Astropedology is the study of very ancient paleosols and meteorites relevant to the origin of life and different planetary soil systems. It is a branch of soil science (pedology) concerned with soils of the distant geologic past and of other planetary bodies to understand Earth's place in the universe. A geologic definition of soil is "a material at the surface of a planetary body modified in place by physical, chemical or biological processes". Soils are sometimes defined by biological activity but can also be defined as planetary surfaces altered in place by biologic, chemical, or physical processes. By this definition, the question for Martian soils and paleosols becomes, were they alive? Astropedology symposia are a new focus for scientific meetings on soil science.
Advancements in understanding the chemical and physical mechanisms of pedogenesis on other planetary bodies in part led the Soil Science Society of America (SSSA) in 2017 to update the definition of soil to: "The layer(s) of generally loose mineral and/or organic material that are affected by physical, chemical, and/or biological processes at or near the planetary surface and usually hold liquids, gases, and biota and support plants". Despite the meager understanding of extraterrestrial soils, their diversity may raise the question of how science might classify them, or formally compare them with Earth-based soils. One option is to simply use our present soil classification schemes, in which case many extraterrestrial soils would be Entisols in the United States Soil Taxonomy (ST) or Regosols in the World Reference Base for Soil Resources (WRB). However, applying an Earth-based system to such dissimilar settings is debatable. Another option is to distinguish the (largely) biotic Earth from the abiotic Solar System, and include all non-Earth soils in a new Order or Reference Group, which might be tentatively called Astrosols.
Moon
Lunar paleosol composition The surface of the Moon is covered with lunar regolith, a mixture of fine dust and rocky debris made by meteorite impacts, considered the soil of the Moon.prohects.ncsu.edu/project/agronauts/mission3_5.htm Astronauts found few rock samples to pick up on mature regolith surfaces. The rocks had all been broken to fine soil by micrometeorite bombardment over the past billion years. The bulk of lunar regolith is a fine gray soil, breccia and rock fragments of the local bedrock. Continuous attacks by micrometeorites further disintegrate and melt soil particles. This melt, mixed with lithic fragments, forms irregular clusters called agglutinates.curator.jsc.nasa.gov/lunar/letss/regolith.pdf Lunar soil is fine-grained, unconsolidated material that is poorly sorted and has high porosity (41-70%) and relatively low density (0-9-1.1 g/cm3) compared to its constituent particle density (2.9-3.2 g/cm3). The rock fragments found in Lunar paleosol grains vary in size (ranging from silt-sized grains to boulder-sized grains) and include fragments of highland anorthosite and mare basalt from meteorite impacts. Lunar paleosols also contain mineral grains that make up the largest fraction of silt-sized grains (0.06-0.03 mm) in the soil since the crystals in lunar basalts are fine-grained. Paleosols found on the Moon contain many plagioclase and pyroxene minerals (ranging from a few to 40%). Olivine minerals are present but occur much more sporadically and rarely than these other minerals. Grains of ilmenite, spinel, metallic particles of kamacite and taenite, phosphide (schreibersite), and sulfide (troilite) have also been found in Lunar paleosols. However, they are also much less common than plagioclase and pyroxene. A third soil component found in Lunar paleosols, along with rock fragments and mineral grains, are glass particles, of which there are two main kinds: homogeneous glasses and agglutinates. Homogeneous glasses are variable in size (<2 μm-2 cm) and are typically found as dumbbell or teardrop-shaped glass particles. These particles are hypothesized to have formed as volcanic ash or melted rock that resulted from the heat released from meteoroid impacts. Agglutinates are inhomogeneous glass-bonded aggregates that are variable in size (<2 μm-1 mm) and are described as irregular masses of glass and crystals. These glass particles are cemented together by bridges of glassy cement between the minerals and rock fragments and are usually ring or bowl-shaped in appearance, much like a crater. Ring or bowl-shaped agglutinates form from the outward spreading of impact melt that cemented surrounding soil grains. Less common or distinct agglutinate shapes are known to form from the cementation of scattered drops of impact melt or parts of agglutinates that have broken from later impacts. Lunar paleosols closely resemble the parent material in chemical and physical composition. Although these paleosols form more physically than chemically, chemical processes on the Moon have formed thin (20-50 μm) amorphous rings in the soil resulting from the local cooling of vapor from meteoroid impacts. These impacts have created an enrichment of silica (Si) and sulfur (S) and depletion in magnesium (Mg), calcium (Ca), aluminum (Al), and titanium (Ti). Lunar exploration has found all essential minerals for growth of plants in sufficient quantities. Organic matter in the form of amino acids have been detected in lunar samples from the Apollo missions, but isotopic and molecular evidence of these compounds suggests terrestrial contamination as the source.
Lunar soil development Some proposed mechanisms for Lunar paleosol formation include micrometeoroid bombardment, soil mixing resulting from charge-separation induced by solar energy, thermal fatigue or soil churning caused by changes in temperature, and soil wing sputtering. Larger scale impacts would affect soil formation on the Moon by interrupting this process by releasing ejecta blankets. Lunar soil formation is based on the degree of reworking by micrometeoroids, and there are several developmental stages of formation. The first stages consist of a coarse-grained and poorly-sorted blanket of impact ejecta. In the next stages, micrometeoroid bombardment breaks down the material to even finer grains, which increases the ratio of agglutinates. This micrometeoroid bombardment also adds meteoritic metal and reduces iron (Fe^2+) in silicates to metallic iron in the soil. Agglutinates and metal cause the soil to darken in color over a long period. The top of the lunar soil profile is highly enriched in agglutinates and ferromagnetic index, which is thought to reflect an increased redistribution caused by large impacts of micrometeoroid modified soil material through time. The time of formation of lunar paleosols is variable and can take up to hundreds of millions of years in some paleosols. The time for soil formation on the Moon has been calculated based on the influx rate of micrometeoroids of a size range capable of forming agglutinates. These calculations demonstrate that there are comparably much slower soil formation rates on the Moon than on Earth. Extrapolations for the time of soil formation have also been made based on crater production rates that suggest 81 thousand years (ka) for the time for overturning the upper 2 cm of Lunar soil.
Mars
Evidence of paleosols on Mars is derived from both in-situ and orbital remote sensing investigations of the Martian surface. In-situ chemical / mineralogical analyses (Mars Science Laboratory) and images (Mastcam, MAHLI) from the Curiosity rover in Gale Crater on Mars have shown similarities with soil horizons and soil structures found on Earth. Morphological evidence includes soil features such as gradational alteration and bedding disruption by dilational cracks and veins, consistent with soluans of desert soils, blocky angular ped structures, sand wedges, a shallow gypsic (By) horizon, and vesicular structure. Structures like those found in Earth's desert soils caused by microbial gas production after rains (vesicular structure) have been recognized on Mars, but definitive evidence of life on Mars has yet to be obtained. Mineral abundances within the paleosols show olivine weathering to smectite and a modest depletion of phosphorus. Such hydrolytic weathering is comparable to weathering found on Earth. The chemical and morphological features of the Yellowknife Bay paleosols are a new line of evidence for late Noachian (3.7 ± 0.3 Ga) paleoclimate on Mars, and are interpreted as forming under a hyperarid frigid paleoclimate. Radiometric dating suggests the paleosols at Yellowknife Bay are 3.7 billion years old (± 0.3 billion years) and reveal a change from possible "warm and wet" conditions of the Early Noachian (~4.1-3.9 Ga) to an extreme arid and cold climate with limited soil formation.