Splotch / Atmospheric blast

Some bodies explode in the atmosphere and never reach the surface. Their shock wave leaves a roundish radar-dark spot on the ground. These are radar splotches.

Splotches are dark spots without the crater we would expect at their centre. The remains of the body may have reached the surface, but they had slowed down so much that they no longer made a crater. Dark spots also occur with craters. There the incoming body managed to break through the atmosphere to the surface; the smallest craters are 2–3 km across.

A bright, diffuse halo usually surrounds the dark splotch. The dark interior is 10–70 km across. The bright outer ring can be followed to a diameter of up to 200 km (Soderblom and Chadwick 1992).

Splotches and rings occur in every combination of dark and bright radar brightness. This suggests that the interaction between atmosphere and surface is complex. It is especially complex when the body has broken up and its fragments explode at different times.

Such interactions during flight can also create thin parabolic patterns on Mars. The plate shows an unusual pattern, too. It looks strikingly like Pac-Man, the computer-game character. Ejecta blankets with a missing segment are well known; an oblique impact causes them. Because of the dense atmosphere, the missing segment in the impact direction appears even at fairly high entry angles, higher than on other bodies.

The shock wave was strongest in the central dark area. There it pulverised the rock surface. Farther out it only cracked or churned up the debris. This made the surface rougher there, hence the brighter radar signal (Zahnle 1992).

Splotches are found mainly in lower-lying areas. So they appear where the impacting body travelled farther through the atmosphere, or where the air pressure or temperature is higher.

Volcanic pyroclastic flows create similar diffuse, but bright, radar spots.

About 400 splotches have been found on Venus. This is fewer than expected, since small impacting bodies should be more common than large ones. Yet there are only 400 splotches for a total of 935 normal craters. Perhaps splotches, which mark fine-grained dust, fade over time. Double splotches (splotches close to each other) make up 14%. This matches the share of double craters on Earth and the share of binary asteroids (Cook et al. 2003).

On Earth, the traces of superbolides (fireballs) match these features. One example is the Tunguska event (Illés-Almár 2003), which flattened 80 million trees over 2000 km² of taiga. On a surface without vegetation, such an atmospheric impact would have left a radar splotch. A similar event took place over Antarctica in 2004.