Science Summary

As the design of the instrument progresses, it is informed by the impacts of design choices on the ability to make key scientific measurements. Here we summarize the science measurements considered in instrument design.

Exoplanets

1. Planet Formation Physics:

PSI will directly constrain the detailed physics of planetary accretion.

Key Questions: What are the detailed physics of planetary accretion? How quickly do forming planets accrete? How steady or episodic is planetary accretion? What is the geometry of infalling material (e.g. spherical, boundary layer, magnetospheric)? What are the physical and chemical properties of circumplanetary disks, and what does this imply for moon formation? How do forming planets interact with protoplanetary disks and with each other?

Brief Description of Measurements:

• Measure accretion rates for a sample of forming planets with a range of semimajor axes

• Observe the formation location of a sample of planets to constrain disk-planet interactions and angular momentum transfer (and for comparison to older populations)

• Measure the geometry of the accretion flow onto a sample of forming planets

• Constrain the physical and chemical properties of circumplanetary disks (e.g. mass, chemical content)

• Search for and detect RV signals caused by exo-moons around known exoplanets

2. Atmospheric and Surface Composition and Evolution:

PSI will map molecular abundances, clouds, and surface properties for rocky and giant planets.

Key Questions:

1. What detailed physical processes shape the composition, dynamics, and evolution of planetary surfaces and atmospheres?

2. What processes shape the global structure and temporal variability of planetary atmospheres?

3. What are the constituents of clouds and how do they shape the bulk properties of planetary atmospheres?

4. What processes shape the vertical structures of planetary atmospheres?

5. What are the atmospheric and surface compositions of rocky planets around nearby M stars, and what does this imply for habitability?

Brief Description of Measurements:

• Measure molecular abundances of (giant) exoplanets with known masses (e.g. Gaia-identified systems)

• Monitor rotation rates and cloud variability in known giant exoplanets

• Measure atmospheric escape for highly irradiated planets

• Measure thermal emission and planetary energy budgets for rocky planets also detected in reflected light

• Characterize the atmospheric composition and temperatures of rocky planets in the HZs of the nearest M-dwarfs

• Searching for biosignatures on rocky planets in the HZs of the nearest M-dwarfs

• Map the surfaces (e.g. discriminating between continents and oceans) of the nearest rocky worlds in reflected light, including ocean glint

3. Comparative Planetology:

PSI will measure the locations and compositions of thousands of planets.

Key Questions:

1. What is the distribution of planet masses, atmospheric compositions, and orbital properties, and what does this imply about how planets form and evolve?

2. How do planetary system architectures evolve at early times?

3. What does the distribution of giant planet compositions imply about compositions and formation of potentially unseen rocky planets?

4. What does the distribution of super-Earth and ice giant compositions imply about their formation?

5. Is our solar system dynamically common?

Brief Description of Measurements:

• Fill in the mass function of giant planets at a wider range of semimajor axes and for a wider range of stellar properties (e.g. metallicity, mass).

• Characterize the morphology and prevalence of exozodiacal dust around a sample of stars

• Characterize the atmospheric properties (e.g. C/O ratios, metallicities, molecular abundances) of giants as a function of semimajor axis and stellar properties

• Constrain planetary orbital architectures (e.g. obliquities, eccentricities) for a wide range of ages and stellar properties

• Characterize the atmospheric properties of super-Earths and ice giants as a function of semimajor axis and stellar properties

• Measure isotopologues of e.g. 13CO, HDO, CH3D in the atmospheres of planets for a wide range of semimajor axes and stellar properties

Solar System

1. Giant planet atmospheres

PSI will measure the 3D structure and dynamics of planetary atmospheres at spacecraft-quality resolution (20 km at Jupiter, 130 km at Neptune).

Key Questions:

1. How do discrete atmospheric features (vortices, waves, convective storms) evolve in three dimensions and interact with their environments? What are the links between dynamical flows and compositional and/or thermal gradients?

2. What objects are the giant planets accreting in the present day, and what processes govern impact physics? How do chemical and particulate signatures evolve over time, and how can impacts be used to trace atmospheric circulation?

3. How do atmospheres respond to vastly different timescales of forcing by solar energy, magnetospheric interaction, and internal dynamics? How do auroral morphology and H3+ spectral emission trace magnetospheric dynamics? How do cloud activity, haze properties, and composition vary on seasonal timescales? What non-seasonal climate cycles operate on the giant planets, and what drives them?

Brief Description of Measurements:

• Observe a few discrete auroral features, convective storms, impact debris fields, polar vortices, and long-lived weather features (imaging and low-res IFU covering the full feature in each case; hi-res spectra in select locations; PSI-Blue, -Red, -10).

• Compile a complete catalog of diverse features from first-light item above.

