Saturn Atmospheric Science Goals

In 2009, the US National Science Academies established a committee to develop a planetary science strategy for the coming decade.  Along with an international team of planetary scientists, I helped to write white papers surveying atmospheric science goals for the four giant planets in our solar system.  Given the page-constraints for these white papers, and the huge volume of feedback we received, we decided to provide supplementary material for the Decadal Survey 2013-2023 committee.


The pages below provide a list of outer planet atmospheric science themes and investigations to summarise the white papers. Any comments and suggestions should be directed to me, and the pages will be continually updated as new ideas and topics arise.  The lists aren’t meant to be exhaustive, and are biased towards atmospheric science.

A.  Dynamics and Circulation of the Troposphere and Stratosphere

1.Continuous monitoring of wind patterns for studies over a variety of time scales (from hours to seasons)

a.Dayside visible and near-IR imaging with 30 km resolution to determine the 2D horizontal wind patterns (zonal and meridional) and their changes with time, particularly in response to thermophysical changes within discrete features (storms, etc.).

b.Thermal-IR mapping to measure temperatures and thermal windshear gradients.  Repeat on a weekly/monthly timescale to monitor how the thermal field changes in response to localised dynamics and seasonal changes.

c.Sub-mm sounding of molecular lineshapes to directly determine wind speeds in the mesosphere, stratosphere and upper troposphere.

d.Combine wind fields and thermal fields to measure potential vorticity globally and over a range of timescales to study the seasonal variability.

2.Determine the rotation rate of Saturn (the length of the day) to determine the stability of the zonal jet streams and the belt/zone structure

a.Long-term monitoring of the Saturn Kilometric radiation to establish the causes of variability.

b.Measurement of the shape of the gravitational field.

c.Numerical modelling to determine the rotation rates from stability arguments.

3.Determine the depth of the zonal wind patterns on Saturn

a.Microwave remote sensing and gravitational field mapping to search for evidence of deep zonal wind patterns below the cloud tops.

b.Multiple entry probes/mini probes to track the atmospheric motions to 20 bar or deeper.

c.5-micron imaging of discrete cloud features (cold spots) to measure the wind field at the 2-4 bar level.

4.Study the importance of moist convection of multiple condensibles in shaping the observed cloud-top meteorology

a.Determine the spatial distribution of H2O and NH3 below the cloud tops using entry probes, microwave remote sensing or radio occultation studies.

b.Measure the spatial distribution and power of lightning.

5.Monitor time-varying phenomena in Saturn’s troposphere over a range of timescales

a.Rapid responses required for emergent features such as large storms, equatorial outbreaks of cloud activity, or other rapidly evolving features.  Monitor on hourly timescales across a broad spectral range.

b.Long-term monitoring of the semi-annual oscillation of the temperature field and modulation of composition as a result of wave activity (monthly-yearly timescales).

c.Monitor the ephemeral nature of slowly moving thermal waves in thermal-IR mapping over weekly timescales.

d.Establish the stability of features such as the ribbon wave, string of pearls, north polar hexagon and polar cyclonic vortices via continual visible and IR monitoring as the seasons change.

6.Study the structure and composition of Saturn’s polar vortices

a.High inclination orbits to provide nadir-views for thermal mapping, near-IR mapping and visible imaging of the poles.

b.Track the expected disappearance of the southern hemisphere polar stratospheric warm hood (70-90S) and the emergence of a similar structure in the spring hemisphere.

c.Determine the factors responsible for the stability of the north polar hexagon, and the reason for absence/ephemeral nature of polygonal waves in the southern hemisphere.

B.  Tropospheric Composition and Chemistry

1.Identify seasonal variations in the upper tropospheric distribution of NH3 and disequilibrium species (PH3, AsH3, GeH4), and track these changes with time.

a.Mid-IR and far-IR mapping (100 km spatial resolution) of the global distribution of PH3 and NH3, with regular repeats (yearly) to track the evolution of asymmetries.  Tie asymmetries to aerosols to study photochemical shielding.

b.Near-IR high spectral resolution mapping (100 km resolution) of PH3, AsH3, GeH4 and NH3 (yearly repetition) to study asymmetries at deeper levels (1-3 bar), and relate these to photochemistry and vertical eddy mixing.

c.Far-IR mapping of ortho/para hydrogen fractions with yearly repetition to study the relation between para-H2 conversion and aerosols.

2.Search for and map previously unidentified species in the troposphere

a.High far-IR and sub-mm spectral resolution mapping of HCN, CS, etc.

b.Constrain isotopic ratios 12C/13C and 14N/15N (among others) within the troposphere.

3.Study the atmospheric composition below the cloud tops

a.Microwave remote sensing to constrain the abundances of NH3, H2S and H2O below the condensation levels.

b.Multiple entry probes for in situ measurement of the tropospheric composition.

C.  Clouds, Aerosols and Hazes on Saturn

1.Map the spatial distribution of clouds and hazes in the troposphere and stratosphere to monitor how this evolves seasonally

a.Near-IR mapping over monthly timescales on both the dayside (1-4 microns) and nightside (4-5 microns) to determine the aerosol properties (vertical structure, absorption, scattering properties, composition, optical depth) and their evolution with time.

b.Microwave remote sensing of the vertical cloud structure (NH3, H2S and H2O clouds) and how these evolve with time.

c.Visible, near-IR and UV mapping of high-latitude polar hazes and the relation to auroral activity.

