Документ взят из кэша поисковой машины. Адрес оригинального документа : http://selena.sai.msu.ru/Symposium/South%20Pole.doc Дата изменения: Mon Oct 6 18:10:03 2008 Дата индексирования: Thu Feb 27 20:28:36 2014 Кодировка: Поисковые слова: asteroid

"South Pole - Aitken" Basin on the First Images

of the Lunar Far-side

V.I.Chikmachev and V.V.Shevchenko

Sternberg State Astronomical Institute,Universitetskii pr. 13,Moscow,Russia

As the completion of a global topographic survey of the surface of
Solar System bodies has shown, the ring structure adjacent to the southern
region of the Moon is the largest crater of the Solar System in terms of
absolute size - 2500 km in diameter with an average depth of 12 km (Spudis
et al., 1994).
The relative size of this structure is so large that, if the
traditional point of view on the process of impact cratering is adopted,
the cavity originally formed in this giant structure could uncover
materials to the depth of the lunar mantle (Head et al.,1993).
Even only these circumstances show that studying this multiring
structure, which does not yet have an approved name (but is tentatively
called the South Pole - Aitken basin), is of fundamental importance. Since
the diameter of the ring structure observed at present reaches 1.4 times
the radius of the Moon, the reconstruction of this impact - structure
formation mechanism is undoubtedly a topical problem in the investigation
of the evolution of planetary surfaces. It is obvious that detailed
information on the South Pole - Aitken basin topography is necessary for
developing realistic models of the formation of this structure.
Additional interest in the South Pole - Aitken basin topography arose
after the results of the Clementine bistatic radar experiment and then the
Lunar Prospector neutron - spectrometer results suggested the presence of
lunar ice deposits in permanently shaded regions ("cold traps") , which
presumably exist near the poles of the Moon (Shevchenko, 1998).
The first relief maps for the marginal zone of the Moon were
constructed by Hayn (1914) as early as at the beginning of the current
century. As a rule , altitudes were reference to the most probable circles
best describing the lunar limb at a given libration. Later, Nefed'ev (1958)
and Watts (1963) made attempts to summarize the results of separate
profiles measurements to form a unified system by using the parameters of
the lunar - disk asymmetry obtained from observations. From these maps it
was already possible, knowing in advance of the existence of the basin, to
trace a part of this gigantic ring structure (fig.1).

[pic]
Fig.1. Map by Nefed'ev of marginal zone in the South-Pole region.
The heavy solid line shows the giant-basin rim.

Head (1993) and Spudis (1994) suggested in their reviews that the
existence of the structure identified later as the South Pole - Aitken
basin was first predicted by Hartman and Kuiper (1962) on the basis of a
relief analysis of the mountain ridges observed in the libration zone, i.e.
after the first images of the lunar far-side had been obtained.
Then , Head (1993) and Spudis (1994) pointed out that a number of
publications of the results of limb-profile measurements from the images
returned by Zond-6 and Zond-8 detected a depression (more than 2000 km in
diameter and up to 5-7 km deep) in the region currently known to be
occupied by the South Pole-Aitken basin (for example, Rodionov et al.,
1971, 1976).
However, the earliest images of this structure - the largest in the
Solar System - were obtained when the lunar far-side was first photographed
in 1959. The plan location of this structure, detected in four images as a
darker area at the edge of the visible disk , was determined from a central
darkening, 1500 km in diameter and centered at 179(E, 50(S (Barabashov et
al., 1961; Lipsky, 1962; Breido et al., 1962; Lipsky, 1965). This structure
was named Mare Ingenii on the map, which was produced from images obtained
by Luna-3 in 7 October 1959 (fig.2).

[pic]
Fig.2. Part of the map produced in 1960 from the lunar far-side
images obtained by Luna-3 in 1959.

The current parameters of the basin were determined from the images
returned by Galileo and from the Clementine images and laser - altimetry
results. According to these data, the central part of the basin is 1400 km
in diameter and it is centered at 180(E,50(S.
Thus, the first identification of the basin by Lipsky et al. in 1959
was reasonable enough. In the first descriptions of the western part of the
structure, it was noted that its surface is covered by numerous craters and
crater maria. This is confirmed by the present-day images of the South -
Aithen basin floor [see the scheme of the secondary ring structures of the
South Pole - Aitken basin (Head, 1993)].
Interpreters could not find obvious signs of the giant-basin contours
in the Lunar Orbiter images obtained in the second half of the 1960s. As a
result, the boundaries of the entire structure were corrected and the name
Mare Ingenii was given to only a small dark structure in the north-western
part of the basin, about 270 km in diameter.


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