Effect of COVID-19 global lockdown on our Moon

ABSTRACT

Systematic investigation of lunar night-time temperatures can possibly be thought as a stable platform to study Earth's radiation budget and climate change as advocated earlier by several researchers. In this study, we report an interesting observation possibly of changing Earth's climate as experienced by the Moon, utilizing a rare and novel context of COVID-19 global lockdown. Lunar night-time surface temperatures of six different sites on the Moon's nearside were analysed during the period 2017–2023. Results showed an anomalous dip in the lunar night-time surface temperatures for all the sites during April–May 2020, the strict COVID-19 global lockdown period, when compared to the values of the same period during the previous and subsequent years. Since the terrestrial radiation has also showed a significant reduction during that time, the anomalous decrease observed in lunar surface temperatures is attributed to the COVID-19 global lockdown effect. Therefore, our study shows that the Moon has possibly experienced the effect of COVID-19 lockdown, visualized as an anomalous decrease in lunar night-time surface temperatures during that period. These results can be substantiated further from Moon-based observatories in future, thereby making them potential tools for observing Earth's environmental and climate changes.

radiation: dynamics, atmospheric effects, Planetary systems, Earth, Moon, planets and satellites: surfaces

1 SUMMARY

Earth and Moon exhibit an interlinked history, as they have shared the same cosmic environment since their origin. Earth, having a rapidly changing climate, is now incapable of reproducing our evolutionary history. On the other hand, Moon preserves a rich archive of our past and provides a unique platform to study the changes associated with the Earth–Moon system. Recent studies have shown that the radiation emitted from Earth can have significant impact on the lunar surface temperatures (LST; Huang 2008; Song et al. 2017). Therefore, systematic investigation of lunar night-time temperatures can possibly be thought as a stable platform to study Earth's radiation budget and climate change. Our present study reveals that the changes in terrestrial radiation (TR) have a measurable influence on the lunar surface, visualized as an anomalous decrease in lunar night-time surface temperatures during the strict global lockdown period, April–May 2020.

2 INTRODUCTION

Due to the lack of an atmosphere, the Moon receives a high amount of heat and radiation from the Sun and thus exhibits a significant diurnal variation in its surface temperature. This effect becomes further prominent due to the presence of a highly insulating surficial layer with globally varying thickness (Durga Prasad et al. 2022). On the other hand, Earth also receives sunlight, a part of which is absorbed by its atmosphere while the rest radiates back to space, which is known as TR or Earthshine (Glenar et al. 2019). During daytime, the lunar nearside attains an energy balance between incoming solar radiation, TR and internal heat flow. However, during night-time, the Moon receives radiation only from Earth, and thus it can influence the night-time surface temperatures (Song et al. 2017). This effect was observed by Clementine spacecraft navigation camera, which imaged the partially illuminated lunar limb by sunlight reflected from Earth (Hahn et al. 2002). Huang (2008) showed that the lunar nearside surface is more sensitive to TR when compared to the received solar radiation at night-time. It was numerically shown that a 4 mW/m² change in the Total Earth Irradiance could vary the lunar night-time surface temperature by 0.37 K, while a change of 4 W/m² in Total Solar Irradiance could lead to only 0.28 K change in LST during daytime. The results from Moon and Earth Radiation Budget Experiment (MERBE) have shown that analysing lunar irradiance with improved calibration techniques can possibly predict Earth's climate instabilities in future (Matthews 2018). The study by Glenar et al. 2019 showed that the terrestrial spectral signature, including both the solar reflectance band and thermal band, could affect lunar temperatures and therefore can be a potential energy source in prospecting the cold trapping of volatiles at Permanently Shadowed Regions (PSRs). Kloos & Moores, 2019, has modelled the illumination received at lunar poles and estimated that Earthshine could illuminate 40 per cent–45 per cent of the permanently shadowed area at lunar poles and could have more coverage at lower latitude PSRs. It is also proposed that Earthshine along with other illumination sources such as starlight, interplanetary photons etc. under the absence of solar irradiance can largely affect the surface temperatures and thus the volatile stability on the Moon (Kloos et al. 2021). Lucey et al. 2021 suggested that Earthshine should also be taken into consideration when interpreting spectral data for PSR surfaces. In view of the above discussions, it is evident that lunar night-time surface temperatures can act as a proxy to understand the variation in TR. The work presented in this paper is an attempt to visualize such changes probably as seen by the Moon, utilizing a rare and novel context of COVID-19 lockdown.

