# The atmospheric structure of the Ice Giant planets from in situ measurements by an entry probe

Francesca Ferri, Giacomo Colombatti, Alessio Aboudan, Carlo Bettanini, Stefano Debei, Ari Matti Harri, Jean Pierre Lebreton, Alice Le Gall, Athena Coustenis, Karen Aplin

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Francesca Ferri, Giacomo Colombatti, Alessio Aboudan, Carlo Bettanini, Stefano Debei, et al. The atmospheric structure of the Ice Giant planets from in situ measurements by an entry probe. EPSC-DPS Joint Meeting 2019, Sep 2019, Genève, Switzerland. pp.EPSC-DPS2019-660-1. insu-03563832

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Submitted on 10 Feb 2022

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# The atmospheric structure of the Ice Giant planets from *in situ* measurements by an entry probe

Francesca Ferri<fnref n="1" />, Giacomo Colombatti<fnref n="1" />, Alessio Aboudan<fnref n="1" />, Carlo Bettanini<fnref n="1" />, Stefano Debei<fnref n="1" />, Ari Matti Harri<fnref n="2" />, Jean Pierre Lebreton<fnref n="3" />, Alice LeGall⁴, Athena Coustenis⁵, Karen Aplin<fnref n="6" />,

## Abstract

Atmospheric entry probes offer a unique opportunity for sounding atmospheric regions not reachable from remote sensing observations. *In situ* measurements during the entry and descent allow for investigating the atmospheric composition, structure and dynamics down deep into the atmosphere (e.g. Galileo probe at Jupiter [1], Huygens probe at Titan [2]).

In the framework of the opportunity for a NASA-ESA joint mission to Uranus, Neptune and their moons, we are proposing an Atmospheric Structure Instrument (ASI) for an entry probe at the Ice Giant planets.

## 1. Introduction

The atmosphere of Uranus and Neptune, the Ice Giants of our Solar System, were probed by the Voyager 2 during its flyby in 1986 and 1989 respectively. The atmospheric temperature profiles were primarily retrieved from the Voyager radio occultations [3], [4]; while both ground based and Voyager thermal infrared, solar and stellar occultation observations further constrained the thermal structure (e.g. [5]).

<figure><img src="image_2.png" /><figcaption>Figure 1: Entry, Descent into the Ice Giant atmosphere</figcaption></figure>

An atmospheric entry probe will allow for sounding the atmosphere of the Ice Giants down deep, below the 1 bar level, investigating the atmospheric composition, structure and dynamics by *in situ* measurements.

## 2. The Ice Giant ASI: science objectives and expected results

On the heritage of the Huygens ASI experiment at Titan [6,7], the Ice Giant ASI will consist of a multi sensor package designed to measure the physical quantities characterizing the Uranus' or Neptune's atmosphere during the entry and descent of the probe into the planet. The key measurements will be acceleration, pressure, temperature and electrical properties all along the probe descent down deep into the atmosphere in order to investigate the atmospheric structure, dynamics and electricity.

The atmospheric profile along the probe trajectory will be retrieved by the measurements of the deceleration of the entry probe and by direct measurements of the pressure and temperature during the descent under parachute. The resulting atmospheric thermal structure constrains the atmospheric stability, dynamics and its effect on the atmospheric chemistry.

The variations in the density, pressure and temperature profiles provide information on the atmospheric stability and stratification, on the presence of wind, thermal tides, waves and turbulence in the atmosphere. The estimation of the temperature lapse rate can be used to identify the presence of the condensation and aerosols and cloud layers, to distinguish between saturated and unsaturated, stable and conditionally stable regions. Measurements of the atmospheric electrical properties along the descent could contribute to the

<footnote n="1">¹Università degli Studi di Padova, CISAS “Giuseppe Colombo”, Padova, Italy,</footnote>

<footnote n="2">²Finnish Meteorological Institute (FMI), Helsinki, Finland,</footnote>

<footnote n="3">³LPCE2, Orleans, France, ⁴LATMOS, France, ⁵LESIA Obs. Paris-Meudon, France,</footnote>

<footnote n="6">⁶University of Bristol, UK, (francesca.ferri@unipd.it)</footnote>
study of the moist convective processes, clouds
formation and characterization, and allow for
detection of possible electrical discharges, i.e.
lightning. Moreover IG-ASI will contribute also to
the analysis of the atmospheric composition,
providing the primary engineering function by
establishing the descent trajectory, and probe altitude
and velocity for correlating all the other probe
experimental data.

3. Summary and Conclusions

Following the success of the Huygens probe at Titan,
we are looking for envisaging a similar joint effort
and collaboration as per the NASA-ASI-ESA
Cassini-Huygens mission, in order to explore the Ice
Giant planets. At present NASA-ESA collaborative
studies are in progress in order to identify potential
mission configurations and a possible European
contribution to a joint mission to Uranus, Neptune
and its moons. One of the potential elements is an
entry probe into Uranus' or Neptune's atmosphere.
On the heritage of the Huygens HASI, we are
proposing a similar instrument to be flown on board
of the atmospheric probe.

The scientific objectives, measurements and expected
results for an Atmospheric Structure Instrument (ASI)
of an entry probe at the Ice Giant planets will be
presented and discussed in the framework of the
opportunity for an NASA-ESA joint mission to the
Ice Giants.

Acknowledgements

We acknowledge all the support our national space
agencies (ASI, CNES, FMI, UKSA) and ESA are and
will ensure and grant us in order to achieve this
international space endeavor.

We would like also to thank the ESA M* Ice Giant
CDF (Critical Design Facility) study team for any
technical data and information that they will supply
on the atmospheric entry probe for the definition of
the mission configuration, timeline and IG-ASI
operations and duty cycle.

References

[1] Seiff, A., Kirk, D. B., Knight, T. C. D., Mihalov, J. D., et al. Structure of the Atmosphere of Jupiter: Galileo Probe Measurements *Science*, **272**, DOI: 10.1126/science.272.5263.844, 1996

[2] Lebreton, J.P., O. Witasse, C. Sollazzo, et al. An overview of the descent and landing of the Huygens probe on Titan *Nature*, **438**, 758-764, 2005

[3] Lindal, G. F., J. R. Lyons, D. N. Sweetnam, et al. The atmosphere of Uranus: Results of radio occultation measurements with Voyager 2. *J. Geophys. Res.* **92**, 14,987–15,001, 1987.

[4] Lindal, G. F., J. R. Lyons, D. N. Sweetnam, et al. The atmosphere of Neptune: Results of radio occultation measurements with the Voyager 2 spacecraft. *Geophys. Res. Lett.* **17**, 1733–1736, 1990.

[5] Marley M. and C. P. McKay, Thermal Structure of Uranus' atmosphere *Icarus* **138**, 268–286, 1999.

[6] Fulchignoni, M., F. Ferri, F. Angrilli, et al. The characterisation of Titan's atmospheric physical properties by the Huygens Atmospheric Structure Instrument (HASI), *Space Science Reviews*, **104**, 395-431, 2002.

[7] Fulchignoni, M, F, Ferri, F. Angrilli, et al. *In situ* measurements of the physical characteristics of Titan's atmosphere and surface *Nature*, **438**, 785-791, doi:10.103038/nature04314, 2005