Hypervelocity impacts in the laboratory on hot rock targets

The variation of impact crater size in rock has been investigated as a function of target temperature in the range 150 K - 1150 K. Three rock types were used: limestone, sandstone and basalt. A total of thirty impacts were observed, at a typical impact speed of 5 km s-1 with a 0.8 mm diameter stainless steel spherical projectile. The three rocks behaved in two ways. The craters in limestone and sandstone initially grew in size, until a maximum was reached at around 500 K. Crater size then fell again as temperature increased further. For basalt however, crater size fell as temperature increased, reaching a constant level above 800 K. This strongly suggests that crater sizes seen in experiments on Earth, should not be taken as typical, rather they are a function of rock temperature. It proved difficult to relate crater size to target strength, as data in the literature on rock strength vs. temperature were in some cases contradictory.