2.Determine the vertical structure, shape and composition of the cloud and aerosol particles

a.Multiple entry probes to sample the clouds and hazes in situ, to determine their composition and the vertical density distribution.

b.The vertical altitudes of cloud tracers will be vital in determining the altitudes for the cloud-tracked zonal jet system.

3.Study the relation between cloud and haze distributions and discrete atmospheric phenomena

a.Track the changes in cloud properties in relation to thermophysical changes to the temperature and composition (e.g. the episodic emergence of cloud activity at the equator, or convective storms).

b.Correlate the spatial distribution of lightning with the emergence of convective cloud activity.

D.  Stratospheric Composition and Photochemistry

1.Map the seasonal variations in hydrocarbon distributions (and hence photochemical rates) throughout the stratosphere, and the impact of dynamical redistribution of this material.

a.Mid-IR high-resolution spectroscopic mapping of ethane, acetylene and higher order hydrocarbons.  Regular repeats (~yearly) of full maps to track seasonal evolution.

b.Near-IR observations of stratospheric hydrocarbons,

c.UV mapping of high-altitude hydrocarbons, yearly repetition to track evolution.

d.Couple photochemical models with dynamical models to understand the spatial distribution of these hydrocarbons.

2.Determine the spatial distribution of stratospheric material from exogenic sources (e.g. HCN, OCS, CO, H2O)

a.Study the influx rates of material both from the rings and the interplanetary environment, and the spatial modulation of this material by the magnetic field.

3.Study the possibility of stratospheric warmings at high latitudes and their associated phenomena.

E.  Upper Atmospheric Processes and Interaction with the External Environment

1.Determine the general circulation of the mesosphere and thermosphere and the existence of zonal jets at these high altitudes

a.Radio occultation and limb-sounding studies of high altitudes to determine the vertical temperature structure.  Use multiple latitudes and repeat with a sufficient spatial sampling to track changes to the circulation with time.

b.Sub-mm sounding of molecular lineshapes to determine windspeeds in the mesosphere.

c.Compositional measurements via solar/stellar occultations for high-altitude hydrocarbons, hazes, etc., for use as tracers of the circulation.  Map these distributions as they vary with time (monthly timescales).

2.Study the influence of vertically propagating waves on the heating mechanisms for the thermosphere and ionosphere

a.Vertical temperature profiles (radio occultations, stellar/solar occultations, sub-mm sounding) to detect and characterise vertically propagating waves.

b.Determine how auroral energy is redistributed with latitude throughout the ionosphere.

3.Study the three dimensional structure and morphology of the Saturnian aurora, and their influence on the upper neural atmosphere

a.Determine the chemical and haze composition of the polar atmosphere, and how this differs from mid-latitudes.

b.Map the temperature of the upper atmosphere to determine the influence of auroral energy deposition versus radiative heating.  Repeat periodically to search for evolution of the thermal field in response to e.g. solar wind activity.

c.Determine the altitude distribution of auroral energy deposition.

d.Map the H3+ emission and H2 glow in the near-IR and UV, respectively, and repeat over multiple timescales (days to years) to study the evolution of the aurora with season.

4.Determine the influence of the rings on atmospheric properties

a.Map the thermal structure of the troposphere and stratosphere over seasonal timescales to study the influence of ring shadowing on temperatures and radiative cooling.

b.Map clouds, aerosols and gaseous composition to determine the influence of ring shadowing on tropospheric chemistry and dynamics.

c.Map the presence of stratospheric oxygen species to understand the direct connection between ring material and the composition of the middle-atmosphere.

F.  Formation, Evolution and Internal Structure of Saturn

1.Determine the bulk composition of Saturn below the cloud-tops to place constraints on the mass of rocky/icy material accreted during formation:

a.In situ sampling at multiple locations to determine the relative enrichments in heavy elements (C, O, N, S, P, As, Ge, etc.) over the solar composition.

b.High spectral resolution near-IR observations to determine the tropospheric abundance of PH3, AsH3, GeH4, CH3D, 13CH4, 15NH3 etc.

c.In situ measurements of isotopic ratios (12C/13C, D/H, 14N/15N, 18O/16O)

d.In situ measurement of Noble gases (He, Ne, Ar, Kr, Xe, etc.)

e.Mid-IR determination of 12C/13C and 14N/15N ratios from 7-14 micron region with high spectral resolutions.

f.Microwave remote sensing of the 1-100 bar region of Saturn’s troposphere to determine the deep abundances of volatiles (NH3, H2S and H2O) below their condensation altitudes.

g.Radio occultation studies (possibly UHF) to probe the vertical distribtution of NH3, H2S and H2O in the upper troposphere.

2.Determine the basic shape of Saturn and the deep zonal jet profile.

a.Measure high order gravitational and magnetic field moments to determine the planet’s shape.

b.Use microwave remote sensing and gravitational field measurements to search for evidence of zonal winds at depth.

3.Investigate the rate of influx of material from exogenic sources, and the impacts of such influx on planetary accretion models.

a.Determine the rate at which Saturn suffers impacts from micrometeorites and asteroidal/cometary collisions.

b.Study the relation ship between the magnetic field and materials from Saturn’s rings, to determine how and where the rings can modify the atmospheric composition (e.g. stratospheric oxygen-bearing species).