A new look at Apollo 17 LEAM data: Nighttime dust activity in 1976

• Housekeeping and science data from the Apollo 17 LEAM instrument were analyzed.
• Excessive temperatures (>60 °C) during the lunar day were observed.
• During lunar night 20 potential dust impact events were found.
• Half of the events occurred in three statistically significant bursts.
• 1976 LEAM data do not indicate strongly enhanced dust activity at the terminator.

One of the unresolved enigmas from the Apollo era is the existence and characteristics of highly electrically charged dust floating above the lunar surface. Potential evidence for this hypothesized phenomenon came from the Lunar Ejecta and Meteorites (LEAM) experiment on Apollo 17. The LEAM instrument consisted of three sets of multi-coincidence dust sensors facing different directions. Recently, new arguments were raised (O'Brien, 2011) that the signals recorded by LEAM may be caused by interferences from heater current switching, which occurred most frequently near sunrise and sunset. In order to shed light on this controversy a new look into the LEAM data was initiated within the Colorado Center for Lunar and Dust and Atmospheric Studies (CCLDAS) team of NASA's Lunar Science Institute (NLSI). The purpose of this analysis is to verify the earlier analysis by Berg et al. (1975), and to find evidence for impacts of interplanetary meteoroids in the LEAM data available to us. A second goal is to find in the LEAM house keeping data evidence for excessive power switching and correlated signals in the LEAM science data. The original analysis by Berg et al. (1975) covered LEAM data during 22 lunations (~22 months) in 1973 and 1974. This data set is no longer available. For the present study, we had access to LEAM data for only about 5 lunations (140 days) in 1976. We analyzed the housekeeping data and observed excessive heating from about 24 h after sunrise until about 24 h before sunset. We defined sunrise and sunset when the LEAM temperature measurement reached −20 °C above which significant solar heating was apparent. For about 9 days around lunar noon the temperatures were so high that LEAM was switched off. During the times of excessive heating LEAM became very noisy. We limit our current analysis to about 24 h before sunset to about 24 h after sunrise when the LEAM temperatures were moderate <60 °C. This carefully analyzed data set of 74.6 days constitutes about 75% of the periods when LEAM was switched on in 1976. We did not find a systematic correlation between the infrequent heater switches and the occurrence of signals. During the lunar night the temperature was quite stable at approx. −25 °C. One TOF dust impact event and 19 potential dust events were recorded by all three sensors during the periods when the instrument was at moderate temperatures. This corresponds to an average event rate of 0.25/day. While nine events are compatible with a random occurrence the other 10 events occurred in three statistically significant bursts within about 1 h or less after another. Two bursts occurred within the middle of lunar night and one burst of three events was recorded by the West sensor just an hour before sunrise. The background rates are compatible with impact rates recorded by the dust instruments onboard the Pioneer 8 and 9 spacecraft in interplanetary space. Based on our definition of sunrise and sunset, the 1976 LEAM data do not indicate strongly enhanced dust activity at the terminator.