Apurva V. OZA Detection and Dynamics of Satellite Exospheres

160 Pages·2017·21.66 MB·English

1
THE`SE DE DOCTORAT DE
l’UNIVERSITE´ PIERRE ET MARIE CURIE
Sp´ecialit´e
Plan´etologie et Instrumentation Spatiale
´ ˆ
Ecole Doctorale Astronomie et Astrophysique Ile de France (AAIF 127, Paris)
Pr´esent´ee par
Apurva V. OZA
Pour obtenir le grade de
DOCTEUR de l’UNIVERSITE´ PIERRE ET MARIE CURIE
Sujet de la th`ese :
Detection and Dynamics of Satellite Exospheres
Soutenue le 28 Septembre, 2017
Devant le jury compos´e de :
Dr. Fran¸cois Leblanc Directeur de th´ese
Dr. Jean-Jacques Berthelier Co-Directeur de th´ese
Pr. Bruno Sicardy President de jury
Pr. Joachim Saur Rapporteur
Dr. Iannis Dandouras Rapporteur
Dr. Olivier Witasse Examinateur
Dr. Anna Millilo Examinateur
Dr. Alain Doressoundiram Examinateur
et
Pr. Robert E. Johnson Invit´e
l’UNIVERSITE´ PIERRE ET MARIE CURIE
Abstract
Astronomy & Astrophysics Ecole Doctorale 127
Institut Pierre Simon Laplace: LATMOS
Doctor of Philosophy
by
3
Je pr´esente une analyse multidisciplinaire sur le comportement et les effets d’une
exosph`ere surface-bounded (li´ee `a la surface), qui tourne de mani`ere synchrone
autour de son corps primaire. Je d´efinie ce type d’exosph`ere dans cette th`ese comme
une exosph`ere satellitaire. Si l’exosph`ere satellitaire poss`ede une population thermique
de compos´es volatils fortement coupl´ee `a la temp´erature de surface, elle sera capable
de subir un cycle diurne pendant une p´eriode orbitale. Ce cycle diurne exosph´erique
n´ecessite que les esp`eces volatiles soient g´en´eralement non adsorbantes et aient une
dur´ee de vie proche de la p´eriode orbitale du satellite. Je fournis la premi`ere preuve de
l’existence d’un tel cycle d’exosph`ere diurne satellitaire sur deux des lunes glac´ees de
Jupiter: Europe et Ganym`ede. La preuve est appuy´e par une comparaison approfondie
entre les observations d’aurores d’oxyg`ene proches de la surface dans le lointain
ultraviolet par le t´elescope spatial Hubble (HST) et les simulations Monte Carlo 3-D
des exosph`eres d’O2 proches de la surface. Apr`es avoir inclus l’effet de la rotation et
les donn´ees les plus r´ecentes sur la pulv´erisation d’ions magn´etosph´eriques dans le
mod`ele, les exosph`eres simul´ees ont g´en´er´e des bourrelet atmosph´eriques plus ´etendus
pendant le cr´epuscule. Je d´ecris en d´etails ce cycle en r´esolvant de mani`ere analytique
l’´equation de conservation de masse 1-D pour une atmosph`ere cr´ee et d´etruite tout au
long de son orbite. Je d´emontre que, en raison du chauffage de la surface du satellite
par le Soleil, la densit´e atmosph´erique maximale devrait atteindre un pic pr`es du
cr´epuscule.Ce sc´enario de l’´evolution de l’exosph`ere aidera `a r´epondre `a des questions
non r´esolues concernant les hypoth`eses dans les exosph`eres de satellites connues telles
que: l’adsorption sur les grains de r´egolite, les interactions mol´eculaires avec des ions
ou d’autres esp`eces traces, l’´echappement exosph´erique, la migration exosph´erique et
