A. Formation, Evolution and Internal Structure of Neptune
1.Determine the elemental enrichment of Neptune to constrain formation theories.
a.In situ sampling of Neptune to at least the 10 bar level to determine vertical profiles of CH4, NH3 and H2S clouds in the upper troposphere, etc. Well-mixed regions of NH3 and H2O are expected to exist at pressures of several hundred kbars, inaccessible with present probe technology and microwave radiometry.
b.In situ sampling to 40 bars to determine the chemistry of NH3, H2S and NH4SH below the condensation cloud levels.
c.In situ sampling or microwave radiometry to 100 bars to investigate the aqueous chemistry in the H2O cloud.
d.In situ determination of isotopic ratios (12C/13C, D/H, 14N/15N, 18O/16O)
e.In situ measurement of Noble gases (He, Ne, Ar, Kr, Xe, etc.)
f.Detection and upper limits on disequilibrium species (PH3, AsH3, GeH4) to constrain enrichment from in situ sampling or high-sensitivity spectroscopy in the 4-5 micron region.
g.Radio wavelengths (1-100 cm) to probe the composition beneath 100 bars.
2.Measure the internal structure and mass distribution profile of Neptune
a.Measurement of high-order gravitational and magnetic field moments to understand the vertical distribution of rock, ice and gas from the core of the planet through to its outer layers. Repeat for a range of latitudes and longitudes. Requires an orbiter at close proximity.
b.Laboratory simulations of ices representative of the interior structure of Neptune.
3.Precise determination of Neptune’ self-luminosity and internal energy.
a.Spectroscopic measurements of the emission of thermal energy across the electromagnetic spectrum to determine the energy balance, and search for variability since Voyager 2.
b.Global mapping of thermal emission to determine the locations of maximal radiative cooling (e.g. the importance of high latitude polar emission).
4.Determine the nature and location of the magnetic field generation region
a.Measure high-order moments of the planetary magnetic field at a wide range of latitudes and longitudes to determine the location of the dynamo (potentially at 70% of the planetary radius).
b.UV and IR imaging of auroral regions to map the spatial variability of energy deposition, modulated by the magnetic field.
B. Circulation and Dynamics of the Troposphere and Stratosphere
1.Determine the spatial distribution of tracers for atmospheric motion
a.High spectral resolution microwave and IR studies of CO, CH4, NH3, H2S, and H2O distributions to constrain planet-wide circulation systems.
b.Far-IR spectroscopy to map the spatial distribution of the ortho/para-H2 ratio as a tracer for vertical mixing in the troposphere and stratosphere. Regular monitoring of this ratio to determine how vertical mixing rates vary with time.
c.Near-IR and visible measurements of cloud opacity and structures, used to trace strong convective updrafts and redistribution of material.
2.Investigate the implications of seasonal forcings experienced by Neptune (165 year orbital period) for comparison with Saturn
a.Study asymmetries in cloud and aerosol properties as tracers of vertical mixing and convective instabilities using near-IR and UV spatio-spectral imaging. Repeat observations on 2-5 year timescales to search for seasonal changes.
b.Global thermal mapping in the mid and far-IR to determine the response of the temperature field to the seasonal forcings, and establish radiative time constants for the atmosphere.
c.Radio occultations and solar/stellar occultations for high vertical-resolution studies of the temperature field. Repeat at multiple latitudes to determine asymmetries in vertically-propagating wave activity and implications for atmospheric stability.
d.High latitude observations in UV, visible, near-IR and thermal to study polar vortices and zonal organisation. Temporal monitoring of Neptune’s south polar hotspot as the degree of insolation varies over time.
3.Determine the zonal and meridional circulation of Neptune, and the depth of the belt/zone structures and jet streams, and the reason for Neptune having the fastest equatorial winds in the solar system:
a.Track visible cloud features on timescales of hours to measure zonal wind speeds at the cloud tops, dayside imaging at 200 km spatial resolution.
b.Monitor discrete cloud features over daily-weekly timescales to study meridional motions.
c.Search near-IR wavelengths to find contrasts for cloud tracking at multiple atmospheric levels (e.g. will 5 um studies reveal contrasts at depth as they have for Jupiter and Saturn?)
d.Determine the zonal organisation of Neptune near the poles to understand whether polar vortex phenomena are common to all of the outer planets (i.e. does Neptune exhibit polar vortices?)
e.Thermal measurements of the vertical temperature structure to determine the vertical windshear, and hence extrapolate cloud-tracked winds to lower pressures in the stratosphere.
f.Meridional variability in the temperature field to look for spatial variations in the emission of internal energy left over from formation.
g.Search for perturbations to magnetic and gravity field measurements to identify temperature and density contrasts in the deep interior, to study the depth of the zonal wind field and the relation to interior convection.
