Gravity Data Reveal Origin of Cryptomaria

doi:10.21203/rs.3.rs-3594613/v1

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Gravity Data Reveal Origin of Cryptomaria

https://doi.org/10.21203/rs.3.rs-3594613/v1

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Incomplete studies exist on the identification and formation processes of lunar cryptomaria, which stand as crucial hubs of late lunar magma ocean activity and early volcanism, presenting a mystery in their orgin. Here, we show 29 delineated cryptomaria regions, based on remote sensing data. A thorough investigation of gravity anomalies within these proposed cryptomaria regions was undertaken. Through this analysis, we successfully identified and classified cryptomaria by integrating topographic features and chemical compositions. Our findings reveal that the formation of ancient cryptomaria is linked to large impact events. This establishes a clear correlation between impact events and the formation timeline and spatial distribution of cryptomaria. Older cryptomaria owe their existence to these impact events, while the younger cryptomaria is primarily located in the interiors and margins of maria, with their basalts originating from the lateral spread of these maria. Furthermore, the study of maria thickness remains a contentious subject, and we anticipate that forthcoming investigations, consolidating data from various sources, will ultimately resolve this matter.

Earth and environmental sciences/Planetary science/Rings and moons

Earth and environmental sciences/Planetary science

Lunar cryptomaria, the light plains on the luanr surface, are defined as mare basalts covered by highly reflective ejecta from impact events^1–3. The formation of cryptomaria may be linked to the earliest volcanic activity on the Moon. While most maria formed between 3.3 and 3.8 Ga with a peak in volcanic activity around 3.5 Ga^4,5, the cryptomaria is believed to have formed even earlier, dating back as far as 4.35 Ga^6,7. Therefore, exploring the formation, origin, and characteristics of the cryptomaria can enhance our comprehension of lunar volcanism history and thermal evolution. Simultaneously, it represents a crucial research component in unraveling the time and space distribution characteristics of lunar volcanism. With Chang'e 6 poised to land in the lunar farside South Pole–Aitken (SPA) Basin for a series of scientific tasks within the southern part of the Apollo Basin, a notable area of cryptomare⁸, it is evident that the investigation into cryptomaria will remain a prominent subject of study in the future.

Remote sensing data provides a wealth of information crucial for understanding the surface characteristics of the cryptomaria. One common approach for identifying cryptomaria regions is through the analysis of dark-halo craters (DHCs) in visible images on the lunar surface¹. These DHCs effectively strip away the highly reflective material covering the cryptomaria, thereby exposing the dark basalt beneath to the lunar surface, both within and around the DHCs^1,9,10. Additionally, the results from the analysis of DHCs depths indicate that the thickness of the cryptomaria ranges from a few meters to several kilometers^3,10,11. Previous work⁷ utilized remote sensing data to examine 29 proposed cryptomaria regions (Fig. 1) on the Moon and identified 20 regions of cryptomaria based on pyroxene features observed at DHCs. They delineated the boundaries of each cryptomaria and analyzed the morphological characteristics of its surface, concluding that the cryptomaria covers approximately 1.8% of the lunar surface. Radar wavelengths have the capability to detect information from a few meters to tens of meters below the surface of the moon. The use of Chang'E-2 microwave radiometer and ground-based radar^12,13 has offered fresh perspectives in mapping the extent of the cryptomaria.

However, remote sensing data are limited in their ability to detect the lunar crust and its depths in the cryptomaria region. For a more comprehensive understanding of the lunar interior, gravity anomaly data play a crucial role in providing reliable information about the subsurface characteristics^14–16, such as the volume and thickness of basalts in the cryptomaria¹⁷. These estimates have led to an increase in the total amount of volcanic deposits on the lunar surface, raising it from 16.6% to a range of 17.9–19.5%. This thickness results obtained from the gravity data are greater than those estimated from the analysis of DHCs. Furthermore, the depth of the cryptomaria within the same region derived from gravity data substantially exceed those acquired from DHCs, with depths of up to 8 km in the former and only 1.8 km in the latter^3,18. This implies a divergence between certain outcomes obtained from gravity data and those gleaned from remote sensing data. Consequently, it becomes imperative to leverage the gravity anomaly data to conduct a comprehensive analysis of the global cryptomaria region, establishing correlations between its internal characteristics and its surface, spatial, and temporal distribution.

