” moniker. The only way to detect it is by its effect on other particles.
One main dark matter particle candidate is the hypothetical weakly interacting massive particles, or WIMPs. They don’t emit light and rarely bump into other particles.
So in recent decades, special detectors have been set up to catch the fleeting flash of light or heat produced if a WIMP happens to smash into a nucleus of normal matter.
One of the oldest and best-known is the dark matter project (DAMA), deep in a mountain, at Italy’s Gran Sasso National Laboratory.
In 1998, the DAMA group reported “pulses” of dark matter that fluctuated as we orbited the sun, which they said was evidence that dark matter was in “clumpy” in our solar system, and not consistently spread throughout.
It’s a controversial claim, and more than 20 years on, no-one’s confirmed the DAMA group’s findings — or any other direct detection claim, for that matter.
High-tech detectors to … rocks?
So what can old rocks do that high-tech detectors can’t?
The new paper outlines how pure crystals of ancient minerals could serve as ready-made particle detectors.
Because WIMPs only weakly interact with normal matter, most would pass through the planet unimpeded.
But on occasion, a WIMP travelling through Earth’s rock could carry enough energy to knock a nucleus in a crystal and shove it aside a tiny bit.
The movement might only be a few billionths of a metre, but it will still leave a defect in the lattice, which stays frozen for hundreds of millions of years.
And given enough time, a mineral might accumulate enough of these defects, which could then be imaged by pelting the crystal with X-rays.
The idea first came to physicists, including Andrzej Drukier, a few decades ago, but for a bunch of technical reasons, it simply wasn’t going to work back then.
Dr Drukier put it on the backburner while he spent 25 years building medical instruments.
But when he recently revisited the concept, he realised that what was once impossible could be within reach.
“What we did was realise that we now had 30 years of solid progress in nanoimaging and boreholes going deeper and deeper, with Russian programs digging very deep holes,” Dr Drukier, now at Stockholm University, said.
You couldn’t use any old rocks as dark matter “palaeo-detectors”, though.
For instance, particles spat out by radioactive decay could also produce defects in a crystal’s lattice, which is “a big problem in all existing detectors”, Dr Drukier said.
So he and his colleagues suggested analysing rocks with very few or no radioactive elements, taken from the bottom of boreholes where they’d been protected by more than 10 kilometres of overlying rock.
This would minimise the effects of radioactive decay and other background noise.
Dr Drukier said he and his colleagues hope to conduct feasibility studies in the next year or two, where they’ll artificially create crystal defects in such minerals and image them.
What else can old rocks tell us?
One benefit of using minerals is the wide range of chemical elements found in crystals, Dr Drukier said.
The big detectors constructed underground tend to be filled with pure elements and compounds, such as xenon or sodium iodide.
But if WIMPs don’t interact with the nuclei of those elements and compounds, well, we won’t see anything.
More elements means casting a wider net to snare a dark matter particle’s signature.
So should we ditch the big expensive detectors?
Not yet, Dr Bignell said. Looking for traces of dark matter in rocks is unlikely to yield results on their own, but could be used to confirm or complement other detection methods.
Aside from trying to figure out what dark matter is made of, tracking its interactions in old rocks could also paint a picture of dark matter within our galaxy, the Milky Way.
It takes around 230 million years for our solar system to circle the galaxy once.
“One of the really cool things with this, that no other technique can do, is that you can use rocks of different ages to probe the history of the Earth’s exposure to dark matter,” Dr Bignell said.
“Then we could retrace where dark matter has been on our journey around the Milky Way.”