• Build up multi-year central-meridian resolved spectral scans of Uranus and Neptune (PSI-Blue + PSI-10, continuous low-spectral-res + regular sampling at hi res).

• Obtain full time series showing how specific features evolve over their full intrinsic lifetimes: auroral phenomena (minutes to hours), convective storm eruptions (hours to months), impact debris fields (days to weeks), vortices (days to years).

2. Small Bodies

PSI will characterize active processes and collisional histories in various small body populations across the solar system.

Key Questions:

1. What are the occurrence rates and formation mechanisms of multiple systems within different populations and families? Are statistics biased by undetected close binaries or small satellites? What is the nature of compositional differences between components in multiple systems? How do compositional differences among components of multiple systems—or across the surface of resolved objects—trace conditions in the early solar system, and surface processing over time?

2. How are ring systems formed and maintained around small bodies? Are small body ring systems recent or primordial? How are ring systems stabilized in weak gravitational fields?

3. How do active processes such as outgassing and disruption evolve? What controls outgassing variability on rotational and orbital timescales? What does the physics of cometary nuclear jets reveal about internal structure and composition? What do disruption events tell us about internal structure of asteroidal and cometary bodies?

4. What is the nature of interstellar objects (ISOs), and how diverse are they? What are the basic properties of ISOs such as size, shape, rotation rate, composition, and density? How do ISO composition, structure, and history control outgassing processes?

Brief Description of Measurements:

• Search for low-res spectral differences within known multiple systems and ringed objects (ex., Chariklo at a distance of 16 AU has a diameter of 20 mas and a ring diameter of 70 mas).

• Obtain time-series imaging and spectra of near-nucleus coma morphology during cometary outgassing

• Obtain time-series imaging of cometary/asteroidal disruption process

• Obtain resolved imaging of ISO, spectral imaging of active ISO

• Conduct targeted surveys for binaries in multiple small body populations

• Conduct full survey of small bodies uniquely resolvable by PSI to search for surface inhomogeneity

3. Surfaces

PSI will characterize the composition of satellite surfaces and measure both seasonal variability and transient variability due to geological activity (water plumes; lava lakes), on scales of tens of km.

Key Questions:

1. How do CO₂, N₂, CH₄ migrate seasonally on icy satellites and KBO’s? Does the distribution and abundance of surface ices change seasonally on the outer Solar System’s icy bodies, as predicted by models? Is the appearance of the surfaces of these objects as seen by spacecraft fly-bys representative of their typical appearance?

2. What are the physical processes underlying Io’s bright volcanic hot spots, and what does this imply about Io’s internal structure? (a) What is the temperature distribution across Io’s lava lake crusts? From the temperature distribution, the crustal overturn velocity and consequently the lava composition and volatile content can be calculated. (b) What temperature are Io’s magmas at? By resolving Io’s eruptions into the actively erupting component and the spreading, cooling flow component, we can put the strongest constraints on the magma composition of the erupting component. This in turn reveals the composition and state of Io’s mantle, which are directly linked to the process of tidal heating. The spatial scales of Io’s volcanoes are ~100’s of km, and PSI’s resolution will be 50-100 km over 2-5 microns.

3. What is the composition and variability of the plumes of the icy ocean moons? (a) On what timescales are Enceladus’ plumes variable? This informs our understanding of the mechanisms driving the plumes. Enceladus’ plumes are known to be variable on diurnal timescales, and Cassini showed hints of a longer-term modulation on that variability, but post-Cassini follow-up is now needed to confirm the latter. (b) Do Europa’s plumes exist, and if so what is their density, composition, and variability? All detections to date have been very low SNR, and the only direct H2O detection was additionally unresolved. A spatially-resolved, decent-SNR detection could finally confirm their existence.

Brief Description of Measurements:

• Map temperatures within erupting volcanic centers on Io: 1. Resolve the temperature distribution across the lava crust on the lava lake Loki Patera. 2. Resolve an actively erupting volcano into its high-temperature erupting component and the cooling lava flow component, and measure the temperatures of both. Measurement: 2-5 micron IFS, low spectral resolution, ideal FOV of ~1.5” or larger.

• Map CH4, CO2, and N2 on Triton and Pluto. 1-2.5 micron low-resolution IFS covering broad bands of these ices.

• Build up a long-term timeline of the surface ices on outer Solar System bodies to detect seasonal variability of present - same as the above measurement but with a temporal component.

• Detect and resolve H₂O emission from water-world geysers. Measurement: 2.5-4.5 high-spectral-resolution (on-disk/off-disk) measurements at Europa and Enceladus, targeting the 3-micron region.

• If H2O detected above: search for other potential plume species such as CH4, NH3, HCN using the same technique.