Evolution Of Planetary Surfaces Research Articles

Retrieving soil surface roughness with the Hapke photometric model: Confrontation with the ground truth

Surface roughness can be defined as the mean slope angle integrated over all scales from the grain size to the local topography. It controls the energy balance of bare soils, in particular the angular distribution of scattered and emitted radiation. This provides clues to understand the intimate structure and evolution of planetary surfaces over ages. In this article we investigate the capacity of the Hapke photometric model, the most widely used in planetary science, to retrieve surface roughness from multiangular reflectance data. Its performance is still a question at issue and we lack validation experiments comparing model retrievals with ground measurements. To address this issue and to show the potentials and limits of the Hapke model, we compare the mean slope angle determined from very high resolution digital elevation models of volcanic and sedimentary terrains sampled in the Asal-Ghoubbet rift (Republic of Djibouti), to the photometric roughness estimated by model inversion on multiangular reflectance data measured on the ground (Chamelon field goniometer) and from space (Pleiades images). The agreement is good on moderately rough surfaces, in the domain of validity of the Hapke model, and poor on others.

Dust Phenomena Relating to Airless Bodies

Airless bodies are directly exposed to ambient plasma and meteoroid fluxes, making them characteristically different from bodies whose dense atmospheres protect their surfaces from such fluxes. Direct exposure to plasma and meteoroids has important consequences for the formation and evolution of planetary surfaces, including altering chemical makeup and optical properties, generating neutral gas and/or dust exospheres, and leading to the generation of circumplanetary and interplanetary dust grain populations. In the past two decades, there have been many advancements in our understanding of airless bodies and their interaction with various dust populations. In this paper, we describe relevant dust phenomena on the surface and in the vicinity of airless bodies over a broad range of scale sizes from $\sim10^{-3}~\mbox{km}$ to $\sim10^{3}~\mbox{km}$ , with a focus on recent developments in this field.

Formation Timescales of Amorphous Rims on Lunar Grains Derived From ARTEMIS Observations

AbstractThe weathering of airless bodies exposed to space is a fundamental process in the formation and evolution of planetary surfaces. At the Moon, space weathering induces a variety of physical, chemical, and optical changes including the formation of nanometer‐sized amorphous rims on individual lunar grains. These rims are formed by vapor redeposition from micrometeoroid impacts and ion irradiation‐induced amorphization of the crystalline matrix. For ion irradiation‐induced rims, however, laboratory experiments of the depth and formation timescales of these rims stand in stark disagreement with observations of lunar soil grains. We use observations by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) spacecraft in orbit around the Moon to compute the mean ion flux to the lunar surface between 10 eV and 5 MeV and convolve this flux with ion irradiation‐induced vacancy production rates as a function of depth calculated using the Stopping Range of Ions in Matter model. By combining these results with laboratory measurements of the critical fluence for charged‐particle amorphization in olivine, we can predict the formation timescale of amorphous rims as a function of depth in olivinic grains. This analysis resolves two outstanding issues: (1) the provenance of >100 nm amorphous rims on lunar grains and (2) the nature of the depth‐age relationship for amorphous rims on lunar grains.

Application of planetary analog mechanical properties to subsurface geological investigations

Investigations of the mechanical properties of planetary analog materials are of great importance to improve future mission success and to enhance our understanding of the history and evolution of planetary surfaces and subsurfaces. Determining the stratigraphy of the subsurface using drills and penetrometers implies knowing the mechanical properties of the regolith. Therefore, we investigated the mechanical properties of Martian regolith analogs (JSC Mars-1 and an unaltered basaltic soil from Mojave desert). Penetration force is dependent on the bearing capacity factor (Nq) and the angle of internal friction which is found to be function of the level of compaction and increases with relative density. The bearing capacity factor (Nq) showed an exponential trend with the angle of internal friction. For Mojave soil, (Nq) increased from 64 to 338 with φ increasing from 32 to 41.3° while (Nq) increased from 161 to 3347 with φ increasing from 39.4 to 54.7° for JSC Mars-1. Based on estimation of the bearing capacity factor and the angle of internal friction, we predicted the force of penetration under the Martian gravity using a 1.2cm diameter penetrometer. Under a bulk density of 1240 and 1500kgm−3 in JSC Mars-1 and Mojave soil, 460 and 200N is the estimated force to reach a depth of about 20cm deep. Based on this investigation, distinguishing between different regolith layers on the subsurface of Mars is possible and observable using penetration force.

Buried impact basins, the evolution of planetary surfaces and the Chicxulub multi-ring crater

Impact craters are distinctive landforms on Moon, Mars, Venus and other bodies of the Solar System. In contrast, the Earth has few craters, due to the dynamic nature of the planet, where craters and other geological structures are destroyed, modified or covered. Planetary missions have also shown that in other worlds where craters are numerous and well preserved, the crater record has been modified, through the identification of buried structures. Studies of the concealed crater record have major implications for the crater-size frequency distribution and crater-counting chronologies. On Earth, Chicxulub is an example of a large multi-ring buried basin. Its study provides clues for the investigation other planetary surfaces. In addition, geophysical surveys have unravelled its deep 3-D structure, providing data and constraints for new planetary missions.

