Investigating Europa's Habitability with the Europa Clipper

The habitability of Europa is a property within a system, which is driven by a multitude of physical and chemical processes and is defined by many interdependent parameters, so that its full characterization requires collaborative investigation. To explore Europa as an integrated system to yield a complete picture of its habitability, the Europa Clipper mission has three primary science objectives: (1) characterize the ice shell and ocean including their heterogeneity, properties, and the nature of surface-ice-ocean exchange; (2) characterize Europa's composition including any non-ice materials on the surface and in the atmosphere, and any carbon-containing compounds; and (3) characterize Europa's geology including surface features and localities of high science interest. The mission will also address several cross-cutting science topics including the search for any current or recent activity in the form of thermal anomalies and plumes, performing geodetic and radiation measurements, and assessing high-resolution, co-located observations at select sites to provide reconnaissance for a potential future landed mission. Synthesizing the mission's science measurements, as well as incorporating remote observations by Earth-based observatories, the James Webb Space Telescope, and other space-based resources, to constrain Europa's habitability, is a complex task and is guided by the mission's Habitability Assessment Board (HAB).

High atmospheric metal enrichment for a Saturn-mass planet

Atmospheric metal enrichment (i.e., elements heavier than helium, also called "metallicity") is a key diagnostic of the formation of giant planets^1-3. The giant planets of the solar system exhibit an inverse relationship between mass and both their bulk metallicities and atmospheric metallicities. Extrasolar giant planets also display an inverse relationship between mass and bulk metallicity⁴. However, there is significant scatter in the relationship and it is not known how atmospheric metallicity correlates with either planet mass or bulk metallicity. Here we show that the Saturn-mass exoplanet HD 149026b^5-9 has an atmospheric metallicity 59 - 276 times solar (at 1σ), which is greater than Saturn's atmospheric metallicity of ~7.5 times solar¹⁰ at >4σ confidence. This result is based on modeling CO₂ and H₂O absorption features in the thermal emission spectrum of the planet measured by JWST. HD 149026b is the most metal-rich giant planet known, with an estimated bulk heavy element abundance of 66±2% by mass^11,12. We find that the atmospheric metallicities of both HD 149026b and the solar system giant planets are more correlated with bulk metallicity than planet mass.

Juno's Close Encounter With Ganymede-An Overview

The Juno spacecraft has been in orbit around Jupiter since 2016. Two flybys of Ganymede were executed in 2021, opportunities realized by evolution of Juno's polar orbit over the intervening 5 years. The geometry of the close flyby just prior to the 34th perijove pass by Jupiter brought the spacecraft inside Ganymede's unique magnetosphere. Juno's payload, designed to study Jupiter's magnetosphere, had ample dynamic range to study Ganymede's magnetosphere. The Juno radio system was used both for gravity measurements and for study of Ganymede's ionosphere. Remote sensing of Ganymede returned new results on geology, surface composition, and thermal properties of the surface and subsurface.

Understanding Hypervelocity Sampling of Biosignatures in Space Missions

The atomic-scale fragmentation processes involved in molecules undergoing hypervelocity impacts (HVIs; defined as >3 km/s) are challenging to investigate via experiments and still not well understood. This is particularly relevant for the consistency of biosignals from small-molecular-weight neutral organic molecules obtained during solar system robotic missions sampling atmospheres and plumes at hypervelocities. Experimental measurements to replicate HVI effects on neutral molecules are challenging, both in terms of accelerating uncharged species and isolating the multiple transition states over very rapid timescales (<1 ps). Nonequilibrium first-principles-based simulations extend the range of what is possible with experiments. We report on high-fidelity simulations of the fragmentation of small organic biosignature molecules over the range v = 1-12 km/s, and demonstrate that the fragmentation fraction is a sensitive function of velocity, impact angle, molecular structure, impact surface material, and the presence of surrounding ice shells. Furthermore, we generate interpretable fragmentation pathways and spectra for velocity values above the fragmentation thresholds and reveal how organic molecules encased in ice grains, as would likely be the case for those in "ocean worlds," are preserved at even higher velocities than bare molecules. Our results place ideal spacecraft encounter velocities between 3 and 5 km/s for bare amino and fatty acids and within 4-6 km/s for the same species encased in ice grains and predict the onset of organic fragmentation in ice grains at >5 km/s, both consistent with recent experiments exploring HVI effects using impact-induced ionization and analysis via mass spectrometry and from the analysis of Enceladus organics in Cassini Data. From nanometer-sized ice Ih clusters, we establish that HVI energy is dissipated by ice casings through thermal resistance to the impact shock wave and that an upper fragmentation velocity limit exists at which ultimately any organic contents will be cleaved by the surrounding ice-this provides a fundamental path to characterize micrometer-sized ice grains. Altogether, these results provide quantifiable insights to bracket future instrument design and mission parameters.

Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus

Saturn’s main ring system is associated with a set of small moons that either are embedded within it or interact with the rings to alter their shape and composition. Five close flybys of the moons Pan, Daphnis, Atlas, Pandora, and Epimetheus were performed between December 2016 and April 2017 during the ring-grazing orbits of the Cassini mission. Data on the moons’ morphology, structure, particle environment, and composition were returned, along with images in the ultraviolet and thermal infrared. We find that the optical properties of the moons’ surfaces are determined by two competing processes: contamination by a red material formed in Saturn’s main ring system and accretion of bright icy particles or water vapor from volcanic plumes originating on the moon Enceladus.

Prevalent lightning sferics at 600 megahertz near Jupiter's poles

Lightning has been detected on Jupiter by all visiting spacecraft through night-side optical imaging and whistler (lightning-generated radio waves) signatures^1-6. Jovian lightning is thought to be generated in the mixed-phase (liquid-ice) region of convective water clouds through a charge-separation process between condensed liquid water and water-ice particles, similar to that of terrestrial (cloud-to-cloud) lightning^7-9. Unlike terrestrial lightning, which emits broadly over the radio spectrum up to gigahertz frequencies^10,11, lightning on Jupiter has been detected only at kilohertz frequencies, despite a search for signals in the megahertz range ¹² . Strong ionospheric attenuation or a lightning discharge much slower than that on Earth have been suggested as possible explanations for this discrepancy^13,14. Here we report observations of Jovian lightning sferics (broadband electromagnetic impulses) at 600 megahertz from the Microwave Radiometer ¹⁵ onboard the Juno spacecraft. These detections imply that Jovian lightning discharges are not distinct from terrestrial lightning, as previously thought. In the first eight orbits of Juno, we detected 377 lightning sferics from pole to pole. We found lightning to be prevalent in the polar regions, absent near the equator, and most frequent in the northern hemisphere, at latitudes higher than 40 degrees north. Because the distribution of lightning is a proxy for moist convective activity, which is thought to be an important source of outward energy transport from the interior of the planet^16,17, increased convection towards the poles could indicate an outward internal heat flux that is preferentially weighted towards the poles^9,16,18. The distribution of moist convection is important for understanding the composition, general circulation and energy transport on Jupiter.

Jupiter's interior and deep atmosphere: The initial pole-to-pole passes with the Juno spacecraft

On 27 August 2016, the Juno spacecraft acquired science observations of Jupiter, passing less than 5000 kilometers above the equatorial cloud tops. Images of Jupiter's poles show a chaotic scene, unlike Saturn's poles. Microwave sounding reveals weather features at pressures deeper than 100 bars, dominated by an ammonia-rich, narrow low-latitude plume resembling a deeper, wider version of Earth's Hadley cell. Near-infrared mapping reveals the relative humidity within prominent downwelling regions. Juno's measured gravity field differs substantially from the last available estimate and is one order of magnitude more precise. This has implications for the distribution of heavy elements in the interior, including the existence and mass of Jupiter's core. The observed magnetic field exhibits smaller spatial variations than expected, indicative of a rich harmonic content.

Alluvial and fluvial fans on Saturn's moon Titan reveal processes, materials and regional geology

Fans, landforms that record the storage and transport of sediment from uplands to depositional basins, are found on Saturn's moon Titan, a body of significantly different process rates and material compositions from Earth. Images obtained by the Cassini spacecraft's synthetic aperture radar reveal morphologies, roughness, textural patterns and other properties consistent with fan analogues on Earth also viewed by synthetic aperture radar. The observed fan characteristics on Titan reveal some regions of high relative relief and others with gentle slopes over hundreds of kilometres, exposing topographic variations and influences on fan formation. There is evidence for a range of particle sizes across proximal to distal fan regions, from c. 2 cm or more to fine-grained, which can provide details on sedimentary processes. Some features are best described as alluvial fans, which implies their proximity to high-relief source areas, while others are more likely to be fluvial fans, drawing from larger catchment areas and frequently characterized by more prolonged runoff events. The presence of fans corroborates the vast liquid storage capacity of the atmosphere and the resultant episodic behaviour. Fans join the growing list of landforms on Titan derived from atmospheric and fluvial processes similar to those on Earth, strengthening comparisons between these two planetary bodies.

Membrane alternatives in worlds without oxygen: Creation of an azotosome

The lipid bilayer membrane, which is the foundation of life on Earth, is not viable outside of biology based on liquid water. This fact has caused astronomers who seek conditions suitable for life to search for exoplanets within the "habitable zone," the narrow band in which liquid water can exist. However, can cell membranes be created and function at temperatures far below those at which water is a liquid? We take a step toward answering this question by proposing a new type of membrane, composed of small organic nitrogen compounds, that is capable of forming and functioning in liquid methane at cryogenic temperatures. Using molecular simulations, we demonstrate that these membranes in cryogenic solvent have an elasticity equal to that of lipid bilayers in water at room temperature. As a proof of concept, we also demonstrate that stable cryogenic membranes could arise from compounds observed in the atmosphere of Saturn's moon, Titan, known for the existence of seas of liquid methane on its surface.

PROTOSOLAR AMMONIA AS THE UNIQUE SOURCE OF TITAN's NITROGEN

The origin of Titan's nitrogen-rich atmosphere is thought to be ammonia ice, but this has not yet been confirmed. Furthermore, it is uncertain whether the building blocks of Titan formed within the Saturnian subnebula or in the colder protosolar nebula (PSN). Recent measurements of the nitrogen isotope ratio in cometary ammonia, combined with evolutionary constraints on the nitrogen isotopes in Titan's atmosphere provide firm evidence that the nitrogen in Titan's atmosphere must have originated as ammonia ice formed in the PSN under conditions similar to that of cometary formation. This result has important implications for the projected D/H ratio in cometary methane, nitrogen isotopic fractionation in the PSN and the source of nitrogen for Earth's atmosphere.

The far future of exoplanet direct characterization

We describe future steps in the direct characterization of habitable exoplanets subsequent to medium and large mission projects currently underway and investigate the benefits of spectroscopic and direct imaging approaches. We show that, after third- and fourth-generation missions have been conducted over the course of the next 100 years, a significant amount of time will lapse before we will have the capability to observe directly the morphology of extrasolar organisms.

Dynamical habitability of planetary systems

The problem of the stability of planetary systems, a question that concerns only multiplanetary systems that host at least two planets, is discussed. The problem of mean motion resonances is addressed prior to discussion of the dynamical structure of the more than 350 known planets. The difference with regard to our own Solar System with eight planets on low eccentricity is evident in that 60% of the known extrasolar planets have orbits with eccentricity e > 0.2. We theoretically highlight the studies concerning possible terrestrial planets in systems with a Jupiter-like planet. We emphasize that an orbit of a particular nature only will keep a planet within the habitable zone around a host star with respect to the semimajor axis and its eccentricity. In addition, some results are given for individual systems (e.g., Gl777A) with regard to the stability of orbits within habitable zones. We also review what is known about the orbits of planets in double-star systems around only one component (e.g., gamma Cephei) and around both stars (e.g., eclipsing binaries).

