| Abstract: | Title:
Uranus and Neptune as methane planets: Producing icy giants from
refractory planetesimals
Abstract:
The most distant planets in the solar system, Uranus and Neptune, are
known as 'ice giants'. This term has been deemed appropriate because
according to the prevailing belief of the last several decades, they
contain roughly twice as much water as rock. In a new paper, we
challenge this view and question the dominance of water. Modern
observations of outer solar system bodies rather show them to be
refractory-, and not ice-rich. Therefore, how can one form ice giants
from ice-poor building blocks? As a first step to solve the composition
tension problem, we develop a new technique to generate hundreds of
thousands of random computer-generated models for the internal structure
and composition of Uranus and Neptune. Mapping the space of possible
compositions, we show that these planets cannot be rock giants instead
of ice giants, thereby eliminating this as a potential solution.
Alternatively, we propose an entirely new solution to form ice giants
from ice-poor planetesimals. Contemporary observations show that the
refractory materials inside outer solar system planetesimals contain a
large fraction of refractory organics. We use several different codes in
order to show that these organics are chemically transformed by reacting
with the hydrogen in the growing atmospheres of Uranus and Neptune,
during and after their accretion, thereby generating copious amounts of
methane. This would make Uranus and Neptune methane-giants, and since
methane is an ice, the term 'ice giants' turns out to be strictly
appropriate. We show that there is an excellent agreement between random
models of Uranus and Neptune with methane and chemistry-derived models.
We offer a prediction to check our result in an upcoming mission to
Uranus, in the next decade. |