Constraints on Mimas' interior from Cassini ISS libration measurements

Resonant capture of Saturn's arcs

Obtained images from the Cassini spacecraft have unveiled several small satellites in the vicinity of Mimas' orbit: Aegaeon, Methone and Anthe. Methone and Anthe are situated within arcs of material, while Aegaeon orbits within an arc near the inner edge of Saturn's G ring. The presence of these arcs and moons is consistent with their confinement by corotation eccentric resonance (CER) with Mimas. Aegaeon is captured in an inner 7 : 6 mean motion resonance with Mimas, while Methone and Anthe are, respectively, captured in outer 14 : 15 and 10 : 11 resonances with Mimas. Here, we estimate the probabilities of capturing these small satellites into their respective CERs with Mimas as the orbit of the latter evolves through tidal effects. We will discuss the potential implications of this work, in particular the constraints it may provide on Mimas' orbital evolution.This article is part of the theme issue 'Major advances in planetary sciences thanks to stellar occultations'.

Tidal Deformation and Dissipation Processes in Icy Worlds

Tidal interactions play a key role in the dynamics and evolution of icy worlds. The intense tectonic activity of Europa and the eruption activity on Enceladus are clear examples of the manifestation of tidal deformation and associated dissipation. While tidal heating has long been recognized as a major driver in the activity of these icy worlds, the mechanism controlling how tidal forces deform the different internal layers and produce heat by tidal friction still remains poorly constrained. As tidal forcing varies with orbital characteristics (distance to the central planet, eccentricity, obliquity), the contribution of tidal heating to the internal heat budget can strongly change over geological timescales. In some circumstances, the tidally-produced heat can result in internal melting and surface activity taking various forms. Even in the absence of significant heat production, tidal deformation can be used to probe the interior structure, the tidal response of icy moons being strongly sensitive to their hydrosphere structure. In the present paper, we review the methods to compute tidal deformation and dissipation in the different layers composing icy worlds. After summarizing the main principle of tidal deformation and the different rheological models used to model visco-elastic tidal response, we describe the dissipation processes expected in rock-dominated cores, subsurface oceans and icy shells and highlight the potential effects of tidal heating in terms of thermal evolution and activity. We finally anticipate how data collected by future missions to Jupiter's and Saturn's moons could be used to constrain their tidal response and the consequences for past and present activities.

Geologic Constraints on the Formation and Evolution of Saturn's Mid-Sized Moons

Saturn's mid-sized icy moons have complex relationships with Saturn's interior, the rings, and with each other, which can be expressed in their shapes, interiors, and geology. Observations of their physical states can, thus, provide important constraints on the ages and formation mechanism(s) of the moons, which in turn informs our understanding of the formation and evolution of Saturn and its rings. Here, we describe the cratering records of the mid-sized moons and the value and limitations of their use for constraining the histories of the moons. We also discuss observational constraints on the interior structures of the moons and geologically-derived inferences on their thermal budgets through time. Overall, the geologic records of the moons (with the exception of Mimas) include evidence of epochs of high heat flows, short- and long-lived subsurface oceans, extensional tectonics, and considerable cratering. Curiously, Mimas presents no clear evidence of an ocean within its surface geology, but its rotation and orbit indicate a present-day ocean. While the moons need not be primordial to produce the observed levels of interior evolution and geologic activity, there is likely a minimum age associated with their development that has yet to be determined. Uncertainties in the populations impacting the moons makes it challenging to further constrain their formation timeframes using craters, whereas the characteristics of their cores and other geologic inferences of their thermal evolutions may help narrow down their potential histories. Disruptive collisions may have also played an important role in the formation and evolution of Saturn's mid-sized moons, and even the rings of Saturn, although more sophisticated modeling is needed to determine the collision conditions that produce rings and moons that fit the observational constraints. Overall, the existence and physical characteristics of Saturn's mid-sized moons provide critical benchmarks for the development of formation theories.

Long-Term Evolution of the Saturnian System

Here we present the current state of knowledge on the long-term evolution of Saturn's moon system due to tides within Saturn. First we provide some background on tidal evolution, orbital resonances and satellite tides. Then we address in detail some of the present and past orbital resonances between Saturn's moons (including the Enceladus-Dione and Titan-Hyperion resonances) and what they can tell us about the evolution of the system. We also present the current state of knowledge on the spin-axis dynamics of Saturn: we discuss arguments for a (past or current) secular resonance of Saturn's spin precession with planetary orbits, and explain the links of this resonance to the tidal evolution of Titan and a possible recent cataclysm in the Saturnian system. We also address how the moons' orbital evolution, including resonances, affects the evolution of their interiors. Finally, we summarize the state of knowledge about the Saturnian system's long-term evolution and discuss prospects for future progress.

