Arabia Quadrangle - Layers

Layers

Many places in Arabia are shaped into layers. The layers can be a few meters thick or tens of meters thick. Recent research on these layers by scientists at California Institute of Technology (Caltech) suggest that ancient climate change on Mars caused by regular variation in the planet's tilt, or obliquity may have caused the patterns in the layers. On Earth, similar changes (astronomical forcing) of climate results in ice-age cycles.

A recent study of layers in craters in western Arabia revealed much about the history of the layers. Although the craters in this study are just outside the boundary for the Arabia quadrangle the findings would probably apply to the Arabia quadrangle as well. The thickness of each layer may average less than 4 meters in one crater, but 20 meters in another. The pattern of layers measured in Becquerel crater, suggests that each layer was formed over a period of about 100,000 years. Moreover, every 10 layers were bundled together into larger units. The 10-layer pattern is repeated at least 10 times. So every 10-layer pattern took one-million years to form.

The tilt of the Earth's axis changes by only a little more than 2 degrees; it is stabilized by the relatively large mass of our moon. In contrast Mars's tilt varies by tens of degrees. When the tilt (or obliquity) is low, the poles are the coldest places on the planet, while the equator is the warmest—as on Earth. This causes gases in the atmosphere, like water and carbon dioxide, to migrate pole ward, where they freeze. When the obliquity is higher, the poles receive more sunlight, causing those materials to migrate away. When carbon dioxide moves from the poles, the atmospheric pressure increases, maybe causing a difference in the ability of winds to transport and deposit sand. Also, with more water in the atmosphere sand grains may stick and cement together to form layers. This study of the thickness of layers was done using stereo topographic maps obtained by processing data from the high-resolution camera onboard NASA's Mars Reconnaissance Orbiter.

Recent research leads scientists to believe that some of the craters in Arabia may have held huge lakes. Cassini Crater and Tikonravov Crater probably once were full of water since their rims seem to have been breached by water. Both inflow and outflow channels have been observed on their rims. Each of these lakes would have contained more water than Earth's Lake Baikal, our largest freshwater lake by volume. The watersheds for lakes in Arabia seem to be too small to gather enough water by precipitation alone; therefore it is thought that much of their water came from groundwater.

Another group of researchers proposed groundwater with dissolved minerals came to the surface, in and later around craters, and helped to form layers by adding minerals (especially sulfate) and cementing sediments. Upon close examination, Arabia layers appear to have a slight tilt. This tilt supports formation with the action of a rising water table. A water table generally follows the topography. Since the layers slope slightly down toward the northwest, the layers may have been created by groundwater, rather than a single large sea that has been suggested.

This hypothesis is supported by a groundwater model and by sulfates discovered in a wide area. At first, by examining surface materials with Opportunity Rover, scientists discovered that groundwater had repeatedly risen and deposited sulfates. Later studies with instruments on board the Mars Reconnaissance Orbiter showed that the same kinds of materials exist in a large area that included Arabia.

• Layers in Gill crater, as seen by HiRISE under HiWish program.

• Close-up of layers in Gill crater, as seen by HiRISE under HiWish program.

• Close-up of layers on edge of a mesa in Gill crater, as seen by HiRISE under HiWish program.

• Layers under cap rock of a pedestal crater, as seen by HiRISE under HiWish program. Pedestal crater is within the much larger Tikhonravov Crater.

• Close-up of some layers under cap rock of a pedestal crater, as seen by HiRISE under HiWish program.

• Close-up of some layers under cap rock of a pedestal crater and a dark slope streak, as seen by HiRISE under HiWish program.

• layers in a butte in Arabia, as seen by HiRISE under HiWish program.

• Layers in Arabia, as seen by HiRISE under HiWish program.