Earth’s atmosphere contains a small but measurable amount of helium, roughly 5.24 parts per million by volume. That makes it the second most abundant noble gas in our air after argon, though it trails far behind nitrogen and oxygen. The interesting part is not really whether helium is there, but how it got there, why there is so little of it, and how humans are quietly changing the balance.
Where Atmospheric Helium Comes From
Almost all the helium in our atmosphere is helium-4, and most of it was born underground. Uranium and thorium atoms scattered through Earth’s crust and mantle undergo radioactive decay, and each alpha particle they emit is essentially a helium-4 nucleus. Over billions of years, this process has steadily pumped helium out of rocks and into the air above.158th U.S. Rock Mechanics/Geomechanics Symposium. Helium Gas Release by Rocks Undergoing Crushing Some of this helium seeps up slowly through soil and sediment. Some reaches the surface more dramatically, venting through fault zones and volcanic systems.
A much smaller share of atmospheric helium is helium-3, a lighter isotope with a different origin story. Some helium-3 traces back to the formation of the planet itself, primordial gas that has been locked in the deep mantle since Earth first assembled from the solar nebula. Mantle plumes and mid-ocean ridge volcanism gradually bring small quantities of this ancient helium-3 to the surface.2Geochemistry, Geophysics, Geosystems. Primordial Helium‐3 Exchange Between Earth’s Core and Mantle There is also a cosmic contribution: interplanetary dust particles raining into the atmosphere carry extraterrestrial helium, though modeling suggests only about 0.5% of the infalling dust mass survives atmospheric entry at temperatures cool enough to retain its helium.3Geochimica et Cosmochimica Acta. Atmospheric entry heating and helium retentivity of interplanetary dust particles
Why There Is So Little of It
If helium has been seeping out of Earth’s interior for billions of years, you might expect a lot more of it overhead. The reason there is not comes down to a simple physical fact: helium atoms are light enough and fast enough to escape Earth’s gravity entirely. At the top of the atmosphere, where air thins out to nearly nothing, some helium atoms reach speeds that carry them past the escape velocity and off into space for good.
This thermal escape process, sometimes called Jeans escape, has been studied for decades. A classic estimate put the average rate of thermal helium escape at about 60,000 atoms per square centimeter per second, averaged over a full solar cycle.4Reviews of Geophysics. The escape of helium from the Earth’s atmosphere Collisions with other atmospheric molecules slow the process slightly compared to the idealized calculation; detailed modeling that accounts for a background of oxygen finds the actual escape flux is reduced by about 10% for helium.5Planetary and Space Science. Evaporation of hydrogen and helium atoms from the atmospheres of Earth and Mars
Thermal escape is not the only way helium leaves. A second mechanism, charge exchange, also matters. In the upper atmosphere, helium ions trade their charge with nitrogen and oxygen molecules, and the resulting fast-moving neutral helium atoms can fly away. This process shows large swings between day and night and between high and low solar activity, but the global average does not shift by more than about a factor of three across a solar cycle. Researchers have concluded that charge exchange is a significant contributor to Earth’s overall helium loss.6Planetary and Space Science. Helium escape from the Earth’s atmosphere: The charge exchange mechanism revisited
A Steady-State Atmosphere
With helium constantly entering from below and leaving from above, scientists have long asked whether the two flows balance out. The evidence strongly suggests they do. Estimates of how much helium Earth’s crust and mantle release, compared with how much escapes to space, point to a roughly steady-state helium atmosphere, one where the total amount stays approximately constant over geological time scales.7Chemical Geology: Isotope Geoscience section. Terrestrial helium degassing fluxes and the atmospheric helium budget: Implications with respect to the degassing processes of continental crust
This balance is not guaranteed by any simple physical law. It depends on specific rates of radioactive decay in the crust, on mantle convection patterns that bring deep gases to the surface, and on conditions in the upper atmosphere that control escape. The fact that it works out to a near-equilibrium is one of those quiet coincidences in planetary science. If Earth had slightly weaker gravity or a much thinner atmosphere, helium would escape faster and the concentration would be even lower. If radioactive decay rates were higher, the atmosphere would hold more.