3 COVID-19 LOCKDOWN & EARTH'S RADIATION BUDGET

Recently there has been a global scenario where the TR was significantly reduced due to international shutdown because of global outbreak of COVID-19 pandemic. During late 2019, a respiratory syndrome, SARS, popularly known as COVID-19, was identified in China (Zhu et al. 2020) and rapidly spread across the nations. This caused large-scale death rates internationally, and World Health Organization declared this disease a global pandemic on March 11, 2020. Restrictions were established over various nations, such as travel constraints, economic closures, curfews, and social distancing. China imposed a lockdown on January 23, 2020 and subsequent lockdowns were introduced in other countries during the following eight months (Haider et al. 2020; Lancet 2020; Pachetti et al. 2020; Ren 2020; Anil & Alagha 2021). Strict global lockdown was imposed during April–May 2020 which brought the entire world to a shut down. Beyond June 2020, the countries slowly started relaxing restrictions restarting their industrial and economic activities. Anthropogenic emissions were considerably decreased during the strict lockdown period. Numerous studies were conducted to evaluate the global emission and air quality changes during this lockdown period, and several claimed that there had been a reduction in global radiative fluxes and pollutants. Ming et al. (2021) found out that there was a 32 per cent reduction in aerosol optical depth and a 7 per cent reduction in clear-sky reflection over the East Asian Marginal seas during the lockdown period. Mazhar et al. (2021) found a significant reduction in Net Solar Radiation, Net Thermal Radiation, and Net Radiation during the global lockdown. Furthermore, various global and regional studies also investigated similar effects of COVID-19 imposed lockdown on Earth's Radiation Budget and stated a clear reduction in TR and air pollution across the world (Berman & Ebisu 2020; Broomandi et al. 2020; Diamond & Wood 2020; Pathakoti et al. 2020; Timmermann et al. 2020; Choi & Brindley 2021; Fiedler et al. 2021; Gettelman et al. 2021  a, b; Latha et al. 2021; Reifenberg et al. 2021; Srivastava et al. 2021; Zhang et al. 2021; Asutosh et al. 2022; Lawand et al. 2022; Singh & gokhale 2022; Yang et al. 2022).

3.1. Impact of terrestrial radiation on LST

When the sunlight reaches the surface of Earth, some portion of the radiation is absorbed by the surface and the atmosphere, and the rest is reflected in shortwave (0.2 < λ < 5 |$\mu$|m). The planet is heated during this process and emits an amount of infrared radiation (λ > 5 |$\mu$|m), which is denoted as Outgoing Longwave Radiation (OLR). The trapped greenhouse gases also strengthen this re-emitted OLR in the atmosphere, which is produced due to human activities such as industrial pollution, transportation, and burning of fossil fuels. These shortwave and longwave radiations form critical components of TR propagating out to space (Barry & Chorley 2009). Further, it has been confirmed that the amount of TR is governed by fluctuations in atmospheric processes and anthropogenic effects.

