On the surface of many of the icy moons in our solar system, scientists have documented strike-slip faults, those that occur when fault walls in the ground's crust move past one another sideways, as is the case at the San Andreas fault in California. Two recently published studies led by University of Hawaiʻi at Mānoa earth and space scientists document and reveal the mechanisms behind these geologic features on the largest moon of Saturn, Titan, and Jupiter's largest moon, Ganymede.
Conducting these types of geologic investigations prior to launch and arrival of space exploration missions, helps identify interesting locations for lander exploration and maximizes what can be learned from extraterrestrial icy moons.
"We are interested in studying these features on icy moons because that type of faulting can facilitate the exchange of surface and subsurface materials through heating processes, potentially creating environments conducive for the emergence of life," said Liliane Burkhard, lead author of the studies and research affiliate at the Hawaiʻi Institute of Geophysics and Planetology in the UH Mānoa School of Ocean and Earth Science and Technology.
When an icy moon moves around its parent planet, the gravity of the planet can exert tidal forces. Rather than creating high and low tides as in Earth's ocean, on an icy moon, the tidal pull puts stress on the icy surface and can drive geologic activity such as strike-slip faulting.
Titan, a frozen ocean world
The extremely cold temperatures on the surface of Titan mean that water ice acts as rock that can crack, fault, and deform. Evidence from the Cassini spacecraft suggests that tens of miles below the frozen surface, there is a liquid water ocean. Further, Titan is the only moon in our solar system with a dense atmosphere, which, uniquely, supports an Earth-like hydrological cycle of methane clouds, rain, and liquid flowing across the surface to fill lakes and seas, placing it among a handful of worlds that could potentially contain habitable environments.
The NASA Dragonfly mission will launch in 2027, with a planned arrival on Titan in 2034. The novel rotorcraft lander will conduct several flights on the surface, exploring a variety of locations to search for the building blocks and signs of life.
In their investigation of the Selk crater area on Titan, the designated initial landing site for the Dragonfly mission, Burkhard and her co-author explored the potential for strike-slip faulting. To do this, they calculated the stress that would be exerted on Titan's surface due to tidal forces as the moon orbits Saturn and tested the possibility of faulting by examining various characteristics of the frozen ground.
"Our prior research indicated that certain areas on Titan might currently undergo deformation due to tidal stresses. However, the conditions we've determined to be necessary for strike-slip fault displacement appear to be unlikely in the Selk crater region," said Burkhard. "Consequently, it's safe to infer that Dragonfly won't be landing in a strike-slip ditch!"
In a second recently published study, Burkhard and her colleagues also examined the geology of Ganymede, the largest moon of Jupiter and larger than planet Mercury, to investigate its history with tidal stress. In particular, the team looked at a region called Nippur Philus Sulci, where new terrain overlays older terrain. During the investigation of intermediate aged and younger parts of this area, the team found the direction of their slip features to have different alignments. This suggests that features in the youngest terrains may have formed through processes other than high tidal stress.
"We can conclude that Ganymede has had a tidal 'midlife crisis,' but it's youngest 'crisis' remains enigmatic," Burkhard added.
