The ionosphere of Ganymede

doi:10.1016/S0032-0633(00)00154-9

Planetary and Space Science

Volume 49, Issues 3–4, March 2001, Pages 327-336

, ,

https://doi.org/10.1016/S0032-0633(00)00154-9 Get rights and content

Abstract

We consider the distribution of plasma density in the atmosphere of Ganymede and present electron density profiles inferred from data of the Plasma Wave instrument of Galileo. To study the question of ionospheric plasma, we present a zero-dimensional local rate equation model of the source and loss functions and the atomic and molecular processes we assume to be taking place. We conclude from the model that Ganymede has a bound ionosphere composed mainly of molecular oxygen ions in the polar regions and of atomic oxygen ions at low latitudes and that protons are absent everywhere. This implies that the plasma observed to be flowing out along the open flux tubes connected to the polar cap is composed of ions of atomic oxygen. We predict that Ganymede is surrounded by a corona of hot oxygen atoms. The model neutral atmosphere has a composition similar to that of the ionosphere and is exospheric everywhere. Our calculated neutral column density is consistent with values of Ganymede ultraviolet auroral brightness observed by means of the Hubble Space Telescope.

Introduction

The icy Galilean satellites of Jupiter have atmospheres. The atmosphere of Europa has been modeled numerically in detail in 3-D by Saur et al. (1998) and their results are consistent with observation. Ganymede is significantly larger than the other satellites and, more importantly, has an intrinsic magnetic field as discovered by the Galileo spacecraft (Kivelson et al., 1996; Gurnett et al., 1996).

The earliest indication of a possible atmosphere at Ganymede was a stellar occultation observation by Carlson et al. (1973), who found an atmospheric pressure of the order of a microbar. Observations by the UVS (Ultraviolet Science) telescope of Voyager 1 placed an upper limit on the pressure about five orders of magnitude lower (Broadfoot et al., 1981). Hubble space telescope (HST) observations by Hall et al. (1998) are consistent with the lower density, which is in contradiction to the early ionosphere/atmosphere model of Yung and McElroy (1977). Kumar and Hunten (1982) showed that the atmosphere can have two possible states that differ in pressure by a factor of 10⁵. The uncertainties in the H₂O vapor pressure extrapolations are, according to Kumar and Hunten (1982), sufficient to leave indeterminate the differentiation between the high and low density states of the system. The latter is a regime in which the generation rate of the constituents of the atmosphere is easily accommodated by thermal escape. As we shall see below, the HST observations fit the low-density state model of Kumar and Hunten (1982).

In this paper, we shall attempt to evaluate the distribution of plasma density in the exospheric ionosphere in both the polar cap and the equatorial regions of Ganymede. We also present new Galileo plasma wave science (PWS) measurements that have implications for the nature of the ionosphere. A simple steady-state analytic model is developed for Ganymede, which is consistent with the findings of PWS and HST but raises severe questions with respect to published interpretations of Plasma Science (PLS) data. We find the latter to be inconsistent with the physical processes that must be taking place at and near the surface of Ganymede.

Section snippets

The observations

In this section, we shall first present the PWS data obtained during the Ganymede 1 and Ganymede 2 (hereafter G1 and G2, respectively) encounters and then discuss the published Galileo Ultraviolet Science (UVS) and PLS data.

We also quote data and inferences from the energetic particle detector (EPD), radio occultation studies, ground based observations and HST findings. The implications of the combined data sets will be shown to be consistent with our model. The horizontal and vertical

Composition and sources

The existence of an indigenous atmosphere entails the presence of an ionosphere as well. Sputtering, even in the absence of major sublimation, can deliver molecules to the space above the surface and, if conditions are favorable, support a significant atmosphere. As mentioned above, the existence of a molecular oxygen atmosphere on both Europa and Ganymede has been established (Hall 1995, Hall 1998). In this section, we explore the sources and the physical mechanisms that lead to the

Ion densities

We consider two separate regions on the surface of Ganymede, the polar cap region, for which the latitude λ>45° and the lower latitude regions equatorward of this limit. Temperature maps published by Orton et al. (1996) show a range of temperatures from about $150 K$ in the subsolar equatorial zone to below $90 K$ near the poles and in the pre-dawn sector. For the low temperatures in the polar cap region, all sputtering products can be expected to recondense immediately with the exception of

The closed field line region

During encounter G8, the spacecraft entered a region of closed field lines as indicated by the detection of trapped energetic electrons (Williams et al., 1997) and the magnetometer measurements by Kivelson et al. (1998). In this region, the energetic particle spectra change drastically (Eviatar et al., 2000). The magnetospheric thermal plasma density can be expected to be very low in this region, because of the inaccessibility of closed drift paths to injected Jovian particles (Eviatar et al.,