les cons´equences endog`enes sur l’exosph`ere, tr`es ´enigmatiques. Au cours de cette th`ese,
deux types d’observations ont ´et´e rapport´ees concernant l’activit´e cryovolcanique
d’Europe. Je pr´esente les premi`eres simulations orbitales de tous les produits `a base
d’eau connus sur Europe et compare leurs densit´es aux observations ´enigmatiques de
HST. En particulier, j’ai ´egalement cherch´e `a r´econcilier les r´esultats avec l’influence
suppos´ee de la force de mar´ee de Jupiter. Selon les estimations de mar´ees thermiques
pr´esent´ees dans cette th`ese, des variations locales, `a l’´echelle du centim`etre, de la
temp´erature de surface allant jusqu’`a 100 K devraient ˆetre pr´esentes. Le chauffage
calcul´e est coh´erent avec le premier type d’observations d’Europe, indiquant un
cryovolcanisme explosif hautement transitoire alimentant l’exosph`ere avec de grandes
quantit´es d’eau. Cette source de vapeur d’eau, via la dissociation par impact
d’´electrons, peut g´en´erer la population d’hydrog`ene atomique requise de „ 1011H{cm2,
manquant dans nos simulations, pour correspondre `a la d´ecouverte r´ecente d’une
couronne d’hydrog`ene sur Europe. Observ´es en transit, le deuxi`eme type d’observation
de sortes de jets fins ´etait de composition inconnue et je fournis des densit´es de colonne
mises `a jour de deux gaz traces: NaCl et SO , qui devraient ˆetre pr´esents sur Europe
2
en raison d’un volcanisme au moins effusif ou peut-ˆetre mˆeme d’une sublimation
accrue. Au moment de la r´edaction de ce manuscrit, des campagnes d’observation de
la mol´ecule NaCl sont r´ealis´ees par une autre ´equipe utilisant ALMA, mais pour
l’instant aucune d´etection n’a ´et´e faite. La d´etection de la mol´ecule SO2, par ALMA,
doit ˆetre tr`es utile pour concilier les observations ´evasives de HST et d´eterminer
l’influence endog`ene de l’oc´ean en sous-surface d’Europe sur son exosph`ere.
Finalement, ces gaz traces ne seront jamais compl`etement caract´eris´es `a moins qu’un
nouveau spectrom`etre hautement sensible ne soit d´evelopp´e pour sonder l’´energie des
mol´ecules pr´esentes dans n’importe quelle exosph`ere. Pour cette raison, j’ai pass´e trois
ans `a travailler en parall`ele, au LATMOS, pour caract´eriser une nouvelle source
d’ionisation, un canon `a ´electrons `a nanotubes de carbone (CNTeg), le premier de son
genre. J’ai test´e pr`es d’une douzaine de puces de nanotubes de carbone, dont six
´emettent des courants d´esir´es „ 100µAmps stables `a 1 microAmp pendant plusieurs
jours. J’ai d´evelopp´e un code qui a simul´e, avec une r´esolution de 10 microm`etres, les
trajectoires d’´electrons soumises `a des champs ´electriques mod´er´es. Le code a confirm´e
que la source d’ionisation fonctionne comme pr´evu pour augmenter son niveau de
pr´eparation technologique. Ce sera la premi`ere d´emonstration de la nanotechnologie
dans la spectrom´etrie de masse neutre, qui peut s’av´erer ˆetre l’une des technologies les
plus performantes (P ă 10 milliWatts) dans l’avenir de l’instrumentation spatiale.