A recently formed ocean inside Saturn's moon Mimas

Moons potentially harbouring a global ocean are tending to become relatively common objects in the Solar System1. The presence of these long-lived global oceans is generally betrayed by surface modification owing to internal dynamics2. Hence, Mimas would be the most unlikely place to look for the presence of a global ocean3. Here, from detailed analysis of Mimas's orbital motion based on Cassini data, with a particular focus on Mimas's periapsis drift, we show that its heavily cratered icy shell hides a global ocean, at a depth of 20-30 kilometres. Eccentricity damping implies that the ocean is likely to be less than 25 million years old and still evolving. Our simulations show that the ocean-ice interface reached a depth of less than 30 kilometres only recently (less than 2-3 million years ago), a time span too short for signs of activity at Mimas's surface to have appeared.

Extremophiles in Earth's Deep Seas: A View Toward Life in Exo-Oceans

Humanity's search for extraterrestrial life is a modern manifestation of the exploratory and curious nature that has led us through millennia of scientific discoveries. With the ongoing exploration of extraterrestrial bodies, the potential for discovery of extraterrestrial life has expanded. We may better inform this search through an understanding of how life persists and flourishes on Earth in a myriad of environmental extremes. A significant proportion of our knowledge of extremophiles on Earth comes from studies on deep ocean life. Here, we review and synthesize the range of environmental extremes observed in the deep sea, the life that persists in these extreme conditions, and the biological adaptations utilized by these remarkable life-forms. We also review confirmed and predicted extraterrestrial oceans in our solar system and propose deep-sea sites that may serve as planetary field analog environments. We show that the clever ingenuity of evolution under deep-sea conditions suggests that the plausibility of extraterrestrial life is much greater than previously thought.

The NASA Roadmap to Ocean Worlds

In this article, we summarize the work of the NASA Outer Planets Assessment Group (OPAG) Roadmaps to Ocean Worlds (ROW) group. The aim of this group is to assemble the scientific framework that will guide the exploration of ocean worlds, and to identify and prioritize science objectives for ocean worlds over the next several decades. The overarching goal of an Ocean Worlds exploration program as defined by ROW is to "identify ocean worlds, characterize their oceans, evaluate their habitability, search for life, and ultimately understand any life we find." The ROW team supports the creation of an exploration program that studies the full spectrum of ocean worlds, that is, not just the exploration of known ocean worlds such as Europa but candidate ocean worlds such as Triton as well. The ROW team finds that the confirmed ocean worlds Enceladus, Titan, and Europa are the highest priority bodies to target in the near term to address ROW goals. Triton is the highest priority candidate ocean world to target in the near term. A major finding of this study is that, to map out a coherent Ocean Worlds Program, significant input is required from studies here on Earth; rigorous Research and Analysis studies are called for to enable some future ocean worlds missions to be thoughtfully planned and undertaken. A second finding is that progress needs to be made in the area of collaborations between Earth ocean scientists and extraterrestrial ocean scientists.

Evolution of Saturn's Mid-Sized Moons

The orbits of Saturn's inner mid-sized moons (Mimas, Enceladus, Tethys, Dione, and Rhea) have been notably difficult to reconcile with their geology. Here, we present numerical simulations coupling thermal, geophysical, and simplified orbital evolution for 4.5 billion years that reproduce observed characteristics of their orbits and interiors, provided that the outer four moons are old. Tidal dissipation within Saturn expands the moons' orbits over time. Dissipation within the moons decreases their eccentricities, which are episodically increased by moon-moon interactions, causing past or present oceans in the interior of Enceladus, Dione, and Tethys. In contrast, Mimas' proximity to Saturn's rings generates interactions that cause such rapid orbital expansion that Mimas must have formed only 0.1-1 Gyr ago if it postdates the rings. The resulting lack of radionuclides keeps it geologically inactive. These simulations can explain the Mimas-Enceladus dichotomy, reconcile the moons' orbital properties and geological diversity, and self-consistently produce a recent ocean on Enceladus.

Assessing the Ecophysiology of Methanogens in the Context of Recent Astrobiological and Planetological Studies

Among all known microbes capable of thriving under extreme and, therefore, potentially extraterrestrial environmental conditions, methanogens from the domain Archaea are intriguing organisms. This is due to their broad metabolic versatility, enormous diversity, and ability to grow under extreme environmental conditions. Several studies revealed that growth conditions of methanogens are compatible with environmental conditions on extraterrestrial bodies throughout the Solar System. Hence, life in the Solar System might not be limited to the classical habitable zone. In this contribution we assess the main ecophysiological characteristics of methanogens and compare these to the environmental conditions of putative habitats in the Solar System, in particular Mars and icy moons. Eventually, we give an outlook on the feasibility and the necessity of future astrobiological studies concerning methanogens.