How old is the imbrium basin

The scientists also found that several other basins on the moon may have been created by oblique impacts from protoplanets. For instance, they estimated that the Moscoviense and Orientale basins on the moon's far side may have been created by asteroids about 62 and 68 miles and km across, respectively — larger than previous estimates.

All in all, the researchers suggest that protoplanets may have once been common in the asteroid belt. Material from protoplanets smashing into the moon could help explain why lunar samples that NASA's Apollo missions brought back to Earth had high levels of meteoritic rock. This was especially true of the Apollo 16 mission, which landed downrange from the Imbrium impact.

The scientists added that collisions with these protoplanets might explain many of the impacts that battered the moon and the rest of the inner solar system during a violent era known as the Late Heavy Bombardment between 4. Thousands of chunks that sheared off the Imbrium Asteroid and other protoplanets would have kept going after impact, flying off into space to continue striking other bodies.

Schultz and Crawford detailed their findings online today July 20 in the journal Nature. Follow Charles Q. Choi on Twitter cqchoi. Original article on Space. It has been difficult to establish the exact link of the samples to the Serenitatis Basin ever since the Apollo 17 collection was brought back, because it was not easy to discern between samples formed by the Imbrium event and those formed by Serenitatis. Find out more about OU research in space science. Contact details.

Find your personal contacts including your tutor and student support team:. In order to evaluate the formation ages, we first defined the mare units based on spectral features.

Second, we made crater size—frequency distribution CSFD measurements of the units and determined their depositional ages. Finally, the timing of the tectonic deformations was constrained based on the crosscutting relationships between tectonic structures and the mare units.

The degradation levels of small craters Moore et al. Concentric systems of the mare ridges in Mare Imbrium. Red lines indicate the mare ridges, gray circles indicate prominent craters, and brown areas indicate highland materials.

PL: Promontorium Laplace. R: Montes Recti. C: Mons Chajorra. AV: Alta Vista. Yellow spots indicate the locations of lobate scarps.

Orange and blue lines indicate grabens and sinuous rilles, respectively. Yellow boxes indicate the areas where CSFD measurements were taken. Crater ages of the units in Ga are indicated as well. The CSFDs for the units. Craters — m in diameter were used. Dashed lines indicate the diameter ranges used to fit the production function.

They uniformly cover the entire lunar surface. The DTMs describe altitudes from the spherical surface with a radius of In order to screen out the long-wavelength topography and to enhance short-wavelength tectonic topography, the present selenoid was subtracted from the DTMs.

The MI obtained spectral data at four visible and five near-infrared spectral bands with spatial resolutions of 20 and 60 m, respectively Ohtake et al.

Those images have a spatial resolution of 0. We used the differences in spectral features to define the mare units and conducted the CSFD measurements on each unit. We defined the stratigraphic mare units in the study area based on the spectral ratio images made from the MI images at , and nm wavelengths. The spectral bands were chosen to enhance the contrasts in maturity, Ti content and Fe content of the basalts Fischer and Pieters ; Lucey et al.

Figure 2 c shows a mosaic of spectral ratio images, where relatively mature or low-Ti mare basalts are indicated by purplish or reddish colors and immature, high-Ti or high-Fe basalts by bluish ones.

The circular yellowish spots indicate fresh craters and their ejecta blankets in this image. We defined nine units from A through I. The spectral ratio image shows that the mare in the study area is clearly divided into two regions with warm and cold colors Fig. This contrast has long been recognized since the Apollo era e. The southern border of Unit F and the eastern border of Unit H are not clear because the spectral features gradually change. The hatched areas in Fig.

Several researchers made CSFD measurements in our study area e. Their divisions coincide with ours more or less, except for units in Sinus Iridum. The surface of the bay can be subdivided into several units, but it is difficult to trace their boundaries due to the low contrasts in spectral features and to the NNW trending ray with a width of 20 km Fig.