Conclusions

We have presented observations of the radial profile of the electron density in the near-Ganymede polar cap region from the G1 and G2 flybys. The n/B ratio computed from these profiles is indicative of a polar cap outflow. We have developed a simple rate equation model that predicts an ionosphere on Ganymede comprised of oxygen ions, with molecular oxygen dominant in the polar region and atomic oxygen in the equatorial zone. The predicted neutral composition in both regions is similar. Our

Acknowledgements

We gratefully acknowledge helpful discussions with W.S. Kurth, M.A. McGrath, R.E. Johnson, P.D. Feldman, D.F. Strobel, L.J. Paxton and M.E. Brown. We are grateful to M.G. Kivelson for her gracious permission to use the magnetometer data upon which Fig. 4 is based. The work at Tel Aviv University and at the Max-Planck-Institut für Aeronomie was sponsored in part by the German–Israel Foundation for Basic Research (GIF) under grant I-562-242.07/97. The research at the University of Iowa was

References (47)

A. Bar-Nun et al.
Ejection of H₂ O, O₂, H₂ and H from water ice by 0.5–6 kev H⁺ and Ne⁺ ion bombardment

Surf. Sci.

(1985)
S.A. Budzien et al.
Solar flux variability and the lifetimes of cometary H₂ O and OH

Icarus

(1994)
R.E. Johnson
Polar “caps” on Ganymede and Io revisited

Icarus

(1997)
H.U. Schmidt et al.
Cometary gas and plasma flow with detailed chemistry

Comput. Phys. Commun.

(1988)
E.M. Sieveka et al.
Thermal and plasma induced molecular redistribution on the icy satellites

Icarus

(1982)
E.F. van Dishoeck et al.
The dissociation of OH and OD in comets by solar radiation

Icarus

(1984)
Y.L. Yung et al.
Stability of an oxygen atmosphere on Ganymede

Icarus

(1977)
Alexander, C.J., Bolton, S.J., Carlson, R., Ip, W.H., Spencer, J.R., Upward flux of sublimation from the surface of...
C.A. Barth et al.
Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede

Geophys. Res. Lett.

(1997)
Book, D.L., 1977. Revised and enlarged collection of plasma physics formulas and data. Memo. Rep. 3322. Naval Res....

A. Broadfoot

Overview of the Voyager ultraviolet spectrometry results through Jupiter encounter

J. Geophys. Res.

(1981)

M.E. Brown et al.

Observations of Ganymede's visible aurora

Bull. Amer. Astron. Soc.

(1999)

W.M. Calvin et al.

O₂ on Ganymede: Spectral characteristics and plasma formation mechanisms

Geophys. Res. Lett.

(1996)

R.W. Carlson et al.

An atmosphere on Ganymede from its occultation of SAO-186800 on 7 June 1972

Science

(1973)

P.C. Cosby

Electron-impact dissociation of oxygen

J. Chem. Phys.

(1993)

A. Eviatar et al.

Distribution of density in an ion-exosphere of a non-rotating planet

Phys. Fluids

(1964)

A. Eviatar et al.

Trapped energetic electrons in the magnetosphere of Ganymede

J. Geophys. Res.

(2000)

P.D. Feldman et al.

HST/SSTIS ultraviolet imaging of polar aurora on Ganymede

Astrophys J.

(2000)

L.A. Frank et al.

Outflow of hydrogen ions from Ganymede

Geophys. Res. Lett.

(1997)

D.A. Gurnett et al.

Evidence for a magnetosphere at Ganymede from plasma-wave observations by the Galileo spacecraft

Nature

(1996)

D. Hall et al.

Detection of an oxygen atmosphere on Jupiter's moon Europa

Nature

(1995)

D.T. Hall et al.

The far-ultraviolet oxygen airglow of Europa and Ganymede

Astrophys. J. Lett.

(1998)

A.R. Hendrix et al.

Ganymede's ozone-like absorber: Observations by the Galileo ultraviolet spectrometer

J. Geophys. Res.

(1999)

Cited by (55)

Jovian auroral radio source occultation modelling and application to the JUICE science mission planning

2021, Planetary and Space Science

Occultations of the Jovian low frequency radio emissions by the Galilean moons have been observed by the PWS (Plasma Wave Science) instrument of the Galileo spacecraft. We show that the ExPRES (Exoplanetary and Planetary Radio Emission Simulator) code accurately models the temporal occurrence of the occultations in the whole spectral range observed by Galileo/PWS. This validates of the ExPRES code. In addition to supporting the analysis of the science observations, the method can be applied for preparing the JUICE moon flyby science operation planning.
Preliminary estimation of the detection possibilities of Ganymede's water vapor environment with MAJIS