Fossil Fuels Are Tipping the Balance
That steady state held for a very long time, but human activity is now visibly nudging it. Fossil fuels, particularly natural gas, contain small amounts of helium trapped alongside methane and carbon dioxide. When we extract and burn those fuels, the helium goes into the atmosphere as a byproduct. For years, this effect was assumed to be too small to detect against the large background of existing atmospheric helium. That changed with a 2022 study that used high-precision mass spectrometry to measure the ratio of helium-4 to nitrogen in archived air samples spanning nearly five decades. The results were clear: helium-4 concentrations have risen measurably, increasing at an average rate of about 39 billion moles per year between 1974 and 2020.8Nature Geoscience. Increasing atmospheric helium due to fossil fuel exploitation
Thirty-nine billion moles per year sounds enormous, but it is small relative to the total atmospheric inventory. The concentration change amounts to a tiny fraction of the existing 5.24 ppm, enough to detect only with extremely sensitive instruments. Still, the finding matters for a couple of reasons. It confirms that the helium cycle is not immune to human disruption, and it opens the door to using atmospheric helium as an independent tracer for fossil fuel emissions, a cross-check on carbon dioxide measurements.
Two Isotopes, Two Stories
The helium in our atmosphere comes in two isotopic flavors, and their ratio tells scientists a surprising amount about where the gas originated. Helium-4, by far the dominant isotope, is the product of radioactive decay. Helium-3 is rarer and mostly primordial, having formed during the Big Bang and been incorporated into Earth during planetary accretion. The ratio of helium-3 to helium-4 in the atmosphere has been established at about 1.393 × 10⁻⁶, meaning there is roughly one helium-3 atom for every 720,000 helium-4 atoms.9Journal of Geophysical Research: Atmospheres. Direct determination of the helium 3 content of atmospheric air by mass spectrometry
That ratio is not set in stone, at least not on long time scales. Measurements of helium trapped in historical metallurgical slag from Japanese refineries dating back to the 1600s suggest the atmospheric helium-3 to helium-4 ratio was about 4% higher centuries ago than it is in modern air. The decline fits with the expectation that burning fossil fuels releases disproportionate amounts of helium-4 (which accumulates in underground reservoirs from uranium and thorium decay), diluting the helium-3 share. Surface air samples also show a geographic gradient: the ratio is slightly lower at northern latitudes where most fossil fuel extraction and consumption occurs.10Geochimica et Cosmochimica Acta. Atmospheric helium isotope ratio: Possible temporal and spatial variations
More recent work has tried to pin down whether this isotopic shift is ongoing over just the last century. High-precision measurements of air trapped in stainless steel containers dating from 1910 to 2016 found no statistically significant change in the isotopic ratio over that period, with the trend consistent with zero within measurement uncertainties.11Geochemical Perspectives Letters. Atmospheric helium isotopic ratio from 1910 to 2016 recorded in stainless steel containers This does not necessarily contradict the longer historical record; the century-scale shift may simply be too small for even modern instruments to resolve over a hundred-year window. The total helium-4 concentration is rising detectably, but the isotopic ratio changes so slightly per year that confirming the trend requires either longer time spans or even more precise techniques.
Helium as an Earthquake Gauge
One of the more unexpected applications of atmospheric helium science has nothing to do with the atmosphere itself, at least not directly. Geologists have found that helium seeping out of the ground changes in response to earthquakes, and these changes carry information about what is happening deep in the crust.
After the 2014 Nagano earthquake in Japan, gas samples collected near the main fault showed an increase in the helium-3 to helium-4 ratio, while samples from areas farther from the epicenter showed a decrease of up to about 30%. The interpretation is that faulting can either crack open rocks that have been storing radiogenic helium-4, or it can open pathways for mantle-derived helium-3 to rise through newly permeable fault zones. Which signal you see depends on where you are relative to the fault.12Applied Geochemistry. Release of mantle and crustal helium from a fault following an inland earthquake
The 2016 Kumamoto earthquake in southwestern Japan provided the first quantitative link between the amount of helium released into groundwater and the volumetric strain change measured by seismological instruments. The correlation was strong enough that researchers proposed groundwater helium could function as a kind of natural strain gauge, potentially useful for detecting stress buildup in regions where conventional monitoring equipment is sparse.13PubMed Central. Groundwater helium anomaly reflects strain change during the 2016 Kumamoto earthquake in Southwest Japan Work on the Irpinia Fault Zone in Italy has extended this idea further, showing that crustal helium-4 degassing from fault zones occurs throughout the seismic cycle and varies in ways that may reflect changes in the stress field related to earthquake nucleation.14Communications Earth & Environment. Earthquakes control the impulsive nature of crustal helium degassing to the atmosphere
None of this amounts to earthquake prediction, which remains one of geoscience’s most stubbornly unsolved problems. But helium monitoring offers something genuinely useful: a geochemical window into crustal processes that complements the seismometers and GPS stations that dominate current monitoring networks. In remote areas where installing dense instrument arrays is impractical, sampling groundwater or soil gas for helium anomalies could fill a real gap.
