Марстағы кратерлер: қатты қалдықтар мен мұздың әсері

Specific type of impact crater

Rampart craters are a specific type of impact crater which are accompanied by distinctive fluidized ejecta features found mainly on Mars. Only one example is known on Earth, the Nördlinger Ries impact structure in Germany. A rampart crater displays an ejecta with a low ridge along its edge. Usually, rampart craters show a lobate outer margin, as if material moved along the surface, rather than flying up and down in a ballistic trajectory. The flows sometimes are diverted around small obstacles, instead of falling on them. The ejecta look as if they move as a mudflow. Some of the shapes of rampart craters can be duplicated by shooting projectiles into mud. Although rampart craters can be found all over Mars, the smaller ones are only found in the high latitudes where ice is predicted to be close to the surface. It seems that the impact has to be powerful enough to penetrate to the level of the subsurface ice. Since ice is thought to be close to the surface in latitudes far from the equator, it does not take a large impact to reach the ice level. Based on images from the Viking program in the 1970s, it is generally accepted that rampart craters are evidence of ice or liquid water beneath the surface of Mars. The impact melts or boils the water in the subsurface producing a distinctive pattern of material surrounding the crater. Ryan Schwegman described double layered ejecta (DLE) craters as showing two distinct layers of ejecta that appear to have been put in place as a mobile, ground hugging flow. His measurements suggest that ejecta mobility (the distance ejecta travels from the crater rim) typically goes up with increasing latitude and may reflect ice concentration. That is the higher the latitude, the greater the ice content. The lobateness (curved shape of the perimeter of ejecta) usually goes down with increasing latitude. Furthermore, DLEs on sedimentary ground seem to display higher ejecta mobility than those on volcanic surfaces. A detailed discussion of various kinds of Martian craters, including double layer ejecta craters (rampart craters) can be found in a 2014 paper by David Weiss and James Head.

Single layered ejecta craters are one type of rampart crater. They have one ejecta lobe that extends 1 to 1.5 crater radii from the rim of the crater. They have an average diameter of 10 km. Although present at all latitudes, they are most common near the equator. Their average size increases the more distant from the equator. It has been suggested that these types of craters are produced by impact into icy ground. Specifically, it is an impact that does not go entirely through the icy layer. The increase in size away from the equator is explained by a possible greater thickness in the icy layer away from the equator.

In the Mariner and Viking mission a type of crater was found that was called a "pancake crater." It is similar to a rampart crater, but does not have a rampart. The ejecta is flat along its whole area, like a pancake. Under higher resolutions it resembles a double layer crater that has degraded. These craters are found in the same latitudes as double layer craters (40–65 degrees). It has been suggested that they are just the inner layer of a double layer crater in which the outer, thin layer has eroded. Craters classified as pancakes in Viking images, turned out to be double layer craters when seen at higher resolutions by later spacecraft.