Argon-40 in Uranus and Neptune may trace their origins more than their rock

A new model puts the argon-40 that Uranus and Neptune inherited from the solar nebula at anywhere from 1 to about 1,220 times the nebula’s own level, enough at the high end to drown out the few parts per billion made by radioactive potassium in their rocky interiors.

Peer reviewed · Modelling study · No argon-40 has been measured on either planet

Planetary scientists have hoped that one gas, argon-40, could reveal how much rock is hidden inside Uranus and Neptune, because radioactive potassium in rock slowly turns into it. A team led by Olivier Mousis of the Southwest Research Institute (SwRI) now calculates that argon-40 inherited when the planets formed could swamp that rock signal. Their paper, published on 11 November in The Planetary Science Journal, concludes that the gas may say more about where the planets’ building blocks formed. The catch: these are model estimates, and no spacecraft has yet measured argon on either planet.

Evidence at a glance

Is this a big deal?

It is a useful warning, not a discovery. Earlier work proposed argon-40 as a gauge of rock inside giant planets; this paper says the gauge works only in some formation scenarios and lists the extra measurements a probe would need. It does not tell us what Uranus or Neptune are made of.

What scientists found

Argon-40 in a planet’s air can come from two places. Some is primordial: gas that was already in the solar nebula, the cloud of gas and dust the planets formed from, and that was swept into the planet. The rest is radiogenic: made when potassium-40 in rock decays. Potassium-40 has a half-life of about 1.25 billion years, so roughly 92% of the original supply has decayed in the 4.5 billion years since the planets formed (a SciQuest calculation from the paper’s values).

For a planet with as much rock as the authors assume for Uranus and Neptune, radioactive decay would add argon-40 equal to about 1.9 to 13 parts per billion of the atmosphere, depending on how much potassium the rock holds. If the planets had captured argon at the nebula’s own ratio, the primordial share would be only about 0.63 parts per billion, and the rock signal would win easily.

The model says that ratio may not hold. Combining the carbon already seen in the planets’ methane with the model’s argon-to-carbon ratios in solid ice, the team estimates primordial argon-40 at about 1 to 610 times the nebula’s level in Uranus and about 1 to 1,220 times in Neptune. Above roughly 100 times, the authors write, the inherited argon overwhelms the radiogenic signal.

Mousis put the conclusion this way in the SwRI release: “Taking this under consideration, we have concluded that atmospheric argon-40 measurements probably tell us more about the planets’ formation environments and history than about their rocky or icy interiors.”

How they found it

The team, from SwRI, the Space Science Institute, Caltech and NASA’s Jet Propulsion Laboratory, built the estimate in two halves. The radiogenic half is bookkeeping: rock mass, a meteorite-like potassium content (or one 10 times lower), the decay rate, and an assumption that 30% of the argon made inside reaches the atmosphere. The authors stress that 30% is a reference value, not a prediction.

The primordial half comes from a time-dependent model of the nebula. It follows grains of ice as they drift towards the Sun and vapour as it spreads outwards. Argon gets locked into solids in two ways. It can freeze as pure argon ice, at about 22 kelvin (−251 °C), or become trapped in cages of water ice called clathrates, which form at about 35 kelvin (−238 °C).

In the model, argon vapour that diffuses outward across the argon “ice line” refreezes there. Over the first 3 million years, solids around 23 to 24 astronomical units from the Sun (1 au is the Earth–Sun distance) end up with up to 13 times more argon relative to carbon than the nebula as a whole. Solids that formed elsewhere could carry less than a tenth of the nebula’s ratio. The argon ice line in this model sits at 25 to 30 au, close to where Uranus and Neptune orbit today.

The upper estimates follow from that peak. Uranus’s methane implies carbon at about 38 to 47 times the nebula’s level, and Neptune’s at about 63 to 94 times. Multiply the highest carbon value by the 13-fold argon boost and you get 13 × 47 ≈ 611 for Uranus and 13 × 94 ≈ 1,222 for Neptune, matching the paper’s upper limits of about 610 and 1,220 (SciQuest’s check; the paper does not print this step).

A kitchen-table way to see the problem

Imagine trying to work out how many candles burned in a closed room by measuring the smoke, when you do not know how hazy the room was before anyone lit a match. If the room started clear, the smoke is a good candle counter. If it started thick with haze, the candles barely change the reading.

The analogy breaks down in one way that makes the real problem harder: haze and smoke look different, but an argon-40 atom from the nebula is identical to one made by decay. Researchers have to separate them indirectly, using argon-36, which is never made by potassium decay, and other gases.

