By Brendt C. Hyde and Ian Nicklin
As rocks from space come through the Earth’s atmosphere they are travelling at speeds as high as 70 km/s. At these speeds, air in front of large space rocks gets compressed and, in turn, heats the rocks surface forming a melt layer. This heating combined with frictional heating (air molecules smashing into the rock) and ablation (material being ripped off of the rock) causes the rock to become more rounded and aerodynamic. This process can also lead to ‘thumbprint’ impressions in the rock referred to as regmaglypts (See figure 1). By the time a meteorite hits the Earth’s surface, the melted surface has cooled to a glassy layer referred to as a fusion crust and regmaglypts and other evidence of melting are solidified in place.
In some cases, a meteorite will orient itself while it is travelling through the atmosphere. This occurs because some orientations are simply more aerodynamic then others. The leading side of these meteorites develop a smooth surface with evidence of molten material streaming towards the ‘back’ of the meteorite. This process gives the meteorite the shape of a nose cone, similar to those seen on the front of airplanes or on the leading edge of a rocket. These nose cones can unfortunately break up in the atmosphere or when the meteorite hits the Earth. They can even be degraded by weathering on the Earth’s surface over time. However, occasionally we are treated to a nice intact nose cone.
The ROM has a number of these ‘oriented nose cones’ in its collection and one spectacular example just happens to be this months ‘meteorite of the month’ (See figure 2). Visitors to the museum can also see this specimen on display in the Teck Suite of Galleries: Earth’s Treasures. This meteorite happens to be a type of meteorite referred to as a chondrite. However, the nose cone feature can occur on many types of meteorites. Similar melt features, a little less distinct, can be seen on the martian meteorite NWA 3171 also on display. Next time you are at the ROM take another look at the meteorite cases. Now that you’ve seen some examples of melt features, you will undoubtedly be able to find more.