Absolute Model Ages Research Articles

Very recent and wide-spread basaltic volcanism on Mars

New spacecraft data provide increasing evidence for a dynamic environment on presentA¢Â�Â�day Mars. Exogenic processes such as impact cratering, mass wasting processes, and active dune migration have all been observed to modify the surface. No traces of current endogenic activity have been found yet, but some studies point to very localized volcanism in the last few millions of years. However, no systematic study of young volcanic surfaces had been performed so far. We present absolute model age determinations of plains volcanism on Mars as derived from impact crater sizefrequency distributions. Extended areas in Tharsis, the largest volcanoA¢Â�Â�tectonic region on Mars, have been resurfaced by lava flows in the last few tens of millions of years. We also present results on the rheologic properties of these lava flows, inferred from morphometric measurements. Yield strengths are in the range of 100A¢Â�Â�300 Pa, and viscosities reach values of 10^2 to 10^3 Pa s, indicating basaltic compositions. The results imply that Mars retained until recently, and probably still retains, enough internal heat to produce wideA¢Â�Â�spread plainA¢Â�Â�style volcanism, producing lowA¢Â�Â�viscosity lava flows throughout large parts of Tharsis

Mars: The evolutionary history of the northern lowlands based on crater counting and geologic mapping

The geologic history of planetary surfaces is most effectively determined by joining geologic mapping and crater counting which provides an iterative, qualitative and quantitative method for defining relative ages and absolute model ages. Based on this approach, we present spatial and temporal details regarding the evolution of the Martian northern plains and surrounding regions. The highland–lowland boundary (HLB) formed during the pre-Noachian and was subsequently modified through various processes. The Nepenthes Mensae unit along the northern margins of the cratered highlands, was formed by HLB scarp-erosion, deposition of sedimentary and volcanic materials, and dissection by surface runoff between 3.81 and 3.65 Ga. Ages for giant polygons in Utopia and Acidalia Planitiae are ∼ 3.75 Ga and likely reflect the age of buried basement rocks. These buried lowland surfaces are comparable in age to those located closer to the HLB, where a much thinner, post-HLB deposit is mapped. The emplacement of the most extensive lowland surfaces ended between 3.75 and 3.4 Ga, based on densities of craters generally > 3 km in diameter. Results from the polygonal terrain support the existence of a major lowland depocenter shortly after the pre-Noachian formation of the northern lowlands. In general, northern plains surfaces show gradually younger ages at lower elevations, consistent local to regional unit emplacement and resurfacing between 3.6 and 2.6 Ga. Elevation levels and morphology are not necessarily related, and variations in ages within the mapped units are found, especially in units formed and modified by multiple geological processes. Regardless, most of the youngest units in the northern lowlands are considered to be lavas, polar ice, or thick mantle deposits, arguing against the ocean theory during the Amazonian Period (younger than about 3.15 Ga). All ages measured in the closest vicinity of the steep dichotomy escarpment are also 3.7 Ga or older. The formation ages of volcanic flanks at the HLB (e.g., Alba Mons (3.6–3.4 Ga) and the last fan at Apollinaris Mons, 3.71 Ga) may give additional temporal constraint for the possible existence of any kind of Martian ocean before about 3.7 Ga. It seems to reflect the termination of a large-scale, precipitation-based hydrological cycle and major geologic processes related to such cycling.

An improved crustal magnetic field map of Mars from electron reflectometry: Highland volcano magmatic history and the end of the martian dynamo

We apply improved kinetic modeling of electron transport in the martian thermosphere to fit pitch angle distributions measured by the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer (MAG/ER), together with appropriate filtering, binning, averaging and error correction techniques, to create the most reliable ER global map to date of crustal magnetic field magnitude at 185 km altitude, with twice the spatial resolution and considerably higher sensitivity to crustal fields than global maps of magnetic field components produced with MAG data alone. This map compares favorably to sparsely sampled dayside MAG data taken at similar altitudes, insofar as a direct comparison is meaningful. Using this map, we present two case studies. The first compares the magnetic signatures of two highland volcanoes, concluding that the comparatively greater thermal demagnetization at Syrtis Major compared with Tyrrhena Patera is likely due to a higher ratio of intruded to extruded magmas. The second uses the map along with topographic data to compare the magnetic signatures and crater retention ages of the demagnetized Hellas impact basin and magnetized Ladon impact basin. From this comparison, we determine that the martian global dynamo magnetic field went from substantial to very weak or nonexistent in the absolute model age time interval 4.15 ± 0.05 to 4.07 ± 0.05 Ga ago.