In this study, we investigate the gravity anomalies in 29 proposed cryptomaria regions on the Moon using the most recent gravity model to uncover the internal characteristics of the cryptomaria (Fig. 1b). The identification and classification of these cryptomaria regions are accomplished by integrating morphological features, chemical component characteristics, and lunar crust thickness. We propose a classification model, enabling the segregation of cryptomaria into two distinct types based on their originating magma source. This uncovers the correlation between internal characteristics of the lunar crust and the surface tectonics of cryptomaria, which is of key importance for comprehending the formation process of cryptomaria and investigating the total amount of mare basalts.

2.1. Gravity in cryptomaria

In this work, we investigated the gravity of 29 possible cryptomaria locations across the globe. As shown in Fig. 1 and Table 1, these 29 investigated regions are proposed by previous work, of which DHCs and mafic-rich soils are present in 19 cryptomaria regions⁷. Mascons are present in 10 of the 19 cryptomaria regions, and the maximum values of gravity anomalies for mascon in these regions are from 56.3 mGal to 141.9 mGal (Supplementary Note 1 and Supplementary Fig. 1). It is obvious that these mascons occur in the centre of craters or basins, in the form of sub-circular or ring shapes. Moreover, in large impact basins, positive and negative gravity anomalies alternate around a centre. It is noteworthy that several mascons are present in Regions Schiller-Schickard and South Pole-Aitken and that these are located in the center of the crater (Note 2). Regions Schiller-Schickard and South Pole-Aitken are distinctly different compared to other cryptomaria regions. Most of the cryptomaria are located within the mascon basin, while the cryptomaria in Schiller-Schickard and South Pole-Aitken are distributed both inside and outside the mascon basin. Moreover, these results suggest that mascons are found in all of these cryptomaria regions, and these are located in the centre of impact craters or large impact basins. This also suggests an intrinsic correlation linking cryptomaria, mascons, and craters.

Table 1

List of proposed cryptomaria.
Id	Region^a	Latitude	Longitude	Observed cryptomaria^b	Mascon^c	Type of cryptomaria^d
1	Australe	-53.31	104.33	Y	N	Diffuse-type
2	Balmer	-17.28	71.43	Y	Y	Impact-type
3	Cleomedes	27.50	55.50	Y	Y	Impact-type
4	Dewar	-2.13	167.40	Y	Y	Impact-type
5	Hercules	45.80	44.10	Y	N	Diffuse-type
6	Humboldtianum	59.27	78.44	Y	Y	Impact-type
7	Langemak	-9.93	119.48	Y	Y	Impact-type
8	Lomonosov-Fleming	17.71	107.84	Y	N	Diffuse-type
9	Mare Frigoris	50.68	17.85	Y	N	Diffuse-type
10	Marginis	10.33	94.42	Y	N	Diffuse-type
11	Mendel-Rydberg	-50.18	-94.92	Y	Y	Impact-type
12	Milne/Tsiolkovskiy	-31.14	112.77	Y	Y	Impact-type
13	Schiller-Schickard	-45.28	-53.18	Y	Y	Both
14	Smythii	-2.93	86.86	Y	Y	Impact-type
15	South Pole-Aitken	-47.69	-157.90	Y	Y	Both
16	Taruntius	5.49	46.45	Y	N	Diffuse-type
17	Van de Graaff	-27.80	170.80	Y	N	Diffuse-type
18	W. Humorum	-28.00	-47.74	Y	N	Diffuse-type
19	W. Procellarum	3.93	-70.75	Y	N	Diffuse-type
20	Bel'kovich	60.93	87.57	N	Y	Impact-type
21	Casatus	-72.69	-30.46	N	N
22	De Forest	-76.91	-163.10	N	N
23	Deslandres	-32.17	-5.63	N	Y	Impact-type
24	Korolev	-4.20	-157.50	N	N
25	Maurolycus	-42.03	14.09	N	N
26	Mendeleev	6.07	141.01	N	Y
27	Schrodinger	-74.58	132.78	N	Y	Impact-type
28	Walther	-32.92	0.67	N	Y	Impact-type
29	Zucchius	-61.42	-50.71	N	N

^a Due to the spatial distribution, Milne and Tsiolkovskiy are treated as one region, while Deslandres and Walther are treated as two regions in this work.

^b Results of the identification of cryptomaria in previous work⁷. Y indicates identified as cryptomaria, and N indicates not identified as cryptomaria.