When the COVID-19 pandemic hit the world and lockdowns were imposed in many countries, anthropogenic activities were reduced significantly and, thus, the greenhouse emissions. This led to a reduction in cloud cover and pollutants over many nations; hence, the outgoing TR decreased proportionally. The solar energy received at the top of the atmosphere of Earth for a mean solar distance is found to be around 1366 ± 2 W/m², wherein one-third of the incoming radiation is reflected back (https://ceres.larc.nasa.gov/science/). This radiation, along with thermal emission, particularly in the longwave, reaches the nearside of the Moon at lunar night-time to cause an increase in the surface temperature in the absence of solar insolation. Previous studies have estimated that an average TR of 60–150 mW/m² reach lunar surface, which could correspond to 6–10 K change in the surface temperatures in thermal equilibrium (Glenar et al. 2019). Even though this radiative flux exhibits slight seasonal variation, at any given instant the flux remains nearly constant, unless there is a change in anthropogenic processes, which contributes to the flux rate. However, during the lockdown, the shortwave radiation exhibited a decrease of around 17 per cent–20 per cent due to the global depletion in clouds and aerosols, and longwave radiation was also correspondingly decreased owing to the reduced greenhouse emissions. Since the lunar surface is an amplifier of TR signature (Huang 2008; Song et al. 2017; Glenar et al. 2019; Kloos & Moores 2019), it is expected that this mechanism would probably affect the lunar surface, and thus, if it had, the lunar night-time temperatures would also exhibit a relative change during the same period. While several inter-dependent factors [such as the anthropogenic emissions – aerosols and greenhouse gases, cloud cover, albedo, reflected solar radiation from TOA (top of the atmosphere) and solar activity] affect the Earth's outgoing TR, only COVID-19 lockdown driven factors deemed to be responsible for the observations reported here, as all other factors have been shown to have no influence on the signature observed in our study. Anthropogenic emissions and aerosols, the key source for enhanced outgoing TR, were found to be considerably decreased during the strict lockdown period of April–May 2020 (Berman & Ebisu 2020; Reifenberg et al. 2021; Sanap 2021; Srivastava et al. 2021; Gettelman et al. 2021a; Yang et al. 2022). Another source of variation in outgoing TR is the reflected solar radiation from the top of the atmosphere. However, an increased solar radiation (insolation) at the surface, particularly during April–May 2020, attributed to strict COVID lockdown has been reported (Van Heerwarden et al. 2021) which implies a reduced outgoing TR also as reported in other studies (Mazhar et al. 2021). The effect of solar activity can be readily ruled out as a downward trend in flux is observed systematically for 5–6 yr during a solar cycle and not found to exhibit an abrupt decrease as observed during April–May 2022. Also, the solar minima occurred in the beginning of solar cycle 25 around 2019 (https://www.swpc.noaa.gov/products/solar-cycle-progression), while the decrease in TR is observed only in 2020 ruling out any solar cycle effect on the outgoing TR. While all other possible factors are ruled out, strict COVID lockdown during April–May 2020 seems to be the only reason for the observed decrease in terrestrial TR and in turn for the anomalous dip observed in lunar night-time surface temperatures as reported here. Even though the COVID-19 pandemic has given the world a hard time socially and economically, this global state has given us a natural and unique opportunity to analyse this large variation in TR from a planetary perspective.

4 METHODOLOGY

Several efforts have been made to find the relationship between TR and lunar night-time surface temperatures (LST; Huang 2008; Song et al. 2017; Gläser & Gläser 2019; Ye et al. 2021). Since there are no direct observations from the Moon, an unambiguous explanation is not feasible at present. As earlier studies have shown that TR has an impact on LST (Huang 2008; Song et al. 2017), there is a reason to believe that it can be reduced considerably during the lockdown period, where the entire global activity came to a standstill. Pursuing it further necessitates the measurement of surface temperature variations during the lockdown period. Such a data is currently available only from Diviner Lunar Radiometer (DLRE) instrument onboard NASA's Lunar Reconnaissance Orbiter (LRO). DLRE is a nine-channel radiometer that provides global LST data at a spatial resolution of 180 × 300 m for an altitude of 50 km. It has a spectral range of 0.35–400 μm and operates in a nadir-pointing push broom configuration (Paige et al. 2010). While the first four channels are helpful for observation of solar radiation and surface geology, the last four channels are meant for surface temperature measurement – to characterize daytime, night-time and polar temperatures on the Moon. Night-time surface temperatures from DLRE for six diverse sites on the Moon were analysed through 2017–2023, covering the periods before, after and during the COVID-19 lockdown, to look for any lockdown-induced subtle signature.