l’UNIVERSITE´ PIERRE ET MARIE CURIE
Abstract
Astronomy & Astrophysics Ecole Doctorale 127
Institut Pierre Simon Laplace: LATMOS
Doctor of Philosophy
by
5
I present a multidisciplinary analysis on the behavior and consequences of a
surface-bounded exosphere synchronously rotating about its primary, defined in this
dissertation as a satellite exosphere. Should the satellite exosphere possess a thermal
population of volatiles strongly coupled with the surface temperature, the satellite
exosphere will be capable of experiencing a diurnal cycle over an orbital period. This
exospheric cycle requires that the volatiles be generally non-adsorbing and have a
lifetime close to the satellite’s orbital period. I provide the first evidence of the
existence of such a diurnal satellite exosphere cycle on two of Jupiter’s icy moons:
Europa and Ganymede. The evidence was surmounted by an in-depth comparison
between the near-surface far-ultraviolet oxygen aurorae observations by the Hubble
Space Telescope(HST) and 3-D Monte Carlo simulations of the near-surface O2
exospheres. The simulated exospheres, after including the critical effect of rotation,
and the most updated knowledge of magnetospheric ion sputtering to date, generated
atmospheric bulges peaking at dusk local time. I further describe this cycle by
analytically solving 1-D mass conservation for an atmosphere being built and
destroyed throughout its orbit, ultimately demonstrating that due to time-dependent
solar heating of a satellite’s surface alone, the maximum column density should indeed
peak near dusk. This confirmation of exospheric evolution will help answer unsolved
questions regarding the assumptions in known satellite exospheres such as, but not
limited to: adsorption on to regolith grains, molecular interactions with ions or other
trace species, exospheric escape, exospheric migration, and most enigmatic of all the
endogenic consequences on the exosphere. During this dissertation, two sets of
observations were reported regarding the potential for cryovolcanic activity at Europa.
I therefore present the first orbital simulations of all known water-products at Europa,
and compare their densities to the enigmatic HST observations. In particular I also
sought to reconcile the supposed influence of Jupiter’s tidal force. According to tidal
heating estimates presented in this dissertation, local, centimeter-scale, surface
temperature variations of up to „ 100 K should be present. The calculated heating is
consistent with the first set of Europa observations, indicative of highly-transient
explosive cryovolcanism populating the exosphere with prodigious amounts of water.
This source of water vapor, via e- impact dissociation may source a required atomic
hydrogen population of „ 1011H{cm2 , missing in our simulations, to match the recent
discovery of a hydrogen corona at Europa. The second set of narrow jet-like features
observed in transit were of unknown composition, and I provide updated column
densities of two trace gases: NaCl and SO , which should be present at Europa due to
2
at least effusive volcanism or even enhanced sublimation. At the time of writing this
manuscript, the hunt for the former molecule NaCl has already been carried out by
another team using ALMA. The detection of the latter molecule SO2, by ALMA, I
argue should be most useful in reconciling the evasive HST observations and
determining the endogenic influence of Europa’s subsurface ocean on its exosphere.
Lastly, these trace gases will never be fully characterized unless a new highly sensitive
Contents 6
spectrometer is developed capable of probing the energy of the molecules present in the
exosphere. For this reason, I have spent three years in parallel, at LATMOS
characterizing a novel ionization source, a carbon nanotube electron gun (CNTeg), the
first of its kind. I tested almost a dozen carbon nanotube chips, six of which emit
desired currents „ 100 microAmps stable to within 1 microAmp over several days. I
developed a code with 10 micrometer resolution, which simulated the electron
trajectories subject to moderate electric fields, and confirmed that the ionization
source functions as expected boosting its technology readiness level. This will be the
first demonstration of nanotechnology in neutral mass spectrometery, and may prove
to be one of the most power efficient (P ă 10 milliWatts) technologies in the future of
space instrumentation.