^c The mascon can be identified based on Fig. 1b. Y indicates identified as cryptomaria, N indicates not identified as cryptomaria. Y indicates the presence of mascon, and N indicates the absence of mascon.

^d Results of the cryptomaria classification in this work.

No mascons are detected within the other 9 cryptomaria regions, and Supplementary Fig. 2 illustrates the gravity anomaly characteristics of these 9 regions. None of these light smooth plains have mascons within them, but they are all adjacent to maria outcrop area (Supplementary Note 3). This observation suggests that the mare basalts in these cryptomaria regions did not form in-situ but are more likely to have been diffused by maria activity in the surrounding vicinity. Even though cryptomaria are present in Crater Taruntius and Crater Van de Graaff, gravity data findings reveal no indication of high-density material in the lunar crust within these regions. As a result, the likelihood of intrusive formation of mare basalt is considered improbable. Rather, it is more plausible that these basalt formations originated from the surrounding mare areas of these craters.

The results of gravity anomalies indicated that some of cryptomaria are located in the interior of craters or basins and correlate with impact-formed mascons. The other part of cryptomaria is correlated with mascons and is not associated with impact craters, but is adjacent to maria outcrop area. Of these, the cryptomaria in Regions Balmer, Cleomedes, Dewar, Humboldtianum, Langemak, Mendel-Rydberg, Milne, Smythii, and W. Humorum are associated with mascons. the cryptomaria in Regions Australe, Hercules, Marginis, Lomonosov-Fleming, Mare Figoris, Taruntinus, Van de Graaff, and W. Procellarum are not associated with mascons. And the cryptomaria in Regions Schiller-Schickard and South Pole-Aitken include are a combination of these two conditions.

2.2. Gravity in uncertain cryptomaria

Of the 29 proposed cryptomaria regions worldwide, 10 are unable to be confirmed as cryptomaria⁷. Surface features revealed by remote sensing data are not sufficient to determine the presence of cryptomaria in these regions^7,19. Consequently, our study delved into the gravity anomalies of these 10 regions, aiming to gain fresh insights into their internal characteristics. The results of gravity anomalies in these 10 uncertain cryptomaria regions are shown in Supplementary Figs. 3 and 4. This results indicated the five uncertain cryptomaria regions, Bel’kovich, Deslandres, Walther, Mendeleev and Schrodinger, are located inside crater and there is a mascon in the center of the crater. And the gravity anomalies of these mascons are 30.3 mGal, 99.6 mGal, 26.7 mGal, 71.8 mGal, 113.9 mGal. Region Bel’kovich is a light smooth plain located on the northwestern in Crater Bel’kovich, and it is also located in the gravity anomaly ring of to Crater Humboldtianum. Region Deslandres and Walther are located in Crater Deslandres and Walther, and to their northwest is Mare Nubium. Region Mendeleev is located at low latitudes on lunar farside. There is a distinct mascon in the center of Crater Mendeleev, and a faint gravity anomaly ring on the edge of this basin. Region Schrodinger is located at south pole on lunar farside, and there is a distinct mascon in the center of Crater Schrodinger. No DHCs or mafic-rich soils were detected in these five regions to classify them as cryptomaria. However, similar to certain cryptomaria regions (Supplementary Fig. 1), these light smooth plains are situated within the crater and coincide with the presence of mascons. The remaining five uncertain cryptomaria regions are situated within craters (Supplementary Fig. 4), with Crater De Forest being the smallest, measuring 55.37 km in diameter, and Crater Korolev being the largest, with a diameter of 418.85 km. The five uncertain cryptomaria regions exhibit light smooth plains, but they do not contain mascons. The global distribution map (Fig. 1) reveals that all five non-cryptomaria regions are situated within the lunar highlands, far from the low-reflectivity lunar maria.

