Instead of craters, when meteorites land on snow, they make snow carrots.
Early one February morning in 2013, a bright meteor and several flashes were seen near the Ural Mountains in western Russia. The 20-meter-diameter (65 ft) meteoroid broke apart in the atmosphere, releasing most of its energy about 20 to 40 kilometers (12 to 25 miles) up. A few days later, geologists collected 450 small fragments averaging 3 to 6 centimeters wide from an area 40 kilometers (25 miles) south of Chelyabinsk. They added up to 4 kilograms (9 lbs) of meteorites.
Most of these were found in funnel-shaped holes in the snow. While the larger bits pierced down to 70 centimeters below, the smaller fragments got stuck in “snow carrots.” For the bottom 15 to 25 centimeters, the walls of these holes narrowed into a cone, and at the tip of the carrots, the meteorites were surrounded by a dense shell of coarse-grained snow. There are two main hypotheses for snow carrot formation: The fluffy snow became compacted by the impact, or the warm fragments caused partial snow melt.
To study the dynamics and thermal evolution of these fragments, an international team led by Robert Luther of Museum für Naturkunde modeled catastrophic fragmentation followed by penetration into snow. Seconds after the Chelyabinsk meteor exploded, the individual fragments lost 90 percent of their mass and decelerated down to 2 kilometers (1.2 miles) per second. After spending a few minutes free falling, they hit the snowy surface with a velocity of 28 to 74 meters (92 to 243 ft) per second.
These simulations revealed that the fragments would have had plenty of time to cool down. By the time they smacked into the surface, they should have reached atmospheric temperatures of minus 20 degrees Celsius. That means the snow carrots likely formed from mechanical forces.
As the projectile encountered snow, it pushed the material out of the way, forming a funnel-shaped crater. Fluffy, porous snow next to and underneath the fragment became compressed—at initial compaction speeds of 80 meters (262 ft) per second—which increased the density by up to 18 percent across the funnel’s 3.4-centimeter-thick walls. Over the next 10 milliseconds, Science explains, the denser snow gradually slowed the meteorite down and the walls of the funnel narrowed accordingly.
Their findings [pdf] were presented at the 46th Lunar and Planetary Science Conference in Texas last week. In this snapshot series, red corresponds to higher density and blue to lower density. The time between each frame is 2 milliseconds.
Images: C. Lorenz (top), R. Luther et al. (middle)