8. Mars: the red planet
Highland craters, lowland plains, and remnant magnetism
A world divided
The two hemispheres of Mars have distinctly different terrain. The planet’s southern half is extremely ancient, generally elevated, and highly cratered. Like the lunar highlands, most of the craters in the southern landscape on Mars date back to an intense bombardment by meteorites early in its history, estimated to about 3.9 billion years ago. The northern hemisphere, by contrast, consists mainly of younger, lower-lying, flat plains that are predominantly of volcanic origin and have been greatly transformed over the eons. It is as if the two hemispheres somehow fused together to form a divided world.
Heavily cratered highlands
The cratered scars of impacting meteorites can be found all over Mars, but the craters are more densely concentrated in the southern hemisphere where the terrain resembles the lunar highlands. The largest meteorites have gouged huge impact basins out of the Martian surface, throwing up mountains along their rims. The smooth floors of the biggest impact basins are light-colored, circular features that have been observed with ground-based telescopes. They retain their classical designations made more than a century ago - such as Argyre for the “silver” island at the mouth of the Ganges river and Hellas, the Greek word for Greece. The giant Hellas basin, some 2.3 million meters across, is covered with white frost in southern winter, forming a brilliant white disk seen from Earth.
The inquisitive, close-up eyes of spacecraft detect numerous craters that are smaller than the impact basins. Some of these craters become frosted in the southern autumn and winter. Large craters on Mars are named after astronomers and scientists who have studied Mars, including Schiaparelli; smaller craters are named after villages on Earth.
The ejected material surrounding some craters on Mars can be explained by supposing that the ground contained water or ice when they were formed. The flowing splashed pattern looks like that formed when you drop a pebble in the mud. On Mars, the heat of impact may have melted or vaporized water ice frozen in the Martian ground just below the surface, like the layers of permafrost underlying the Arctic landscapes of Earth. Or the impact might have released liquid water from the ground beneath the permafrost. The steam and liquid water then acted as a lubricant for the flowing debris and the muddy material sloshed outward like a wave until it dried and stiffened, or became cool and refroze.
Lowland volcanic plains
The extensive lowland plains that dominate the northern hemisphere of Mars are relatively flat and sparsely cratered. Most of them appear to be covered with lava flows, and are thus of volcanic origin. The plains of Mars are designated by the names of lands, followed by the Latin planita, meaning “a level surface or plain”. But they are not completely smooth. Volcanoes rise up in some of them, mesas and buttes in others; boulders or dunes give them a small-scale texture.
The Latin term planum, meaning “plateau or high plain” follows the name of flat elevated regions, in contrast to the a low-lying planita. Most of the planitae are located in the northern hemisphere, while the plana are found just south of the equator. Another Latin name, terra, is used to designate an extensive land mass in the older, heavily cratered highlands.
Leftover magnetic fields
Earth has a global, dipolar magnetic field, like a bar magnet, with a north pole, a south pole and magnetic fields that loop between them. It is generated by the dynamo action of currents in its hot, molten metallic core. In contrast, there is not a global magnetic field on Mars, so its internal dynamo is now either extinct or much weaker than Earth’s. The Martian magnetic field exists only in local regions on the surface. Most likely, the local fields are fossil remnants of an early time when the Martian dynamo was sufficiently vigorous to magnetize the planet’s crust and maintain a dipolar field that is now imprinted in the ancient rocks.
The dichotomy between the northern and southern hemispheres of Mars is retained in its leftover magnetism. The magnetic fields that have survived from the planet’s active youth are located within the heavily cratered southern hemisphere, and the northern lowlands are now largely free of magnetism. Magnetic fields were therefore present when the highlands were formed. But when the impacts stopped and the lowland plains originated, there was no magnetic field left – only the remnants currently preserved in the ancient highland crust.
The remnant magnetic fields are also missing from the very large, southern impact basins, such as Hellas and Argyre. When these impacts occurred, the internal dynamo must have been shut down, and the global fields were no longer present. Otherwise, the impacted material in the planet’s crust would have been magnetized.
The magnetic fields that have survived on Mars are banded into a striped, bar code pattern of alternating magnetic polarity, each stripe extending up to 2 million meters from east to west. The magnetic field in one band points out of the planet, and the adjacent one points in; next to that it points out again and so forth. These data remind us of reversals in the Earth’s magnetic polarity that have been recorded in volcanic lava which has hardened into rock. The volcanic rock preserves the direction of the magnetic field at the time it solidified, and rocks of different ages indicate that the terrestrial magnetic field has regularly flipped or reversed direction, a few times every million years.
(page 6 of 10)
Copyright 2010, Professor Kenneth R. Lang, Tufts University