As a result, the division of the mare units by Hiesinger et al. We used the uniform and high-resolution MI images of this region and divided the bay into seven units. The images have higher resolution than the data set used by the previous studies. Among the seven units, three hatched areas in Fig. The CSFD measurement was taken in the portion of the region where secondary craters are few in each mare unit.

The lambert azimuthal equal-area projections of the TC images were used for crater counting to measure the surface area accurately. We used the production and chronology functions proposed by Neukum and Neukum et al. The former is expressed as. This value is given by fitting the production function Eq. Figure 3 shows the CSFDs in the units. We carefully drew the boundaries of the crater counting areas in the mare units to exclude portions with steep slopes or abundant secondary craters.

As a result, the model ages of Units I, H and G were estimated as 3. Unit F was 3. Units B, D and A were 3. Unit C was 2. The youngest unit, Unit E, was estimated as 2. The production function was fitted to the distributions for the craters with D between 0. The upper limit of D is determined from the fact that there are few craters larger than 1 km in the counting areas.

Considering the confidence intervals, the ages of the mare units determined in previous studies Hiesinger et al. The discrepancies in the age of Unit E can be explained by the differences in the ranges of crater sizes targeted in the CSFD measurements. Our result is consistent with Morota et al. We counted craters larger than m in diameter, a few times smaller than the threshold used by the previous studies.

Hiesinger et al. Bugiolacchi and Guest also used these images, but they counted craters larger than m instead of m. Qiao et al. Small craters are easily obliterated by thin and younger lavas, but larger craters with high rims are not. The rim heights of 20 and 30 m correspond to the crater diameters of and m, respectively Pike , p.

Accordingly, the number densities of smaller and larger craters tend to indicate younger and older ages, respectively, if smaller ones have been obliterated by younger lavas. Therefore, an obliteration by lavas with a total thickness of several tens of meters explains the older age in Hiesinger et al.

Since craters larger than m were rare in our counting areas, the obliteration did not appear as a jump in the CSFD plot in Fig. Secondary craters in Unit E were infrequent and were distinctive from primary craters.

We counted a larger number of craters and used higher-resolution and lower solar angle images, which advantageous to crater counting, than the previous studies Bugiolacchi and Guest ; Hiesinger et al. Therefore, though the results of the estimated ages of Units F, H and I disagree with the previous studies Bugiolacchi and Guest ; Hiesinger et al.

However, many secondary craters were observed in Units F, H and I. We carefully excluded them, but possibly made an overestimation of the ages owing to their contamination.

The ages of the mare units in the study area. Solid circles with error bars indicate the ages determined in this study. The bars depict the plus—minus standard deviations. Open circles indicate the ages determined in previous studies; H, Hiesinger et al.

In cases where their divisions of the mare units are significantly different from ours, their ages are not shown in this diagram.

Three-dimensional topographic models on which spectral ratio images are mapped. Intersecting area of Ridge Systems A and B. Solid contour lines indicate the altitude. The contour interval is 50 m. The vertical exaggeration is Locations lacking in the MI data are indicated by the N—S trending white stripe.

The dashed line indicates the northwestern border of Unit E. Craters ruptured by thrust faults. The crater size is about 80 m in diameter. The larger one is located at the south of Montes Recti There are a number of mare ridges, grabens and lobate scarps in the study area, which we describe in the following subsections. We constrained the formation ages of the ridges by the ages of the mare units dammed and deformed by the ridges. Since the mare basalts were so inviscid Basaltic Volcanism Study Project that they made level surfaces at the time of deposition, a mare unit that is dammed by a ridge is younger than the formation of the ridge, and a deformed one is older than the formation of the ridge.

Bryan and Schaber applied this technique to ridges in southwestern Imbrium, where high-resolution images taken by the Apollo missions were available. In addition, the high-resolution TC and NAC images allowed us to determine the formation ages of the lobate scarps and grabens, by using the degradation levels of small craters Moore et al. We used the informal names of the islands, Chajorra and Alta Vista, which were introduced by Lee , for convenience Fig.

