Lunar Capture

Origins of the Earth's Moon

Evidence on the Moon

There are two major visible surface features of the Moon. (Atlas of the Moon) The lighter areas are primarily the highlands or the original lunar crust. Meteorites have bombarded this old crust since the beginning of time. The darker areas are the mare or low lying basalt regions. This is where ancient multi-ringed impact basins have been filled with “lava.”

Lunar flood basaltsGreat lunar flood basalts are seen as the dark splotches clearly visible on the Moon. These dark basalts form the face of “the old man on the Moon.” They were originally thought to be lunar seas and were called mare, the Latin word for ocean. They are actually vast seas of melted lunar crust, much like Earth’s lava, which flowed into the great meteor craters of the Moon.

Unlike the highlands, the mare are only lightly cratered. This dichotomy of surface cratering plays an important part in telling the story of a Permian tidal heating event.

Lunar ChartLarge impact craters known as multi-ring impact basins form the “saucers” in which the mare lavas settled. These ancient basins are presumed to be the remains of the last large impacts of lunar formation. Thus these basins are a record of the final planetesimal impact history of the Moon’s formation process.

The subsequent filling of these basins with lava to form the mare has provided a major point of contention with lunar scientists. Several hypotheses have been proposed including the lava forming as a result of the impact event itself (18), lava welling up from inside as a result of radioactive heating (19) and an early debate of the basins actually being filled with dust not lava (20).

Did you know?

Moon LandingIt was not until the first Moon landing that a dust origin for the filling of the mare basins was finally set aside. The seriousness with which this model was taken is clear in NASA’s planning; the large disked feet on the first lunar lander were designed to support the spacecraft on the dust surface of the Moon and the landing sites were carefully picked to avoid landing in a sea of dust.

Information about what the mare basins looked like prior to their filling can still be seen. Craters that formed on the floors and walls of these large impact basins prior to their being flooded with basalts are discernible under the lava. The rims of some of these craters can be seen above the lava surface and the surface features of the lava itself give evidence of the cratering below.
Lunar CratersCopernicus

Moon-Earth Graph

Age of the Mare and Cratering

Impact basinsThe isotopic dating of the lunar rocks retrieved by Apollo date the mare formation at 3.2 to 3.8 billion years ago. Science has produced no adequate theory for a heat source to account for the melting of the Moon’s surface that could produce the mare “flows” other than the primordial heat of formation. The Moon has generally been considered to be a cold lifeless body and the Moon’s crust is thought to have solidified early in its life span. Meanwhile, the multi-ring impact basins record the impact history of the lunar surface up to the formation of the last of these giant impacts.

Although the lunar mare is assumed to be very old, it has few craters. To account for this anomaly the model of intense bombardment (MIB) was created. The MIB is assumed to be similar throughout solar system with a possible exception in the Saturn system.

The assumption upon which the isotopic dating of the lunar mare are based must have one or more errors which have led to the incorrect belief that the lunar mare are ancient basalt surfaces.

Recent findings on Mars

Since the lunar mare are the only surfaces that have been specifically dated isotopically, they are used to date other surfaces throughout the solar system through crater counting. Mars’ surface, dated by the lunar mare, was therefore also assumed to be billions of years old since there are similar crater densities on both sufaces. A factoring system is used to translate between planetary bodies which takes into account gravitational pull, atmospheric densities and such. (Before and after look at impact craters)

Relief Map of Mars
Map of Mars

Recent cratering information from Mars reveals a much younger Martian surface than has previously been assumed (21), contradicting the lunar isotopic dating. Astonishing images from the Mars Global surveyor show 20 new craters on the Martian surface in a recent 7-year period (22 , new craters); according to the MIB, no new craters should have formed on Mars during this time period. Amazingly these 20 craters appeared on only the 5% of the Martian surface that was surveyed, 95% of the Martian surface was not even covered.

Impact cratersEvidence from Mars’ crater Zunil adds further contradictory evidence. Studies of Zunil confirm that secondary cratering is actually much more prevalent than has been assumed given the lightly cratered lunar mare. This cratering evidence on Mars indicates that many more “secondary” craters are formed with each impact of a primary crater. Only primary impacts are typically used to date a surface and if the secondary cratering rate is accurate it would mean that there are actually many fewer primary craters on the lunar mare than has been generally thought. Given the “known” cratering rate of Earth, that means that the mare should be even younger, not older, than the pre-Apollo thinkers believed. It was determined that if the mare were ancient (3.5 b.y.) surfaces then secondary cratering could not be very effective, since there were so few craters on the mare. This assumption is being directly challenged by the evidence being found on a number of planetary surfaces now being studied through out the solar system.

The “cratering paradox” is that the unexpectedly high Martian cratering rate indicates a need for adjustment to reduce the age of the lunar mare.

Recent Findings on Europa

Crater Pwyll on EuropaJupiter’s moon Europa lends some interesting insights into our own moon. Europa’s icy surface holds only a few primary craters and its constant resurfacing makes its surface unique for observing cratering impacts. Because primary impacts are few, the distribution of secondary craters formed by primary impact can be readily discerned on Europa.

The number of secondary craters generated by each primary impact on Europa is much greater than previously believed; fewer than 40 primaries (greater than 10km) have produced 10 million secondaries larger than 100m (23,24). This confirms the findings from Zunil crater on Mars.

Implications for the Earth-Moon System

Since the lunar mare were used to date the Martian surface, any “discovered” increase in the Martian cratering rate must be addressed as it pertains to the lunar mare cratering rate. A ‘greater’ rate of impacting bodies, as found on Mars, means the finely tuned flux between the Earth, Moon, and Mars must be recalibrated. The importance of secondary cratering as noted for other bodies can only exacerbate the situation in terms of its impact on lunar mare cratering and the dating of the lunar mare.

These findings point toward much younger surfaces and a more dynamic solar system than has been previously imagined. Yet the estimated age of the mare surface has not been revised significantly since the preliminary examination of Apollo 11 lunar samples in 1969 (25).