Although engineering capability and safety are important considerations, the primary goal when choosing an optimal landing site on the Moon for a science mission is to maximize the scientific return.
A research article recently published in Journal of Geophysical Research: Planets by Zhao et al.  describes two candidate landing sites in consideration for the Chang’e 5 mission, a lunar mission planned by the China National Space Administration, originally slated to launch in 2017 but which has now been delayed. One of the most exciting aspects of Chang’e 5 is that it aims to return up to 2 kilograms of samples to Earth. But from which site should these samples be taken?
In the frame is Mons Rümker, a distinctive rise in northern Oceanus Procellarum that is the location of a number of enigmatic domes.
Zhao et al. use an impressive array of data from spacecraft launched by Japan, India and the US to describe the geology of potential landing locations in this area.
Their observations support the interpretation that the rise and domes are the consequence of extrusive volcanism, some of which might be relatively young (about 3 billion years ago, from the Eratostenian period). One of the two landing sites (Site A) is on one of these potentially young volcanic domes; the second (Site B) is on an area of lineated terrain on the northern part of the rise that is interpreted as ejecta from Iridum crater.
Both sites are of great scientific interest, but a choice between the two must be made. Experience of previous lunar landings suggest that a few factors typically come into play when seeking the best site.
First, it is desirable to explore somewhere with a diverse suite of processes have acted, resulting in a range of landforms and materials. Diversity maximizes what can be explored at one location, but can come with some cost in added complexity; the amount of landing site complexity that a mission should accept is not a straightforward call.
Second, having a reasonable hypothesis for the sequence of geologic events and processes that affected a landing site is important for being able to understand in situ measurement once the data have been gathered. In situ observations can then anchor our knowledge of lunar geology from remote sensing. There are obvious tensions here. If a landing site is too complex, it may prove very hard to untangle, limiting both the direct science return of the mission and the ability to broadly extrapolate its findings. If it is too simple, some amount of science return is left on the table.
Third, on a planetary surface with previous landing sites, such as the Moon, it is typically worth going somewhere new and different. The argument for this is that planetary surfaces are big, and landing sites are small, so exploring the same place or same kind of place twice may lead to missed opportunities for serendipitous discoveries. On the other hand, going back to the same place a second time might be smart if the value of what was learned during previous exploration outweighs the lost opportunities for unexpected discoveries, though the vigorous debate about sending Mars 2020 rover back to the same location as Mars Exploration Rover Spirit illustrates how this can be controversial.
Based on Zhao et al.’s analyses, both of the candidate Chang’e 5 landing sites in their paper score highly on the criteria discussed above: the sites are likely to be diverse, have a reasonably tractable geologic context, and are novel. Site A would be particularly exciting because no prior lunar landing sites have been on a steep-sided dome, and exploring whether these landforms are indeed young is of substantial interest. As acknowledged in the paper, Site B is a bit more confusing, and the origin of its materials is more uncertain: for example, past work has suggested there may be pyroclastic materials at this location but Zhao et al.’s study did not support this conclusion.
Regardless of where the next landed missions go, however, Zhao et al.’s paper is an excellent reminder of how much we still stand to learn by going back to the Moon’s surface and how exciting the prospect is for lunar sample return for the first time since the 1970s. NASA has recently announced a workshop to be held in January 2018 to examine other candidate landing sites on the Moon that we might explore in the coming decade.
—Caleb I. Fassett, Associate Editor for JGR: Planets and NASA Marshall Space Flight Center, email: [email protected]