2.3. Gravity anomalies, impact crater size and crust thickness

As depicted in Fig. 1, a notable observation is the association of most proposed cryptomaria with craters and mascons. To explore this correlation further, we conducted a statistical analysis on the crater diameters, lunar crust thicknesses at the impact sites, and gravity anomalies within the proposed cryptomaria. As shown in Fig. 2 and Table 2, there is a correlation between the diameter of the impact crater, the lunar crust thickness and the gravity anomaly. Generally, larger impact craters tend to exhibit relatively smaller lunar crust thickness. This is a trend, but there are instances of relatively small impact craters with small lunar crust thickness. Craters De Forest, Bel'kovich, and Taruntius, which have a diameter of approximately 56 km, exhibit a minimum lunar crust thickness ranging from 18 to 19 km. As depicted in Fig. 1, these small craters are situated within older and larger craters. Moreover, Crater Korolev, with a diameter of 418.85 km, exhibits a lunar crust thickness of 37.06 km. This relatively thicker crust can be attributed to its location at the lunar farside. The crust on the farside tends to be thicker compared to the nearside. This indicates that the thickness of the lunar crust is notably influenced by impact events, with larger impacts typically resulting in thinner crusts. Additionally, the local geological conditions also affects the thickness of the lunar crust, and the lunar dichotomy suppresses the effect of impact events on the thickness of the lunar crust on the lunar farside.

Table 2

Characteristics of crater in the proposed cryptomaria.
ID	Crater	Diameter (km)	Thickness (km)	Gravity (mGal)
2	Balmer	541.68	15.53	124.1
3	Cleomedes	136.02	28.94	63.5
4	Dewar	240.85	28.94	77.1
6	Humboldtianum	593.17	3.18	83.2
7	Langemak	104.14	40.94	17.2
11	Mendel-Rydberg	637.9	9.53	77
12	Milne	250.79	18.35	141.9
13	Schickard	200.15	31.06	56.6
13	Schiller-Zucchius	352.06	9.53	63.6
14	Smythii	886.77	5.65	82.7
15	Baldet_1	182.24	13.77	39.4
15	Oppenheimer	205.51	13.06	52.5
15	Apollo	475.98	2.82	63.2
16	Taruntius	56.84	19.05	0
17	Van de Graaff	145.74	22.24	0
20	Bel'kovich	56.53	19.06	30.3
21	Casatus	103.04	33.88	0
22	De Forest	55.37	18.35	0
23	Deslandres	216.51	21.18	99.6
24	Korolev	418.85	37.06	0
25	Maurolycus	112.52	27.53	0
26	Mendeleev	331.36	24.00	71.8
27	Schrödinger	312.03	7.77	113.9
28	Walther	128.22	28.24	26.7
29	Zucchius	62.99	31.77	0

Impact events can indeed influence the formation of mascons within cryptomaria. As shown in Fig. 2, craters with small diameters or thick crusts tend to have no mascon signatures. However, when excluding these regions, there is no significant correlation between the gravity anomalies of mascons and the size of craters or crust thickness in other proposed cryptomaria. Among the 5 uncertain cryptomaria depicted in Supplementary Fig. 4, none of them exhibit mascons. These regions have crust thicknesses greater than ~ 19 km and crater diameters smaller than ~ 112 km. A notable case is Crater Korolev, which stands out with its diameter of 418.85 km and location in the lunar highlands, and the thicker crust is the main reason that prevents it from forming a mascon. Craters Taruntius and Van de Graaff are considered to be cryptomaria regions, but they do not exhibit mascons. The diameters and crust thicknesses of these two craters align with the uncertain cryptomaria regions illustrated in Supplementary Fig. 4. These findings indicate that small-sized impact craters do not provide favorable conditions for magma intrusion into the lunar crust, thereby inhibiting the formation of mascons and consequently preventing the in-situ development of cryptomaria. The regions depicted in Supplementary Fig. 1 are identified as cryptomaria based on their DHCs and chemical composition^7,19. Within these regions, both craters and their associated mascons are present. However, according to the statistical analysis shown in Fig. 2, there is no significant correlation observed between the values of gravity anomalies of the mascons and the size of the craters or the thickness of the crust (Supplementary Note 4). The results in the regions depicted in Supplementary Fig. 1 suggest that large basin impact events are essential for the formation of masscons and that the cryptomaria developed in situ are situated within these mascon basins. Additionally, some regions classified as uncertain cryptomaria (Supplementary Fig. 3) exhibit gravity characteristics, crater sizes, and crust thicknesses that are comparable to those of certain cryptomaria regions (Supplementary Fig. 1). This strongly suggests that these uncertain regions are indeed cryptomaria.

