Potential synergies between LADEE and future missions to the lunar surface seem most appropriate with respect to dust measurements. The level 1 requirement for the LADEE dust investigation is the following:
18.104.22.168 Detect or set new limits for the spatial and size distribution of atmospheric dust over an altitude range from 3 km up to 50 km, with a height resolution of 3 km,at a minimum detectable density of 10-4 grains/cc, for grains from 100 nm to at least 1 micrometer in radius. These observations shall be conducted at the temporal and spatial scales for Ar as outlined in 22.214.171.124
There are three possible outcomes from this investigation;
- LADEE detects an active dust population of lunar origin over the altitude range between 3-50 km.
- LADEE detects an active dust population of lunar origin over a lower altitude range, at a minimum of 3 km above the surface.
- LADEE is unable to obtain a definitive detection of an active dust population of lunar origin at any time during the mission.
For all three outcomes above, a potentially important future measurement to make from the surface would be of the low altitude component of Lunar Horizon Glow (LHG), which was first detected by the video cameras on the Surveyor series of lunar landers Figure 1. The most plausible explanation for the bright feature observed near the horizon is the forward scattering of sunlight by dust <10 um in size and levitating about 1-3 m above the surface. This event lasted more than 3 hours, and was far brighter than expected if the dust was merely the secondary ejecta from typical meteoric influx, indicating that some other process could be at work.
Figure 1: Surveyor 7 lander video images of LHG on the western horizon shortly after lunar sunset.
Measurements of LHG after LADEE will verify that there is indeed a low altitude dust population, and could establish whether this is part of any larger, more global population or represents a separate, local process. Correlating LHG with local time could also prove very useful in determining under what conditions dust levitation occurs, and validate one of LADEE’s core science goals, to determine if electrostatic charging of the surface is a significant contributor to this phenomenon.
If the outcome of the LADEE measurements is either 2 or 3 above, then there may be a more energetic component to the dynamic lunar dust population. In this case, we expect a “dusty sleet” component that impacts the surface with speeds on the order of hundreds of meters/sec, and may be episodically produced near the terminator regions. This dust population may be distinct from the LHG mentioned above, and require different detection mechanisms. The Lunar Ejector and Meteorites (LEAM) experiment has made a tentative detection of this more energetic dust component, however this interpretation of the data remains controversial to this day. Hence a dedicated, focused instrument to address this question on a future lunar lander could significantly impact our understanding of this phenomenon.
Measurement suggestions for future landers to the lunar surface are summarized as follows: in all cases, a high definition, sensitive camera or spectrometer capable of scanning the sunset horizon in a similar geometry as the Surveyor 7 mission would validate the interpretation of LHG. A dust detector optimized for low speed charged particles in the um size range could be included to characterize the detailed dust properties (size/mass, charge, velocity). If outcomes 2-3 are achieved, a second dust sensor specialized for the detection of high speed particles (“dusty sleet”) traveling at speeds in excess of ~200 meters per second could be included to validate the results obtained by LADEE and also the initial findings by LEAM.
Questions for all potential Lunar Lander teams
1. Where do they want to land?
- This determines what surface-based measurements might be useful
2. How long do they plan to survive?
- Survival of a 14-day night is nontrivial!
3. What instruments are baselined now?
- A reasonable-resolution imager with low-light capability would be awesome for the Surveyor-like horizon glow (which LADEE won’t be able to address)
4. What’s the available payload mass?
5. What is the fee for integrating/testing/operating?
6. What is the schedule for integration, test, launch and landing/ops?