What happens when a meteorite hits snow? Instead of forming classic impact craters, the fragments form strange funnels of dense snow diving into the surface instead. Here's the physics of how "snow carrots" form.

When hunting for meteorite fragments, geologists found them at the bottom of dense funnels of coarse snow they named "snow carrots." This snow carrot has been dug up and flipped upside-down. Image credit: Cyril Lorenz

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When the Chelyabinsk meteorite exploded over Russia in February 2013, it showered rock fragments over the landscape below. In the following days, geologists from the nearby Vernadsky institute tracked down 450 fragments, most only 3 to 6 centimeters large. At a total mass of 4 kilograms, the fragments account for just 0.02% of the original mass of the meteorite: most of the meteorite was lost through ablation as the atmosphere chipped, scoured, and evaporated material.

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The first fragments collected from the Chelyabinsk meteorite explosion in 2013. Image credit: Vernadsky Institute

When the Chelyabinsk meteorite exploded, it was in the heart of a Russian winter. The ground below was buried in roughly 70 centimeters of snow, a compressible, porous blanket over the land. The largest fragments punched through the snow, hitting the frozen ground. The hole's walls contained strangely coarse snow, but otherwise it wasn't particularly noteworthy. But the little fragments did something strange as they got stuck within the snow. Instead of blasting out a hole or a crater, they burrowed into funnelling holes with the meteorite at the bottom. The walls had the same strangely coarse snow, creating a "snow carrot" 15 to 25 deep centimeter with the meteorite firmly encased in a shell of dense snow at the base.

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The smallest fragments created snow carrots, a dense funnel of snow reaching into the ground [left]. The snow is dense enough to firmly grasp the meteorite even when unearthed and inverted [right]. Image credits: Cyril Lorenz [left] and Svetlana Demidova [right]

Puzzled, the researchers had two hypothesis: either the fragments were still hot when they impacted, partially melting the snow, or the fragments carried enough kinetic energy to ram the snow, compacting it as the impact slowed its fall. Either the meltwater recystallized at a higher temperature, forming coarser snow, or the water vapor along the channel walls could have condensed into coarse snow or sublimated into hoarfrost.

To test the hypotheses, researchers at Museum f√ľr Naturkunde, the Institute for Dynamics of Geospheres, and the Planetary Science Institute developed models to investigate the fragments' fall. They presented their results this week at the 46th Lunar and Planetary Science Conference in Texas.

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Model of fragment impacting snow, where red indicates higher density and blue lower density. Image credit: Luther at al.

The models start with a catastrophic disruption of either small 40 gram fragments or larger 10 kilogram chunks burning through the atmosphere. They lost 90% of their mass through ablation, hitting the ground as just 1.3 centimeter to 8.5 centimeter large chunks. For almost every starting scenario, the fragments cooled to ambient temperatures long before hitting the surface. (The only exception? A large fragment starting as a molten sphere of rock and metal.) Unless something strange was happening and the fragments weren't simply in free-fall from the greedy grasp of gravity, an ordinary fragment from the Chelyabinsk meteorite would have been too cool to melt through the snow. At the same time, when those modelled fragments hit snow, the fluffy, porous substance is compressed into a funnel-shaped crater with increasing density along the hole's walls: snow carrots. The density increase is substantial: up to an 18% increase stretching out up to 3.4 centimeters from the crater walls. As theoretical models go, it is a tidy fit for the field observations of the geologists who collected the many tiny fragments of Chelyabinsk.

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Comet particles left funnel-tracks in aerogel during the Stardust mission. Image credit: NASA/JPL

The best bit of this research is that we've seen something similar before: the tiny particles of comet caught by the Stardust mission created similar funnels in light, strong aerogel. This could be the start of understanding impact events of small, dense objects into porous, compressible materials.

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Read more: Snow Compaction During The Chelyabinsk Meteorite Fall. Tip via Science.