3.1. Cryptomaria classification

Cryptomaria are maria regions on the Moon that are covered by highly reflective materials originating from the ejecta of craters or basins. Based on the source of the ejecta, these cryptomaria can be classified into four types: Copernicus-type, Balmer-type, Proximal basin ejecta-type, and Distal basin ejecta-type^1,7,10,20. In this study, the gravity data results are utilized to reveal the origin of mare basalts within the cryptomaria and to classify the cryptomaria accordingly. We have divided the cryptomaria into two main types (Fig. 3): Impact-type and Diffuse-type.

Regions Balmer, Cleomedes, Dewar, Humboldtianum, Langemak, Mendel-Rydberg, Milne, and Smythii (Supplementary Fig. 1) are typical examples of the Impact-type cryptomaria. These regions have experienced significant impact events, ranging from smaller crater formations with diameters of 104.14 km to larger multi-ring basin formations with diameters of 886.77 km. The impact events in these regions have resulted in the thinning of the primitive lunar crust and the uplift of the lunar mantle (Fig. 3a). This geological process has led to the intrusion of mafic magma into the crust²¹. The presence of mascons, as observed in the gravity anomaly map, indicates the existence of materials within the lunar crust that possess a higher density than the average crustal material. The combination of crater and basin features in these regions suggests that the high-density material is mafic magma. These mafic magma can reach the lunar surface through the dikes, filling the bottom of the craters or basins²². Subsequent high-reflectivity ejecta from external sources have covered the low-reflectivity material, resulting in the characteristic appearance of cryptomaria as we observe them today. Within the regions of Cleomedes, Humboldtianum, Mendel-Rydberg, and Smythii, the mare basalts are not completely covered by ejecta, and there are still exposed continuous maria on the lunar surface. Furthermore, some DHCs can strip away the high-reflectivity material from the surface of the cryptomaria, exposing the underlying basalt to the lunar surface.

Regions Australe, Hercules, Lomonosov-Fleming, Marginis, Mare Frigoris, Taruntius, Van de Graaff, W. Humorum, and W. Procellarum (Supplementary Fig. 2) represent typical examples of the Diffuse-type cryptomaria. These regions show no mascons and are situated either at the edge of the maria or within the maria themselves. As illustrated in Fig. 3b, large craters situated within the maria have penetrated the layer of mare basalt covering the lunar surface. The ejecta surrounding these craters has covered the basalt layer, giving rise to the formation of cryptomaria. Alternatively, the perimeters of the maria are coated with ejecta material from craters, resulting in the formation of cryptomaria along the edges of the maria. The formation of craters also leads to a reduction in local topography, creating favorable conditions for the infilling of the crater floor by diffuse mare basalt. Irregular positive gravity anomalies are observed in these regions. Taking into account the geological background of these areas and existing research findings on gravity anomalies^21–24, it is suggested that these gravity anomalies are likely formed due to the presence of mare basalt filling low-lying terrains or magma intrusion into the lunar crust through dikes.

Cryptomaria of both Impact-type and Diffuse-type are found in Regions Schiller-Schickard and South Pole-Aitken. These regions are characterized by the presence of several large impact craters, and cryptomaria can be observed both within and outside of these craters⁷. In this work, the cryptomaria regions on the Moon have been classified into two types: Impact-type and Diffuse-type, based on the source of the mare basalt. This classification provides valuable insights into the origin of basalt in 19 cryptomaria regions. Moreover, the gravity anomaly features within the lunar crust presented by the gravity model can effectively distinguish between these two types of cryptomaria. These gravity anomalies, combined with the spectral and morphological characteristics of the lunar surface, further support and validate the classification of the cryptomaria regions into these two types.

This new classification model enhances our comprehension of cryptomaria formation and paves the way for future investigations. According to the classification results, it is evident that not all cryptomaria are the outcome of ancient volcanic activity. For instance, the formation time of Diffuse-type cryptomaria is closely with the formation time of the surrounding maria (Supplementary Note 3). Impact-type cryptomaria are more likely to represent the oldest volcanic products on the Moon. This consideration is crucial when studying basalt production during various periods of lunar volcanism.

3.2. Cryptomaria identification

A significant concentration of DHCs involved in the excavation of basaltic materials has been observed in 19 out of the 29 proposed cryptomaria regions worldwide (Table 1), establishing them as prominent cryptomaria regions⁷. Conversely, the remaining 10 regions, lacking evidence of DHCs, were not classified as cryptomaria. In this work, we investigate the feasibility of determining whether these regions can be classified as cryptomaria through an analysis of gravity anomalies, topographical features, and chemical content of the lunar surface.

