Hydrogen, the simplest element with a single proton and one electron, is the most abundant element in the universe, making up about 75% of all normal matter. This fundamental element is a building block of stars and is pervasive throughout the cosmos. On Earth, however, its presence is often in compound forms rather than as free molecular hydrogen. This article explores environments on our planet where hydrogen might be scarce or present in minimal concentrations, challenging assumptions about its omnipresence.
Hydrogen’s Widespread Presence on Earth
Hydrogen is exceptionally common on Earth, primarily due to its ability to readily form compounds with other elements. The most significant reservoir is water (H₂O), which covers approximately 71% of the planet’s surface and is fundamental to all known life. Water molecules are constantly cycling through oceans, ice caps, rivers, and the atmosphere.
Beyond water, hydrogen is an integral component of all organic compounds, forming the backbone of carbohydrates, proteins, fats, and DNA essential for living organisms. It is also found in fossil fuels like natural gas (methane, CH₄), coal, and petroleum. While molecular hydrogen (H₂) exists only in trace amounts in the atmosphere, it is present as water vapor and methane. Additionally, hydrogen is incorporated into various hydrated minerals within the Earth’s crust and mantle.
Environments with Minimal Hydrogen
Despite its widespread distribution, certain environments on Earth contain significantly lower concentrations of hydrogen. Extremely arid regions, such as hyper-deserts, exemplify areas where hydrogen, particularly in the form of water or organic matter, is minimal. While not entirely absent, the scarcity of liquid water and biological activity leads to a pronounced reduction in hydrogen-containing compounds in these surface environments.
Deep-seated igneous and metamorphic rocks, especially those formed under high temperatures and pressures without hydrous minerals, also represent environments with low hydrogen content. Minerals like pure olivine or pyroxene are considered nominally anhydrous, lacking water in their primary crystal structure. However, even these minerals can incorporate trace amounts of hydrogen as hydroxyl (OH⁻) groups or fluid inclusions, typically in parts per million (ppm) levels.
Artificially created environments, such as ultra-high vacuum chambers or containers of highly purified noble gases or metals, aim for the lowest possible hydrogen concentrations. Despite rigorous purification processes, achieving absolute zero hydrogen is practically impossible due to its ubiquitous nature and tendency to adsorb onto surfaces. Certain high-temperature volcanic vents and fumaroles, particularly those with low water activity, may also exhibit reduced hydrogen concentrations compared to the surrounding hydrogen-rich volcanic gases.
Hydrogen in Earth’s Deep Interior
The Earth’s deep interior, including its mantle and core, might seem devoid of hydrogen, but scientific research indicates its subtle yet significant presence. Within the mantle, hydrogen is not found as free water but is dissolved within the crystal structures of nominally anhydrous minerals. Minerals like ringwoodite, wadsleyite, and bridgmanite, which are abundant in the mantle, can accommodate hydrogen in the form of hydroxyl (OH⁻) groups. This incorporation is part of the “deep Earth water cycle,” where hydrogen is transported and stored, influencing the mantle’s physical properties.
Under the extreme pressures and temperatures of the deep mantle, molecular hydrogen (H₂) can also have appreciable solubility in various upper mantle minerals, particularly under reducing conditions. The Earth’s core, primarily composed of iron and nickel, is also hypothesized to contain hydrogen as a light element component.
Research suggests that hydrogen contributes to the core’s observed density deficit. Some studies estimate that the Earth’s core could contain a substantial amount of hydrogen, possibly equivalent to 30 to 70 times the amount of water in the modern oceans. This indicates that the core may represent a significant reservoir of hydrogen, influencing its density and the generation of Earth’s magnetic field.
The Concept of Absolute Absence
The scientific understanding of hydrogen’s distribution on Earth suggests that achieving its absolute absence is practically unattainable. Given hydrogen’s fundamental role in water, organic life, and its high reactivity, it constantly cycles through various forms and environments. Even in environments engineered for extreme purity, or in naturally occurring settings considered “hydrogen-poor,” trace amounts often persist.
The question of “where hydrogen is not found” on Earth is therefore more accurately framed as identifying locations of extreme scarcity or negligible concentrations rather than complete non-existence. Hydrogen is a dynamic element, perpetually moving and reacting, making its total exclusion from any natural Earthly environment an immense challenge. The omnipresence of hydrogen, in various chemical forms, underscores its pervasive influence across Earth’s diverse systems.