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Dark wounds on icy moons: Ganymede's subsurface ocean as a dark matter detector

William DeRocco

TL;DR

The paper investigates macroscopic dark matter in the mass range $10^{11}$-$10^{17}$ g as a detector using Ganymede, showing that high-velocity DM can deposit energy along a line, creating cylindrical boreholes; analytic scaling plus iSALE simulations reveal that DM impacts can expose deep subsurface material to the surface, potentially leaving compositionally distinct signatures that JUICE/MAJIS and RIME could detect; the work maps a large, previously unconstrained regime in DM parameter space and proposes observational tests, including exit wounds and depth-dependent compositional changes.

Abstract

Dark matter in the form of macroscopic composites is largely unconstrained at masses of $\sim 10^{11}- 10^{17}$ g. In this mass range, dark matter may collide with planetary bodies, depositing an immense amount of energy and leaving dramatic surface features that remain detectable on geological timescales. In this paper, we show that Ganymede, the largest Jovian moon, provides a prime target to search for dark matter impacts due to its differentiated composition and Gyr-old surface. We study the effects of dark matter collisions with Ganymede first with analytic estimates, finding that in a large region of parameter space, dark matter punches through Ganymede's conductive ice sheet, liberating sub-surface material. This sub-surface material may be compositionally different from the surface ice, providing a key observable with which to discriminate asteroid impacts from those caused by dark matter. We confirm our analytic estimates with dedicated simulations of dark matter impacts using iSALE, a multi-material impact code. We then discuss potential detection prospects with two missions currently en route to the Jovian system, Europa Clipper and JUICE, finding that these missions may have the ability not only to identify signs of life on the Galilean moons, but signs of dark matter as well.

Dark wounds on icy moons: Ganymede's subsurface ocean as a dark matter detector

TL;DR

The paper investigates macroscopic dark matter in the mass range - g as a detector using Ganymede, showing that high-velocity DM can deposit energy along a line, creating cylindrical boreholes; analytic scaling plus iSALE simulations reveal that DM impacts can expose deep subsurface material to the surface, potentially leaving compositionally distinct signatures that JUICE/MAJIS and RIME could detect; the work maps a large, previously unconstrained regime in DM parameter space and proposes observational tests, including exit wounds and depth-dependent compositional changes.

Abstract

Dark matter in the form of macroscopic composites is largely unconstrained at masses of g. In this mass range, dark matter may collide with planetary bodies, depositing an immense amount of energy and leaving dramatic surface features that remain detectable on geological timescales. In this paper, we show that Ganymede, the largest Jovian moon, provides a prime target to search for dark matter impacts due to its differentiated composition and Gyr-old surface. We study the effects of dark matter collisions with Ganymede first with analytic estimates, finding that in a large region of parameter space, dark matter punches through Ganymede's conductive ice sheet, liberating sub-surface material. This sub-surface material may be compositionally different from the surface ice, providing a key observable with which to discriminate asteroid impacts from those caused by dark matter. We confirm our analytic estimates with dedicated simulations of dark matter impacts using iSALE, a multi-material impact code. We then discuss potential detection prospects with two missions currently en route to the Jovian system, Europa Clipper and JUICE, finding that these missions may have the ability not only to identify signs of life on the Galilean moons, but signs of dark matter as well.

Paper Structure

This paper contains 10 sections, 14 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Ganymede
  3. Dark matter impacts
  4. Energy deposition and shock propagation
  5. Expansion and excavation flow
  6. Collapse and late-time material transport
  7. Simulations
  8. Results
  9. Discussion
  10. Conclusion

Figures (4)

  • Figure 1: The region in which dark matter impacts may leave detectable surface features on Ganymede. Gray regions correspond to existing constraints Sidhu2020Picker2025Dessert2022. The light purple region is the region in which the dark matter has sufficient initial energy to penetrate through the 12 km conductive ice layer on Ganymede's surface. The darker purple region corresponds to when the dark matter penetrates the entirety of Ganymede, leaving both an entrance and exit wound. Rough estimates for the approximate number of craters and crater radius from Eqs. \ref{['eq:nimpacts']} and \ref{['eq:cratersize']} are provided on the top and right axes respectively, rounded to one significant figure for readability. JUICE's global spectral mapping will enable the detection of features down to kilometer-scale over the majority of Ganymede's surface Tosi2024Review, providing the resolution and coverage necessary to identify craters of interest throughout the purple-shaded parameter space.
  • Figure 2: Peak pressure as a function of radius from impactor column. Blue dots show the measured values for a simulation of a 1-meter radius dark matter impactor impinging on Ganymede's surface. The red solid curve shows a best-fit power-law to the data while the gray dotted curve shows the prediction from the analytic scaling arguments of Sec. \ref{['sec:impacts']}, indicating good correspondence.
  • Figure 3: Primary results from simulations. The three panels show the evolution of a 1-meter radius dark matter impactor colliding with Ganymede at three times. At $t = 1$ s (leftmost panel), the shock has formed and propagated $\approx 3$ km, leaving behind a transient borehole of radius $\approx 0.3$ km. At $t = 11.42$ s (middle panel), the borehole has begun to collapse from the bottom up, forming a jet of subsurface material. At $t=64.52$ s (rightmost panel), this jet reaches the surface and escapes, potentially leading to observable compositional differences in the final relaxed crater.
  • Figure 4: Simulation results for impactors of varying size at corresponding times post-impact. The results show that both the transient cavity radius and timescale for formation scale approximately linearly with the radius of the impactor. Note the changing scale on the axes between the panels.