Contents
Abstract en Francais 2
Abstract in English 4
List of Figures 10
List of Tables 17
Symbols 20
1 Introduction 22
I Dynamics of Satellite Exospheres. 29
2 Time-Dependent Atmospheric Bulges on Tidally-Locked Satellites 31
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.2 Atmospheric Bulge Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.2.1 Stellar Radiation-Driven Source . . . . . . . . . . . . . . . . . . . . 37
2.2.2 Magnetospherically-Driven Source . . . . . . . . . . . . . . . . . . 38
2.2.3 Stellar Radiation & Magnetosphere-Driven Source . . . . . . . . . 39
2.3 Orbital Evolution of Atmospheric Bulges . . . . . . . . . . . . . . . . . . . 40
2.3.1 Europa’s O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2
2.3.2 Ganymede’s O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2
2.4 Conclusion & Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.5 Dusk Over Dawn Molecular O Asymmetry in Europa’s Exosphere. . . . . 46
2
2.6 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.7 Europa Exosphere Global Model . . . . . . . . . . . . . . . . . . . . . . . 49
2.7.1 Sputtering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.7.2 Surface Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.8 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
2.8.1 Exospheric O Evolution. . . . . . . . . . . . . . . . . . . . . . . . 54
2
2.8.2 Atmospheric O Bulges . . . . . . . . . . . . . . . . . . . . . . . . 56
2
2.8.2.1 Stationary Case: Atmospheric O Bulge at Noon . . . . . 56
2
2.8.2.2 Rotating Case: Atmospheric O Bulge at Dusk . . . . . . 58
2
2.8.2.3 Rotating Case: Surface-Exosphere Coupling of O . . . . 60
2
2.9 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
7
Contents 8
2.9.1 Orbital Evolution of Exospheric and Auroral Oxygen . . . . . . . . 62
2.9.2 O Adsorption on Grains in the Water Ice Regolith . . . . . . . . . 64
2
2.9.3 ComparisonsofOxygenAuroraeandOxygenExosphere: AStrat-
ified Exosphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
2.9.3.1 Disk Emission: r ă 1.0r . . . . . . . . . . . . . . . . . 66
Eu
2.9.3.2 Limb Brightening: 0.8 ă r ă 1.2r . . . . . . . . . . . . 66
Eu
2.9.4 Coronal Expansion: r Á 1.25r . . . . . . . . . . . . . . . . . . . 72
Eu
2.9.4.1 Plasma-Driven Atmospheric Expansion . . . . . . . . . . 72
2.9.4.2 Water-Driven Expansion . . . . . . . . . . . . . . . . . . 73
2.10 Conclusion/Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
2.11 Summary and Future Prospects of Chapter 2 . . . . . . . . . . . . . . . . 77
II Perspectives on Ocean World Exospheres 78
3 Exogenic & Endogenic Water-Product Exospheres: Examining Europa
& Ganymede Ensemble 80
3.1 Introduction to Water-Product Exospheres . . . . . . . . . . . . . . . . . 81
3.2 Water-Product Observations: Water Vapor and Atomic Hydrogen . . . . 82
3.2.1 Water Vapor Plumes . . . . . . . . . . . . . . . . . . . . . . . . . . 82
3.2.2 Europa Exospheric Hydrogen Observations . . . . . . . . . . . . . 84
3.3 Orbital Evolution of Exospheric Water Products . . . . . . . . . . . . . . 84
3.3.1 H O Variability: Leading/Trailing Asymmetries . . . . . . . . . . 85
2
3.4 Perspective on Water Vapor Production Mechanisms . . . . . . . . . . . . 87
3.4.1 Surface Ice-Generated Exospheric H O . . . . . . . . . . . . . . . 88
2
3.4.2 Subsurface: Ocean-Generated Exospheric H O . . . . . . . . . . 93
2
3.4.2.1 Tidal Heating . . . . . . . . . . . . . . . . . . . . . . . . 93
3.5 Final Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
III Detection of Exospheres 96
4 Astronomical Observations of Exospheres 98
4.1 Detecting Exospheres Remotely . . . . . . . . . . . . . . . . . . . . . . . . 99
4.1.1 Direct Imaging of Exospheres . . . . . . . . . . . . . . . . . . . . . 99
4.1.2 Direct Spectroscopy of Exospheres . . . . . . . . . . . . . . . . . . 100
4.1.2.1 Infrared Spectroscopy of Exospheres . . . . . . . . . . . . 100
4.1.2.2 Sub-mm Spectroscopy of Exospheres . . . . . . . . . . . . 100
4.1.3 Transmission Photometry/Spectroscopy Observations . . . . . . . 101
4.2 Searches for Volcanic Volatiles in Europa’s Exosphere in the age of ALMA102
4.2.1 NaCl Vapor at Europa . . . . . . . . . . . . . . . . . . . . . . . . . 102
4.2.2 SO Vapor at Europa . . . . . . . . . . . . . . . . . . . . . . . . . 106
2