In contrast to the presence of DHCs observed in the majority of proposed cryptomaria regions, the five regions depicted in Supplementary Fig. 3 do not exhibit any evidence of DHCs⁷. However, these regions do feature large-sized craters, thin lunar crusts, and notable mascons in their gravity anomalies, indicating an environment that is more favorable for mare basalt eruptions². Furthermore, the presence of a mascon at the center of these craters suggests an uplift of the lunar mantle in these regions. These observations are consistent with the characteristics of Impact-type cryptomaria. Based on the results from the morphology and FeO content maps²⁵, typical volcanic formations such as lunar rimas and grabens are present in certain regions (Supplementary Note 5 and Supplementary Fig. 5). Additionally, there is a noticeable increase in FeO content within and around several fresh small craters. These observations provide further evidence supporting the identification of these areas as Impact-type cryptomaria. In contrast, Region Mendeleev is not considered a cryptomare. Although the impact event in Mendeleev caused uplift of the lunar mantle and intrusion of magma into the crust, the magma did not reach the lunar surface, resulting in the absence of surface volcanic features (Supplementary Note 5).

The five Regions Casatus, De Forest, Korolev, Maurolycus, and Zucchius in Supplementary Fig. 4 have all been subjected to impact events, but no mascons are observed in the gravity anomaly data. This indicates that there was no significant uplift of the lunar mantle at the center of these basins and that the intrusion of mafic magma into the lunar crust is more difficult. Additionally, no discernible DHCs are observed on the surface of these regions, and there are no volcanic structures or features indicating an increase in FeO content in the morphology and FeO content maps. These five regions do not exhibit the typical characteristics of Impact-type cryptomaria. Moreover, they are located in isolated areas within the lunar highlands and are not adjacent to any maria, indicating that they do not qualify as Diffuse-type cryptomaria either. These regions are composed of light plains that are not associated with cryptomaria. The formation of these light plains is primarily attributed to the covering effect of ejecta from craters.

Based on the analysis of gravity anomalies and surface features, the initial list of 29 proposed cryptomaria regions has been revised to 23 regions (Table 1). Six regions, Mendeleev, Casatus, De Forest, Korolev, Maurolycus, and Zucchius, do not exhibit characteristics consistent with cryptomaria. While Region Mendeleev cannot be definitively classified as a cryptomaria, it may represent a mafic magma intrusion into the lunar crust. Among the remaining 23 regions, 12 are identified as Impact-type cryptomaria, and 9 are classified as Diffuse-type cryptomaria. Notably, both types of cryptomaria coexist in Regions Schiller-Schickard and South Pole-Aitken.

3.3. Cryptomaria thickness

In this section, our focus is not on estimating the global basalt thickness of the cryptomaria. Instead, we delve into the feasibility of determining the cryptomaria thickness using gravity data and compare the various thickness estimation methods currently available. Gravity anomaly data can provide insights into density anomalies in the lunar crust, which can be attributed to changes in porosity, magma intrusions, or variations in the bulk crustal composition²³. However, such data is not readily applicable for directly estimating the thickness of the cryptomaria. Cryptomaria are maria covered by high-albedo ejecta, previous work²⁶ have estimated the thickness of maria using gravity anomaly data. However, these estimations could not accurately account for the thickness of basalt mare at the edges of the maria. Therefore, estimating the thickness of Diffuse-type cryptomaria using gravity anomalies alone may not be feasible. Furthermore, previous studies have revealed a significant disparity between the thickness of cryptomaria estimated using gravity data and that estimated using DHCs^3,18 (Supplementary Note 6). The result derived from DHCs are considered more reliable when analyzed in the context of impact simulations and the geological characteristics of the region^26–29.

