Researchers at Southwest Research Institute say Uranus and Neptune may have formed from materials from different parts of the early solar system, despite looking similar.
Their study in Astronomy & Astrophysics draws on carbon monoxide measurements to estimate the planets’ hidden oxygen levels and compare their possible formation histories.
NASA’s Voyager 2 visited each planet once during the 1980s. As a result, scientists still lack direct measurements of their deep atmospheres.
Observations from the Atacama Large Millimeter/submillimeter Array (ALMA) put carbon monoxide in Uranus’s upper troposphere at about 5.6 parts per billion. Neptune’s concentration was roughly 100 times higher.
Researchers believe the gas forms deep inside the planets through water-linked chemistry. Because vertical mixing carries it upwards, scientists can use atmospheric models to estimate oxygen below the clouds.
For Uranus, the team estimated an oxygen-to-hydrogen ratio of 62 to 177 times the Sun’s value. Neptune’s estimated ratio was 222 to 342 times the solar level.
Meanwhile, carbon-to-oxygen ratios ranged from 0.06 to 0.52 for Uranus and 0.02 to 0.12 for Neptune. Researchers varied atmospheric mixing by latitude when producing those estimates.
To investigate these differences, scientists compared their findings with models of the gas and dust disk surrounding the young Sun.
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Neptune’s building blocks may have originated near the water ice line, closer to the Sun. Uranus’s chemical values, however, could fit material originating inside the methane ice line.
Instead of gathering materials from the same region, Neptune may have accumulated ingredients that travelled considerable distances.
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Dr Olivier Mousis, a researcher at the Southwest Research Institute, described the planets as “more like cousins with very different histories”.
Although the findings suggest separate evolutionary paths, the models cannot establish either planet’s birthplace with certainty. Researchers believe these differences could help explain how similarly sized planets form around other stars.
NASA’s proposed Uranus Orbiter and Probe mission could provide further insights into planetary composition. However, scientists still need direct atmospheric measurements to test their estimates of the planets’ internal chemistry.
