Martian regolith

Martian regolith is the fine blanket of unconsolidated, loose, heterogeneous superficial deposits covering the surface of Mars. The term Martian soil typically refers to the finer fraction of regolith. So far, no samples have been returned to Earth, the goal of a Mars sample-return mission, but the soil has been studied remotely with the use of Mars rovers and Mars orbiters. Its properties can differ significantly from those of terrestrial soil, including its toxicity due to the presence of perchlorates.

Definitions On Earth, the term "soil" usually includes organic content. In contrast, planetary scientists adopt a functional definition of soil to distinguish it from rocks. Rocks generally refers to 10 cm scale and larger materials (e.g., fragments, breccia, and exposed outcrops) with high thermal inertia, with areal fractions consistent with the Viking Infrared Thermal Mapper (IRTM) data, and immobile under current aeolian (wind) conditions. Consequently, rocks are classified as grains exceeding the size of cobbles on the Wentworth scale. This approach enables agreement across Martian remote sensing methods that span the electromagnetic spectrum from gamma to radio waves. Soil refers to all other, typically unconsolidated, material including those sufficiently fine-grained to be mobilized by wind. Soil consequently encompasses a variety of regolith components identified at landing sites. Typical examples include: bedform (a feature that develops at the interface of fluid and a moveable bed such as ripples and dunes), clasts (fragments of pre-existing minerals and rock such as sediment deposits), concretions, drift, dust, rocky fragments, and sand. The functional definition reinforces a recently proposed generic definition of soil on terrestrial bodies (including asteroids and satellites) as an unconsolidated and chemically weathered surficial layer of fine-grained mineral or organic material exceeding centimeter scale thickness, with or without coarse elements and cemented portions. Martian dust generally connotes even finer materials than Martian soil, the fraction which is less than 30 micrometres in diameter. Disagreement over the significance of soil's definition arises due to the lack of an integrated concept of soil in the literature. The pragmatic definition "medium for plant growth" has been commonly adopted in the planetary science community but a more complex definition describes soil as "(bio)geochemically/physically altered material at the surface of a planetary body that encompasses surficial extraterrestrial telluric deposits". This definition emphasizes that soil is a body that retains information about its environmental history and that does not need the presence of life to form.

Toxicity

Martian regolith is toxic, due to relatively high concentrations of perchlorate compounds containing chlorine. Elemental chlorine was first discovered during localised investigations by Mars rover Sojourner, and has been confirmed by Spirit, Opportunity and Curiosity. The Mars Odyssey orbiter has also detected perchlorates across the surface of the planet. Perchlorates such as calcium perchlorate were first discovered on Mars in 2008 by the NASA Phoenix lander. The levels detected in the Martian regolith are around 0.5%, which is a level considered toxic to humans. These compounds are also toxic to plants. A 2013 terrestrial study found that a 0.5 g per liter concentration caused:

a significant decline in the chlorophyll content in plant leaves reduction in the oxidizing power of plant roots reduction in the size of the plant both above and below ground an accumulation of concentrated perchlorates in the leaves The report noted that one of the types of plant studied, Eichhornia crassipes, seemed resistant to the perchlorates and could be used to help remove the toxic salts from the environment, although the plants themselves would end up containing a high concentration of perchlorates as a result. There is evidence that some bacterial lifeforms are able to overcome perchlorates by physiological adaptations to increasing perchlorate concentrations, and some even live off them. In 2022, NASA and the U.S. National Science Foundation co-funded a multi-year grant to study the use of the bacteria Dehalococcoides mccartyi to break down perchlorates into harmless chlorides and oxygen. However, the added effect of the high levels of UV reaching the surface of Mars breaks molecular bonds, creating even more dangerous chemicals which in lab tests on Earth were shown to be more lethal to bacteria than the perchlorates alone. This, along with cold temperature, would add to the need to grow plants indoors. The chlorine in Martian perchlorates is thought to originate from volcanoes or aqueous weathering of basalt, and the oxygen likely originates from the atmosphere, possibly with some contribution from minerals. It is hypothesized that perchlorate may be formed either by reactions of chlorine with ozone, or by oxidation at grain surfaces, or by reactions enhanced with chlorine dioxide, or through reactions with free radicals produced by electrostatic discharge in dust storms.

Dust hazard

The potential danger to human health of the fine Martian dust has long been recognized by NASA. A 2002 study warned about the potential threat, and a study was carried out using the most common silicates found on Mars: olivine, pyroxene and feldspar. It found that the dust reacted with small amounts of water to produce highly reactive molecules that are also produced during the mining of quartz and known to produce lung disease in miners on Earth, including cancer (the study also noted that lunar dust may be worse). Following on from this, since 2001 NASA's Mars Exploration Program Analysis Group (MEPAG) has had a goal to determine the possible toxic effects of the dust on humans. In 2010, the group noted that although the Phoenix lander and the rovers Spirit and Opportunity had contributed to answering this question, none of the instruments have been suitable for measuring the particular carcinogens that are of concern. The Mars 2020 rover is an astrobiology mission that will also make measurements to help designers of a future human expedition understand any hazards posed by Martian dust. It employs the following related instruments:

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