The thickness of the cryptomaria can be estimated from the depth of DHCs^3,11,30–32, but this method has limitations. The number of DHCs available for analysis is typically small, and they can only provide localized information about the depth of excavation within the cryptomaria. As a result, the depths obtained from DHCs not represent the average thickness of the cryptomaria. In addition to DHC analysis, flood simulations have been employed to estimate the thickness of cryptomaria^7,33. This modeling approach provides an upper limit on the thickness of the cryptomaria and the thickness of mare basalts formed during different time periods. However, it is important to note that these simulations do not account for the influence of exogenous ejecta on basin filling. The process of ejecta deposition from large craters would have occurred concurrently with basalt emplacement³⁴, potentially affecting the filling of the basins. Furthermore, studies utilizing data from the Chang'E-2 microwave radiometer and ground-based radar^12,13 have provided insights into the shallow subsurface of the cryptomaria. Compared to spectral data, these techniques have the advantage of detecting deeper information, to infer the boundary between the upper ejecta of the cryptomaria and the lower mare basalts. This information has facilitated estimations of the thickness of the cryptomaria, ranging from a few meters to several hundred meters. In general, the estimation of cryptomaria thickness requires the utilization of multiple methods to enhance accuracy and reliability. Sori et al.¹⁷ employed gravity methods to estimate the cryptomaria thickness and subsequently verifying the existence of cryptomaria through the examination of DHCs. By combining various methods, researchers can leverage the complementary strengths of each approach to gain a more comprehensive understanding of the thickness and characteristics of the cryptomaria. We propose that a combination of microwave radiometer data, ground-based radar data, and DHCs features can be employed for estimating the thickness of the cryptomaria. This can be achieved by using the area of DHCs as a calibration reference point and establishing a functional relationship between the excavation depth of DHCs and remote sensing data for inverting the cryptomaria thickness in the region. In cases where the diameter of the DHCs is sufficiently large to penetrate the cryptomaria layer, the thickness of the cryptomaria can be observed from high-resolution images, leading to improved inversion results when used as the calibration point. Additionally, high degree and order gravity field model can provide insights into the lunar subsurface, and future gravity field models may offer enhanced accuracy in revealing the cryptomaria thickness. Utilizing the thickness of the cryptomaria estimated from the DHCs features as a constraint for forward modeling gravity anomalies and comparing this result with existing gravity field models may also present a viable method for improving the accuracy of cryptomaria thickness estimates.

Investigating the thickness of cryptomaria stands as a crucial undertaking in the realm of future lunar volcanism research. The insights garnered from these investigations are indispensable for quantifying the cumulative volume of lunar volcanic products and gaining deeper insights into ancient lunar volcanism.

The 29 proposed cryptomaria regions investigated in this work were suggested in earlier research^7,19 (Fig. 1, Table 1). The latest model of the GRGM1200b RM1³⁵, released by the Gravity Recovery and Interior Laboratory (GRAIL), is employed in our study. We extract data on Bouguer gravity anomalies from degree 60 to 600 from this gravity field model, which covers the entire range of the lunar crust. With this data, we investigate the gravity anomaly characteristics of the 29 proposed cryptomaria regions. Furthermore, the lunar crustal thickness map³⁶ offers information about the thickness of the lunar crust in the proposed cryptomaria regions. The Wide Angle Camera (WAC) Global Morphologic Map, GLD100 WAC Global Digital Terrain Model (DTM), and Clementine UVVIS derived iron abundance data serve as sources of information on the morphology, elevation, and chemical composition of the lunar surface^25,37,38. The identification of the cryptomaria is accomplished by combining the characteristics of the lunar surface and the gravity anomalies within the lunar crust. The identification of the cryptomaria is accomplished by combining the characteristics of the lunar surface and the gravity anomalies within the lunar crust. Additionally, QuickMap (https://quickmap.lroc.asu.edu/) can be utilized to browse lunar data, and QuickMap tools can be used to measure impact crater diameters.

Data Availability Statement

The WAC and DTM data are provided by the LROC team (https://wms.lroc.asu.edu/lroc/view_rdr/WAC_GLOBAL, https://wms.lroc.asu.edu/lroc/view_rdr/WAC_GLD100). The GRGM1200b RM1 is provided by Gravity Recovery and Interior Laboratory (https://pgda.gsfc.nasa.gov/products/75).

Acknowledgment

This work is supported by the National Natural Science Foundation of China (42030110, 41874010). Denggao Qiu is sponsored by China Scholarship Council (202106270073).

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Gravity Data Reveal Origin of Cryptomaria

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Abstract

Figures

1. Introduction

2. Results

2.1. Gravity in cryptomaria

2.2. Gravity in uncertain cryptomaria

2.3. Gravity anomalies, impact crater size and crust thickness

3. Discussion

3.1. Cryptomaria classification

3.2. Cryptomaria identification

3.3. Cryptomaria thickness

4. Methods

Declarations

References

Additional Declarations

Supplementary Files

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