Manannán (Crater)

Manannán is one of the largest and most geophysically diagnostic impact craters on Jupiter's moon Europa, centered near 3.1° N, 239.7° W on the trailing hemisphere. Named after Manannán mac Lir, the Celtic deity of the sea and the Isle of Man, the impact structure spans approximately 30 km in rim diameter (with an inner depression roughly 22 km wide) and was imaged at high spatial resolution (~20 to 220 m per pixel) by the Solid State Imaging (SSI) camera and Near-Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft during the E14 and E15 flybys in 1998.

On an active ocean world whose young icy surface (mean crater retention age of 40 to 90 million years) contains fewer than 40 impact craters larger than 10 km in diameter, Manannán provides a critical boundary condition on the thickness and thermal profile of Europa's outer Ice I_h lithosphere. Unlike smaller bowl-shaped craters (< 10 km) or the classic central-peak crater Pwyll (26 km), Manannán has lost its raised topographic rim and deep cavity floor, exhibiting instead a flattened, disrupted interior occupied by a central "spider-like" radial trough network and smooth, reddish-brown cryolava plains formed during refreezing of a deep subsurface impact melt pocket.

Absence of a Raised Rim and Viscous Relaxation

In standard planetary impact mechanics on cold, rigid icy satellites such as Callisto or Rhea, a 30-km impact crater forms a steep terrace-walled basin 1.0 to 1.5 km deep surrounded by a continuous raised rim standing 300 to 500 m above the surrounding plains, with a prominent central peak uplifted from the basin floor. At Manannán, however, stereo photoclinometry and shadow measurements show that total topographic relief across the 22-to-30-km structure rarely exceeds 100 to 200 m, with the interior floor sitting nearly flush with the surrounding ridged plains and lacking both a coherent raised rim and a topographic central peak (Moore et al., 2001; Schenk, 2002).

In comparative studies of Europan impact structures (Schenk, 2002), Manannán sits squarely at a sharp morphological transition occurring at crater diameters between 24 and 32 km:

• Craters D < 24 km (e.g., Cilix, 15 km; Pwyll, 26 km): Retain conventional raised rims, distinct ejecta blankets, and central peaks, indicating excavation entirely within a cold, brittle-elastic conductive upper ice lid.
• Craters D ≈ 28–32 km (Manannán, 30 km): Exhibit collapsed rims, flat hummocky floors, and smooth refrozen melt ponds without concentric multi-ring graben, indicating that the transient crater cavity (reaching depths of ~6 to 8 km during impact) penetrated down into warm, ductile convective ice with an effective viscosity of η ≈ 10¹³ to 10¹⁵ Pa s.
• Multi-Ring Structures D > 35 km (Tyre, 44 km; Callanish, 33 km): Feature concentric annular graben systems formed when the transient cavity breached completely through the ~15-to-25-km ice shell into the underlying global liquid water ocean.

Cryogenic Melt-Lens Refreezing and Volume Expansion

Inside the central 10-km zone of Manannán, Galileo SSI images resolved a unique stellate system of radiating and concentric extensional fractures—informally termed the "Manannán spider"—centered on an irregular circular trough ~3.5 km wide, surrounded by smooth, low-albedo lobate plains that embay and flood pre-existing double ridges. Numerical hydrocode simulations using the iSALE shock physics code coupled with conductive-convective heat transfer models (Cox & Bauer, 2015; Steinbrügge et al., 2020) explain this central spider and smooth pond network via the thermodynamics of impact melt-pocket refreezing.

When the ~1.5-to-2.0-km projectile struck Europa at ~20–26 km s⁻¹, the shock wave excavated the brittle upper lid and deposited intense thermal energy into the warm ductile ice layer at 4 to 7 km depth. Because the target ice at that depth was already near its melting point (250–265 K) due to tidal heating, the impact generated a coalesced subsurface lens of liquid water (H₂O) and brine measuring 5 to 10 km wide and 1 to 3 km thick directly beneath the crater center.

Unlike silicate impact melts on rocky planets, which shrink by ~8–10% as they crystallize, liquid water expands by ~9% in volume when it freezes into hexagonal Ice I_h. As the Manannán subsurface melt pocket cooled from the top and sides over a period of 10³ to 10⁴ years, the growing ice shell trapped the residual liquid water lens in a closed reservoir. Progressive crystallization generated hydrostatic overpressures exceeding 10 to 30 MPa—far surpassing the ~1 to 3 MPa tensile strength of fractured Europan surface ice.