Acheron Fossae, Mars: Tectonic rifting, volcanism, and implications for lithospheric thickness

The Acheron Fossae region is an ancient crustal block exposed in the northwestern Tharsis province on Mars. It is named after the Acheron Fossae, a particularly well‐developed system of extensional tectonic grabens. This study is the first to describe the tectonic inventory in detail, using topographic and image data from the Global Surveyor and Mars Express missions. We see the extensional tectonics along the east–west trending topographic high of Acheron Fossae as a surface expression of upwelling asthenospheric material that initiated regional uplift, crustal extension and breakup, and associated volcanic activity in Noachian time. The tectonic architecture and the dimensions of the extensional structures are comparable to those of terrestrial continental rifts. An area of elevated topography and strong erosion, which constitutes a regional landmark in the eastern Acheron Fossae region, is interpreted as a rift‐related center of volcanism, caused by local magmatic uprise and involvement of the lithosphere. The extension across the Acheron Fossae reaches values between 1.2 km and 8.7 km, comparable to young continental rifts on Earth. Crater statistics indicate an absolute cratering model age between ∼3.9 Ga and 3.7 Ga for the rifting. The uplift observed on the flanks of the Acheron Fossae indicates a fairly thin and thus hot lithosphere. Using flexural analysis, we have constrained the elastic lithosphere thickness at the time of rifting to 8.9–11.3 km, corresponding to thermal gradients between 28 and 41 K km−1. These heat flows are substantially larger than Noachian heat flow values previously reported, but are consistent with the presence of rift‐related volcanism and a magmatically very active environment.

Ages of rampart craters in equatorial regions on Mars: Implications for the past and present distribution of ground ice

Abstract— We are testing the idea of Squyres et al. (1992) that rampart craters on Mars may have formed over a significant time period and therefore the onset diameter (minimum diameter of a rampart crater) only reflects the ground ice depth at a given time. We measured crater size frequencies on the layered ejecta of rampart craters in three equatorial regions to derive absolute model ages and to constrain the regional volatile history. Nearly all rampart craters in the Xanthe Terra region are ˜3.8 Gyr old. This corresponds to the Noachian fluvial activity that region. Rampart crater formation declines in the Hesperian, whereas onset diameters (minimum diameter) increase. No new rampart craters formed after the end of the Hesperian (˜3 Gyr). This indicates a lowering of the ground ice table with time in the Xanthe Terra region. Most rampart craters in the Valles Marineris region are around 3.6 Gyr old. Only one large, probably Amazonian‐aged (˜2.5 Gyr), rampart crater exists. These ages indicate a volatile‐rich period in the Early Hesperian and a lowering of the ground ice table with time in the Valles Marineris study region. Rampart craters in southern Chryse Planitia, which are partly eroded by fluvial activity, show ages around 3.9 Gyr. Rampart craters superposed on channels have ages between ˜1.5 and ˜0.6 Gyr. The onset diameter (3 km at ˜1.5 Gyr) in this region may indicate a relatively shallow ground ice table. Loss of volatiles due to diffusion and sublimation might have lowered the ground ice table even in the southern Chryse Planitia region afterwards. In general, our study implies a formation of the smallest rampart craters within and/or shortly after periods of fluvial activity and a subsequent lowering of the ground ice table indicated by increasing onset diameter to the present. These results question the method to derive present equatorial ground ice depths from the onset diameter of rampart craters without information about their formation time.

Ages and stratigraphy of mare basalts in Oceanus Procellarum, Mare Nubium, Mare Cognitum, and Mare Insularum

Accurate estimates of mare basalt ages are necessary to place constraints on the duration and the flux of lunar volcanism as well as on the petrogenesis of lunar mare basalts and their relationship to the thermal evolution of the Moon. We performed new crater size‐frequency distribution measurements in order to investigate the stratigraphy of mare basalts in Oceanus Procellarum and related regions such as Mare Nubium, Mare Cognitum, and Mare Insularum. We used high‐resolution Clementine color data to define 86 spectrally homogeneous units within these basins, which were then dated with crater counts on Lunar Orbiter IV images. Our crater size‐frequency distribution measurements define mineralogical and spectral surface units and offer significant improvements in accuracy over previous analyses. Our data show that volcanism in the investigated region was active over a long period of time from ∼3.93 to 1.2 b.y., a total of ∼2.7 b.y. Volumetrically, most of the basalts erupted in the Late Imbrian Period between ∼3.3 and 3.7 b.y., and we see evidence that numerous units have been resurfaced. During the Eratosthenian Period, significantly less basalt was erupted. Depending on the absolute model ages that one can assign to the lunar chronostratigraphic systems, five units might be of Copernican age. Younger basalts are generally exposed in the center of the investigated area, that is, closer to the volcanic centers of the Aristarchus Plateau and Marius Hills. Older basalts occur preferentially along the northwestern margin of Oceanus Procellarum and in the southeastern regions of the studied area, i.e., in Mare Cognitum and Mare Nubium. Combining the new data with our previously measured ages for basalts in Mare Imbrium, Serenitatis, Tranquillitatis, Humorum, Australe, and Humboldtianum, we find that the period of active volcanism on the Moon lasted ∼2.8 b.y., from ∼4 b.y. to ∼1.2 b.y. On the basis of the basalts dated so far, which do not yet include the potentially young basalts of Mare Smythii e.g.,all investigated basins but probably also is the location of some of the youngest basalts on the lunar surface.

Tempe Fossae, Mars: A planetary analogon to a terrestrial continental rift?