4.1 Site selection

Since the Moon is tidally locked with Earth, its nearside always faces the Earth, and the terrestrial thermal signature is more prominent on the nearside (Roy et al. 2014). Therefore, we have selected six distinct sites on the nearside of the Moon with a surface area ranging from 900–2500 km². All selected sites have similar surface roughness and albedo values. The sites selected for the present study are shown in Fig. 1. The details and characteristics of each of the selected sites are given in Table 1. The surface roughness for each site is obtained from the LOLA roughness maps at a spatial resolution 1 km/pixel. The Data LDRM_32_N (50 m baseline) is used and average of the pixel values within the site are calculated.

Figure 1.

Nearside map of the Moon showing the sites selected for the present study. Each individual site is also shown in the blow-ups. The base map is LRO WAC mosaic. (Source: LRO quickmap)

Table 1.

Table showing the locations and characteristics of the six sites on the near side of the Moon, selected for this study.

Site name	Maximum Lat/Lon	Minimum Lat/Lon	Region/Basin	Average surface roughness (m)	Normal albedo
S1	4.98, 302.98	4.01, 302.01	Oceanus Procellarum	0.407	0.152
S2	30, 296	28.5, 294	Oceanus Procellarum	0.425	0.164
S3	25.5, 19	24, 16	Mare Serenitatis	0.442	0.175
S4	35, 334	33.5, 332	Mare Imbrium	0.461	0.162
S5	9.4, 36	8, 34	Mare Tranquillitatis	0.339	0.152
S6	17, 60	15.55, 58	Mare Crisium	0.517	0.174

The sites were selected based on the following criteria:

1. The site should be relatively flat, i.e. should have minimum surface roughness, in order to avoid any topographical effects on surface temperatures.

2. It should be within an equatorial or mid-latitude region so that the incident radiation is overhead/near-overhead or the angle is minimal;

3. The site should be of an optimum size i.e. small enough to analyse the local temperature variation, but at the same time hold maximum DLRE data coverage within.

5 RESULTS AND DISCUSSION

5.1 DLRE analysis

For the present study, we have used Diviner Reduced Data Record (RDR) from NASA-PDS Geosciences Node (https://pds-geosciences.wustl.edu/missions/lro/diviner.htm), since the higher level products do not provide the variation on a diurnal basis. The calibrated brightness temperatures for all the selected sites over the period from January 2017 to February 2023 were taken from the RDR data. From this data, night-time surface temperatures corresponding to 22:00 hrs–04:00 hrs local times were then selected to ensure true night-time temperatures with a minimal or no effect of the day-time solar forcing. The obtained data is further reduced and filtered using an automated python code to obtain only the quality measurements for each site. All the observations acquired during off-nadir configuration are omitted. The quality flags are chosen carefully so that the errors due to calibration, geometry and other miscellaneous processes are ignored. Observations from each thermal channel are separated in order to avoid the overlapping of values, and the maximum temperature is evaluated at all data points. While data from all the four Diviner channels (6, 7, 8, and 9) were initially analysed, only data from channel 7 was used for further analysis due to its sensitivity and relevant range of temperatures covered for the selected sites. The temperature accuracy of the measurement for the observed range is <1 K (∼0.6 K; Paige et al. 2010). Since LRO orbits the Moon with an orbital period of 2 h, it scans a particular point on the surface nearly 24 times a year. All the data from January 2017 to February 2023 is compiled to analyse the temperature variations and plotted in Fig. 2. A decrease in the maximum temperature is observed at all sites during the global lockdown period of April 2020 and May 2020. We observed a night-time temperature change of nearly 8–10 K. A figure showing the night-time surface temperatures obtained from Diviner observations for each of the six sites during 2017–2023 is given in Fig. 3. For better visualization, a trend line for each site is also plotted. All the sites seem to report a range of temperatures from nearly 110–150 K. The reason for these range of values is due to the observations considered for a range of local times (instead of only midnight) for sufficient data coverage and inherent diversity in thermophysical properties at each site. As shown in Fig. 2, an anomalous dip in the night-time temperatures is seen for all the sites exactly during April–May 2020, strict global lockdown phase. It is also seen that all sites recorded relatively lower temperature range (shown as clusters) during April–May 2020 compared to those during previous (2017–2019) and subsequent (2021–2022) years of lockdown. As can be seen in Fig. 2, a trend line is also plotted to clearly depict the observed anomalous dip in the temperatures during the lockdown period. The lockdown was prominent from late March to early June 2020, particularly during April–May 2020. The measured brightness temperatures in all the channels have lower values in this specific period than that of 2017–2019 & 2021–2023, taken at the same month and local time. All the sites exhibit a similar pattern, except for few exceptions.