Helium on Mars and Venus
Earth is not the only rocky planet with helium in its atmosphere, but the amounts elsewhere are strikingly similar in concentration while being very different in origin and fate. Observations of Mars and Venus using extreme ultraviolet spectroscopy have yielded helium mixing ratios in the lower and middle atmospheres of both planets in the neighborhood of 10 parts per million, roughly twice Earth’s concentration but the same order of magnitude.15Icarus. Helium on Mars and Venus: EUVE observations and modeling
What differs sharply is the escape mechanism. Earth loses helium primarily through thermal escape and charge exchange with atmospheric gases. Venus, which lacks a global magnetic field to shield its upper atmosphere, loses helium mainly through ionization and sweeping by the solar wind above the ionopause, along with charge exchange between helium ions and carbon dioxide molecules. Mars, smaller and with even weaker gravity, loses helium more readily than either Earth or Venus. Despite these very different loss pathways, all three planets have wound up with low-single-digit-ppm helium concentrations, a convergence that tells us more about the physics of light-gas escape from rocky planets than about any shared geological history.
The gas giant planets are a different story altogether. Jupiter and Saturn are roughly a quarter helium by mass, reflecting the composition of the primordial solar nebula from which they formed. The contrast could not be starker: rocky planets have trace helium because they cannot hold onto it, while gas giants are made of it.
Measuring Helium at Extreme Precision
Detecting a subtle rise in atmospheric helium, or resolving the isotopic ratio to the precision needed for earthquake monitoring, requires instruments far beyond ordinary laboratory equipment. The workhorse technology is noble gas mass spectrometry, where gas samples are purified, ionized, and sorted by mass. For helium isotope work, the challenge is that helium-3 is fantastically rare. In a typical rock sample, the partial pressure of helium-3 can be vanishingly small, on the order of 10⁻¹⁴ Torr.
Specialized double-collector mass spectrometers address this by measuring helium-3 and helium-4 simultaneously, one on an ion-counting detector and the other on a Faraday cup. Improvements to these systems have pushed the detection limit down to roughly 6 × 10⁻¹⁶ cubic centimeters of helium at standard conditions, an improvement of one to two orders of magnitude over earlier designs, with isotopic ratio precision of about ±10% even at extremely low concentrations.16Journal of the Mass Spectrometry Society of Japan. Highly Sensitive and Precise Measurement of Helium Isotopes Using a Mass Spectrometer with Double Collector System
This kind of analytical firepower is what made the fossil-fuel helium discovery possible. Measuring a change of a few parts per billion against a background of 5.24 parts per million requires not just sensitive instruments but also carefully archived air samples collected under controlled conditions, like the Cape Grim air archive in Tasmania or the vintage stainless steel containers used in isotopic ratio studies. Without those decades-old samples, the atmospheric trend would be invisible.
Helium in the Ocean
The atmosphere is not the only reservoir where Earth’s helium shows up. The ocean absorbs helium from the air at the surface and also receives helium from below, through hydrothermal vents at mid-ocean ridges and through the seafloor more generally. Deep ocean waters consistently contain more dissolved helium than you would expect from simple equilibration with the atmosphere, a phenomenon geochemists call “excess helium.”
Early work on dissolved gas saturation in the Atlantic and Pacific showed that much of the excess helium in deep water can be explained by injected air, tiny bubbles forced into the water column by wave action and mixing, which dissolve under pressure. This “atmospheric component” accounts for all of the excess helium in Atlantic deep water and about 60% in the Pacific.17Earth and Planetary Science Letters. Dissolved gas saturation anomalies and excess helium in the ocean The remaining Pacific excess points to a contribution from the mantle, delivered via the hydrothermal systems of the East Pacific Rise and other active spreading centers. The helium-3 to helium-4 ratio of this deep-sea excess is enriched in helium-3 compared to the atmosphere, a fingerprint of mantle origin that has become one of the standard tools for tracing ocean circulation patterns and identifying hydrothermal plumes on the seafloor.
Oceanographers use these helium signals to track how deep water masses form, where they travel, and how quickly they mix. A parcel of water that last saw the surface decades ago carries a dissolved helium signature that acts as a kind of timestamp and origin label. This makes helium one of the more versatile natural tracers in earth science, useful simultaneously for understanding the planet’s interior, its oceans, and its atmosphere.