2020, Planetary and Space Science

The exosphere of Ganymede is the interface region linking the moon’s icy surface to Jupiter’s magnetospheric environment. Its characterization is of key importance to achieve a full understanding of the ice alteration processes induced by the radiation environment. Several scientific instruments that will operate on board the upcoming Jupiter Icy Moons Explorer (JUICE) mission, selected by ESA in the context of its Cosmic Vision programme, have the potential to study Ganymede’s exosphere. Among them, the Moons And Jupiter Imaging Spectrometer (MAJIS) will have the chance to investigate the composition of the moon’s exospheric components and the emission of water molecules. The exospheric water density profile, as obtained from current models, is a crucial parameter for the estimation of the expected signal to noise ratio related to the actual measurement. In lack of an adequate number of Ganymede’s observations from past missions, there is a general difficulty in constraining current exosphere models which are based, in general, on different scenarios and considerations and often show large discrepancies in the estimated spatial distribution of the neutral environment. In this work, we make a preliminary estimation of the expected IR emission from exospheric water molecules, using different modelled density profiles, and we speculate on the possibility of JUICE/MAJIS to detect it. An exercise on the potential plume detection capabilities of MAJIS is also performed. The first necessary step for performing these calculations is a rough comparison of the existing models of Ganymede’s water vapor exosphere. We discuss the characteristics of the neutral environment as derived from different exospheric models available in literature, the role of the ion-surface interactions in the H₂O exosphere generation, and the related implications also in view of future observations. We then use the model outputs to estimate different scenarios for the expected non-Local Thermal Equilibrium (non-LTE) emission from these molecules. The results of this study can be of help during the JUICE observation planning phase.
Constraining Ganymede's neutral and plasma environments through simulations of its ionosphere and Galileo observations

2020, Icarus

Citation Excerpt :

In summary, the ionospheric composition is a subject of debate. Simplistic efforts to characterize the ionosphere have been made by Eviatar et al. (2001b) and Cessateur et al. (2012). More recently, we have developed the first 3D multi-species test particle model of Ganymede's ionosphere (Carnielli et al., 2019).

Ganymede's neutral and plasma environments are poorly constrained by observations. Carnielli et al. (2019) developed the first 3D ionospheric model aimed at understanding the dynamics of the present ion species and at quantifying the presence of each component in the moon's magnetosphere. The model outputs were compared with Galileo measurements of the ion energy flux, ion bulk velocity and electron number density made during the G2 flyby. A good agreement was found in terms of ion energy distribution and bulk velocity, but not in terms of electron number density.

In this work, we present some improvements to our model (Carnielli et al., 2019) and quantitatively address the possible sources of the discrepancy found in the electron number density between the Galileo observations and our ionospheric model. We have improved the ion model by developing a collision scheme to simulate the charge-exchange interaction between the exosphere and the ionosphere. We have simulated the energetic component of the O₂ population, which is missing in the exospheric model of Leblanc et al. (2017) and added it to the original distribution, hence improving its description at high altitudes. These improvements are found to be insufficient to explain the discrepancy in the electron number density. We provide arguments that the input O₂ exosphere is underestimated and that the plasma production acts asymmetrically between the Jovian and anti-Jovian hemispheres. In particular, we estimate that the O₂ column density should be greater than 10¹⁵ cm⁻², i.e., higher than previously derived upper limits (and a factor 10 higher than the values from Leblanc et al. (2017)), and that the ionization frequency from electron impact must be higher in the anti-Jovian hemisphere for the G2 flyby conditions.
First 3D test particle model of Ganymede's ionosphere

2019, Icarus

Citation Excerpt :

The goal is to check the robustness of our conclusions using different magnetospheric models. Panel a) of Fig. 8 shows the electron number density profile along the G2 trajectory as measured by: the PWS instrument (Eviatar et al., 2001) (black curve), an ionospheric simulation driven by fields from the hybrid model of Leclercq et al. (2016) (red curve) and an ionospheric simulation, driven by fields from the MHD model of Jia et al. (2009) (blue curve). The electron number density was obtained by summing the ion density over all ionospheric species, assuming plasma neutrality.

We present the first three-dimensional multi-species ionospheric model for Ganymede, based on a test particle Monte Carlo approach. Inputs include the electromagnetic field configuration around the moon from the magnetospheric models developed by Leclercq et al. (2016) and by Jia et al. (2009), and the number density, bulk velocity and temperature distributions of the neutral exosphere simulated by Leblanc et al. (2017). According to our simulations, O₂⁺ is the most abundant ion species, followed by O⁺, H₂⁺ and H₂O⁺. For O⁺ and O₂⁺, the majority of ions produced impact the moon's surface, while for the other species the majority escapes Ganymede's magnetosphere. For all ion species, the escape occurs either in the direction of corotation of the Jovian plasma or through the Alfvén wings.

