Methane's Behavior Hints at Ultradeep Diamonds in Uranus and Neptune

Hydrocarbons are a huge part of our lives on Earth. They make up the oil and gas that heat our homes and fuel our cars. Yet the hydrocarbon methane (CH4), which is one of the most abundant molecules in the universe, has puzzled scientists for years. When on other planets, methane's behavior is hard to track. Now, scientists have learned a little bit more about this hydrocarbon, helping them understand the processes and conditions found in planetary interiors. In fact, the new findings may hint at the possibility of ultradeep diamonds in the interiors of Uranus and Neptune.

"Our knowledge of physics and chemistry of volatiles inside planets is based mainly on observations of the fluxes at their surfaces," said Sergey Lobanov, lead author of the study, in a news release. "High-pressure, high-temperature experiments, which simulate conditions deep inside planets and provide detailed information about the physical state, chemical reactivity and properties of the planetary materials, remain a big challenge for us."

Methane's melting behavior is only known below 70,000 times normal atmospheric pressure. Yet much more extreme and harsher conditions exist within the interior of planets. In fact, previous studies have indicated little information about methane's chemical reactivity under pressure and temperature conditions similar to those found in the deep Earth. This led researchers to believe that methane is the main hydrocarbon phase of carbon, oxygen and hydrogen-containing fluid in some parts of the Earth's mantle.

In order to learn a bit more about methane, the researchers used high-pressure experimental techniques in order to examine methane's phases and reactivity under a range of temperatures and pressures mimicking environments found beneath Earth's surface. They found that at pressures reaching 790,000 times normal atmospheric pressure, methane's melting temperature was still below 1,900 degrees Fahrenheit. This suggested that methane is not a solid under any conditions met deep within most planets. In fact, its melting point is even lower than melting temperatures of water and ammonia.

At temperatures above about 1,700 degrees, though, methane becomes chemically more reactive. It first disassociates into elemental carbon and hydrogen and then light hydrocarbon molecules begin to form. Pressure also affects the composition of the carbon-hydrogen system.

The findings have implications for the chemistry that occurs beneath the surface of Earth and other planets. More specifically, research hints that the icy gas giants of Uranus and Neptune might form ultradeep diamonds within their mantles.

The findings are published in the journal Nature Communications.