Prospecting the Moon’s Poles

To plan for the development and use of the Moon’s polar resources, we need detailed information to identify the state and distribution of its volatile substances.  Obtaining such knowledge about this part of the Moon will require surface mobility to examine and characterize deposits on meter-to-kilometer scales.  In recent years, remote sensing has relayed new and exciting information about the environment and deposits of the poles – resources that will give us new capabilities – including the presence of water ice.  Despite acquiring these new measurements, we still have only a partial picture of the nature and origin of these potential resources.

A surface rover, equipped with extensive instrumentation and powered by a long-lived power source (such as a nuclear-powered radioisotope thermal generator or RTG) – that over the course of several months to years could traverse into and out of deep craters and explore both sunlit and dark areas – is the ideal way to explore the polar regions.  Unfortunately, such a large and capable mission tends to be very expensive (on the order of about $1 billion).  With such a hefty price tag (driven primarily by the need for an RTG), that mission isn’t likely, so we must gather the information we seek through the development of a less ambitious, lower cost mission.

A low-cost, limited-duration mission might land in a sunlit landing site, where it would then carefully traverse into the shadowed areas to map and measure the volatile content in both places.  This is the idea behind NASA’s Resource Prospector (RP) mission, currently being studied by the Human Exploration and Operations Mission Directorate.  The RP mission is designed to be relatively low-cost (less than $250 million) and short (on the order of one week duration), exploring near the lunar south pole.  It will not answer all of our questions about the volatiles that exist at the lunar poles, but it can address the most pressing ones, including how much water ice is present at a single locality, how it is distributed laterally and vertically in the lunar soil, and how heterogeneous the ice deposits are on scales of hundreds of meters.  We will obtain ground truth for a single, selected area near the poles – not all of the information needed for development but enough to at least begin to understand our technology needs at the system level.

The RP mission is built around a payload named RESOLVE (Regolith and Environment Science and Oxygen and Lunar Volatiles Extraction).  This payload is made up of several different instruments designed to measure the volatile content in the lunar soil and to demonstrate the possibility for extraction of same.  Before significant amounts of water ice at the poles had been confirmed, RESOLVE had been designed to work with lunar soils of low volatile content (such as ordinary mare regolith).  However, with the new data, we have modified the instruments of RESOLVE to measure the polar volatiles and demonstrate how we can access those materials by collecting and heating a sample to characterize the emitted gases.

While data from the LCROSS lunar impactor mission indicates that water ice is the dominant volatile substance in polar regolith, other species are present in lower concentration, including ammonia, methane, carbon monoxide, sulfur dioxide and some simple organic molecules.  Exotic elements such as mercury may also be present.  The RESOLVE package takes soil samples, heats them to drive off volatile materials, and then a mass spectrometer-gas chromatograph and near-infrared (IR) spectrometer measures the species present and their concentration.  We hope to collect the water vapor driven off during heating to demonstrate how water ice may be processed to support future activities on the Moon.  In addition, there is also an experiment in hydrogen reduction of regolith to extract oxygen, whereby hydrogen passed over a heated soil sample reduces iron oxides into native metal and water.  This process is usable on any lunar soil, including the volatile-poor ones found near the equator.  In essence, RESOLVE is a miniaturized chemical laboratory and processing plant combined into one small package.

To obtain soil samples for analysis, it will be necessary for the RESOLVE payload to visit various locales hundreds of meters around the landing site.  Thus, it must be mounted on some type of device that will give it access to the regolith.  The choice for the RP mission is a rover utilizing a subsurface sample acquisition device.  Possible subsurface samplers include: a shallow regolith drill which obtains soil samples from depths of a few centimeters, down to a meter or more (if the drill bit is hollow, a core sample could be extracted from it); an auger that brings up cuttings for analysis, but detailed stratigraphic information is lost; a device called a mole that “burrows” its way into the soil, turning up buried soil as it proceeds.  The specific technique to obtain subsurface materials has yet to be chosen.

To learn how water ice is distributed vertically in the Moon, we would retrieve samples from a variety of carefully controlled depths.  For its lateral distribution, the rover can travel across the terrain of the lunar poles measuring the bulk hydrogen of the soil using a neutron spectrometer and potential water and hydroxyl on the surface with the near-IR spectrometer.  Another mission goal is to understand the variation of volatile content between sunlit and in dark areas.  In the sunlit areas, we do not expect water to be present on the surface but here the temperatures at shallow depths (i.e., below a few tens of centimeters) might be cold enough for water ice to be stable.  If so, we need to understand how much is present and how (in addition to the water ice found in the extremely cold, dark areas) it might serve as a water source.

At the moment, the RP mission is still being defined.  Originally, Canada was going to provide the rover and subsurface drill but they withdrew from the project last fall because they could not commit to a planned launch date.  The current strawman has NASA developing and providing a rover “in-house,” but at which center and with what type of rover is still undecided.  Because the rover and analysis package must successfully soft-land on the Moon, the mission also requires a lander.  Though nothing is finalized, NASA is in discussion with JAXA, the Japanese space agency to supply a lander (one they had intended to use on their SELENE-2 mission).  Another possibility (if a suitable one can be developed and flight qualified in time) is the procurement and use of a commercial lunar lander.

The RP mission has the potential to provide us with first-order data on lunar polar volatiles, their distribution in the surface and subsurface, and how they vary in concentration and state.  It will not answer all of our questions about the nature and inventory of polar water but it is a good start toward gathering this necessary strategic knowledge.  Since NASA is doing the mission largely in-house, with little participation from the scientific and engineering community outside of the agency (and as of yet, no lander and no rover), one might be excused for thinking that RP is largely a fictional mission.  But I am told on good authority that the money to fly this mission has been identified and that the RP mission is on track for a 2019 launch.

Hopefully, a successful RP mission will generate new interest in the location and accessibility of the Moon’s polar volatiles, as well as the challenge of prospecting and using lunar water in future space activities.  Once we’ve identified the locations and understand the various states of these volatile lunar materials, we will be primed and ready to create an affordable space faring capability – using the resources of the Moon to build incremental, extensible transportation and habitation systems.

Paul D. Spudis | READ MORE

Paul D. Spudis (1952-2018) was a senior staff scientist at the Lunar and Planetary Institute in Houston, Texas, and a prolific author on the subject of the moon. His books include The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources.