Meteorites might be more likely to strike near the equator

Geoffrey
Evatt was snowmobiling in Antarctica when he spotted an outlandish feature. A
black rock stood so starkly against the diamantine ice that even the untrained
eye would have known it was not from this world, but a meteorite. “You’ll never
get over that high of finding the first one,” he says.

Not
that it was unexpected. Before heading to Antarctica, Evatt, an applied
mathematician at the University of Manchester in England, and his colleagues calculated
where they might find the alien rocks. Two summers spent snaking up and down
their chosen spot netted 120 in total — matching their prediction and giving
them the confidence to use their calculations (plus additional ones of fireball
trajectories) to create a global tally. The results, reported online April 29
in Geology, reveal that more than 17,000
impacts occur across the globe every year, with the majority
of meteorites hitting low latitudes
.

“The
punchline is that if you want to go and see these fireballs streaking across
the sky, it’s best to be near the equator,” Evatt says.

When
it comes to counting meteorites, though, Antarctica is an easy target. Most
meteorites collected so far have been found on the continent — thanks to the
fact that a single dark rock can be spotted easily enough against a white
background. Knowing how many impacts occurred within a specific region lets
researchers extrapolate that number to the rest of the planet, much like how
collecting rainwater in a bucket allows weather forecasters to determine how
much rain fell over a larger area.

But
Antarctica does present one major complication: The ice doesn’t remain still; it
ebbs and flows. As it moves toward the ocean, the ice carries meteorites that
fell elsewhere on the continent toward local stranding zones, eddies within the
ice. Over time that ice sublimates, turning into vapor, and reveals older,
hidden meteorites. Scientists have long collected meteorites within these
zones, but it’s impossible to know which meteorites surfed there versus which
crash-landed — and when each group arrived.

To
tease out the number of meteorites that fall onto a stranding zone every year,
Evatt and colleagues calculated the ice’s movement, as well as a number of
other factors, including the rate of snow accumulation and ice sublimation.

In
theory, multiplying the number of crashes by the total amount of area not
covered in the study could produce a global estimate. Indeed, this is what
previous studies have done. But that method is accurate only if meteorites
strike other regions with a similar intensity. Turns out, they may not. By
incorporating orbital mechanics — how Earth’s gravity pulls in passing material
— into the calculations, the team found that meteorite rates vary drastically
by latitude. The number of strikes at the poles is roughly 65 percent of what might
be expected at the equator, the analysis indicated. (Interestingly, the global
tally still falls in line with previous estimates, albeit with much smaller
error bars.)

To
verify the finding, the team broke down by latitude data from NASA’s Center for
Near Earth Object Studies, or CNEOS, which records fireball events across the
globe. That analysis revealed a similar intensity trend — a peak in the rate of
meteorites at the equator with a diminished rate toward the poles. But Matthew
Genge, a planetary scientist at Imperial College London not involved in the
study, worries that there is too little fireball data to ground the team’s
complicated calculation. In short, he argues that although the latitude trend
is certainly visible in the data, delete a few data points and it disappears.

CNEOS
director Paul Chodas, however, who was also not involved in the research, says
that variation in where meteorites strike makes sense. The reason is simple:
Most meteorites arrive from the asteroid belt, which circles the sun in the
same plane as Earth and is therefore positioned close to the equator. Genge
agrees that this should be the case, but he is not sure the difference should
be as extreme as what Evatt’s team reports.

The
best path forward, Genge argues, will be to use additional fireball sightings
from NASA, along with new systems in the works to track meteorite impacts, to
see if the trend holds up.  

That
data will not only reveal the best locations to look for these brilliant
streaks of light, but also the best locations to avoid them. That could help inform
where to best place such long-term survival resources as the Global
Seed Vault
, a storage facility built to ensure that crop seeds survive
disasters. Luckily, the bunker is already located at 78 degrees N in Norway’s
Svalbard archipelago. 

The ice-flow analysis could also give scientists a huge leg up when it comes to finding these solar system relics — speeding up the discovery of new clues about the formation of the early solar system and the inner rocky planets, including our own (SN: 4/18/18).

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