1 Introduction

The lunar regolith records how the flux of material in near-Earth space has changed over at least 4 billion years (e.g., Hörz et al. 1991; Lucey et al. 2006). It can be used to infer changes in solar and cosmic radiation (Pepin et al. 1980 and papers therein), wider galactic processes (Crawford et al. 2010), the micrometeorite flux (Zolensky et al. 2006), and the population of larger impactors that may have significantly altered the surfaces of the Earth and Moon (Joy et al. 2012). The first step in determining the evolutionary record in the regolith is to establish the ages of different regolith samples.

The ages (e.g., antiquity) of lunar regolith samples can be estimated from a bulk sample’s trapped 40Ar/36Ar ratio. Originally produced by the decay of 40K in the lunar interior, some 40Ar was degassed into the lunar exosphere, ionized, and re-implanted in the regolith; this excess 40Ar is a trapped component, as it did not form in situ through radioactive decay (Eberhardt et al. 1972). Similarly, 36Ar, which is implanted by the solar wind, is also a trapped gas. The abundances of trapped 40Ar within a regolith sample is normalized to 36Ar as an indicator of the point in time of the last exposure to space (i.e., the solar wind), before closure of the system through burial by an ejecta blanket or basalt flow. Because the 40Ar flux has decreased over time, at approximately the same rate as 40K decay has occurred within the lunar crust, then the trapped 40Ar/36Ar ratio implanted into surface soils has also decreased. Ancient lunar regolith, therefore, will record a larger trapped 40Ar/36Ar (ArTr) ratio than modern regolith. The variation of ArTr with time has been used to estimate the ages of lunar regolith samples (e.g., Eugster et al. 1980, 1983, 2001; McKay et al. 1986, 1989; Eugster and Polnau 1997). The relationship between ArTr and time was recently recalibrated (Joy et al. 2011, after Eugster et al. 2001) to produce the equation:

$${\text{t = 1}} . 2 1 0 3 {\text{ ln(Ar}}_{\text{Tr}} )\; + \;0.7148$$
(1)

where t is the model age in billions of years. This model age represents the regolith breccia closure age (tc), which is the last time grain-size components of the breccia were exposed to solar wind and may be the formation time of the breccia. The ratio can also be used as a measure of the time period in the past when a soil was last exposed at the surface (i.e., appearance age, ta, after Heymann et al. 1975), which is similar to the regolith breccia closure age. What we call an appearance age is not the amount of time that modern soils have resided at the surface; implanted Ar does not give a good estimate for the total elapsed exposure time for mature soils, because implanted Ar saturates.

Solar 36Ar and lunar atmosphere 40Ar are implanted into soil grains only while they reside at the very surface. Most of the implanted Ar is contained within the fine-grained soil component and constructive particles like agglutinates. In the simplest case, this fine-grained soil is simultaneously generated by impacts and exposed at the surface, and the ArTr ratio would give the time for both. However, many soils may have experienced multiple stages of impact-generation and surface exposure occurring at different times; for these, the ArTr ratio would reflect some degree of averaging of those various times. However, in mature, fine-grained soils implanted Ar is in saturation, and a period of later exposure would tend to replace Ar implanted earlier with Ar implanted later. Furthermore, the last period of surface exposure likely produced much of the fine-grained and agglutinate component. Thus, we expect most (and for some soils all) of the implanted Ar to reflect the most recent, significant exposure to space. However, for some soils the measured ArTr ratio may be somewhat increased because of residual Ar from earlier exposure, and the measured ratio would suggest a somewhat older age compared to the last surface time. This is probably the greatest source of uncertainty in the appearance age of a particular soil (not necessarily a soil breccia) and why examining several soils from different locales is desirable.