Why it matters to you

This changes nothing in daily life. It is a question about origins: whether the two outermost planets are mostly ice or mostly rock, and where their ingredients came together. It also shapes how a future ice-giant probe would be equipped, since argon-40 alone would leave its meaning open.

What the evidence does not show

It does not show how much argon-40 Uranus or Neptune contain. The SwRI release says the study “measured” the planets’ argon-40, but the paper is a set of calculations; no instrument has sampled either atmosphere’s argon.

It does not show that argon-40 is useless as a rock gauge. The paper says it can still constrain the rocky component when the primordial share is small and enough gas has escaped from the interior. An argon level close to the nebula’s own value would point that way.

The authors list their own weak points. How efficiently argon leaks from the deep interior is unknown, and layered interiors could cut it to 10% or less. Complete degassing would raise the radiogenic share about 3.3 times, though they say large primordial enrichments would still dominate. If much of the carbon in the planets’ building blocks was locked in solid organic material rather than ices, the predicted argon boosts would shrink. The nebula’s starting argon-40 level is also uncertain; the paper takes it from a meteorite and notes other published estimates several times higher.

How this fits what we already knew

Voyager 2 is the only spacecraft to have visited Uranus (1986) and Neptune (1989), and both visits were flybys that could not sample the deep atmosphere. Methane measured from afar shows both planets are rich in carbon, the starting point for this study. On SciQuest we reported in October on a separate carbon-to-oxygen study suggesting the two planets may have different origins; this paper adds argon as a second formation clue.

In 2024, Francis Nimmo and colleagues argued that the large heavy-element share of Uranus and Neptune might boost their radiogenic argon-40, making it a possible rock gauge. This paper borrows their 3 parts-per-billion detection limit and their low-potassium case. Jupiter offers the only real comparison: the Galileo probe found argon there at only about 2 to 3 times the nebula’s level, but its mass spectrometer did not detect argon-40.

Independent comment

SciQuest has not yet obtained comment from a researcher outside the study team; the peer reviewers’ views are not public. We will add independent comment if we receive it.

How other headlines framed it

• SwRI, also reposted by Phys.org, led with the interior question and described the work as measuring the planets’ argon-40, although the paper models it.
• The Brighter Side of News kept a hedged headline about clues to the rocky interiors and stated in the text that no atmosphere was sampled.

What happens next

The test needs a probe that falls into an ice giant’s atmosphere with a sensitive mass spectrometer. The US National Academies’ 2022 planetary science decadal survey ranked a Uranus Orbiter and Probe as its highest-priority new flagship mission; SwRI describes it as a proposed NASA mission concept. The paper does not name a mission, but says future probes are expected to reach at least about 10 bars of pressure, below where any argon cloud could form.

The authors recommend measuring argon-40 together with argon-36, nitrogen, the nitrogen-15 and carbon-13 isotope ratios, and carbon below the clouds. They also suggest returning to Jupiter with a more sensitive instrument to pin down the nebula’s original argon-40.

For a sense of how far away that is, see our guide to how long it takes to reach each planet.

In one sentence

Argon-40 in Uranus and Neptune may turn out to record where their building blocks formed more than how much rock they hold, and a future entry probe would need argon-36 and other gases measured alongside it to tell the two apart.

Read next

• Uranus and Neptune may have different origins, a carbon-to-oxygen study suggests
• Gravity on Uranus: how strong is it compared with Earth?
• Gravity on Neptune: how strong is it compared with Earth?

For all eight planets side by side, see gravity on other planets.

Sources and disclosure

1. Mousis O, Hofstadter MD, Glein CR, Lunine JI, Canup RM. “Primordial and Radiogenic Argon 40 in Uranus and Neptune.” The Planetary Science Journal 7:258, 11 November 2026. CC BY 4.0.
2. Southwest Research Institute. “SwRI team studies interior composition, origins of ice giants.” News release, 11 November 2026.
3. National Academies of Sciences, Engineering, and Medicine. Release on Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023–2032, 19 April 2022.
4. NASA Science. Voyager 2 mission page.
5. NASA/JPL. PIA01492: Voyager 2 image of Neptune.

The study was funded by SwRI’s internal research programme, with part of the work done at JPL under a NASA contract, according to the paper.

Disclosure: This article was drafted with AI assistance and checked against the sources above. The decay fraction, the parts-per-billion values for inherited argon and the 13 × 47 and 13 × 94 check are SciQuest calculations from values in the paper; the graphic is a SciQuest original drawn from the paper’s numbers. Spotted an error? See our corrections policy.