Mapping Melts on the Moon

Small deposits of impact melt can help explain early lunar history, if you know where the material came from.

The 47th Lunar and Planetary Science Conference (LPSC), a large international event held every spring in Houston, Texas, once again showcased a wide variety of the subjects that make up modern planetary science. My own contribution followed up on some previous work, locating and identifying deposits of shock-melted material produced by the formation of giant multi-ring impact basins. These features were created early in lunar history and because they are found globally, their relative and absolute ages can inform us about the cratering history of the Moon.

When an impact crater forms, a small zone near the point of impact is vaporized and melted by the intense shock pressures created by the collision. This melt (called impact melt) is an important product for two reasons: 1) its chemical composition represents an average of the target rocks, which allows us to deduce the make up of the pre-impact curst; and 2) it is the material whose radiogenic isotopes are “re-set”, which allows us to determine exactly when an impact occurred. Our understanding of the time scale of lunar history is determined by the radiometric dating of rocks, which explains why shock-melt from large impacts are prime targets for study.

The problem with ancient basins (a basin is any impact crater larger than 300 kilometers in diameter) on the Moon is that because they are so old, they have been heavily modified—partially buried by other crater and basin deposits and filled with volcanic lava flows. Impact melt is concentrated in the center of a crater. Since most basins are subsequently filled with mare lava, few exposed melt sheets survive for us to sample.

For the last few years, my students and I have been working on mapping the occurrence of melt sheet features that have survived. Melt may survive burial in one of two ways. First, some basins are not completely filled with lava, retaining their original configuration for more than 3.8 billion years. The classic example of this type of basin is the spectacular Orientale basin, almost 1,000 km across on the western limb of the Moon. (Side note: Why is a basin on the western limb called “Mare Orientale” (Eastern Sea)? Because in the old days of telescopic study, the lunar east and west convention referred to Earth, not lunar, coordinates. The “Orientale” limb of the Moon was on the left (east) when facing the Moon, with north at top. Hence, the Eastern Sea. Yes, crazy—but then, no one in the early 20th Century expected people would be flying to the Moon). The Orientale basin is nearly perfectly preserved, with just a small amount of mare lava flooding the innermost basin. Large regions of the interior are covered by wrinkled, cracked and fissured deposits—the remnant of the basin impact melt sheet (see photo below). Thus, Orientale is a basin whose melt can be directly sampled.

Mapping Melts on the Moon
Impact melt deposits of the Crisium (left) and Orientale (right) basins. The Orientale melt sheet is well known but the Crisium basin melts are newly discovered. The cracked and fissured surfaces are diagnostic of a melt sheet origin.

As the youngest lunar basin, the absolute age of Orientale is already constrained by the ages of the oldest, post-basin units on the Moon (old mare lavas, dated at 3.8 billion years, abbreviated “Ga”). What we really need to know are the ages of older and middle-aged basins. Although all lunar basins have been dated on a relative scale (i.e., the Nectaris basin is older than the Imbrium basin), only Imbrium (3.85 Ga) and (possibly) Serenitatis (3.88 Ga) have absolute dates, and both of those are uncertain. So we are left with the problem of having no widespread exposures of the impact melt of the basins we most want to date.

However, once again, the Moon has obliged us. In all basins, small amounts of material ejected from the central structure appear to be impact melt. Although we cannot be certain of its origin until someone lands there, impact melt has some distinctive morphological properties—as mentioned above, it typically has a cracked surface texture, visual evidence of past liquid flow, and possibly a contrasting composition to its surrounding terrain. I have mapped and found evidence for ejected melt from at least three basins (Orientale, Imbrium and possibly Nectaris). At the conference, I reported on some newly recognized deposits of impact melt for the Crisium basin (see the map below) on the eastern limb of the Moon (i.e., opposite Orientale!).

Mapping Melts on the Moon
New geological map of the Crisium impact basin. The newly described melt deposits are shown in bright red along the margins of the mare-highland contact in Mare Crisium.

The Crisium basin is a large (~1,000 km diameter) impact feature centered at 17°N, 59°E. It has a broadly elliptical outline and a polygonal rim shape. It has been proposed that the basin appears elongated because of a pre-existing smaller basin in the east, but a more plausible explanation is that Crisium is a result of an oblique impact by a projectile traveling from west to east, and sheared off, such that its upper half impacted separately downrange of the main basin. Such an impact would create an elliptical feature, elongated in the east-west direction. The inner basin of Crisium is nearly filled with lava, but during mapping, we noticed some small “islands” of pre-mare rock that had not been covered (left side of the photo, bright red (Nm) in the geological map). Study of these islands (called “kipukas” in Hawaii) showed that they were of highlands composition and had a cracked and fissured surface. This relation is nearly identical to that seen at Orientale. We suggested that these features were parts of the original basin floor—the Crisium basin impact melt sheet.

This new finding is significant in two ways. If we could visit these sites on the Moon, we could obtain direct samples of impact melt from the Crisium basin, a large feature that sampled the entire lunar crustal column in this region. Study of its chemical and mineral composition could help us better understand the complex igneous history of the Moon’s crust. In addition, the Crisium basin is of intermediate age (older than Imbrium but younger than Nectaris) and determining its absolute age could help us understand whether the Moon experienced an impact “cataclysm” or massive increase in the rate of impacts around 4 Ga. Answering this question is important not only for deciphering lunar history, but for the interpretation of the cratering histories of all the terrestrial planets, including the Earth.

I have written previously on some of the problems attendant with using samples to reconstruct the history and evolution of the Moon. It’s easy to pick up rocks but doing science requires that we collect samples of known geological context. Unless we fully understand what the samples represent, we cannot make broad generalizations that allow us to reconstruct planetary geological history. The current focus of much of the lunar sample community is on sampling the melt sheet of the enormous South Pole-Aitken basin, the largest (2,600-km diameter) basin on the Moon. Although we know it is the oldest basin in relative terms (all other basins lie on top of it), we do not know its absolute age. It could be as young as 3.9 Ga, in which case a cataclysm is required, or it could be as old as 4.3 Ga, which lessens the possibility of a cataclysm.

Because this feature is so old, the geological context of samples from it is unclear. It is better to sample other, less degraded basins in an attempt to decipher the early cratering history of the Moon, largely because their contexts will be clear, as the original units are better preserved. Finding exposed impact melt at Crisium means that it is worth searching for additional melt deposits from other lunar basins. By finding these exposures, we can plan for future missions to either return samples to Earth or date the rocks onsite with automated equipment. By compiling a set of ages from many basins distributed widely over the Moon, we will understand more about the earliest evolution of the planets.

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