This method of determining sample chronology is attractive for several reasons. First, the calculation can be used to determine model ages for both regolith breccias and soils. Moreover, no other age-dating technique has been reliably used to derive brecciation age or soil appearance age. The unique contribution of the ArTr ages are such that data can be (and are here) acquired on many more samples than other age data and, thus, can be compared and contrasted without the uncertainties of smaller population datasets. Furthermore, often the ArTr ages can be related to a sample characteristic (e.g., maturity index, space exposure duration records) in ways that other datasets cannot.

Joy et al. (2011) used the updated calibration to determine the model ages of 18 Apollo 16 regolith breccias. Here, we use that same method to calculate the model ages of an additional 191 lunar samples using published Ar data. These include material returned by the Apollo and Luna missions (Fig. 1a, c), which provide a measure of regolith ages on the nearside of the Moon, and lunar regolith breccia meteorites (Fig. 1b) that provide a global distribution of regolith ages.

Fig. 1
figure 1

Representative soil and regolith samples in this study: a regolith breccia 61295 was chipped off this boulder on the rim of Plum Crater (~40 m diameter). The sample was collected from the protruding knob below the right foot of the gnomon, which is ~62 cm tall. Detail of AS16-114-18412. b Allan Hills 81005 regolith breccia lunar meteorite collected by the US Antarctic Search for Meteorites program in January, 1982 with 1-cm cube for scale. Detail of S82-35869. c Apollo 11 Lunar Module Pilot Edwin “Buzz” Aldrin hammers a drive tube into the soil to recover sample 10005. The solar wind experiment is nearby. Detail of AS11-40-5964

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2 Methods

We extracted ArTr values from the literature and used them in Eq. 1 to calculate closure (breccia) and appearance (soil) ages (Fig. 2); these values, along with their uncertainties, can be found in the electronic supplementary material of the Online Resource (Table ESM-1). Literature ArTr data that do not have accompanying uncertainty information are assigned an uncertainty of 30 % following McKay et al. (1986) and Joy et al. (2012). In some cases, prior studies report measured 40Ar and 36Ar values, but not the trapped values; for these samples, we calculated the ArTr ratio (Table ESM-2; Fig. 2). To do so, the expected radiogenic 40Ar component was calculated using (1) an approximate sample age (Table ESM-2) based on the youngest reported age of a given regolith component (e.g., a basalt clast), (2) the bulk K of the sample, and (3) the 40K decay constant (Steiger and Jäger 1977). This radiogenic component is then subtracted from the total measured 40Ar to yield the trapped 40Ar abundance, which is used to generate ArTr; this ratio is subsequently applied to Eq. 1 (ESM-2). Although the decay constant of Steiger and Jäger (1977) was used here, applying other values (e.g., Renne et al. 2010; Schwarz et al. 2011) produce similar ArTr values. Calculated ArTr ratios are assigned a 30 % uncertainty following McKay et al. (1986) and Joy et al. (2012) and can be found Table ESM-2 of the Online Resource.

Fig. 2
figure 2

Stacked histogram of a regolith breccia and b soil model ages. Soil samples in b include surface and trench soils as well as those collected from drive tubes and drill cores. Bin size is 0.25 Ga. Data and data sources are provided in the electronic supplementary material located in the Online Resource Tables ESM-1, ESM-2 and ESM-3

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3 Results

A total number of 194 discrete samples are represented in Fig. 2, with multiple analyses for 32 samples (see Table ESM-1 and ESM-2). Twenty-four calculated ages are not plotted in the figure, because the calibration yields a negative model age; these samples are listed in Online Resource Tables ESM-1 and ESM-2 for reference. The younger portion of the model calibration (Joy et al. 2011) may not be adequately suited to dating the youngest samples, because that part of the calibration curve was defined by only 3 samples, the youngest of which has a 21Ne-defined exposure age of 0.05 Ga (Eugster et al. 2001 and references therein). Thus, samples with negative model ages are not considered further, but nonetheless are regarded as the youngest samples in this study.

