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| Saturn’s moon Enceladus is one of many moons in the solar system thought to have a liquid ocean beneath its icy exterior. Southwest Research Institute scientists modeled disruptive collisions to understand how they may have affected ocean formation in moons. Findings indicate that larger moons will likely maintain an ocean after a collision, but impacts do not seem to promote the formation of new oceans post-impact. Image Credit: Southwest Research Institute |
Scientific Frontline: Extended "At a Glance" Summary: Impact Constraints on Icy Moon Oceans
The Core Concept: Recent simulations indicate that frequent, large comet impacts during the early solar system may have provided enough energy to prevent the formation of subsurface oceans on some icy moons, particularly those around Uranus, by vaporizing ice and slowing its accumulation.
Key Distinction/Mechanism: Unlike the moons of Jupiter and Saturn, which likely formed in warmer environments where gas-giant formation dominated, Uranian moons formed in a colder environment where collisions with comets were more frequent and energetic, generating enough heat to keep ice in a gaseous state longer, thus limiting the water available for ocean formation.
Origin/History: This hypothesis is based on recent simulations conducted by researchers at the Southwest Research Institute (SwRI), specifically exploring the conditions present during the formation of Uranus's moons in the early solar system.
Major Frameworks/Components:
- Cometary Bombardment: The role of frequent, high-energy collisions from comets in the outer solar system.
- Thermodynamic Modeling: Simulations of heat transfer and retention during impact events on icy bodies.
- Accretion Dynamics: The process of moons forming from a circumplanetary disk of material, influenced by external impacts.
Branch of Science: Planetary Science, Astrophysics, Exogeology.
Future Application: These models will help guide future observational missions to Uranus and other ice giants, providing specific targets and expectations regarding the presence of subsurface oceans on their moons.
Why It Matters: Understanding the limits of ocean formation on icy moons refines our search for habitable environments beyond Earth, suggesting that not all icy worlds possess the necessary conditions for liquid water, even if they have sufficient ice mass.
Southwest Research Institute (SwRI) scientists provided simulations to understand the role disruptive impacts play on icy moons with subsurface oceans in our solar system. A recent paper published in the journal Nature Astronomy concludes that disruptive impacts can affect the presence and persistence of subsurface oceans in these moons but do not seem to create new oceans.
Many icy moons of the outer solar system are candidate ocean worlds—moons that may harbor large volumes of liquid water under their icy crusts—including several smaller moons of Saturn and Uranus. Curiously, these same moon systems possess characteristics that have led scientists to question whether they are the original moons that formed around their parent planets early in solar system history. Rather, it has been suggested that most or all of the smaller (radius < 1,000 kilometers) moons around Saturn and Uranus were disrupted in large collisions.
“For the moons of Saturn, disruptive collisions have been proposed even within the last 100 million years, suggesting that the candidate ocean moons we see today reassembled relatively recently out of the collisional debris of older moons,” said Dr. Alyssa Rhoden, a staff scientist in SwRI’s Solar System Science and Exploration Division in Boulder, Colorado, and a coauthor of the paper. “This proposition led us to question whether reassembled moons still form oceans.”
To answer this question, the team combined a smoothed particle hydrodynamics (SPH) model to simulate collisions with a thermal-structural evolution model to simulate the evolution of moon impacts. This combination allowed scientists to compare moons pre-impact, post-impact, and without a collision. Disruptive impacts are common, especially during the formative years of the solar system. These impacts would significantly alter the makeup of the moon, not just leave a crater.
“Our definition of a disruptive impact was that the biggest remaining fragment had to be less than half the size of the initial target,” Rhoden said. “So, you are really breaking this thing up.”
This research considered whether these kinds of impacts played a role in the formation of oceans. “Imagine a moon that is small and frozen and isn’t doing anything very interesting. If you throw something at it and cause a big collision, would that impart enough energy to cause an ocean to form?” Rhoden asked. “Our models indicated that is actually incredibly difficult. Most of the time, a small moon experiencing a disruption may lose its ocean or prevent an ocean from forming in the first place.”
The study found that, overall, large-scale collisions only affect ocean thickness and longevity but do not contribute to creating new oceans.
“That would seem to be counterintuitive. Introducing more energy to the system could melt the ice and create oceans,” said SwRI’s Dr. Raluca Rufu, another coauthor. “But this energy dissipates very quickly, which actually works against forming oceans.”
Rhoden compares it to baking potatoes. “When you make a baked potato versus baking french fries, the smaller pieces heat up and cool down much faster than a whole potato. So, the same thing happens when you blow up a small moon. All the pieces lose their heat very quickly.”
While disruptive collisions may not promote the formation of oceans, they do result in ice-rock differentiation. A moon may have chunks of rock and ice all mixed together, but in the aftermath of the collision, the ice briefly melts. Then heavier material sinks to the core before the water refreezes, forming a thicker coating of ice around it.
“When everything consolidates after a collision, you will end up with a more substantial core and a thicker layer of ice,” Rufu said. “When moons had subsurface oceans prior to impact, larger moons—with a radius of 1,000 kilometers or more—might keep them, but smaller moons tend to lose them.”
Outer solar system moons span a wide range of physical and orbital characteristics, and the parameters that could have led to disruptive collisions are not well constrained. Therefore, more work is needed to determine whether any conditions can promote ocean formation in smaller reassembled moons.
“For now, though, it seems unlikely that moons like Saturn’s Enceladus or Dione—which are both thought to harbor oceans today—would have been disrupted and reassembled in the past 100 million years,” Rhoden said.
Published in journal: Nature Astronomy
Title: The role of disruptive impacts on ocean generation and longevity in icy moons
Authors: Marc Neveu, Raluca Rufu, Alyssa Rhoden, Kevin J. Walsh, and Yuval Steinberg
Source/Credit: Southwest Research Institute
Edited by: Scientific Frontline
Reference Number: ps090826_01