See Part 1: Compelling Planetary Science Missions in this series of posts.
My first selection for the most compelling planetary science mission on the Decadal Survey list is the Lunar Polar Volatiles Explorer (LPVE). From the Executive Summary:
The Lunar Polar Volatiles Explorer concept involves placing a lander and rover (with an instrument payload) in a permanently sun-shadowed lunar polar crater. The rover will carry a suite of science instruments to investigate the location, composition, and state of volatiles. While previous orbital missions have provided data that support the possibility of water ice deposits existing in the polar region, this LPVE concept seeks to understand the nature of those volatiles by direct in-situ measurement. A prospecting strategy is employed to enable lateral and vertical sampling only where higher hydrogen concentrations are detected, thus eliminating the criticality of statistically significant numbers and distributions of samples required by stochastic approaches.
As with most or all of the Decadal Survey mission concepts, there are more and less capable variants of this mission. For example, some instruments that are in more capable variants could be dropped for a more affordable but less capable mission, and the rover power system could be based on batteries or ASRGs.
Lunar volatiles are of scientific interest
because they record not only those released from the interior of the Moon during its geologic evolution, but also species derived from the solar wind, cosmic dust, and comets. Thus, the volatiles in the cold traps provide a record of the evolution of the Moon, the history of the sun, and the nature of comets that have entered the inner solar system over the last several billion years.
They are also of interest as potential resources for later exploration missions or even lunar and cis-lunar space infrastructure development.
The LPVE mission seeks to answer questions about the distribution, chemical and isotopic composition, physical form, and deposition rate of the volatiles. We don't know the distribution of the volatiles, so we need a mobile explorer so we can test multiple locations. That's where the rover comes in. A neutron spectrometer on the rover is used to identify locations in the regolith with hydrogen. The rover positions itself at the locations. It's able to drill 2 meters into the regolith. Instruments like another neutron spectrometer and an imager can be put in the drill hole to assess any volatiles there. This allows the rover to identify the best sample locations within the hole. The rover is able to retrieve samples from the drill hole and bring them to a gas chromatograph / mass spectrometer that heats them for analysis.
In addition to these "core" capabilities, "priority 2" instruments include X-Ray diffraction to measure the mineralogy of the retrieved samples, ground-penetrating radar and surface imaging for geological context, and a mass spectrometer to measure the lunar exosphere.
The fully-capable mission variant with an ASRG would be expected to last over a year and to be able to travel nearly 200 km. It would be able to take 460 samples. Battery variants would last a few days, be able to travel a few km, and be able to take about 20 samples. Since this is my most compelling mission pick, I would be inclined to go for the full instrument suite and ASRG power supply in this case. The difference in mission cost (at least in the estimates presented in the report) is minor, and the increased capability is significant. The fully capable mission cost is estimated to be $1.132B in FY15 dollars; the battery mission cost is estimated to be $0.972B. That's a lot of money in either case, but all of the missions in the Decadal Survey list are in the more expensive New Frontiers or Flagship mission classes. If any funds are available from the Robotic Precursor line in upcoming years, one might imagine that funding line contributing an instrument or 2, making it easier for Planetary Science to run a fully capable LPVE mission.
In addition to the science, "astronaut scouting", and resource potential of this mission, I find the idea of a capable rover moving across hundreds of kilometers while drilling into the dirt to be compelling at a more basic level. It seems that this sort of mission speaks to the handyman or "Dirty Jobs" part of our nature. It just looks like a lot of fun.