Figure 2.

Diviner temperature measurements obtained for 2017–2023 for all sites. A drop in temperatures during April 2020–May 2020 can be clearly differentiated from that for the remaining years.

Figure 3.

Figure showing the temperatures obtained during January 2017–December 2022 for all sites. Average trendlines of all sites show a dip during April–May 2020 (strict global lockdown period), which is highlighted in the plots.

The minimum night-time temperatures recorded for each site for the months April–May during 2017–2023 is given in Table 2. It is seen that all sites recorded relatively lower temperatures during April–May 2020 compared to those during 2017–2019 and 2021–2023. For a better clarity, year-wise night-time temperatures of all sites, combined for each site, is shown in Fig. 4. It shows the annual variation for each site through 2017–2023. A regression line is also plotted for comparison of COVID lockdown induced trend with that of the others. It can be clearly seen that a reverse trend occurs in 2020 particularly due to COVID induced reduced temperatures in April–May, while the trend is almost identical for all other years, both before and after the COVID lockdown period. This contrary result can clearly be attributed to the effect of decrease in the surface temperature due to effective global lockdown during April–May 2020.

Figure 4.

Comparison of annual variation trend in the night-time surface temperature for all sites combined. An inverse trend particularly during COVID-19 strict global lockdown period during the year 2020 is also seen.

Table 2.

Lowest temperatures recorded in channel-7 for each site during the months of April–May, 2017–2022.

Site name	2017 Lowest T (K)	2018 Lowest T (K)	2019 Lowest T (K)	2020 Lowest T (K)	2021 Lowest T (K)	2022 Lowest T (K)
Site-1	119.6	118.4	128.4	107.0	135.1	143.8
Site-2	106.3	114.8	104.3	96.2	125.4	131.7
Site-3	116.5	102.2	118.5	102.7	117.9	122.6
Site-4	117.0	107.1	115.1	102.3	118.8	130.2
Site-5	128.0	118.5	119.4	108.6	110.9	112.1
Site-6	115.1	117.0	114.2	100.6	114.5	140.0

5.2 Correlation of the observations to other possible factors

As an anomalous decrease in the lunar night-time surface temperatures exactly during the COVID-19 lockdown period is observed, the effect of other possible factors such as solar activity and seasonal flux variation have also been investigated. Results show that none of these factors have any influence on the observed signature thus supporting our findings to be only due to COVID-19 lockdown.

5.2.1. Seasonal variation

As the lunar subsolar latitude changes from −1.54⁰ to +1.54⁰ within a period of a year (∼355.26 Earth days), only a slight variation in the solar flux is observed on the Moon. Even though this variation is less because of the low axial tilt, it has significant effect on PSRs and volatile transport on the lunar surface at high-latitudes and poles (Williams et al. 2019). March 2020 was the lunar winter minimum in the Northern hemisphere, and March 2021 and March 2022 were also winter times, but not minimum (https://www.lpi.usra.edu/lunar/tools/lunarseasoncalc/). However, this effect is minimal/none in equatorial and mid-latitudes, and therefore we expect the seasonal variation at the selected sites to be negligible. If this effect was significant, then it would have been reflected in 2021–2022 data also, which is not the case as can be clearly inferred from Fig. 2. Therefore, the seasonal effect is ruled out to influence the present unique observation.

