Smooth Young Plains

The smooth plains appear to be the youngest terrain on Mercury, and they make up about 40 percent of the area photographed by Mariner 10. The photomosaic on page 102 from Mariner 10 is of the southern half of Mercury's Shakespeare quadrangle, named for the ancient Shakespeare crater located on the upper edge to the left of center. This portion of the quadrangle covers the Mercurian surface from 20°

The chaos region antipodal to Caloris Basin may have been caused by the convergence of shock waves generated by the impact that formed the basin.

Mercury's Shakespeare quadrangle, named for the ancient crater on the upper edge of the mosaic to the left of center, stretches from 20° to 45° north latitude and from 90° to 180° longitude.

(NASA/JPL/Mariner 10)

to 45° north latitude and from 90° to 180° longitude. Bright ejecta rays radiating away from craters cut across and are superimposed on all other surface features, indicating that the source craters are the youngest topographic features on the surface of Mercury. Other parts of this quadrangle are almost bare of craters.These plains could be the result of resurfacing by volcanic activity, since this is the most common process for creating smooth plains in the terrestrial planets. The action of water can create flat plains, such as those in the Northern Hemisphere of Mars, which are thought to be the basin bottoms of oceans dried up long ago. Mercury's obvious lack of water and its immense temperatures courtesy of the Sun make volcanic activity the most likely resurfacing process.

Though these are called young plains, they are only "young" in comparison to the ancient cratered areas.The better-known geologic history of the Moon and models of the early solar system show that there was a period of intense bombardment of the early planets, up to about 3.8 billion years ago. From studying the surfaces of other planets it is thought that even aside from the Late Heavy Bombardment, which was a particularly intense cratering period, cratering rates have subsided over time in the inner solar system (see the sidebar "The Late Heavy Bombardment," on page 103). Geologists who study cratering in images from other planets (often this discipline is called photogeology) have produced graphs of cratering rates versus time, as best they can without radiometric

The Late Heavy Bombardment

T here was a period of time early in solar system development when all the celestial bodies in the inner solar system were repeatedly impacted by large bolides. This high-activity period might be anticipated by thinking about how the planets formed, accreting from smaller bodies into larger and larger bodies, and so it may seem intuitive that there would be a time even after most of the planets formed when there was still enough material left over in the early solar system to continue bombarding and cratering the early planets.

Beyond this theory, though, there is visible evidence on Mercury, the Moon, and Mars in the form of ancient surfaces that are far more heavily cratered than any fresher surface on the planet (Venus, on the other hand, has been resurfaced by volcanic activity, and plate tectonics and surface weathering have wiped out all record of early impacts on Earth). The giant basins on the Moon, filled with dark basalt and visible to the eye from Earth, are left over from that early period of heavy impacts, called the Late Heavy Bombardment.

Dating rocks from the Moon using radioactive isotopes and carefully determining the age relationships of different craters' ejecta blankets indicate that the lunar Late Heavy Bombardment lasted until about 3.8 billion years ago. Some scientists believe that the Late Heavy Bombardment was a specific period of very heavy impact activity that lasted from about 4.2 to 3.8 billion years ago, after a pause in bombardment following initial planetary formation at about 4.56 billion years ago, while other scientists believe that the Late Heavy Bombardment was the tail end of a continuously decreasing rate of bombardment that began at the beginning of the solar system.

In this continual bombardment model, the last giant impacts from 4.2 to 3.8 billion years ago simply erased the evidence of all the earlier bombardment. If, alternatively, the Late Heavy Bombardment was a discrete event, then some reason for the sudden invasion of the inner solar system by giant bolides must be discovered. Were they bodies perturbed from the outer solar system by the giant planets there? If they came from the outer solar system, then more of the material was likely to be water-rich cometary material. If as much as 25 percent of the Late Heavy Bombardment was cometary material, it would have contributed enough water to the Earth to create its oceans. If this model is correct for placing water on the Earth, then a further quandary must be solved: Why didn't Venus receive as much water, or if it did, where did the water go?

dates for planetary surfaces other than the Moon's. These curves allow us to make estimates of the ages of terrain that has craters. Even the smooth young plains of Mercury have some craters, and judging from cratering rates, they were formed about 3.7 billion years ago. These are immensely aged when compared to surface materials on Earth, most of which are younger than 500 million years, but they are still a young feature on the very old surface of Mercury.