3.1 Regolith Breccias

The oldest regolith breccias in this study (tc ≥ 3.3 Ga) were collected from landing sites that are located on Imbrium basin ejecta (i.e., Apollo 16 and 14). Samples with closure ages 3.3 to 2.0 Ga are primarily derived from the Apollo 16 site (Fig. 2a). Most of the younger samples (tc < 2.0 Ga) are derived from younger mare surfaces, with the widest age range shown by the Apollo 15 breccias (tc ~ 2.7 to 0.01 Ga; Fig. 2a); the 4 oldest regolith breccias from Apollo 15 (tc ~ 2.7 to 2.1 Ga; Fig. 2a) were collected from the rim of Spur Crater (~90 m diameter) on the Apennine Front and contain abundant green pyroclastic glass beads. Additionally, only Apollo 15 has regolith breccias with tc < 0.5 Ga (Fig. 2a). In contrast, Apollo 11 regolith breccias have a tightly clustered tc range of 1.9 to 1.1 Ga (Fig. 2a). We also include 2 samples from Apollo 12 (tc ~ 1.9 and 0.8 Ga) and Apollo 17 (tc ~ 1.8 and 1.7 Ga). The younger Apollo 12 regolith breccia (tc ~ 0.8 Ga; Fig. 2a) is a KREEPy sample (i.e., rich in K, REE, and P), which likely contains Copernicus crater ejecta material (Meyer et al. 1971; Bogard et al. 1994; Wentworth et al. 1994) and may not represent a regolith sample derived entirely of local material at the Apollo 12 site. Two regolith breccia particles from the Luna 16 drill core have ages of ~0.8 and 0.5 Ga (Fig. 2a). Lunar meteorites (Figs. 1b, 2a) span a shorter range of closure ages (3.7 to 1.0 Ga) than the full set of Apollo regolith breccias (3.9 to 0.01 Ga).

3.2 Soils

Most surface and trench soil samples have appearance ages (ta) of <2.0 Ga (Fig. 2b), which is also true for soils from the drill cores and drive tubes; we have plotted surface and trench soils together with the cores and tubes in Fig. 2b, because they have similar model ages of the time period in the past when the soil was last exposed at the surface. Soils from older terrains (i.e., Apollo 14 and 16) compose the majority of samples with ta ~ 2.0 to 1.5 Ga (Fig. 2b). Those younger than 1.5 Ga can be found at all landing sites. Apollo 17 surface and trench soils have the greatest variance with 2 distinct groups illustrated in Fig. 3: an older group with model ages of 3.2 to 2.7 Ga and a younger group encompassing 1.8 to 0.2 Ga. The older Apollo 17 trench soils (≥2.7 Ga) were collected from a depth of 5 to 8 cm within a meter-long trench on the south rim of Shorty Crater (~110 m diameter) and are composed largely of orange and black pyroclastic glass; these soils have the oldest model ages in this study. In contrast, most of the Apollo 17 soils with model ages <1.0 Ga were collected in the central valley or along the base of the North Massif; most of these samples were collected at depths <5 cm. Apollo 17 soils with model ages 1.8 to 1.0 Ga were also predominantly collected at depths <5 cm, but at least two samples were collected from ~15 cm at the bottom of two trenches (73141 and 79261). The oldest Apollo 15 soil sample (15421), represented by three separate analyses (ta = 2.2 to 2.0 Ga), is also associated with substantial amounts of pyroclastic glass and was collected from the upper few cm on the rim of Spur crater in conjunction with the oldest Apollo 15 regolith breccias (e.g., 15427). Nearly all of the Apollo 15 soils are surface samples (i.e., within the top few cm) with the exception of 15031, which was collected at the bottom (~30 cm) of a trench; the deep drill core (15001–15006) was collected ~10 m away.

