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?

the hafnium would have decayed in the meantime. There is enough excess tungsten in Martian meteorites to show that the Martian core formed within 15 million years after planetary accretion, before all the hafnium in the mantle decayed. This estimate of the time of core formation lends support to the magma ocean hypothesis by showing that all the heat of core formation would be available in a short period of time to heat the planet.

A second radiogenic element system, strontium-neodymium, is contained entirely by silicate minerals: Neither strontium nor neodymium dissolve into the iron-nickel core materials. By measuring the isotopes of these elements, it is possible to make a measurement of when the silicate crust of the planet formed, and the answer is again about 20 million years after the solar system formed.Thus the core and crust of Mars formed at about the same time, very early in the life of the solar system.This is an important constraint for scientists making hypotheses about the formation of Mars: Their theories must allow for early, simultaneous formation of the crust and core.

The author and her colleagues at Brown University, Marc Parmentier and Sarah Zaranek, are studying in detail the process of solidification of a magma ocean. Since all the terrestrial planets probably experienced a magma ocean of some size early in their formation, theories of the processes of solidification may make important predictions about later planetary development. A cooling, crystallizing magma ocean would solidify from the bottom up because of the influence of pressure.At high pressures magma can crystallize at higher temperatures than it can at low surface pressures. Pressure acts to force the atoms together into denser forms, and crystals are almost always denser than the liquid they crystallize from.A magma ocean would be convect-ing rapidly and keeping its temperature relatively constant from top to bottom, and so it will crystallize first at the high pressures at its bottom.

As crystals form at the bottom of the magma ocean and build up layers of solid, the remaining liquid in the magma ocean is being fractionated and thus is constantly changing.The results of high-pressure experiments in many laboratories around the world, notably Yingwei Fei's laboratory at the Carnegie Institute of Washington, can be used to predict which minerals will form from the magma ocean at a given pressure, temperature, and liquid composition. If the entire silicate mantle of Mars formed the magma ocean, the ocean would have been about 1,240 miles (2,000 km) deep.At its bottom the pressure would be about 24 million atmospheres (24 GPa) (for more, see the sidebar "What Is Pressure?" on page 48).The first minerals to crystallize would have been majorite, an alumina-poor relative of garnet, and magne-siowustite, which is simply a mixture of iron and magnesium oxide. These minerals are only stable at high pressures and though they can be created in the laboratory in special high-pressure furnaces, they have only been found in natural samples as tiny inclusions in diamonds.

At a pressure of about 14 billion atmospheres (14 GPa), which is reached at a depth of about 680 miles (1,100 km) in the Martian magma ocean, majorite and magnesiowustite would no longer be stable, and the crystallizing minerals would change to garnet, olivine, and pyroxene. Here an unusual thing happens:At these depths olivine and pyroxene are less dense than the silicate liquid and will float, though at lower pressures they are more dense than their coexisting liquid and will sink. As the magma ocean continues to crystallize, then, the garnet will sink while the olivine and pyroxene remain floating in the liquid. Eventually as fractionation continues the olivine and pyroxene will also sink, and in the end, the magma ocean will have fully solidified. The critical consideration at this point is the density profile of the resulting solid Martian cumulate mantle (the solids produced by fractionating a liquid are called cumulates). If higher density materials lie on top of lower density materials, the higher density materials will tend to flow down until the cumulates are ordered from highest density at the bottom to lowest density at the top (for more, see the sidebar "Rheology, or How Solids Can Flow" on page 62).Two factors control the density of the cumulate pile.The first is the minerals that make it up, since the differing crystal structure of minerals will produce different densities. The second is the evolution of the liquid magma composition as fractionation proceeds. All the minerals in these magma ocean models contain both magnesium and iron. All the minerals also prefer to incorporate magnesium into their crystal structures rather than iron, and so fractionation preferentially enriches iron in the liquid. As fractionation proceeds, the minerals are forced to incorporate more iron, because the magma is more and more depleted in magnesium. Iron is more than twice as dense as magnesium. The later minerals to crystallize, therefore, are denser than the early minerals, because they contain more iron.

The author's research group has calculated the effects of mineral type and composition on the density of the cumulate stack resulting from Martian magma ocean crystallization and can predict how the cumulates will flow to reach a gravitationally stable order with the densest material at the bottom.The densest cumulates, as it turns out, are those that crystallize last near the surface.These cumulates sink to the bottom of the mantle in great slow-moving solid drips. The next densest material is a part of the original lowermost cumulates, followed by the layer of garnet that is produced while olivine and pyroxene float. This layer of garnet will sink slightly to lie at the top of the densest material.The overturn of cumulates into a stable stratification (densest at the bottom and lightest at the top) will be complete within a few million years of solidification.

Here is a possible answer to the apparent depletion of alumina in Mars's mantle: Garnet, of all the minerals that will crystallize from the magma ocean, contains the most alumina. When it sinks to near the bottom of the cumulate pile, it carries with it most of the alumina in the magma ocean, leaving the rest of the mantle depleted. The later mantle melting that produced the volcanic rocks on Mars's surface is most likely to have come from shallower depths, from which it would carry a record of an alumina-depleted source. In bulk, though, the mantle of Mars would have a similar alumina budget to the mantle of Earth.

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