Evolution and Differentiation of Large Icy Moons

Abstract (Swedish)

Purpose: In preparation of several future missions with the objective to research the
large icy moons of the Solar System, it is necessary to have a cohesive understanding
of the conditions that might lead to the formation of liquid oceans in these moons.
The purpose of this work is to create a numerical model for simulating the temperature evolution of the cores of large icy moons.
Method: Assuming 1-D spherical geometry, this work constructs a numerical model
determining the energy and consequent temperature evolution in the rocky cores of
large icy moons under the influence of radiogenic heating from 40K, 235U, 238U and
232Th. Both conductive and convective heat transfer methods were considered with
different fractions of potassium present to account for the potential leaching of this
element into the water layers above. The endothermic process of dehydrating clay
minerals was also implemented in this model.
Results: Using the code the cores of Titan, Europa, Rhea and Mimas were simulated. It was determined that both core radius and remaining potassium fraction
after leaching have significant effects on temperature evolution, with the highest possible temperatures reached in Titan with 100% of the potassium remaining at 3100
kelvins. However, using the often quoted fraction of 30% of the remaining potassium
in the rocky cores, the temperatures in the cores of Titan and Europa are significantly lower, though continuing to increase at present day. When considering longer
timescales, the core of Titan can undergo melting and subsequent metal-silicate differentiation even at the fraction of primordial potassium set to 30% of its original
value.
Conclusions: Even with low amounts of potassium remaining in the rocky cores of
Titan and Europa, the presence of an ocean on top of them remains very likely. Furthermore, radiogenic heating alone is sufficient to lead to full differentiation into a
metallic core, a silicate outer core and an ice mantle, provided the potassium fraction
in the core is sufficient.

Popular Abstract

In the early 17th century when Galileo Galilei first decided to point a telescope at
Jupiter, he noticed four faint objects placed around it on the same line. As he spent
more and more nights viewing them he realised that instead of moving in the same
direction as Jupiter these four objects in fact orbited the planet itself. This proved
that it is possible for another body to be the centre of gravity for others, other than
the Earth, challenging the widely spread tenets of that time.
These four objects are now known as the Galilean moons of Jupiter and three of them
are classified as large icy moons because they consist in large part of ice, in addition
to rocks. It is often thought that the rocky parts of these moons are gathered into a
rocky core, leaving a thick ice mantle on top, but it is also possible for the rock and
ice to be mixed up either throughout the entire moon or as an extra layer between a
pure rocky core and pure ice mantle.
Since humans inevitably view the Universe from an Earth-centric point of view, we
are fairly certain that liquid water is essential for the existence of life. This makes
the large icy moons extremely interesting to planetary astronomy because if any of
those moons are heated enough, it is possible for the ice to partially melt. This would
form a liquid ocean somewhere in the thick ice mantle, making the large icy moons
incredibly interesting to those in search of extraterrestrial life.
Large icy moons can be heated in several different ways, but the most important one
by far is the heating from radioactive elements present in the rocky part of the moon.
Since these generate heat they are often called radiogenic elements and since they are
very long-lived, they have been heating the moons up to the present day since their
formation 4 billion years ago! However, it is thought that during formation the ice
was melted and well-mixed with the rocks and the water, which froze soon thereafter,
absorbed a portion of one of the radiogenic elements: potassium. This process is
called potassium leaching and because of this the ice mantle does not only get heated
from the rocks below but also from within, making the formation of an ocean that
much more likely!
This project involved writing a code to simulate these processes for several different
large icy moons from the time of their formation to the present day and beyond.The
most important parameter in these simulations is temperature since it defines the
conditions in the ice, allowing for the possible formation of liquid oceans. In this
work all large icy moons are concluded to be likely to host an ocean either now or in
the future, with the largest ones like Titan even possibly having a metallic core having
differentiated from the silicates similarly to how the rocks and ice differentiated.

• author: Tomberg, Piia
• supervisor: Thomas Ronnet
• organization: Lund Observatory - Undergoing reorganization
• course: ASTM31 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• publication/series: Lund Observatory Examensarbeten
• report number: 2021-EXA184
• language: English
• id: 9058864
• date added to LUP: 2021-06-28 09:33:32
• date last changed: 2021-06-28 09:33:32