A new look at Apollo 17 LEAM data: Nighttime dust activity in 1976
Introduction
As part of the Apollo Lunar Surface Experiments Package (ALSEP) the Lunar Ejecta and Meteorites (LEAM) Experiment (Berg et al., 1973, Anon., 1975a, Anon., 1975b, Anon., 1972) was deployed by the Apollo 17 astronauts on December 11, 1972 in the Taurus-Littrow area (Fig. 1) about 200 m west of the Landing Module (LM). Recently the Lunar Reconnaissance Orbiter (LRO) photographed the Apollo 17 landing site and the ALSEP station. In order to characterize the lunar dust environment the LEAM experiment measured by three sensors the speed, radiant direction, momentum, and kinetic energy of incident particles. All three sensors were contained in a single box standing on 4 legs and were connected to the ALSEP central station by a cable. LEAM started measurements after the return of the landing module and continued to make observations for about 3.5 years.
Section snippets
The LEAM instrument
The goal of the LEAM experiment was to record micrometeoroids bombarding the lunar surface and to detect secondary particles that had been ejected by bigger meteoroid impacts. Three classes of micron-sized cosmic dust particles were expected to be registered by LEAM: lunar ejecta, interplanetary dust from comets and asteroids, and interstellar grains.
The LEAM experiment is located at 20.164°N latitude and 30.774°E longitude on the moon (Berg et al., 1975). LEAM consisted of three sensor systems
Previous data analysis
Once LEAM started to operate it became clear that its observations contradicted expectations. Based on previous measurements in interplanetary space by Pioneer 8 and 9, for example, the expected rate of interplanetary dust particles was a few impacts per day. Instead, LEAM registered up to hundreds of impacts per day, which swamped any signature of the expected primary impactors of either interplanetary or interstellar origin. Most puzzling was the fact that these events registered in the front
Purpose of the renewed LEAM data analysis
The purpose of this analysis is to repeat the analysis of early LEAM data from 1973 and 1974 by Berg et al. (1975) with data from 1976 and to find evidence for impacts of interplanetary meteoroids in the LEAM data available to us. A second goal is to find in the ALSEP and LEAM housekeeping data evidence for the claim by O'Brien (2011) that the LEAM results published previously were false and could be explained by excessive power switching and correlated signals in the LEAM science data. O'Brien
The 1976 LEAM data set
As part of NASA's lunar data recovery project the Missing ALSEP Data Focus Group provided us with 140 days of LEAM data from 1976 (day 61–200, i.e. March–September) corresponding to about 5 lunations in 287 files. Each file contains a list of lines of 41 words each (totaling ~4 Million lines): 4 words Date/Time information, 1 word Main Frame counter, 31 words LEAM Science Data, and 5 words LEAM HK values. In a first step we identified gaps (totaling about 7 days) and removed overlaps in the
Housekeeping data analysis
Several ALSEP HK measurements monitor the thermal environment. Sunrise, sunset, and periods of solar illumination are clearly visible during the five lunations (lunar days) in 1976. The curves of some temperature measurements (e.g. ALSEP HK word 28 AT-4 Thermal Plate-2 Temp) indicate close control within a narrow temperature range by heaters and/or active radiators while others (e.g. ALSEP HK word 27 AT-l Sunshield-L Temp) display wide temperature range with obviously little or no control.
Science data analysis
In a first step we calculated the sum of all LEAM science channels (Fig. 7). The times when the instrument shuts-off are clearly identified as the sum reaches the maximum total value of ~369. During the times when the instrument was switched on there were peaks in the sum of all science data >200 DN that were regularly spaced, these peaks were identified as test signals (see below). During a total period of 77 days and 3 h the instrument temperature (Fig. 4) was lower than 60 °C. We flag the
Test pulses
Before we can interpret the LEAM science data test pulses have to be extracted from the data set. There are two ways to identify test pulses in the LEAM data set (Fig. 8):
- (1)
by the total of all science channels, because all IDs are 15, all or most film strips are triggered, and elapsed times are >25; and
- (2)
by the number of new DN values between 18 and 24 in the individual channels, because most electronic channels were triggered by a test pulse.
At times of enhanced noise rates, especially during the
Noise rates
Dust impacts should show-up as coincident signals on both front film and front collector channels. Therefore, the individual trigger of only one channel is considered a noise event. During lunar night the noise rates of individual LEAM channels are a few times 10 events per 24 h (Fig. 9). The rates increase towards morning and reach about 100 per day after sunrise. However, Sensor 2 has two very noisy channels that trigger at an anomalously high rate: Channel 13 (DJ-14) Front Film Accumulator
Candidate and potential dust events
Impacts of dust particles have the signature that both the front film and grid (collector) channels record in coincidence the charge signals which advance the front film accumulator by one. In addition both the front film ID and front collector ID indicate which sensor element was hit (cf. Table 1) neither values should be 0. For big impacts multiple strips can trigger, and also a Front Film Pulse Height PHA is measured. Both high-speed impacts and impacts of slow highly-charged particles will
Summary and conclusions
The LEAM instrument was exposed to strong variations in environmental conditions. Excessive heating was observed from about 24 h after sunrise until about 24 h before sunset. For about 9 days around lunar noon the temperatures were so high that LEAM was switched off. Only during the last lunation in 1976 LEAM stayed on. However, during the times of excessive heating LEAM became very noisy and no detailed analysis of the data has been executed as of yet. During the lunar night when the temperature
Acknowledgment
This work was supported by NASA Lunar Science Institute's funds to the Colorado Center of Lunar Dust and Atmospheric Studies. The authors thank Dr. Yosio Nakamura for providing access to the LEAM data and for his help in deciphering it. Valuable information and comments were given by Lynn Lewis, Chairman of NASA Lunar Science Institute Focus Group on Recover of Missing ALSEP Data. The authors thank Ralf Srama and Brian O'Brien for helpful comments.
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