geology of mars cratering image
Crater Morphology
Ejecta Blankets
Crater Degradation

Ejecta Blankets
Compared with the Moon and Mercury, ejecta blankets and fields of secondary craters are less prominent on Mars. Rayed craters are also rare. However, the morphology of crater ejecta on Mars is quite unique. Ejecta blankets of lunar craters are usually blocky near the rim and grade outward with increasingly more fine grain particles until the blanket merges with the surrounding area. These features are consistent with the ballistic emplacement of the ejecta. Many Martian craters, however, have ejecta deposits that appear to have flowed over the surrounding surface like mudflows.

 figure 3.2

These craters are known as rampart craters, fluidized craters, or splash craters. Crater Yuty is a spectacular example of a rampart crater (Figure 3.2) . The ejecta consist of several relatively thin sheets with tongue-shaped fronts. It appears that the debris flowed outward like huge splashes of mud. A ridge was formed at the front of each ejecta lobe. To the right (or east), the ejecta seemed to flow uphill onto a degraded rim of an earlier crater. To the south, a smaller and older crater separated two large lobes, but was eventually filled with liquefied debris. Liquid water might have been incorporated into the materials excavated from the crater, creating a fast moving ejecta mudflow. Water might have been produced if the ice in the regolith was melted upon impact. (Note that regolith represents solid materials lying on top of the bedrock, including soil, rock fragments, and other materials.)

Apparently, ejecta from those Martian craters became fluid, allowing it to flow after excavation. Craters smaller than 15 km in diameter usually have a single ejecta sheet, extending out to about one crater radius (Figure 3.3) . Fronts of ejecta lobes often formed abrupt ridges or escarpments.

 figure 3.3

< back    1    2    3    next >

geology of mars