Backyard lunar mineral prospection, Part III: Towards explaining some TLPs based on local minerology & geomorphology

2024 October 9

Mark Kidger & Juan José Godoy Carrera

One of the legacies of the Apollo missions and the fleet of unmanned probes that have followed them is the recognition that the Moon is far more active than previously believed, with seismic activity deep in the lunar interior and some tectonic activity at shallower depths, as well as gas release from below the surface. Here, we review lunar activity in the context of reports of the observation of transient lunar phenomena (TLPs), attempting to separate myth from reality. We suggest that a significant fraction of reported TLPs could have an explanation in local geology or geomorphology. In a further paper we will examine one particularly well-known TLP report to see if there is a plausible explanation based on local geology.

Introduction

The prevailing view of lunar internal activity has changed significantly since, in 1952, Harold Urey declared that the Moon is a geologically dead body and has been almost since its formation (Urey, 1952).¹ The Apollo missions revealed that the lunar interior shows a low, but still much greater than expected pre-Apollo, level of activity. Similarly, there is a continuous low level of seismic events from impacts, producing fresh craters.

Among the revelations of the Apollo programme and later orbital missions were:

• that the Moon shows both shallow and deep moonquakes, with the strongest deep quakes approaching Richter 6;
• that its core is likely to be partially molten;
• that the shallow moonquakes are due to tectonic activity;
• that there is both continuous and sporadic gas release from the lunar interior.

Note that, while most detected moonquakes were deep events around the boundary between the solid mantle and the liquid / partially liquid core, there were a small number of shallow events too, 28 in total, at depths estimated to be between 50 and 200 km (Watters et al., 2017).² The relative proportions of events detected by the Apollo seismometers were ~90 per cent deep quakes, ~10 per cent meteoroid impacts and ~1 per cent shallow quakes. The shallow moonquakes are apparently somewhat concentrated around large impact basins, with the strongest detected reaching Richter 4.2. Unlike deep moonquakes, the shallow quakes are seemingly uncorrelated with tidal stresses and thus apparently caused by residual tectonic activity (Nakamura et al., 1979).³

That the Moon shows some level of internal activity is, therefore, in no doubt. That this activity can be detected remotely is much less certain. One of the enduring mysteries of lunar research is that of transient lunar phenomena (TLPs) and their possible reality. Over the years, opinion has swung back and forth as to the possibility that any part of the (now) known residual lunar activity could be observable from the Earth as TLPs. However, with one exception, it has now swung against the reality of TLPs as manifestations of lunar activity (e.g. Sheehan & Dobbins, 1999).⁴ Today, the debate is more centred on whether or not at least some reported TLPs have any other physical explanation in lunar science (e.g. Lena & Cook, 2004; O’Connell & Cook, 2013),^5,6 or if they belong to the realms of pseudoscience and a case of observers seeing what they wanted to see (Mobberley, 2006).⁷ Even so, a search for the term ‘transient lunar phenomena’ in the NASA ADS publication database reveals 156 refereed publications since 1964 that reference the term in their abstract and 204 – including 52 since the turn of the century – that reference TLP somewhere in the text.

Ironically, the only type of TLP that is now accepted and whose reality is firmly established by science is impact flashes (e.g. Avdellidou et al., 2021),⁸ although these were formerly treated with great scepticism, even by supporters of the reality of TLPs (e.g. Moore, 1968).⁹ Such impacts on the lunar disc can be detected from Earth as flashes of light, visible for a few tenths of a second. The European Space Agency (ESA) Near-Earth object Lunar Impacts and Optical TrAnsients (NELIOTA) programme has, to date, detected 187 impact events in 277 hours of lunar observations (a detection rate of 0.66 per hour of observation).¹⁰ The Marshall Space Flight Center Lunar Impact Monitoring Program lists 443 potential impacts between 2005 and 2020,¹¹ as well as 35 potential events reported by independent observers,¹² but does not quote figures for its detection rate.

So, more than 600 such events have been confirmed by imaging in just the two main impact monitoring programmes. One particularly bright event, observed during the total lunar eclipse of 2019 Jan 19, was recorded by multiple observers (Madiedo et al., 2019).¹³ Several such flashes have been observed on Jupiter too, most memorably that of the impact of Comet Shoemaker–Levy 9. However, these events on the Moon and Jupiter are almost certainly distinct from the apparently similar flashes observed on Mars, e.g., the famous event in Edom Promontorium in 1958 (Unattributed, 2001; Sincell, 2001),^14,15 which repeated in 2001 June and was observed continuously for nearly 90 minutes (Fienberg, 2001).¹⁶ These Martian flashes are certainly not impact events. Initially thought to be due to reflections from ice crystals in the Martian atmosphere, it now seems more likely that the cause is a specular reflection of the Sun from a geological formation on the surface, particularly as the observing geometry of 1958 was identical to that of the event observed in 2001 June. Such specular reflections could potentially explain some longer-duration flashes on the Moon too – if present – which might last as much as a few tens of minutes, depending on the reflecting surface.

Related to TLPs, and also phenomena that can be considered confirmed, are the so-called ‘permanent blinks’. These were detected by moonblink devices, such as those operating under NASA contract (Trident, 1965; Cameron, 1966) in which red and blue filters alternated at ~2 Hz,^17,18 and the simpler and cheaper-to-produce Sartory moonblink device used by the BAA Lunar Section (Sartory, 1965).¹⁹ Certain lunar features – e.g., an area of Mare Frigoris north-west of Plato, or the crater Fracastorius – were found to provide permanent blinks. In other words, as the moonblink filter changed from red to blue and back again, the feature flashed between brighter and darker, showing that the device was detecting significant colour differences in a particular area. These differences, however, were not transient, but always present and, thus, indicate important localised differences in the composition and minerology of the lunar surface. These are generally different in scale to the large-scale compositional differences detected in lunar maria and discussed in Papers I and II, which are due to the considerable differences in the composition of the many lava flows that filled these basins at different times (Kidger & Godoy Carrera, 2022a; Kidger & Godoy Carrera, 2022b).^20,21 In Figure 1, we can see one such small area of clearly different colour within Mare Frigoris. While most of Mare Frigoris (which, at 1.1 per cent, has by some distance the lowest titanium abundance of any of the lunar maria; Sato et al., 2017),²² is reddish, this bay is strongly blue and, through the strong colour contrast, would always show up as brighter than the rest of the mare in a blue filter and darker in the red filter, providing observers with a permanent blink.

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