4.2.2.1 Sputtered SO Exosphere . . . . . . . . . . . . . . . . . . 106
2
4.2.2.2 SO Venting . . . . . . . . . . . . . . . . . . . . . . . . . 110
2
4.2.2.3 Description & Consequences of Possible SO Observa-
2
tions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
4.3 Detecting the Presence of a Toroidal Atmosphere . . . . . . . . . . . . . 118
4.3.1 EscapingExosphereMechanismstoPopulateaToroidalAtmosphere118
Contents 9
4.3.2 Detecting Neutral Tori . . . . . . . . . . . . . . . . . . . . . . . . . 119
4.4 Conclusions on Ocean World Perspectives . . . . . . . . . . . . . . . . . . 120
5 In-Situ Observations & Technology 122
5.1 Mass Spectrometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
5.2 Introduction: A Carbon Nanotube Electron Gun. . . . . . . . . . . . . . . 125
5.3 CNTegs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
5.3.1 Carbon Nanotube Field Emission . . . . . . . . . . . . . . . . . . . 126
5.3.2 CNTeg emission runs . . . . . . . . . . . . . . . . . . . . . . . . . . 128
5.3.3 CNTeg energy characterization . . . . . . . . . . . . . . . . . . . . 129
5.4 Simulations of Primary Electron Emission Under Electric Fields. . . . . . 135
5.5 Secondary Electron Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
5.6 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
5.6.1 Electron Energy Characterization . . . . . . . . . . . . . . . . . . . 138
5.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
Bibliography 1
List of Figures
2.1 Satellite coordinate system for a planet and rotating satellite. The center body is
the planet along with two orbital positions of the satellite depicted. Black vectors
represent the fixed, observers frame, where y in black indicates the incoming stellar
φs
radiationfluxvector. φ istheanti-stellarinsolationvector,definedassuchinorderto
s
synchronizethetwoframes,whereDusk: φs “3π{2andDawn: φs “π{2. Bluevectors
represent the rotating, satellite frame where φ is the satellite longitude whose origin
is the subplanetary point. We define the origin of the satellite system (blue circle) as
φ0 “φ`π to effectively compare to observations such that the subobserver longitude
is synchronized with the planetary longitude at midnight during satellite eclipse. As
the satellite rotates over an arbitrary timescale ∆t, the satellite will have rotated a
∆φ “Ω∆t. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
obs
2.2 Asimpletemperature-dependentsourceratetreatedasastellarradiation-
driven exosphere source flux Φ in our model. Values are scaled to the
T
most recent simulations for Europa’s O exosphere Φ “ 3¨108 O cm´2
2 0 2
s´1, evaluated at the antijovian point, beginning at φ “ 0, the origin
orb
of our planet-satellite system. Surface temperatures are results from the
thermal model employed in Oza et al. 2017A, where albedo and thermal
inertia effects are included. . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.3 Dusk-over-dawn asymmetry ratio, R, versus β “ Ω{ν. Ratios are deter-
mined by known orbital periods and estimated neutrals lifetimes. It is
seen that if Ω „ ν the asymmetry is maximum. The dash-dotted blue
line represents a possible upper limit to the radiation-driven dusk-over-
dawn asymmetry, as it is computed at the terminator following Eqn. 2.7.
The small circles represent various satellites in the solar system. Red,
cyan, black, and gray are the Galilean satellites (Io, Europa, Ganymede,
Callisto respectively). The magenta and green points are the Saturnian
satellitesDioneandRhearespectively. Thesolidbluelineisahemispheri-
cal average of the radiation-driven dusk-over-dawn asymmetry; more rep-
resentative of the observed asymmetry. . . . . . . . . . . . . . . . . . . . 36
2.4 Average dusk-over-dawn asymmetry ratio, ă R ą, versus φ for the
orb
magnetospherically-drivenandtemperature-dependentmagnetospherically
driven sources as indicated by ă R ą and ă R ą respectively.
mag Tmag
The blue squares are Monte Carlo simulations from Oza et al. 2017 in-
dicated a non-adsorbing O2 population. The black squares are the dusk-
over-dawn oxygen aurorae data by HST. The black dotted line represents
a temperature-enhanced case where we triple the temperature enhance-
ment factor fpφ q to confirm the increase in asymmetry towards dusk.
s
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
10