4.Measure the albedo of Neptune to constrain energy budget calculations
a.Visible and near-IR measurements of the planet’s albedo with a wide range of solar phase angles and incidence angles.
5.Observe the frequency, morphology and stability of discrete convective events (storms, plumes, turbulence) to understand the nature of small-scale convection
a.Determine the three-dimensional structure of discrete cloud-features (which are known to be ephemeral in nature) using multi-wavelength imaging from the UV to the IR.
b.Compare the optical properties of the bright, reflective material seen in the near-IR with the surrounding atmosphere to constrain the composition and the mechanisms generating the cloud features.
c.Determine the stability of the cloud features, and the processes maintaining/dissipating the discrete activity.
6.Determine the spatial distribution of discrete cloud activity to study the underlying mechanisms and their seasonal variability
a.Long term multi-wavelength imaging of the regions of discrete cloud activity to study changes in cloud and aerosol altitudes, atmospheric temperature and composition.
b.Numerical simulations to reproduce the seasonal variations in the brightness of these features in response to the changing stability of the atmosphere.
c.Continuum radio imaging to determine the NH3, H2S, and H2O distributions at depth, for use as a proxy for cloud locations.
d.In situ sampling of regions of discrete cloud activity, versus regions of more sluggish motion, to study the thermophysical differences in atmospheric properties between the different regions.
e.In situ sampling to 40 bars to determine the chemistry of NH3, H2S and NH4SH below the condensation cloud levels.
f.In situ sampling or microwave radiometry to 100 bars to investigate the aqueous chemistry in the H2O cloud.
C. Composition of the Troposphere and Stratosphere
1.Determine the spatial distribution of volatiles in the upper troposphere
a.Far-IR and mid-IR spectroscopy to determine the vertical distribution of NH3.
b.Microwave radiometry and radio occultation studies for the vertical NH3 and H2S profiles, but this will not probe the well-mixed region for bulk N/H and S/H content.
c.Place constraints on the spatial distribution of H2S, NH3, and H2O from radio, microwave and sub-mm observations.
d.In situ sampling of Neptune to at least the 10 bar level to determine vertical profiles of CH4, NH3 and H2S clouds in the upper troposphere; to 40 bars to determine the chemistry of NH3, H2S and NH4SH below the condensation cloud levels; and radiometry to 100 bars to investigate the aqueous chemistry in the H2O cloud.
2.Study the inventory and optical properties of cloud and hazes
a.In situ sampling of aerosol and haze layers to determine optical properties, vertical distribution, size, shape, number density and optical depth.
b.Pressures approaching 40 bars are needed for NH3, H2S and NH4SH cloud chemistry.
c.Pressures approaching 100 bars are required to constrain H2O clouds and aqueous chemistry in the liquid-water cloud.
d.Reflection spectroscopy of the clouds and hazes at multiple phase angles and solar incidence angles to determine optical properties (size, scattering properties, composition) of the aerosols.
3.Measure the spatial distribution of stratospheric hydrocarbons and hazes to constrain photochemical models for Neptune
a.Mid-IR imaging and spectroscopy to search for meridional gradients (asymmetries) in the stratospheric composition.
b.Monitor the hydrocarbon distributions on yearly intervals to search for seasonal variability.
c.Determine the composition of Neptune hazes using solar/stellar occultations at UV and IR wavelengths, or in situ sampling.
4.Detect and map the distribution of trace species and disequilibrium species horizontally, vertically and with time
a.High sensitivity near and thermal-IR spectroscopy to place upper limits on the abundance of PH3, AsH3 and GeH4, which are disequilibrium species on Jupiter and Saturn.
b.Map the spatial distribution of CO using sub-mm spectroscopic maps of the troposphere.
c.High-resolution IR and submillimeter spectroscopy to search for previously undetected species in the troposphere and stratosphere.
D. Upper Atmosphere Circulation and Aurora
1.Confirm the presence and longevity of Neptune aurora
a.No direct detection of aurora from the ground, only detection from Voyager 2. Require better observations of the morphology of the aurora, in the IR (from the ground, possibly using adaptive optics) and in the UV.
2.Monitor the morphology of Neptune aurora with time to constrain models of the ionosphere and magnetosphere of the planet
a.Acquire regular maps of the polar regions of Neptune, in both the UV and the near-IR (H3+ emission).
b.Compare aurora at north and south pole to understand seasonal differences and constrain the unique morphology of the magnetic field.
3.Determine the importance of vertically propagating waves on the heating of the mesosphere and thermosphere
a.Radio occultations, solar/stellar occultations and sub-mm sounding to determine the vertical temperature structure at multiple latitudes to search for evidence of wave activity.
b.Constrain the general circulation of the upper atmosphere and its latitudinal redistribution of energy and tracers.
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