Meteorite impact locations may seem easy to spot, as giant craters on the Earth’s surface show where these distant objects finally came to an abrupt halt. But this is not always the case.
Sometimes these impact scars heal, masked by layers of dirt and vegetation, or wear off again by the elements over time. Scientists have now found a way to discover these hidden fall sites.
Imagine a large chunk of space rock approaching its final destination on Earth. Meteoroids can enter Earth’s atmosphere at 72 kilometers per second (160,000 miles per hour), but they do start to slow down as they travel through our relatively dense atmosphere.
The beautiful light in the sky as a meteor flies overhead is due to “ablation” – when the layers and layers of the meteoroid evaporate as a result of high-speed collisions with air molecules.
Then, if the space rock reaches the ground, it collides with the Earth, creating destructive cones, impact craters, and other telltale signs that a meteorite has fallen here.
It is an intense geological process with concurrent high temperatures, high pressures and fast particle velocities. One of the things that happens during this intense process is that upon impact, a plasma is formed – a type of gas in which atoms decay into electrons and positive ions.
“The impact is tremendous,” says geologist Gunther Kletechka of the University of Alaska at Fairbanks.
“And as soon as contact with this speed occurs, there is a change in kinetic energy into heat, steam and plasma. Many people understand that there is heat, perhaps some melting and evaporation, but people do not think about plasma.”
The team found that all of this plasma did something odd to the rock’s normal magnetism, leaving the impact zone where the magnetism was about 10 times less than natural magnetization levels would normally be.
REM is the amount of natural magnetism found in rocks or other sediments.
As Earth’s sediment gradually settled after it was laid, tiny particles of magnetic metal within it lined up along the planet’s magnetic field lines. These grains then remain trapped in the solidified rock in their orientation.
This is a very low amount of magnetization – about 1–2 percent of the rock’s “saturation level”, and you cannot tell using a normal magnet, but it is definitely present and can be measured fairly easily with geological equipment.
However, when a shock wave occurs – for example, when a meteorite hits – there is a loss of magnetism as the magnetic grains get a nice burst of energy.
“The shock wave provides energy in excess of the energy (> 1 GPa for magnetite,> 50 GPa for hematite) needed to block the remanent magnetic induction in individual magnetic grains,” the researchers write in a new study.
Usually, the shockwave passes and the stones return to their original level of magnetism almost immediately. But, as the team found in the 1.2 billion-year-old Santa Fe collision structure in New Mexico, magnetism never returned to its normal state.
Instead, they speculate, the plasma created a “magnetic shield” that kept the grains compressed, and the grains were just sort of randomly oriented. This led to a drop in magnetic intensity to 0.1 percent of the saturation level of the rock, which is 10 times less than the natural level.
“We present support for a recently proposed mechanism in which the appearance of a shock wave can create magnetic shielding, which allows magnetic grains to be held in a superparamagnetic-like state shortly after exposure to the shock wave, and leaves individual magnetized grains in a random orientation, significantly reducing overall magnetic strength.” – the team writes.
“Our data not only sheds light on how the collision process can reduce magnetic paleointensity, but it also inspires a new direction in collision site research, using paleointensity reduction as a new impact indicator.”
Hopefully this new discovery will mean scientists have another tool in their belt when it comes to finding impact sites, even those that don’t show the usual signs of impact, such as collapsed cones or craters.
Research published in Scientific reports…