To validate our model, the output of our simulations, performed under the Galileo G2 flyby conditions, are compared to the observations. These include the electron density derived by the plasma wave instrument (PWS), the ion energy spectrogram measured by the plasma analyzer (PLS) and the associated plasma moments (Frank et al., 1997a).

On the one hand, the electron density found by our model is consistently underestimated throughout the flyby, being at least one order of magnitude lower compared to observations. We argue that the prime reason for this discrepancy comes from the exospheric density, which may be underestimated. On the other hand, we find a remarkably good agreement between the modeled ion energy spectrogram and that recorded by PLS, providing a validation of the test particle model. Finally, we compare the modeled plasma moments along the G2 flyby with those analyzed by Frank et al. (1997a). The data seems to be more consistent with an ionosphere dominated by O₂⁺ instead of H⁺ or O⁺, as suggested previously in the literature. This supports our finding that O₂⁺ is the dominant ion species close to the surface.
On the orbital variability of Ganymede's atmosphere

2017, Icarus

Ganymede's atmosphere is produced by radiative interactions with its surface, sourced by the Sun and the Jovian plasma. The sputtered and thermally desorbed molecules are tracked in our Exospheric Global Model (EGM), a 3-D parallelized collisional model. This program was developed to reconstruct the formation of the upper atmosphere/exosphere of planetary bodies interacting with solar photon flux and magnetospheric and/or the solar wind plasmas. Here, we describe the spatial distribution of the H₂O and O₂ components of Ganymede's atmosphere, and their temporal variability along Ganymede's rotation around Jupiter. In particular, we show that Ganymede's O₂ atmosphere is characterized by time scales of the order of Ganymede's rotational period with Jupiter's gravity being a significant driver of the spatial distribution of the heaviest exospheric components. Both the sourcing and the Jovian gravity are needed to explain some of the characteristics of the observed aurora emissions. As an example, the O₂ exosphere should peak at the equator with systematic maximum at the dusk equator terminator. The sputtering rate of the H₂O exosphere should be maximum on the leading hemisphere because of the shape of the open/close field lines boundary and displays some significant variability with longitude.
Surface charging of thick porous water ice layers relevant for ion sputtering experiments

2016, Planetary and Space Science

We use a laboratory facility to study the sputtering properties of centimeter-thick porous water ice subjected to the bombardment of ions and electrons to better understand the formation of exospheres of the icy moons of Jupiter. Our ice samples are as similar as possible to the expected moon surfaces but surface charging of the samples during ion irradiation may distort the experimental results. We therefore monitor the time scales for charging and discharging of the samples when subjected to a beam of ions. These experiments allow us to derive an electric conductivity of deep porous ice layers. The results imply that electron irradiation and sputtering play a non-negligible role for certain plasma conditions at the icy moons of Jupiter. The observed ion sputtering yields from our ice samples are similar to previous experiments where compact ice films were sputtered off a micro-balance.

View all citing articles on Scopus

View full text

The ionosphere of Ganymede

Abstract

Introduction

Section snippets

The observations

Composition and sources

Ion densities

The closed field line region

Conclusions

Acknowledgements

Surf. Sci.

Icarus

Icarus

Comput. Phys. Commun.

Icarus

Icarus

Icarus

Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede

Geophys. Res. Lett.

Overview of the Voyager ultraviolet spectrometry results through Jupiter encounter

J. Geophys. Res.

Observations of Ganymede's visible aurora

Bull. Amer. Astron. Soc.

O2 on Ganymede: Spectral characteristics and plasma formation mechanisms

Geophys. Res. Lett.

An atmosphere on Ganymede from its occultation of SAO-186800 on 7 June 1972

Science

Electron-impact dissociation of oxygen

J. Chem. Phys.

Distribution of density in an ion-exosphere of a non-rotating planet

Phys. Fluids

Trapped energetic electrons in the magnetosphere of Ganymede

J. Geophys. Res.

HST/SSTIS ultraviolet imaging of polar aurora on Ganymede

Astrophys J.

Outflow of hydrogen ions from Ganymede

Geophys. Res. Lett.

Evidence for a magnetosphere at Ganymede from plasma-wave observations by the Galileo spacecraft

Nature

Detection of an oxygen atmosphere on Jupiter's moon Europa

Nature

The far-ultraviolet oxygen airglow of Europa and Ganymede

Astrophys. J. Lett.

Ganymede's ozone-like absorber: Observations by the Galileo ultraviolet spectrometer

J. Geophys. Res.

O₂ on Ganymede: Spectral characteristics and plasma formation mechanisms