An enormous amount of methane may be deep within Uranus.
Astronomers taking a closer look at Uranus believe it could
be filled with a lot more methane than we previously thought.
Uranus and Neptune are often referred to as ice giants, with
scientists believing that they are mostly made up of "icy" materials
such as water, methane, and ammonia, surrounding a hot, rocky core. One of the
reasons why astrophysicists believe this is the case is because the region they
formed in would likely have been abundant in the necessary components, hence
the expectation of a lot of water and ice.
"The first models of Uranus and Neptune were
constructed based on the premise that they were formed in the outer parts of a
primitive proto-planetary nebula, where the elemental abundances were
considered solar," the new paper, which has not yet been peer-reviewed,
explains. "Thus, assuming chemical equilibrium, early studies used
calculations of the expected relative molecular abundances to gauge the
potential composition of the planets that formed in this region."
Uranus has not had a whole lot of love from NASA and has
been visited by a probe, Voyager 2, only once. That same probe then journeyed
on to Neptune, providing much better observations of the planets than ground
and space telescopes near Earth allow. When the models of Uranus and Neptune
were compared to our limited observations of the planets, they were found to be
consistent with what we saw close(r) up.
While this might sound like case closed on the composition
of the planets, there was still a bit of an astronomical mystery to solve.
Objects in the Kuiper belt are rich in organic-rich refractory materials, and
are relatively water-poor, suggesting that the planets should contain more
refractory materials than ice-forming materials. So why do earlier models fit
with what we observe? According to the team, there can only be two options:
Ice-poor building blocks can lead through some processes to an ice-rich planet,
or a new model is needed, with the planets having a rockier interior than we
thought.
The team generated hundreds of thousands of models of the
planets, varying the chemical compositions in the initial conditions until
planets similar in mass and structure to Uranus and Neptune emerged. They found
that the models which fit best had an interior containing at least 10 percent
methane, and over 20 percent in models that also have a large water/rock mass
fraction. In these models, methane may be more abundant than water in the
planets' interiors.
"This is a difficulty because CH4 is certainly not that
prevalent in the contemporary solar system, and is normally a very small
fraction compared to water," the team wrote. "We suggest that
organic-rich refractories are sufficiently abundant in outer solar system
planetesimals to drive chemical reactions in the atmospheres of Uranus and
Neptune, which, during the phase of planet growth, may produce the required
methane."
The thick methane layer likely formed in chemical reactions
that took place as carbon-rich planetesimals collided with the growing planet,
and the carbon reacted under intense heat and pressure with hydrogen.
The model would go some way to explaining how an ice-rich
planet formed in an area of our solar system filled with water-poor objects.
While these models may fit, we need more observations of the giants to learn
about their composition.
In short, we're going to need to take a closer look at
Uranus.
The paper is posted on arXiv.