Uranus and Neptune look so much alike that they are often described as twins. Yet their composition could tell two very different stories.
These two planets are the farthest from the Sun. They are similar in size, bluish in color and surrounded by thick atmospheres. They are called ice giants because they contain water, ammonia and methane, among other substances, in forms very different from those we know on Earth.
Uranus and Neptune, with Earth for comparison.
Lunar and Planetary Institute from Houston, TX, USA · CC BY 2.0
For a long time, astronomers thought they had formed from comparable materials in the same broad region of the young Solar System. However, a new study suggests that Neptune has much more oxygen than Uranus deep in its atmosphere.
The researchers studied a gas found higher up in their atmospheres: carbon monoxide. Its abundance provides clues about the chemical elements hidden much deeper down.
The team compared these observations with simulations of how the atmospheres work. In particular, they took into account the movements that transport gases between different altitudes and the differences between regions of the planets. The calculations indicate that Neptune's deep layers would be significantly richer in oxygen.
When the Solar System formed about 4.5 billion years ago, the temperature varied with distance from the Sun. Depending on the location, water and other substances could condense into solid particles or remain in gaseous form. The materials available to build the planets were therefore not identical everywhere.
The researchers compared the estimated compositions with models of the young Solar System. The materials that formed Neptune were located much closer to the Sun than those that formed Uranus. The two planets therefore may not have grown in the same place, and planetary migration may later have placed them in the orbits they occupy today.
This explanation nevertheless requires verification. The calculations depend in particular on the speed at which gases circulate through the atmospheres, which is difficult to determine without on-site measurements. Differences in their subsequent evolution may also have played a role.
Scientists need new observations, and especially a probe capable of studying Uranus up close. The similarities - or lack thereof - between the two planets may conceal an important part of the history of our Solar System.