Our icy planetary relatives: a special slush?

Guest post author, Cecilia Fisher.

The guest post was written by Cecilia Fisher. “I am a first year undergraduate student at UCLA majoring in Anthropology and Arabic. In my spare time, I enjoy singing in the group A Cappella and exploring Los Angeles.

Title: A new perspective on the interiors of ice-rich planets: a mixture of icy rocks instead of ice on top of the rocks

Authors: Alona Vazan, Reem Sari, Ronit Kessel

First Author Institutions: The Open University Astrophysics Research Center, The Open University of Israel

In elementary school, the day your science teacher took out a colored cross-section of the earth was always special. Within that globe, there were four layers: a brown rocky crust, a red mantle, an orange outer core, and a yellow inner core. Each layer has always been separate, and is clearly marked as either a liquid, a solid, or something in between.

Until recently, we thought this concept – where the interiors of planets are rigidly laid out – was universal. Thus, we applied it not only to Earth-like terrestrial planets, but also to ice-rich planets such as Uranus and Neptune. On Uranus and Neptune, scientists hypothesized that an iron core was covered with a rocky mantle covered with an icy crust. Then, that solid mass was surrounded by a gaseous “shell” of hydrogen and helium. Simply put, the scientific community believed that ice and rock were not mixed together, but separated.

The authors of this paper provide a groundbreaking new assertion. They suggest that the rocks and ice do not fall into separate shells or layers, but rather, mix together in the ice-rich interiors of the planets like an unappetizing slushie. To find out why the interiors of ice-rich planets differ from those of terrestrial planets, we must dive into their physical makeup. Whether it is ice or water, cosmic dust, rock or mineral, this composition depends mostly on where the planet formed relative to the star it orbits. For water in particular, its appearance as a liquid, ice or gas depends on where it is formed in relation to the snow line. The snow line is similar to a state border in that it is figurative, but it is an important distinction. Outside the “line,” it’s cold enough that liquids turn into solids. For example, since Uranus and Neptune are outside the line, they have ice instead of water.

shape 1: Artist’s conception of a snow line. In the inner (brown) region, the water is liquid. In the outer area (white/blue) the water is ice. The limit where the color changes is the location of the figurative “snow line”. Photo by A. Angelich Astronomers see a star’s water-snow streak | space | EarthSky.

So, to find how ice and rock actually interact on planets like Uranus, the authors called upon previous experiments within the astrophysics community. Several studies have investigated how the interactions of rock-ice in the interior of planets are affected by different pressures and temperatures. The ultimate goal of these experiments was to determine the combination of temperature and pressure that the mixing process of ice and rock would take. Just like the magma in a volcano must be both hot and powerful to cause an eruption, the planet’s interior must be cold and compact enough to cause internal mixing. In Figure 1 below, the ideal mixture between pressure and temperature for mixing is referred to as SCP, the second critical end point. The figure shows that Earth and Jupiter are mainly present in the complete ice-rock mixture system. However, while Jupiter likely formed in the outer portion of the ice line and is thus ice-rich, Earth is ice-poor.

Figure 2: Ice-rock interaction systems based on experimental laboratory data. The blue solid line and the brown solid line represent Earth-like rock and water ice with a 1:1 (equal) mass ratio. These lines and Ice and rock SCP (black circle) Mark three areas: full mixture of ice and rock (grey shaded area), partial mixture of ice and rock (brown shaded area), and complete miscibility (white area). The water evaporation curve (black dashed) indicates the transition from the liquid state to the gas state (black dashed), until critical water point (black dot). Figure 2 in the paper.

To apply the SCP data to the ice-rich planets, Uranus and Neptune, the authors created two possible different models of what ice-rich interiors might look like. Although they were bound to be drinks, scientists weren’t sure how the ice rock slash mixed with the gas. The first form is an internal structure with the slush gradually distributed throughout – like a latte where the milk is gradually thinned out from the espresso. The second form is a structure in which there is a clear distinction between slushie and gas envelope, just like tootsie pop.

Figure 3: the difference between gradual metal (red and basic envelope (blue) structures. The upper plate is connected to the density of each, and the lower plate to each of them is connected to the fitting. Typical planets are 15 M⊕ where 90% of the mass is ice and rock in equal mass fractions (1:1 ratio), and 10% of the mass is hydrogen and helium in the solar ratio. The planets are located at 10 au from a sun-like star. Figure 1 in the paper.

Before reaching a final conclusion, the authors also looked at how the temperature of the planets evolved over time. Just as a piece of butter softens on a hot summer’s day, planets may change their structure over the billions of years they’ve been around. This process is called thermal evolution. Thermal evolution is important to consider because it may change how the planets react to the temperature on the above graph. Citing previous thermal evolution studies, the authors found that ice and rock, in fact, remain mixed despite years of temperature changes.

Thus, the final results show that in both the gradual distribution (latte) and core envelope (tootsie pop) models of ice-rich planets, ice and rock under high pressures will remain mixed for billions of years.

Importantly, this discovery could help inform future discoveries about ice-rich planets in our solar system and beyond. In fact, shortly after this paper was released, the National Academies of Sciences, Engineering, and Medicine urged NASA to schedule a mission to our largely unexplored, bright blue neighbor Uranus. Excitingly, this mission will check the many moons of Uranus, many of which may contain oceans. If it had oceans, it might have liquid water… and what does that mean? life!

Editing by Astrobit by Sabina Saginbayeva

Premium photo credit: https://recipes.timesofindia.com/us/beverage/non-alcoholic/mocha-slush/rs53975980.cms

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