Co-evolution of atmospheres, life, and climate

After Earth's origin, our host star, the Sun, was shining 20-25% less brightly than today. Without greenhouse-like conditions to warm the atmosphere, our early planet would have been an ice ball, and life may never have evolved. But life did evolve, which indicates that greenhouse gases must have been present on early Earth to warm the planet. Evidence from the geological record indicates an abundance of the greenhouse gas CO(2). CH(4) was probably present as well; and, in this regard, methanogenic bacteria, which belong to a diverse group of anaerobic prokaryotes that ferment CO(2) plus H(2) to CH(4), may have contributed to modification of the early atmosphere. Molecular oxygen was not present, as is indicated by the study of rocks from that era, which contain iron carbonate rather than iron oxide. Multicellular organisms originated as cells within colonies that became increasingly specialized. The development of photosynthesis allowed the Sun's energy to be harvested directly by life-forms. The resultant oxygen accumulated in the atmosphere and formed the ozone layer in the upper atmosphere. Aided by the absorption of harmful UV radiation in the ozone layer, life colonized Earth's surface. Our own planet is a very good example of how life-forms modified the atmosphere over the planets' lifetime. We show that these facts have to be taken into account when we discover and characterize atmospheres of Earth-like exoplanets. If life has originated and evolved on a planet, then it should be expected that a strong co-evolution occurred between life and the atmosphere, the result of which is the planet's climate.

Origin and formation of planetary systems

To estimate the occurrence of terrestrial exoplanets and maximize the chance of finding them, it is crucial to understand the formation of planetary systems in general and that of terrestrial planets in particular. We show that a reliable formation theory should not only explain the formation of the Solar System, with small terrestrial planets within a few AU and gas giants farther out, but also the newly discovered exoplanetary systems with close-in giant planets. Regarding the presently known exoplanets, we stress that our current knowledge is strongly biased by the sensitivity limits of current detection techniques (mainly the radial velocity method). With time and improved detection methods, the diversity of planets and orbits in exoplanetary systems will definitely increase and help to constrain the formation theory further. In this work, we review the latest state of planetary formation in relation to the origin and evolution of habitable terrestrial planets.

Deciphering spectral fingerprints of habitable exoplanets

We discuss how to read a planet's spectrum to assess its habitability and search for the signatures of a biosphere. After a decade rich in giant exoplanet detections, observation techniques have advanced to a level where we now have the capability to find planets of less than 10 Earth masses (M(Earth)) (so-called "super Earths"), which may be habitable. How can we characterize those planets and assess whether they are habitable? This new field of exoplanet search has shown an extraordinary capacity to combine research in astrophysics, chemistry, biology, and geophysics into a new and exciting interdisciplinary approach to understanding our place in the Universe. The results of a first-generation mission will most likely generate an amazing scope of diverse planets that will set planet formation, evolution, and our planet into an overall context.

Stellar aspects of habitability--characterizing target stars for terrestrial planet-finding missions

We present and discuss the criteria for selecting potential target stars suitable for the search for Earth-like planets, with a special emphasis on the stellar aspects of habitability. Missions that search for terrestrial exoplanets will explore the presence and habitability of Earth-like exoplanets around several hundred nearby stars, mainly F, G, K, and M stars. The evaluation of the list of potential target systems is essential in order to develop mission concepts for a search for terrestrial exoplanets. Using the Darwin All Sky Star Catalogue (DASSC), we discuss the selection criteria, configuration-dependent subcatalogues, and the implication of stellar activity for habitability.

A roadmap for the detection and characterization of other Earths

The European Space Agency and other space agencies such as NASA recognize that the question with regard to life beyond Earth in general, and the associated issue of the existence and study of exoplanets in particular, is of paramount importance for the 21(st) century. The new Cosmic Vision science plan, Cosmic Vision 2015-2025, which is built around four major themes, has as its first theme: "What are the conditions for planet formation and the emergence of life?" This main theme is addressed through further questions: 1) How do gas and dust give rise to stars and planets? 2) How will the search for and study of exoplanets eventually lead to the detection of life outside Earth (biomarkers)? 3) How did life in the Solar System arise and evolve? Although ESA has busied itself with these issues since the beginning of the Darwin study in 1996, it has become abundantly clear that, as these topics have evolved, only a very large effort, addressed from the ground and from space with the utilization of different instruments and space missions, can provide the empirical results required for a complete understanding. The good news is that the problems can be addressed and solved within a not-too-distant future. In this short essay, we present the present status of a roadmap related to projects that are related to the key long-term goal of understanding and characterizing exoplanets, in particular Earth-like planets.