5.2.2 Solar activity

In order to correlate our observations with changing solar activity, the temperatures are compared with the sunspot activity during the period. The sunspot activity data is downloaded from NOAA website (https://www.swpc.noaa.gov/products/solar-cycle-progression). The observed sunspot number in the period of 2017–2023 is filtered and plotted against the observed temperatures in Fig. 5. There is progressive sinusoidal oscillation in the sunspot number with minima during 2017–2021which starts increasing beyond. Comparison of sunspot activity and the observed surface temperatures as shown in Fig. 5 show no correlation or trend thereby completely ruling out any effect of the solar cycle on the observed anomalous dip in surface temperatures observed during April–May 2020.

Figure 5.

The complete night-time observations for time range 23.00–04.00 local time within the period 2017–2023 for all sites. The solid line indicates the solar activity variation during the same period. The temperature values are marked inside the ellipse.

Therefore, it can be affirmed that the lunar night-time surface temperatures have definitely shown an anomalous dip during the COVID-19 lockdown period. It is already known that the night-time temperatures are determined mainly by TR, the thermophysical behaviour of regolith and internal heat flow (Hayne et al. 2017, Williams et al. 2017). Considering the variation of thermophysical properties and internal heat flow to be negligibly small over seven annual periods, the observed trend can only be attributed to the considerable variation in TR reaching the nearside surface of the Moon. As there seems to exist no other source to explain the observed anomaly during April–May 2020, the plausible reason is only the strict global COVID-19 lockdown.

6 CONCLUSIONS

For the past two decades, the global scientific community have focused on their steps towards a sustainable presence on the Moon. Recent studies highlight the need for lunar outposts to be accomplished in future. In this paper, we report a unique observation related to the Earth–Moon system and explore the possibility of observing lunar night-time temperatures as a function of TR. The positive effects of the COVID-19 lockdown in terrestrial perspective that occurred during the period April–May 2020 have been presented as experienced by the Moon. Six distinct sites on the lunar nearside were selected, and brightness temperatures were derived from LRO Diviner data. A clear decrease in the lunar surface night-time temperature is observed during the lockdown period. Since the TR was also reduced significantly at that time, it is proposed that the dip in the observed temperatures can be largely attributed to the COVID-19 global lockdown. While this could not be fully established from the present study due to lack of sufficient data or ground truth, the presence of an anomalous dip in the night-time LST exactly during April–May 2020 clearly direct to the effect of reduced TR on the Moon due to global lockdown. In this work, we have utilized a rare and unique opportunity of COVID-19 to carry out our study, which may never occur again. It can also be further substantiated from Moon-based observatories in future as advocated by some researchers. Therefore, Moon-based observatories can become potential tools for observing Earth's environmental changes and needs to be explored extensively.

ACKNOWLEDGEMENTS

We thank the Department of Space, Govt. of India for its support for carrying out this work. A part of this work has been carried out under Govt. of India's DST-INSPIRE fellowship grant and we thank DST for the same. We thank the reviewer for providing useful suggestions for improving the contents of the manuscript. We thank Dr. J.P. Williams for his useful suggestions in processing Diviner RDR data. We also thank Mr. P. Kalyana Reddy for his help during data processing. Diviner data used for this work is freely available in public domain at https://pds-geosciences.wustl.edu/missions/lro/diviner.htm.

DATA AVAILABILITY

The data sets generated and/or analysed during the current study are available at the URL: https://www.prl.res.in/∼durgaprasad/ERB_LST_Data.html.