The Ages of the Lunar Features

An interesting attribute of a lunar feature is the time it was formed. The ages of different features on the Moon are inferred from several sources.

The most precise ages are determined from measurement of isotope ratios in our precious rock samples; association of the samples with specific features establishes their time of formation. Since no samples have been returned from the far side, estimates of the age of events there can only be determined by estimates of ages relative to features of the near side.

When rock samples are unavailable, geologists have resort to less precise measures. For example, sharply defined fea-

n tures are recent because meteorite bombardment softens o o the edges of features by a process called mass-wasting. And 0 counts of crater densities are used to estimate the time a sur-th face has been exposed to bombardment.

A very powerful principle used by geologists is superposi-

Otion. Where one feature overlays another, it is considered to be a younger feature, unless there is evidence to the contrary. ide There are exceptions. For example, an impact may throw a r Si boulder from an older layer of rock onto a younger layer. Far But the general rule is useful, especially in the emplacement he of the ejecta blanket of one basin on top of the ejecta layer T of another. The vertical sequence of layers from different sources is called stratigraphy.

The principle of superposition has other implications than aging. When one layer is deposited on another, the topography of the surface reflects the configuration of the lower layer, as modified by variations in depth of the upper layer. For example, it is sometimes possible to detect the shape of a crater that has been buried by an ejecta blanket. If a crater forms on a slope or rim of an earlier basin, its shape is superimposed on the earlier shape.

Sometimes one event causes another, even though superposition is not involved. For example, the shock wave of a younger impact event can be seen to have caused a landslide at the rim of older nearby crater.

The notes accompanying the photos in this book frequently point out examples of superposition and causality.

Named Age Ranges of the Moon

Systematic study of stratigraphy, together with other clues, establishes a chain of evidence that leads to a relative age range of most of the features of the Moon. These age ranges, called periods, are named for archetype features. In order from the oldest to the younger, they are the pre-Nectarian Period, Nec-tarian Period, Early Imbrian Period, Late Imbrian Period, Eratosthenian Period, and Copernican Period.

The impact of the Nectarian Basin defines the boundary between the pre-Nectarian and Nectarian Periods. The Imbrium Basin event introduces the Early Imbrian Period and the Orientale Basin event (the youngest basin) begins the Late Imbrian Period. The Eratosthenes crater is the archetype of the Eratosthenian Period and Copernicus is the archetype of the Copernican Period. The Early and Late Imbrian Periods are often called epochs.

The far-flung ejecta of these giant basins overlaps and provides a precise means for judging relative ages. A summary of estimates for the absolute age of the periods of lunar history is given in Table 3.1 (Wilhelms, 1987). Of course, these absolute ages have uncertainty ranges, but these are not shown in the table.

After the Orientale event 3.8 billion years ago, there were no further basins formed; in fact, all subsequent craters are smaller than 190 km in diameter. The time of heavy bombardment had passed. The ejecta blankets of the smaller craters formed in later times no longer overlapped, except rarely. Relative ages of the younger features are established by judgements about the degree of erosion of them; they become less sharp as they are hit by the continuing arrival of small impactors. If a feature has a similar degree of erosion to the crater Eratosthenes, it is judged to be of the Eratosthenian Period. If it appears to be as fresh or fresher than the crater Copernicus, it is assigned to the Copernican Period.

The designations of these age periods predate the views of the far side of the Moon and so they are all defined in reference to features on the near side. The relative ages of far side features are based on the stratigraphic relations, feature by feature, from one basin's ejecta to another across the limbs all the way to the far side. For this purpose, especially important basins are Crisium, Orientale, and Imbrium, since their deposits fall on the far side. Features on the far side that are younger than the Orientale Basin are assigned to the Late Imbrian, Eratosthenian, or Copernican Periods depending on the sharpness of their features (mass-wasting is a form of erosion that smoothes features with time) and on counts of subsequent (superposed) craters.

A somewhat controversial indication of age among younger impact features (late in the Eratosthenian Period or early in the Copernican Period) is the presence of rays of bright material forming a star-shaped pattern. One hypothesis is that these rays dim uniformly with age and their presence or absence can be used to establish relative age. This rule has met with contradiction; some craters that appear fresher than others have lesser rays.

Period

Age range (billions of years ago)

Interval (millions of years)

Pre-Nectarian

4.6 to 3.92

680

Nectarian

3.92 to 3.85

70

Early Imbrian

3.85 to 3.80

50

Late Imbrian

3.80 to 3.15

650

Eratosthenian

3.15 to ~1.0

~2,150

Copernican

~1.0 to present

~1,000

Table 3.1. Estimates of absolute ages of the periods.

Table 3.1. Estimates of absolute ages of the periods.

Recent research (Hawke, 2007) suggests that there are two mechanisms for rays to darken and fade. All rays start to darken with time because exposure of the fresh powdered material to the solar wind causes the iron in the grains to aggregate, absorbing radiation. In time, the effect stabilizes, reaching a condition called maturity. The second effect relates to impacts in areas composed of anorthositic highlands near maria. The anorthositic minerals are inherently lighter than the basaltic mare, even after they become mature. Consequently, they only dim as meteoroid bombardment mixes the rays with the substrate, a process that takes much longer than the solar wind effect. These rays are called compositional rays. The time required to thoroughly mix a compositional ray with its substrate so that it fades depends on the thickness of the ray, and therefore on the size of the associated impact feature.

The suggestion has been made that the duration of the Copernican Period be redefined in light of the new understanding. It is proposed (Hawke, 2007) that the start of the Copernican Period be defined to be the time of the impact event of Copernicus, whose rays are now nearly mature but have bright compositional contrast. In the past, Copernicus has served as the archetype crater of the Copernican Period, which was allowed to extend prior to the Copernicus impact event. It will be interesting to see how this proposal is adopted in future geologic research and mapping work. If it is, then craters that are located on the far side, provided they are outside of the South Pole-Aitken Basin and the very small other areas of mare, will be considered Copernican only if they have visible (immature) rays. This is because the composition of those areas of the far side is very uniform, so that the appearance of compositional rays blends into that of the substrate.

Since the discovery of the South Pole-Aitken Basin and the proposal of the Near Side Megabasin, it might be useful to designate new periods to decompose the pre-Nectarian Period. For example, one could define a period between the first formation of the lunar crust to the Near Side Megabasin event, a period between that event and the South Pole-Ait-ken Basin event, and a period between that event and the Nectaris Basin event.

Chapter 4

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