Deciphering the cratering history of the Moon is an important scientific problem. My
NASA has announced that the proposed New Frontiers Moonrise robotic sample return mission is one of three selected for detailed concept study. The objective of this mission is to sample, date and analyze the composition of the impact-generated rocks produced by the largest and oldest crater on the Moon, the South Pole-Aitken (SPA) basin.
The return of surface samples has the potential to answer many important scientific questions. How do we reconstruct the history of a planet from rocks returned from its surface? What are some of the difficulties in such a reconstruction? How well do we really understand the history of the Moon from returned lunar samples? Because context is vital to the correct interpretation of sample return data, these questions must be understood and considered, and underlie the mission strategy.
A great deal can be learned from remote surface measurements, but some properties can only be measured to very high degrees of precision by using returned samples. One key piece of information that is difficult to measure remotely is a rock’s age (measured by its radiometric isotopes). This determination requires a significant sample preparation, handling, and precision measurements; in some dating methods, we must literally take the rock apart, grain-by-grain. The machinery needed to measure isotopic composition tends to be big, massive, and power hungry, all undesirable properties for lunar and planetary payloads.
Geologists collect samples because they cannot bring into the field all the complex and sophisticated equipment used to analyze and describe the physical, chemical and mineral properties of planetary crusts. Samples allow them to conduct many different kinds of measurements in a controlled environment, eliminating external factors that can contaminate results. In addition, samples have long-term value in that they can be stored, archived and examined in detail (sometimes by newly invented techniques) as concepts and understanding change. It is for this reason that lunar sample studies continue to unravel new aspects of the complex history of the Moon 40 years after Apollo 11.
We are able to design a spacecraft to collect rocks and soil and return them to Earth. After analysis, we have lots of data and numbers, but not necessarily any new understanding. Context is important in translating sample data into knowledge.
The geologist in the field must collect samples carefully; field work is not just picking up rocks – it is the attempt to unravel and comprehend the spatial and temporal make-up of planetary crusts. Samples must be representative of the larger, regional geological units they come from. A sample must be of the appropriate size (coarse-grained rocks need larger samples than fine-grained rocks to be representative of their parent units). If possible, we must collect rock samples from outcrop (in place bedrock); rocks obtained from loose pieces on the ground (called “float” by geologists) have uncertain or unknown context and hence, the conclusions we draw from such samples may not apply to regional units. And when done on the Moon or another planetary body, all of this activity must conform to the constraints imposed by the flight system, such as total mass and volume limits for returned samples.
Recently, many countries have flown sensors that have yielded compositional information and globally mapped the Moon. From these data, we can determine chemical and mineral compositions of the geological units of the Moon (which are delineated by extent, morphology and physical properties). When this information is combined with data from returned samples, we can characterize the unit and its history even more fully than traditional field work, where intense, protracted ground study is possible. This is the promise of the new approach – allowing us to combine the low fidelity but broadly distributed data of remote sensing with the highly detailed but narrowly restricted information provided from samples.
However, due to the very nature of the Moon, there are significant geological complications that must be taken into account. Exposed bedrock is rare. A thick cover of regolith is everywhere on the lunar surface. In the highlands (the oldest geological units on the Moon), there may be no bedrock at all, the surface having been thoroughly pulverized into regolith by four billion years of impact bombardment. Consequentially, the context of most Apollo highland samples remains poorly understood. Exquisitely detailed measurements have been made on these rocks but we still cannot be certain about what they represent. Was there a cataclysm at 3.8 billion years ago? Currently, we are left wondering if we have sampled one, a couple, or a dozen basins.
During the Apollo missions, the astronauts did their best to sample and describe the context of representative rocks during collection, but the geological setting of most samples is still guesswork. The location of the samples returned by a robotic spacecraft will be documented to within a fraction of a millimeter. But as they are collected from regolith, their context will remain purely statistical.
By collecting hundreds of relatively small rocks (but still large enough for precision measurement) the argument is made that we will collect the desired SPA basin melt sheet through sheer statistical certainty. I suspect that the mission might well do this. But what about their context? We need to know which of the pebbles collected are from the basin melt sheet. In miniature, this situation duplicates and leaves us with exactly the same issue we currently have with the Apollo samples—which rocks represent the basins we intended to sample? With few exceptions, despite having global remote sensing data to provide context, we still do not know which (if any) impact basins are represented in the collections, which keeps our scientific understanding hobbled by degrees of uncertainty.
A simple “grab” sample from a relatively young and unmodified geological unit on the Moon could solve a major problem. A robotic spacecraft sent to the youngest lava flow on the Moon (dated relatively by crater density) could establish that flow’s absolute age to high precision with a fair degree of certainty. As the age of the targeted geological unit increases, such certainty would decrease as younger events and deposits contaminate and disrupt the continuity of the older units.
The Moonrise mission proposes to sample the oldest preserved terrain on the Moon—the melt sheet floor of the SPA basin. Younger units (craters, basins, and maria) are everywhere in this basin, superposed on top of the SPA melt sheet. Although pieces of the original basin floor may be preserved in places, we will not know in advance what those pieces should look like, leaving us with uncertainty over what was collected from the mission. In short, many samples will be collected, much data will be accumulated, and uncertainty will remain as to what it all means – the same knowledge gap we currently have with the Apollo samples.