40Ar/39Ar age of the Lonar crater and consequence for the geochronology of planetary impacts

AbstractAsteroid impacts play an important role in the evolution of planetary surfaces. In the inner solar system, the large majority of impacts occur on bodies (e.g., asteroids, the Moon, Mars) covered by primitive igneous rocks. However, most of the impacts recorded on Earth occur on different rock types and are poor proxies for planetary impacts. The Lonar crater is a 1.88-km-diameter, Quaternary age crater (Fig. 1) located on the ca. 66 Ma Deccan basaltic traps in Maharashtra (India), and is one of the very few craters on Earth emplaced directly on basaltic lava flows. We carried out 12 40Ar/39Ar step-heating experiments on 4 melt rock samples in order to (1) obtain a precise age for the Lonar crater; (2) study the response of isotopic chronometers during impacts on mafic target rocks; and (3) better understand the dating of extraterrestrial impact craters. We obtained 10 plateau and 9 inverse isochron ages on various aliquots. Combination of selected data into a global inverse isochron yielded an age of 570 ± 47 ka (MSWD = 1.1; P = 0.24). In comparison, previous nonisotopic investigations on rocks thought to be affected by secondary processes yielded a range of much younger ages (ca. 12–62 ka). The measured 40Ar/36Ar trapped values offer a direct comparison with the atmospheric benchmark value and allow us to test the inherited 40Ar* degassing capacity of basaltic impact melt rocks. The 40Ar/36Ar ratio of 296.5 ± 1.7 is indistinguishable from the atmospheric composition and suggests that inherited 40Ar* is absent from the melt rock. This result substantiates diffusion models that predict a near-complete degassing of low-viscosity melt (e.g., basalts) during impact, and demonstrates for the first time that inherited 40Ar* is less problematic for 40Ar/39Ar dating of impact events in basaltic igneous rocks compared to Si-rich rocks. These results provide direct evidence that basaltic melt rocks are excellent candidates for recording the timing of planetary impact events and, as far as dating is concerned, should be the preferred targets of sample recovery by future missions.

Multiring-forming large bolide impacts and evolution of planetary surfaces

In the past few years, meteoritic and cometary impacts have emerged as a major geological agent in the construction and evolution of planetary surfaces. Formation of complex central ring, peak ring and multiring craters involves excavation and melting of large volumes of crustal material. High-resolution geophysical mapping measuring gravity, magnetics, and topography of the Moon and Mars have recently provided information on the subsurface structure of large basins and aided in identifying buried giant craters. The terrestrial crater record has been significantly erased by tectonic, magmatic, and erosion processes and only a small proportion of impact structures remain. Record of multiring craters is limited to three examples: Vredefort, Sudbury and Chicxulub. Deep geophysical surveys and geochemical and isotopic studies of those craters provide means to evaluate the influence of large impacts on the lithospheric and crustal evolution by providing estimates of excavation depth and volume, amounts of material fragmented, ejected, vaporized and melted, and effects on the crustal stratigraphy and crustal thickness. Analyses on the melt from Vredefort, Sudbury, and Chicxulub indicate andesitic composition derived from lower-crustal material. The melt formed inside the lower transient cavity from lower crustal material that was then redistributed and emplaced in upper-crustal levels, resulting in crustal redistribution. Crystalline basement clasts fragmented and incorporated into the breccias show varying degrees of alteration but no significant thermal effects. Ejecta were deposited locally within the crater region and ballistic material and fine ejecta are globally distributed on the planetary surface. Impacts influence the crust–mantle boundary, with Moho uplift. Material from the mantle was not incorporated into the melt and impact breccias, indicating that the excavation cavities were confined to the lower crust. This is also apparently the case for the giant basins on the Moon, including the 2500 km diameter South Pole-Aitken Basin. Considering the numbers of large multiring basins, possible flux of large impacts, and effects on target surfaces, crustal scale redistribution of material during those large impacts has played a major role in the evolution of planetary surfaces.

Outstanding student paper

One presentation in the Planetology Section was named Outstanding Student Paper at AGU's Spring Meeting in Baltimore, Md. Noam Izenberg presented “The Erosion and Deposition Associated With the Great Flood of 1993, Missouri River, Missouri.” He received his B.Sc. with honors from Brown University in 1989 and is planning on defending his Ph.D. dissertation research in late September 1995, at the Department of Earth and Planetary Sciences at Washington University. His main research interest is the evolution of planetary surfaces and the quantitative measurement of surficial change on the Earth and planetary bodies. Currently, he is focusing on the methods of determining catastrophic flood damage using interferometric and polarimetric airborne and satellite radar data, coupled with field analysis.

Cratering in the Solar System

The evolution of planetary surfaces and the evolution of the planets themselves can be determined to a large extent from the number and kinds of craters on them. The cratering phenomenon is examined and some theories of planetary evolution are presented. 22 figs. (GHT)