Lunar and Planetary Institute

# Impact Cratering LabPart II:  Features and Motion of Crater Ejecta

Part I of this lab introduced the mechanics of crater formation and the morphology of simple and complex craters. The formation and structure of impact craters was demonstrated by movies of experimental impacts. The diversity of crater morphologies was illustrated by images of simple and complex craters. Part II of this lab introduces the features and motion of crater ejecta. The lab exercises make use of movies of experimental impacts to demonstrate the motion of ejecta. Calculations of the motion of ejecta will be performed using the ballistic equation.

## Background

An impact crater is much more than a mere hole in the ground. The material excavated from the crater and deposited on the surrounding terrain is called ejecta (Figure 7). Ejecta deposits can be just as diverse as the crater morphology. Differences in the character of ejecta deposits indicate differences in the target material, partitioning of kinetic energy, and planetary environmental factors, such as surface gravity and atmospheric effects. In general, ejecta takes two forms:  a continuous hilly covering of material from the crater rim outward to a distance of about a crater radius (called the continuous ejecta blanket), and swarms of smaller craters (called secondary craters) formed by the impact of individual fragments and clots of fragments thrown from the crater, mostly beyond the continuous ejecta blanket. Crater rays are bright material excavated by small secondary craters. Individual particles of ejecta travel on looping paths called ballistic trajectories, similar to the paths of artillery shells. The distance that ejecta can be thrown on a ballistic trajectory depends both on the angle and velocity at which the material is ejected and on the gravity field of the planet. This distance, d, is called the ballistic range and is mathematically expressed by

d = (V2 sin2θ)/g

where V is the ejection velocity, θ is the ejection angle from the horizontal (surface), and g is the planet's gravity field. (This expression does not take into account the radius of curvature of the planet or satellite, or the decrease in gravity with height.)

 Figure 7. Bright ejecta around a lunar crater. NASA Image AS15-9348.

As you can see from this expression, for a given ejection velocity and angle, ejecta will travel farther on a planet with a weaker gravity field. Also, on the same planet (constant g) and for the same ejection velocity, ejecta will travel farther when ejected at an angle of 45° because the sine of angles greater or less than 90° (2 × 45°) is <1. In large impacts, ejecta can be dispersed over very wide areas of a planet or satellite. On the Moon, some rays and secondary craters extend for thousands of kilometers from their source crater. If a fragment is ejected at a velocity greater than the escape velocity of the planet or satellite, it will leave the planet altogether. Certain meteorites originated from the Moon and Mars, where they were accelerated by impacts to velocities greater than the escape velocities of these bodies.

Ejecta deposits can take several forms depending on the target properties, impact conditions, and other environmental factors. On the airless Moon and Mercury, the ejecta deposits are characterized by a hilly continuous deposit surrounding the crater, called the continuous ejecta blanket. This continuous deposit grades into strings (Figure 8) and clusters of small craters called secondary craters. Secondary craters are formed by the impact of large pieces of ejecta. The ejecta can travel great distances because of the low gravity field (see above equation) and the lack of an atmosphere. Some rays from the lunar crater Tycho extend for over 2000 kilometers. The bright rays on the Moon and Mercury are mostly strings of secondary craters that have disturbed the soil and brightened the surface.

Ejecta deposits are not always evenly distributed around an impact crater. Sometimes there is more ejecta on one side of the crater than the other. Impacts that occur at a relatively low angle spray ejecta preferentially in the direction of impact, the downrange direction. There is often a zone on the uprange side of the crater with little or no ejecta. The shallower the impact, the more asymmetrical the ejecta deposit.

In this activity you will study an experimental impact that divides the ejecta into packets to demonstrate ballistic trajectories of individual particles in the ejecta curtain. You will also make various measurements and analyses, including ejection velocity, asymmetry of ejecta blankets, and extents of outflow ejecta.

 Figure 8. Chain of secondary craters on the lunar surface. Image Credit: NASA/ Goddard Space Flight Center/Arizona State University.