Geoscience Reference
In-Depth Information
SIZE DISTRIBUTION OF ASTEROIDS AND OLD
TERRESTRIAL CRATERS: IMPLICATIONS FOR
ASTEROIDAL DYNAMICS DURING LHB
TAKASHI ITO
∗,†
, ROBERT G. STROM
‡
, RENU MALHOTRA
‡
,
FUMI YOSHIDA
†
and DAVID A. KRING
‡
†
National Astronomical Observatory of Japan
Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
‡
Lunar and Planetary Laboratory, The University of Arizona
Tucson, AZ 85712-0092, USA
∗
tito@cc.nao.ac.jp
Recent progress in asteroid surveys has revealed the fine structures down to
sub-km in diameter of the size-frequency distributions (SFD) of main belt
asteroids (MBAs), as well as near-Earth asteroids (NEAs). These SFDs can be
compared with the SFD of lunar and planetary crater projectiles. The SFD of
the projectiles that created the oldest craters on the lunar highlands, which are
considered a fossil of the Late Heavy Bombardment (LHB) impactors of
∼
4Ga
ago, shows a very good agreement with that of the current MBAs. This fact
indicates that the LHB craters were created by the bombardment of ancient
asteroids ejected from the main belt by a short-term, size-independent event,
such as the radial movement of strong resonances due to the migration of giant
jovian planets. On the other hand, the SFD of the projectiles that have created
younger craters such as those on Mars is very different from that of the MBAs;
instead, it is quite similar to the SFD of NEAs. This newer population of pro-
jectiles might be created by a long-term, size-dependent transportation mech-
anism of asteroids such as the Yarkovsky effect, which preferentially pushes
smaller objects into strong resonances.
1. Introduction
At the dawn of Earth's history, there were intense and cataclysmic impact
events, collectively called the Late Heavy Bombardment (LHB).
1
,
2
The
most intense period of the LHB appears to have occurred
3.9 Gyr ago,
i.e., 500-600 million years after the formation of the Earth-Moon system.
3
,
4
Evidence of this event began to accumulate when Ar-Ar isotopic analyses of
Apollo and Luna samples suggested that several impact basins on the near-
side of the Moon had been produced 3.88 and 4.05 Ga. Additional analysis of
Apollo samples indicated the U-Pb and Rb-Sr systems had been disturbed
nearly uniformly at
∼
∼
3.9 Ga, which was attributed to metamorphism of
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