Fig. 3
figure 3

Variation of Apollo 17 sample model ages with sample type. Surface and trench soils represent two distinct age groups, which are also reflected in the drill core and drive tube samples. Uncertainty bars reflect the uncertainty of the ArTr, propagated through the model age calculation (see Online Resource Tables ESM-1 and ESM-2)

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Drill cores were returned from the final 3 Apollo missions, as well as the Luna 16, 20, and 24 missions (Fig. 2b). We were unable to extract Luna 16 soil data from the literature; nonetheless, we have included 2 small breccia particles recovered from the Luna 16 drill core (Table ESM-2; Fig. 2a). Drive tubes (Fig. 1c), which excavated shallower depths (ranging from ~7 to 68 cm in total length) than the drill cores (ranging from ~224 to 305 cm in total length), were returned from each of the Apollo missions. Soils from the Apollo cores and tubes display similar model ages to the surface and trench soils, despite differences in sample collection depth and method. In particular, the Apollo 17 drill core and drive tube have comparable model ages to the 2 groups of the surface soils, despite the variations in collection depths (Fig. 3). The Luna 20 and 24 cores have equivalent ages to the Apollo cores, tubes, and surface/trench soils, with ta ≤ 1.1 Ga (Fig. 2b).

3.3 Maturity Index

When available, we also report the published maturity index of the samples (Table ESM-3; Figs. 4, 5). The maturity of a sample reflects its exposure to radiation and micrometeoritic processes (Morris 1978). This can be quantitatively measured using the ferromagnetic resonance (FMR) surface exposure index (Is/FeO), which is the ratio of the intensity of the FMR resonance from submicroscopic iron (Is) to the bulk FeO abundance (Morris 1976). The maturity index separates samples into 3 levels of maturation: immature (Is/FeO = 0 to 29), submature (Is/FeO = 30 to 59), and mature (Is/FeO ≥ 60) (Morris 1978). This indicator provides insight to the duration of exposure, but not the moment in time when that exposure occurred. Most regolith breccias are immature (Table ESM-3; Fig. 4a), while most soils are submature to mature (Fig. 4b). This is most readily apparent for the Apollo 16 samples, whose regolith breccias generally have the lowest maturity (i.e., Is/FeO < 10); in contrast, most of the soils are submature to mature (McKay et al. 1986). All regolith breccias and soils with model ages ≥2.0 Ga are composed of immature regolith (Fig. 5). In contrast, samples with model ages <2.0 Ga exhibit a range in maturity index from 3 to 106, with little indication of a relationship with model age (Fig. 5).

Fig. 4
figure 4

Stacked histogram of a regolith breccia and b soil maturity index (Is/FeO) values. Soil samples in b include surface and trench soils as well as those collected from drive tubes and drill cores. Bin size is 10 units. Maturity fields are indicated by dashed lines and represent immature, submature, and mature soils as defined by Morris (1978). Data and data sources are provided in the electronic supplementary material located in the Online Resource Tables ESM-1, ESM-2 and ESM-3

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Fig. 5
figure 5

Comparison of sample model age (Ga) with maturity index (Is/FeO). Maturity divisions (Morris 1978) are indicated by dashed lines. Collisional regimes before 2.0 Ga and after 2.0 Ga both contain diverse sample populations represented by regolith breccias and soils (surface, trench, drive tube, and drill core). Data and data sources are provided in the electronic supplementary material located in the Online Resource Tables ESM-1, ESM-2 and ESM-3. Uncertainty bars reflect the uncertainty of the ArTr, propagated through the model age calculation