As documented by Steinbrügge et al. (2020), this cryostatic overpressurization fractured the overlying central floor in a radial spider pattern and drove late-stage cryovolcanic effusions of pressurized brines onto the surface, forming the smooth, dark lobate plains observed around the central depression. Once the pressurized brine erupted or drained downward into the porous convective sub-layer, the roof of the depleted melt chamber subsided to create the central ring trough alongside pitted secondary features.

Spectroscopic mapping by the Galileo NIMS instrument (1.0 to 5.2 μm) and visible color ratios show that the smooth lobate melt deposits and radial spider fractures inside Manannán—as well as its discontinuous ejecta rays—exhibit distorted water-ice hydration bands at 1.5 and 2.0 μm alongside a strong reddish-brown visible absorption slope. Radiative transfer unmixing indicates that these impact melt extrusions are enriched in hydrated magnesium sulfates (MgSO₄•nH₂O), sulfuric acid hydrate (H₂SO₄•nH₂O), and irradiated sodium chloride (NaCl) brines concentrated during fractional freezing of the impact melt pocket.

Because Manannán preserves solidified samples of mid-crustal brines brought to the surface without requiring full ocean melt-through, it is a high-priority reconnaissance target for NASA's Europa Clipper mission and ESA's JUICE spacecraft. During close flybys in the 2030s, the REASON dual-frequency ice-penetrating radar (9 MHz and 60 MHz) aboard Europa Clipper will sound beneath Manannán to image the frozen dielectric root of the fossil melt chamber and test whether brine pockets remain trapped within the underlying convective ice shell.

Why is Manannán crater almost completely flat instead of deep and bowl-shaped?

When the impactor that formed Manannán excavated 6 to 8 kilometers into Europa's crust, it punched through the cold, rigid upper surface layer into warmer, softer convective ice deeper down. Because warm ice flows like a slow glacier under gravity and because the impact melted a large pocket of liquid water beneath the crater floor, the crater walls and rim slumped inward and relaxed, leaving less than 200 meters of vertical relief.

What formed the "spider" fracture pattern in the middle of Manannán?

The radial, spider-like network of troughs at the center of Manannán formed when a buried pocket of liquid water (melted by the heat of the impact) slowly froze back into solid ice over thousands of years. Because water expands by about 9% when it freezes, the trapped liquid pocket built up intense internal pressure that cracked the overlying ice crust into radial fractures and squeezed salty slush out onto the crater floor.

What does Manannán tell scientists about the thickness of Europa's ice shell?

By comparing Manannán (30 km wide, which reached warm ductile ice but did not punch all the way into the ocean) with larger multi-ring craters like Tyre (44 km wide, which broke completely through into liquid water), planetary geophysicists can bracket the total thickness of Europa's outer ice shell at the time of those impacts to approximately 15 to 25 kilometers.

1. Moore, J. M., Asphaug, E., Morrison, D., Spencer, J. R., Chapman, C. R., Bierhaus, B., Sullivan, R. J., Chuang, F. C., Klemaszewski, J. E., Greeley, R., Bender, K. C., Geissler, P. E., Helfenstein, P., & Pilcher, C. B. (2001). "Impact features on Europa: Results of the Galileo Europa Mission (GEM)." Icarus, 151(1), 93–111. https://doi.org/10.1006/icar.2000.6558
2. Schenk, P. M. (2002). "Thickness constraints on the icy shells of the galilean satellites from a comparison of crater shapes." Nature, 417(6887), 419–421. https://doi.org/10.1038/417419a
3. Cox, R., & Bauer, A. W. (2015). "Impact breaching of Europa's ice: Constraints from numerical modeling." Journal of Geophysical Research: Planets, 120(10), 1708–1719. https://doi.org/10.1002/2015JE004877
4. Steinbrügge, G., Voigt, J. R. C., Wolfenbarger, N. S., Hamilton, C. W., Soderlund, K. M., Young, D. A., Blankenship, D. D., Vance, S. D., & Schroeder, D. M. (2020). "Brine Migration and Impact Melt Resurfacing at Manannán Crater on Europa." Geophysical Research Letters, 47(21), e2020GL090797. https://doi.org/10.1029/2020GL090797