Tempe Terra, the northeastern part of the Tharsis Region on Mars, is characterized by several extensional structures differing in style and age. The Tempe Fossae, a particularly well‐developed system of graben striking N45°E, have been studied for the first time in detail on the basis of Viking Orbiter imagery and Mars Observer Laser Altimeter (MOLA) topographic data. The graben system appears to be unique in the whole region because of its extent, the remarkable width and depth of the graben, the varying graben pattern, and the associated volcanism. Single graben can be up to 35 km wide and 120 km long, while their depth can reach nearly 3000 m. They typically show an asymmetric architecture, often in halfgraben or step fault style. The graben system widens and changes its style along strike from NE to SW from a single, very deep, and narrow graben to a complex set of several shallower, sinuous graben and halfgraben. Crustal extension was measured in the northeastern part from the observable throw and amounts to 2.5–3.1 km. Volcanic structures can be found at several locations along the graben. In the SW the graben system seems to be connected with the Tempe Terra volcanic province marked by flood basalts and plains volcanism. Craters were counted on volcanic units unaffected by extensional tectonics. Crater statistics indicate an absolute crater model age for the upper end of the extensional deformation of ∼3.5 Ga. The dimensions and the characteristics of the mapped graben system resemble those of terrestrial continental rifts, and a comparison between them and the Kenya Rift reveals striking similarities. Therefore the Tempe Fossae are interpreted as a Martian analog to a continental rift associated with an underlying mantle plume. This hypothesis seems to be supported by recent geophysical models based on topography and gravity data from Mars Global Surveyor which indicate regional uplift for Tempe Terra.

Ages of mare basalts on the lunar nearside

The chronology of lunar volcanism is based on radiometric ages determined from Apollo and Luna landing site samples, regional stratigraphic relationships, and crater degradation and size‐frequency distribution data for units largely defined prior to the end of the Apollo program. Here we report on new crater size‐frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins (Australe, Tranquillitatis, Humboldtianum, Humorum, Serenitatis, and Imbrium). Crater size‐frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the Moon. Compared to crater degradation ages, crater size‐frequency ages, performed on spectrally defined units, offer significant improvements in accuracy. Our investigation showed that (1) in the investigated basins, lunar volcanism was active for at least 1.5–2.0 b.y., starting at about 3.9–4.0 b.y. and ceasing at ∼2.0 b.y., (2) most basalts erupted during the late Imbrian Period at about 3.6–3.8 b.y., (3) significantly fewer basalts were emplaced during the Eratosthenian Period, (4) basalts of Copernican age were not found in any of the investigated basins, (5) lunar basin‐filling volcanism probably started within ∼100 m.y. after the formation of the individual basins. We also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins. We found that (1) in all pre‐Nectarian basins (Australe and Tranquillitatis) as well as in the Humboldtianum basin, which is of Nectarian age, the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3.6–3.8 b.y., (2) in the younger basins (Humorum, Serenitatis, and Imbrium) a second peak at 3.3–3.5 b.y. is observed, (3) basalt eruptions younger than 2.6 b.y. occur only intermittently, and (4) in the youngest basins, Serenitatis and Imbrium, we see an extended period of active basin‐filling volcanism (1.5–1.6 b.y.) which is 500 m.y. longer than in the Australe and Humorum and even ∼1.0 b.y. longer than in Tranquillitatis and Humboldtianum.

Kimberlites, mineralogy, geochemistry, and petrology

The Chang'E-5 (CE-5) mission has successfully collected lunar regolith samples from the northern Oceanus Procellarum. The Eratosthenian-aged mare units where the CE-5 landed on, might have been contaminated by distant ejecta delivered by several craters, such as Pythagoras, Sharp B, Harpalus, Copernicus, and other craters. Laboratory dating of distant ejecta in the CE-5 regolith can provide the formation ages of these craters. Therefore, these ejecta materials, the same as local young mare basalts, would serve as new calibration points for lunar chronology curve. Among above craters, the absolute model ages (AMAs) of Pythagoras, Sharp B and Harpalus are not well constrained. Here we used the Crater Size Frequency distribution (CSFD) method to yield their formation ages and the crater density values (N(1)/(km−2)). Using high resolution images, craters on the selected counting areas of their floors, ejecta blankets were identified. Our results indicate that Pythagoras is an Eratosthenian-age crater with an AMA of 2.13 ± 0.16 Ga and a N(1) =1.79 × 10−3 km−2. The Sharp B and Harpalus are both Copernican. The AMA of Sharp B crater is 1.01 ± 0.12 Ga with a N(1) of 8.46 × 10−4 km−2 while the AMA of Harpalus is 1.10 ± 0.04 Ga with a N(1) of 9.22 × 10−4 km−2. The ejecta from Sharp B and Harpalus as well as local mare basalt of the CE-5 site may be served as calibration points for lunar chronology curve, which will fill the age gaps of lunar chronology curve. Based on the estimated ages, we interpreted that Pythagoras ejecta might be beneath the young CE-5 mare unit, while the Sharp B and Harpalus ejecta should overlie the surface of the CE-5 site.