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4 Discussion

4.1 Soil Age Versus Depth

The collection depth of a core/tube sample is not necessarily correlated with the model age, which is likely due to extensive vertical and lateral mixing of the regolith. As an example, illustrated in Fig. 6, the oldest Apollo 15 drill core soils (ta ~ 1.0 and 1.6 Ga) were sampled at depths of 73.5 cm (15005,255) and 77.5 cm (15005,267), respectively (Bogard and Nyquist 1973), but samples from greater depth (80–240 cm) are apparently younger (ta ≤ 0.9 Ga). Young soils with negative ta may be found at varying depths, separated by older soil layers (Fig. 6) that implies thorough regolith gardening. This is also suggested at other sites, such as Apollo 17, where the uppermost portion of the drive tube (74002/1) has a younger age than the lower portions, but also has more agglutinates (McKay et al. 1978), which suggests higher maturity. In contrast, the older and agglutinate-poorer (Taylor et al. 1979) samples in the Apollo 17 drill core (70008) were collected above younger, agglutinate-richer samples (e.g., 70005), suggesting a recent (<0.8 Ga) turnover event. Variation in age does not appear to be completely dependent on collection depth from the cores and tubes. Although the Apollo drill cores have similar variation in ages, the oldest samples are found at varying depths: at the top for Apollo 17, the middle for Apollo 15, and near the bottom for Apollo 16 (Fig. 6). Similarly for the drive tubes, the oldest soil samples vary with depth (Fig. 6). Soils collected from cores and tubes, then, may access paleoregoliths with similar histories to those collected nearer the surface from trenching.

Fig. 6
figure 6

Variation of model age with depth for drill cores and drive tubes. Core depth indicates the depth within the returned core or drive tube and may not represent the actual depth below the lunar surface due to compaction. Core depth of a sample may represent the average of the reported depth range; for example Luna 20 sample L2010 has a depth range of 19–23 cm (Eugster et al. 1975), but is represented here as a single data-point at 21 cm. Note the different scale for Apollo 17 model ages. Drill cores are represented by square symbols and drive tubes are represented by crosses. Error bars reflect the uncertainty of the ArTr, propagated through the model age calculation

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4.2 Multi-component Mixing and ArTr

The older (i.e., >2 Ga) regolith breccias and soils from Apollo 15 and 17 sites generally contain a large amount of pyroclastic glass (Fig. 7), while most of the younger samples (i.e., <2 Ga) contain <8 % pyroclastic glass. However, at least one Apollo 15 soil sample (15300; ~1.3–1.2 Ga) contains nearly 20 % green glass (Fig. 7), and is much younger than a regolith breccia with similar glass content (15426; ~2.7 Ga); this may indicate surface reworking of those soils with lower appearance ages, irrespective of the glass content. In contrast, one of the oldest Apollo 17 samples (74240; ~3.1 Ga) contains only 4 % orange glass (Fig. 7); this suggests that the pyroclastic glass is not primarily responsible for the model age and indicates a secondary cause.

Fig. 7
figure 7

Glass content and calculated model ages of selected glass-rich Apollo 15 and Apollo 17 regolith breccias and soils (See Online Resource Table ESM-4). Data reflects green glass % in Apollo 15 samples and orange + black glass % in Apollo 17 samples

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Another critical similarity between these samples is that they were collected along the rim of an impact crater located near the base of a massif. This may indicate that the high ArTr is due to incorporation of ancient highlands material in addition to the pyroclastic glass, given the close proximity to highlands units (i.e., Apennine Front and South Massif). The process of lateral mixing has long been observed in the Apollo samples (e.g., Wood et al. 1970) and is suspected in the KREEPy Apollo 12 regolith samples, which are thought to have originated from Copernicus crater (~400 km away). Marvin et al. (1971) estimated that 50 % of the material within the Apollo 12 regolith was sourced from a region within 3.1 km, while 95 % of the regolith source area was encompassed within 100 km. These distances, when applied to other Apollo sites, may account for the apparent discrepancy in ages between some materials found closer to massifs than those collected from the relatively young mare surfaces.

4.3 Lunar Surface History

The regolith samples represented in this study span a wide period of lunar surface history from 3.9 Ga to the present day with the largest gap between samples spanning no more than ~0.2 Ga. Thus, we have a reasonably complete temporal record of the regolith from returned samples. An enhanced temporal record, however, in addition to larger sample volumes and broader geographic coverage, are reasonable goals for prospective missions (e.g., NRC 2007). As Fig. 2 illustrates, regolith samples representing diverse ages can be obtained from individual landing sites.