The lava flows seem, from remote sensing, to contain only about 3 percent iron oxide, much less than in terrestrial lava flows. From years of laboratory experiments on how rocks melt, it is known that about the same amount of iron goes into the melt as stays behind in the rock.These experiments indicate, therefore, that Mercury's mantle contains only about 3 percent iron oxide, much less than the Earth's and especially less than Mars's mantle, which is thought to contain about 18 percent iron oxide. Mercury, the Earth, and Mars all formed in the inner solar system and should differ only slightly and systematically in bulk composition, so how can Mercury have so little iron in its silicate mantle? Mercury's large core may have left the mantle depleted in iron.

The volcanic history of the planet remains uncertain, since better-resolved photos are needed to clearly determine the relationships among surface features, and the rest of the planet must also be photographed. What can be seen of Mercury in the existing photographs indicates that volcanism seems to have ended early in Mercury's history, leaving the very old, cratered surfaces pristine. These clean, old surfaces prove that Mercury's geologic activity ceased earlier than any other planet;Venus, Earth, and Mars have all had their crusts completely resurfaced by geologic activity, wiping away the cratering record of the early, violent solar system.Though the mechanism that caused the volcanic activity of Mercury is unknown, researchers have noted that these smooth young plains are much like the volcanic pools on the Moon, visible from the Earth as dark surfaces in impact basins.The volcanic activity on the two planets happened at about the same time, but even on the much better studied Moon, it is not agreed why the large volcanic flows, called mare basalts, occurred ("mare" means ocean, one of their first interpretations).

All the other features on Mercury are cut across by Mercury's most predominant surface feature, called lobate scarps (curved cliffs that meet in relatively sharp angles, creating a scalloped shape).The scarps are between 12 and 300 miles (20 and 500 km) long, and each is hundred of yards in height: These are huge surface features.They can be sinuous when viewed from above, but generally they form smooth arcs.The scarps are approximately evenly distributed across the planet's surface and trend in all directions; they are not parallel or in sets. Analysis of three large scarps (which on Mercury are called "rupes")—Adventure Rupes, Resolution Rupes, and Discovery Rupes—indicate that they are formed by thrust faults. A thrust fault is one in which the land surface has been pressed together laterally, so that one side of the fault moves up and over the other. The scarps are therefore asymmetric in cross section, with a more shallowly sloping side and a steeper, cliff-like side. Discovery Rupes is 220 miles (350 km) long and has a maximum height of about two miles (3 km).

The Michelangelo Quadrangle, which lies in Mercury's southern polar region, contains several large lobate scarps in the lower left side of the image in the figure below. The scarps here cut through existing

Mercury's Michelangelo Quadrangle, near the planet's south pole, shows bright crater ejecta rays that lie across other surface features, indicating that the craters formed more recently than the other features. (NASA/JPL/Mariner 10)

Scarps from Planetary Shrinkage

A planet with crust (thickness exaggerated)

Planet shrinks from cooling (greatly exaggerated)

Planet shrinks from cooling (greatly exaggerated)

A planet with crust (thickness exaggerated)

Brittle crust is forced to break along faults, creating curving fault scarps on the planet's surface

Scarps (cliffs formed by moving along faults) may be created by planetary shrinkage, possibly the result of cooling.

surface features, including several impact craters, indicating that the scarps formed after the features they cut.

Thrust faults indicate that the surface of the planet was in compression, possibly caused when the planet's interior cooled and shrank, early in its history. The mechanism for forming thrust faults through planetary shrinkage is shown in the figure above. These scarps pass through both volcanic and cratered terrain, so they cannot be older than the volcanic plains, at about 3.7 billion years. Remember that the age of the solar system is about 4.56 billion years, so if cooling and shrinking caused these scarps, the planet was apparently still cooling and shrinking over a half billion years after its formation, according to the estimates made by examining the photographs of the surface. Measuring the scarps and adding their effect across the surface of the planet has led some researchers to state that the scarps represent a 0.5—1 percent shrinkage of the planet, which means that Mercury's radius shrank by one to two kilometers. Why the planet would relatively suddenly cool and shrink a half billion years after its initial formation is not well understood.

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