Among them, Ridge Systems A, B and C are part of the concentric systems of the most prominent ring structure of the Imbrium basin Spudis The arches are about km long, 30 km wide and m high relative to the surrounding plain. The hills make a right-stepping en echelon array. He assigned the formation age of the ridges vaguely as sometime in the Eratosthenian Period. We determined the age more precisely. Therefore, the major formation of Ridge System A was sometime after 2. The older limit is defined by the age of Unit C—the youngest of the four deformed units.

A part of the lower slope of the arch is exceptionally blanketed by Unit E Fig. There is no fissure along the ridge that could feed the lavas of Unit E, suggesting that part of the ridge was reactivated and uplifted after the deposition of Unit E. The ridges have heights and widths of — m and several kilometers, respectively.

The southeastern segments of this group coincide with the rim of the Sinus Iridum basin, which was excavated at 3. In the northern part, Units A and B are deformed by this system Fig. The fact that the surface of Unit E does not tilt but becomes thicker toward the ridge system Section 3. The northern part was formed after the deposition of Unit A and before the deposition of Unit E. In the middle part, Ridge System B Fig. The depositional age of the unclassified unit could not be determined because of heavy contamination by secondary craters.

However, the spectral feature of the unit is similar to those of Units A and F. It might indicate that the depositional age of the unit is around 3. Unit E is partly deformed and partly dammed along the foot of the eastern edge of Ridge System B. Therefore, the middle part was tectonically active around the timing of the deposition of Unit E.

Accordingly, most of the southern part is younger than Unit E, but the islands began to be uplifted before the deposition of Unit E. The areas of the islands are too small so that their depositional ages were not determined. Accordingly, the uplift of the northern part began after 3.

The middle part of this system was mainly formed around 2. The southern part of this ridge system is younger than 2. Ridge System C is the northern part of Dorsum Heim and consists of two N-S trending ridges, a prominent one and narrow one, to the southeast of Promontorium Heraclides Figs. They are about m high, several kilometers wide and shorter than km in length.

Units F and I are involved in this ridge system, indicating that the ridges were formed sometime after 3. Each ridge is about m high and 5 km wide. Ridge Systems A and E crosscut at Units D, E and G are affected by this system. Accordingly, the ridge system was formed sometime after 2.

Ridge System F involves Unit D. The depositional age of the unit places the maximum age of the formation at 3. We interpreted that the craters are ruptured by thrust faults constituting mare ridges, because some of the ruptured craters are deformed and some of them are partly hidden by the hanging wall Fig. The ages of ruptured craters are the clues to the timing of tectonic deformations, because the craters must exist before the end of the tectonic activities.

Figure 6 a shows an example of the small ruptured craters. In the image, the thrust sheet covers half of the small crater. The sheet is slightly deformed probably for filling the depression of the crater. Examples of the small ruptured craters are also shown in Additional file 1 : Figure S1. The kilometer-scale crater highlighted in Fig. It is not circular but elliptical in shape. The crater floor is popped up by the faults. To the south of this crater, a crater with a diameter of m lacks an eastern rim, which was destroyed by the over-thrusting or obliterated by talus deposits from the scarp along a ridge.

The ruptured craters are along all the ridge systems in the study area Fig. On the basis of the degradation levels classified by Moore et al. The craters that are 80— m in diameter with gentle depression morphology are classified into the latest stage of Eratosthenian.

In our analysis, the observed craters smaller than m were categorized into shallow bowl-shaped craters, which suggest their formation ages are Copernican Period. Accordingly, all the ridge systems were at least partly active still in the Copernican Period. Note that the observed craters may contain secondary craters. Since it is a qualitative analysis, some of them might be formed in the Eratosthenian Period. It is important to note that there are differences in the linear number densities of large ruptured craters larger than m in diameter; white circles in Fig.

In contrast, there is no difference in the linear number densities of small ruptured craters smaller than m in diameter; yellow circles in Fig. This difference can be explained by the difference in the area density of craters depending on the depositional ages of the units.

There are lobate scarps in the study area Fig.