In the meantime, this newly calibrated set of regolith ages can be used to expand assessments of the impactors that have affected the surfaces of the Earth and Moon. A recent study (Joy et al. 2012) identified chondritic impactors in the interval of 3.77 to 3.35 Ga among Apollo 16 ancient regolith breccias, followed by a more diverse group of impactors between 1.7 and 0.65 and <0.5 Ga. The data in Fig. 2 indicates that more information about the impactors >3 Ga might be gained from Apollo 14 and 17 samples; similarly, the interval of 3 and 1.7 Ga can be examined with samples from all of the Apollo sites. These types of samples are also important because they illuminate the collisional history of the early Earth, which is poorly constrained among terrestrial samples (e.g., Kring 2003). Furthermore, Apollo 15 regolith breccias with closure ages <0.5 Ga could provide details about impactors hitting the Earth-Moon system more recently. Many of these closure ages are reported here for the first time; these represent a critical first step towards unraveling the material flux in the Earth-Moon system throughout time, as the closure ages are essential to the sample selection process.

Regolith breccias have experienced limited surface exposure to the space environment (i.e., most are immature) in contrast to the soils (Figs. 4, 5). The immature nature (Is/FeO ≤ 25) of all regolith samples (breccia and soil) with model ages ≥2.0 Ga indicates a short surface residence time before burial during 3.8 to 2.5 Ga and only marginally longer for 2.5 to 2.0 Ga; this suggests rapid burial during these time periods. In contrast, regolith samples with ages <2.0 Ga show a wider range of maturity (Is/FeO = 3 to 106), implying a variety of surface exposure lengths that do not appear to be dependent on the model age nor the sample collection site. This may reflect different collisional regimes: an older period (~>2.0 Ga) of bigger, more frequent impacts that compact soil into regolith and bury it, and a younger period (<2.0 Ga) dominated by micrometeoritic impacts, resulting in samples becoming more mature before their subsequent closure (Fig. 5). The wide range of maturity among all regolith samples, as well as the contrast between breccias and soils, demonstrates a diverse dataset that can be accessed at an individual site. In particular, the immature nature of most regolith breccias suggests that they may represent a discrete snapshot (i.e., short surface duration) of the lunar surface history, while soils may provide longer-duration histories, and are likely more complicated to unravel. Moreover, a comparison of older (>2.5 Ga), immature regolith breccias with younger, mature ones will reveal differences between short-term (~<5 Ma) and longer-term (~2 Ga) regolith processes.

5 Conclusions

Our study demonstrates that future sample return missions to the Moon, whether human or robotic, can collect temporally constrained paleoregolith records of surface processes (e.g., solar wind, Solar System, galactic; Lucey et al. 2006; Crawford et al. 2007, 2010; Fagents et al. 2010) from both soils and regolith breccias with diverse ages and maturity levels. A near complete history of the lunar surface environment over the past 3.9 Ga has been recorded in regolith breccias (tc ~ 3.9 to 0.05 Ga) and soils (ta ~ 3.6 to 0.03 Ga) from the Apollo and Luna missions as well as lunar meteorites. Ancient regolith breccia samples with model ages >3.3 Ga have been previously used to deduce the types of impactors hitting the surface (Joy et al. 2012), which implies that regolith breccias that closed <3.3 Ga may provide similarly critical information about the bombardment of the Earth-Moon system during geologically recent time periods. The model ages reported here represent critical information for the selection of appropriate samples to examine the compositional variability of impactors in Earth-Moon space over time. This study illustrates the diverse nature of lunar regolith collected at any given Apollo site and implies that augmentations to the dataset, and subsequently our understanding of the bombardment history, could be made by future sample return missions with broader geographic coverage.