Impact Bombardment Throughout the Solar System
Impact events are an intimate part of the formation of planets, both during the initial accretional phase and late in the growth of a planet when giant impact events may dramatically alter the final outcome. Large collisions, for example, have been implicated in the formation of the Moon from the Earth (Figure 1), the stripping of Mercury’s mantle, the northern-southern hemisphere dichotomy of Mars, and the formation of Charon from Pluto. Periods of enhanced impact bombardment of post-accretion planetary surfaces have also been deduced from solar system exploration studies. Apollo, for example, demonstrated that the Moon was heavily cratered sometime during the first ~600 million years of its existence in what has been termed by some to be the period of Late Heavy Bombardment (LHB).
|Figure 1. The leading hypothesis for the origin of the Moon involves a huge collision between the Earth and a planet half its size. Some of that colliding material was added to the Earth, but a large fraction of the impact debris went into orbit around the Earth. The orbiting material accreted together to form the Moon. The CLSE team will be testing that hypothesis by examining the chemical composition of samples from the Moon and the ages of those samples.|
Dusty circumstellar disks around young stars outside our solar system indicate the collisional evolution of young planetary systems can be violent. Initial Spitzer Space Telescope data indicate that some systems show dust signatures well above the average at ages from 100 to 600 million years old. Observations support the idea that the ~350 million-year-old A star Vega and the ~2 billion-year-old G star HD 69830 recently experienced collisions between large planetesimals that have generated these elevated dust signatures.
The consequences of the collisional evolution of young planetary systems, including our own solar system, are profound. It now seems clear that:
- Early bombardment of planets can completely resurface them.
- These impacts can alter the physical and chemical state of (and/or blow-off) planetary atmospheres.
- The bombardment can make surface conditions unpalatable for biogenic processes.
- In contrast, the impacts can also create subsurface environments that are suitable crucibles for pre-biotic reactions and possible habitats for any life that develops.
- The impacting objects and interplanetary dust that accompanies them can deliver important biogenic components like water, carbon, nitrogen, sulfur, and phosphorus).
- The impacting objects may also be the source of important siderophile (iron-loving) element addition.
The Apollo Legacy
While it is generally recognized that the impact cratering rate was more intense early in solar system history, it is not clear how that rate evolved. Some investigators have suggested there was a smooth decline with time, while others have suggested there were one or more episodes of particularly intense activity superimposed on a background decline in the impact rate.
The Apollo and Luna missions provided the first opportunities to investigate this issue. Argon-argon isotopic analyses of Apollo and Luna samples suggested three to possibly six of the impact basins on the nearside of the Moon had been produced between 3.88 and 4.05 billion years ago. Additional analyses of Apollo samples indicated the U-Pb and Rb-Sr systems had been disturbed nearly uniformly at ~3.9 billion years ago, which was attributed to metamorphism of the entire lunar crust by a large number of asteroid and/or cometary collisions in a brief pulse of time, <200 million years long, in what was termed the lunar cataclysm. A growing number of ~3.9 billion-year-old impact melt ages from the Apollo and Luna collections seemed to confirm the pattern. It was suggested that the decline in the impact rate was not smooth, but punctuated by at least one large influx of material.
The hypothesis of an intense period of bombardment ~3.9-4.0 billion years ago is still controversial, however. There are currently several models under consideration. Some investigators have argued for a lunar cataclysm ~3.9-4.0 Ga and a relatively low impact rate between ~4.4 and 4.0 billion years ago (lower curve in Figure 2). They also argued that the duration of the cataclysm may have been as short as 10-20 million years long. Others have argued that the time span of the bombardment may have been longer and/or that the impact rate prior to ~3.9-4.0 billion years ago was relatively high (upper two curves in Figure 2). In all cases, it is generally agreed that there was a significant decrease in the lunar cratering rate after ~3.8 billion years ago when the last basin-forming impact occurred.
Some investigators have suggested that sampling issues, particularly on the Moon, cloud our ability to resolve the impact cratering record prior to ~3.9 billion years ago, and do not accept the notion of a cataclysm on the Moon, asteroids, or any other body in the solar system. There are also interesting discrepancies in existing data. While Apollo samples suggest a relatively abrupt decline in the impact-cratering rate ~3.85 billion years ago, lunar meteorite data and chondritic meteorite data suggest it may have been drawn out until 3.5 to 3.4 billion years ago.
Although the lunar cataclysm hypothesis is one of Apollo’s highlights and remains the number-one science priority of NASA (NRC 2007), it is representative of a broader range of questions. We now understand that impact cratering is the dominant process affecting the lunar surface. There are hints that the Moon’s origin may be intimately tied to a collisional event (the giant impact hypothesis) when the accretion rate was much higher (Figure 2). We have also gleaned from Apollo that impact events have produced a unique lunar surface regolith, which itself is a record of meteoritic and heliophysical processes and the medium with which future lunar surface exploration will be immersed. We have designed an integrated interdisciplinary study of impact processing of the Moon that tackles the highest science priorities identified by the NRC for NASA.
Figure 2. Schematic diagram illustrating the variation among the models of the early impact cratering history on the Moon. While it is generally agreed that there was a decrease in the impact rate following 3.8 Ga (or 3.8 billion years ago), it is still unsettled whether a) there was an anomalously high flux ~3.9 Ga, b) if there was a high flux ~3.9 Ga, how long it lasted, and c) whether the impact rate between ~4.4 and ~4.0 was relatively high or low. Peaks like that shown at 3.9 Ga have also been suggested to occur several times between 4.4 and 4.0 Ga. Modified from Kring (2003).