The search for worlds like our own

The direct detection of Earth-like exoplanets orbiting nearby stars and the characterization of such planets-particularly, their evolution, their atmospheres, and their ability to host life-constitute a significant problem. The quest for other worlds as abodes of life has been one of mankind's great questions for several millennia. For instance, as stated by Epicurus approximately 300 BC: "Other worlds, with plants and other living things, some of them similar and some of them different from ours, must exist." Demokritos from Abdera (460-370 BC), the man who invented the concept of indivisible small parts-atoms-also held the belief that other worlds exist around the stars and that some of these worlds may be inhabited by life-forms. The idea of the plurality of worlds and of life on them has since been held by scientists like Johannes Kepler and William Herschel, among many others. Here, one must also mention Giordano Bruno. Born in 1548, Bruno studied in France and came into contact with the teachings of Nicolas Copernicus. He wrote the book De l'Infinito, Universo e Mondi in 1584, in which he claimed that the Universe was infinite, that it contained an infinite amount of worlds like Earth, and that these worlds were inhabited by intelligent beings. At the time, this was extremely controversial, and eventually Bruno was arrested by the church and burned at the stake in Rome in 1600, as a heretic, for promoting this and other equally confrontational issues (though it is unclear exactly which idea was the one that ultimately brought him to his end). In all the aforementioned cases, the opinions and results were arrived at through reasoning-not by experiment. We have only recently acquired the technological capability to observe planets orbiting stars other than 6 our Sun; acquisition of this capability has been a remarkable feat of our time. We show in this introduction to the Habitability Primer that mankind is at the dawning of an age when, by way of the scientific method and 21(st)-century technology, we will be able to answer this fascinating controversial issue that has persisted for at least 2500 years.

Geophysical and atmospheric evolution of habitable planets

The evolution of Earth-like habitable planets is a complex process that depends on the geodynamical and geophysical environments. In particular, it is necessary that plate tectonics remain active over billions of years. These geophysically active environments are strongly coupled to a planet's host star parameters, such as mass, luminosity and activity, orbit location of the habitable zone, and the planet's initial water inventory. Depending on the host star's radiation and particle flux evolution, the composition in the thermosphere, and the availability of an active magnetic dynamo, the atmospheres of Earth-like planets within their habitable zones are differently affected due to thermal and nonthermal escape processes. For some planets, strong atmospheric escape could even effect the stability of the atmosphere.

Origin and evolution of life on terrestrial planets

The ultimate goal of terrestrial planet-finding missions is not only to discover terrestrial exoplanets inside the habitable zone (HZ) of their host stars but also to address the major question as to whether life may have evolved on a habitable Earth-like exoplanet outside our Solar System. We note that the chemical evolution that finally led to the origin of life on Earth must be studied if we hope to understand the principles of how life might evolve on other terrestrial planets in the Universe. This is not just an anthropocentric point of view: the basic ingredients of terrestrial life, that is, reduced carbon-based molecules and liquid H(2)O, have very specific properties. We discuss the origin of life from the chemical evolution of its precursors to the earliest life-forms and the biological implications of the stellar radiation and energetic particle environments. Likewise, the study of the biological evolution that has generated the various life-forms on Earth provides clues toward the understanding of the interconnectedness of life with its environment.

Titan Radar Mapper observations from Cassini's T3 fly-by

Cassini's Titan Radar Mapper imaged the surface of Saturn's moon Titan on its February 2005 fly-by (denoted T3), collecting high-resolution synthetic-aperture radar and larger-scale radiometry and scatterometry data. These data provide the first definitive identification of impact craters on the surface of Titan, networks of fluvial channels and surficial dark streaks that may be longitudinal dunes. Here we describe this great diversity of landforms. We conclude that much of the surface thus far imaged by radar of the haze-shrouded Titan is very young, with persistent geologic activity.

