Lunar Basins

From The Moon
Jump to: navigation, search

Lunar Basins

(glossary entry)


Description

Lunar Impact Basins are the most important landforms on the Moon. Few are well known, but many exist.

Definitions


Nomenclature

Nearside basins contain maria and so have names derived from their mare names: e.g. the Imbrium basin. When new basins were first discovered by Hartmann and Kuiper (1962) they gave locational names such as “the basin near Schiller”. The US Geological Survey decided – as far as I know without any IAU approval – to name basins after craters on either side of the basin; hence the basin near Schiller became the Schiller-Zucchius basin. This hyphenated system is now standard:
  • Basins filled with maria are named for the mare, and basins that were previously recognized as craters still bear the crater name. Basins located between craters have the names of the two opposite craters with a hyphen in between.


Certainty of Existence

Some impact basins are well defined by multiple rings, central depressions, and surrounding ejecta deposits. Most basins lack some of these characteristics, but still can be relatively confidently identified as basins. Older and more obscure features have greater uncertainty, and Clementine altimetry data has led to the tentative identification of some possible basins that are defined solely as depressions. I here classify basins as certain (1), probable (2) or uncertain (3). However, this terminology may give the impression that some of the 2s and all of the 3s may not in actuality be basins. I doubt if that is correct, but some basins are so poorly imaged that we can not be 100% certain. Some recently proposed basins that have not yet been examined carefully are considered as proposed (4) – they will ultimately be upgraded or removed from the list.

Basin Diameters

Impact basins typically have multiple concentric rings, with one, the rim (Col. H), being . considered equivalent to the rim of a normal impact crater. The rings tabulated here are mostly from Pike and Spudis (199x) and Wilhelms (1987). P&S generally identify more rings than any other lunar scientists, and various of their outer ones are very difficult to see.

Depth

Measurements of impact basin depths have only been practical since the acquisition of altimetry data by the Clementine spacecraft. There are two main sources, papers by Spudis and colleagues (1993, 1995, 1996) and a single paper by Williams and Zuber (1998). Strangely, his most complete listing is in an abstract (Spudis & Adkins, 1996) which gives diameters, depths, volumes and rim heights for 21 basins. In general, the fact that these depths are less than those of Williams and Zuber follows from the averaging of multiple rim heights for each basin by Spudis and Adkins. Still, the worse differences – 6 km vs 3.57 km for Freundlich-Sharonov – suggest that care should be taken in using the data. Column L gives Williams & Zuber depths and Column M gives Spudis and Adkins values.

Rim Height

The only measures of basin rim heights are from Spudis and Adkins (1996) and were determined by subtracting the average surrounding elevation from the average rim elevations.

Mare Thickness

Most nearside basins have a little (Nectaris) to a lot (Serenitatis) of mare on their floors, but the actual measurement of how much is difficult. Estimation of mare thicknesses have been made by a variety of inexact methods including a consideration of the inferred depths of almost lava filled craters and by modeling gravitationally anomalies. All methods are fraught with significant potential errors. I have accepted results from two different methods that seem to span the range. Column P gives the gravitational estimates of Potts and von Frese (2003). This is the only method that has provided depth estimates for both sides of the Moon. But I do not believe that lavas in Imbrium are only 1.1 km deep when we see that the similar sized but unflooded Orientale is about 6 km deep! The crater morphometric method of Williams and Zuber (1998) give values (Col. O) that are geologically reassuring. “Crater morphometric” means assuming the depth to diameter ratios of least flooded basin are the same as the most flooded (and also compensating for subsidence) so that a diameter yields an original depth; subtracting the current depth yields lava flow thickness…we hope.

Mascon Presence

When orbiting spacecraft are pulled sightly nearer the lunar surface than otherwise, we say that there is mascon. A mass concentration occurs where a multi-kilometer deep column of lunar rocks is denser than surrounding rocks. Mascons only occur in some basins, but not all. Mascon basins generally have mare fill, but probably part of the mascon is due to upwarded mantle material under the basin.

Gravity Anomaly

Potts and von Frese (2003b) have calculated the size of gravitational anomaly for most basins. A plus value indicates a mascon, and a negative value, a maslite – a deficiency of mass.

USGS Age

The US Geological Survey developed a stratigraphic system to place all lunar landforms into a positional and time sequence. Here is the sequence:

Superposed Crater Density

Wilhelms (1987, p 148; 179) counted the number of superposed impact craters larger than 20 km on each basin to determine a superposed crater density expressed in the units of number of craters per 106 km2. These densities provide an indication of relative basin age – a higher density indicates an older age. Unfortunately, unavoidable uncertainty is introduced by secondary craters from subsequent basins which may be 20+ km in diameter and may instantly age a basin. An earlier attempt to determine basin relative age sequence by Hartmann and Wood (1971) measured all visible craters superposed on basins and calculated a numerical relative age compared to a mean density of 1.0 for all nearside mare – highlands are saturated with a density of 32. While here is some agreement between the Wilhelms and H&W data there are significant differences (e.g. Apollo) that require investigation.

Relative Ages of Plains and Mare Fill

Nearly all impact basins contain dark mare material and/or lighter-hued smooth plains. Just as they counter craters to determine basin relative age, Hartmann and Wood (1971) determined the relative ages of mare fill and light plains fill in basins. Again 1.0 is the average age of nearside lunar mare and 32 is the number for saturated highlands. The plains have relative ages of 2.5 to 13 and almost certainly are older mare lavas that have been lightened by crater rays and ejecta.

Age Groups

Wilhelms (1987, p 148) classified each Pre-Nectarian basin into age groups, ranging from oldest (Group 1) to youngest (group 9). The classification of at least one basin within each group was based on superposed crater density (Col. T) and/or superposition relations. Other basins were more tentatively assigned to each group according to more subjective morphological clues. Group 1 includes only the South Pole-Aitken basin and the Gargantuan basin, although other ancient basins undoubtedly were formed but are no longer identifiable. These are the two largest basins on the Moon and are saturated by later craters of basin size.
 
  The existence of all Age Group 2 basins was considered by Wilhelms to be uncertain, and topographic data from Clementine has not documented depressions for any of them. They are heavily degraded and are identified by isolated peaks that seem to define circles.Wilhelms (1987, p 179) also classified the 12 known Nectarian age basins into groups, but he numbered them 1 and 2 rather than 10 and 11, which I have done in the table. I have also added a 12th group which contains the 5 youngest basins on the Moon: Imbrium, Orientale and Schrödinger, and Compton and Antoniadi – the latter two being peak ring basins. Antoniadi is often considered a transition between basins and craters, but since it is the best example on the Moon I include it in this basin list.

Basin Age in Billions of Years

Only a few basin ages have been determined by dating of sample collected by Apollo astronauts. And while these radiometric dates can be remarkably precise, often there is only conjecture on the actual origin of the rock dated. Only the age of formation of Imbrium and perhaps Serenitatis can be confidently stated.

Lunar Basins List


Additional Information


LPOD Articles


Bibliography



Retrieved from "http://the-moon.us/index.php?title=Lunar_Basins&oldid=11783"