The Sand Seas of Titan: Cassini RADAR Observations of Longitudinal Dunes

The most recent Cassini RADAR images of Titan show widespread regions (up to 1500 kilometers by 200 kilometers) of near-parallel radar-dark linear features that appear to be seas of longitudinal dunes similar to those seen in the Namib desert on Earth. The Ku-band (2.17-centimeter wavelength) images show approximately 100-meter ridges consistent with duneforms and reveal flow interactions with underlying hills. The distribution and orientation of the dunes support a model of fluctuating surface winds of approximately 0.5 meter per second resulting from the combination of an eastward flow with a variable tidal wind. The existence of dunes also requires geological processes that create sand-sized (100- to 300-micrometer) particulates and a lack of persistent equatorial surface liquids to act as sand traps.

Cassini Ion and Neutral Mass Spectrometer: Enceladus Plume Composition and Structure

The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (<1%) of acetylene and propane also appear to be present. Ammonia is present at a level that does not exceed 0.5%. The radial and angular distributions of the gas density near the closest approach, as well as other independent evidence, suggest a significant contribution to the plume from a source centered near the south polar cap, as distinct from a separately measured more uniform and possibly global source observed on the outbound leg of the flyby.

Rain, winds and haze during the Huygens probe's descent to Titan's surface

The irreversible conversion of methane into higher hydrocarbons in Titan's stratosphere implies a surface or subsurface methane reservoir. Recent measurements from the cameras aboard the Cassini orbiter fail to see a global reservoir, but the methane and smog in Titan's atmosphere impedes the search for hydrocarbons on the surface. Here we report spectra and high-resolution images obtained by the Huygens Probe Descent Imager/Spectral Radiometer instrument in Titan's atmosphere. Although these images do not show liquid hydrocarbon pools on the surface, they do reveal the traces of once flowing liquid. Surprisingly like Earth, the brighter highland regions show complex systems draining into flat, dark lowlands. Images taken after landing are of a dry riverbed. The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent. The number density of haze particles increases by a factor of just a few from an altitude of 150 km to the surface, with no clear space below the tropopause. The methane relative humidity near the surface is 50 per cent.

Cassini Radar Views the Surface of Titan

The Cassini Titan Radar Mapper imaged about 1% of Titan's surface at a resolution of approximately 0.5 kilometer, and larger areas of the globe in lower resolution modes. The images reveal a complex surface, with areas of low relief and a variety of geologic features suggestive of dome-like volcanic constructs, flows, and sinuous channels. The surface appears to be young, with few impact craters. Scattering and dielectric properties are consistent with porous ice or organics. Dark patches in the radar images show high brightness temperatures and high emissivity and are consistent with frozen hydrocarbons.

Compositional maps of Saturn's moon Phoebe from imaging spectroscopy

The origin of Phoebe, which is the outermost large satellite of Saturn, is of particular interest because its inclined, retrograde orbit suggests that it was gravitationally captured by Saturn, having accreted outside the region of the solar nebula in which Saturn formed. By contrast, Saturn's regular satellites (with prograde, low-inclination, circular orbits) probably accreted within the sub-nebula in which Saturn itself formed. Here we report imaging spectroscopy of Phoebe resulting from the Cassini-Huygens spacecraft encounter on 11 June 2004. We mapped ferrous-iron-bearing minerals, bound water, trapped CO2, probable phyllosilicates, organics, nitriles and cyanide compounds. Detection of these compounds on Phoebe makes it one of the most compositionally diverse objects yet observed in our Solar System. It is likely that Phoebe's surface contains primitive materials from the outer Solar System, indicating a surface of cometary origin.

The occurrence of Jovian planets and the habitability of planetary systems

Planets of mass comparable to or larger than Jupiter's have been detected around over 50 stars, and for one such object a definitive test of its nature as a gas giant has been accomplished with data from an observed planetary transit. By virtue of their strong gravitational pull, giant planets define the dynamical and collisional environment within which terrestrial planets form. In our solar system, the position and timing of the formation of Jupiter determined the amount and source of the volatiles from which Earth's oceans and the source elements for life were derived. This paper reviews and brings together diverse observational and modeling results to infer the frequency and distribution of giant planets around solar-type stars and to assess implications for the habitability of terrestrial planets.