Circumplanetary dust dynamics [Elektronische Ressource] : application to Martian dust tori and Enceladus dust plumes / von Martin Makuch

Martin Makuch

Sujets

• Astronomie

Informations

• Publié par: universitat_potsdam
• Publié le: 01 janvier 2007
• Nombre de lectures: 20
• Langue: English
• Poids de l'ouvrage: 9 Mo

Extrait

Aus dem Institut fur¤ Physik der Universitat¤ Potsdam
CIRCUMPLANETARY DUST DYNAMICS:
APPLICATION TO MARTIAN DUST TORI
AND ENCELADUS DUST PLUMES
Dissertation
zur Erlangung des akademischen Grades
doctor rerum naturalium
(Dr. rer. nat.)
in der Wissenschaftsdisziplin Theoretische Physik
eingereicht an der
Mathematisch Naturwissenschaftlichen Fakultat¤
der Universitat¤ Potsdam
von
Martin Makuch
Potsdam, den 15. November 2006













































Elektronisch veröffentlicht auf dem
Publikationsserver der Universität Potsdam:
http://opus.kobv.de/ubp/volltexte/2007/1440/
urn:nbn:de:kobv:517-opus-14404
[http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-14404] Abstract
Our Solar system contains a large amount of dust, containing valuable information about our close cosmic envi-
ronment. If created in a planet’s system, the particles stay predominantly in its vicinity and can form extended dust
envelopes, tori or rings around them. A fascinating example of these complexes are Saturnian rings containing a
wide range of particles sizes from house-size objects in the main rings up to micron-sized grains constituting the
E ring. Other example are ring systems in general, containing a large fraction of dust or also the putative dust-tori
surrounding the planet Mars. The dynamical ? life of such circumplanetary dust populations is the main subject
of our study.
In this thesis a general model of creation, dynamics and death of circumplanetary dust is developed. Endo-
genic and exogenic processes creating dust at atmosphereless bodies are presented. Then, we describe the main
forces in uencing the particle dynamics and study dynamical responses induced by stochastic uctuations. In or-
der to estimate the properties of steady-state population of considered dust complex, the grain mean lifetime as a
result of a balance of dust creation, life and loss mechanisms is determined. The latter strongly depends on the
surrounding environment, the particle properties and its dynamical history. The presented model can be readily
applied to study any circumplanetary dust complex.
As an example we study dynamics of two dust populations in the Solar system. First we explore the dynamics
of particles, ejected from Martian moon Deimos by impacts of micrometeoroids, which should form a putative tori
along the orbit of the moon. The long-term in uence of indirect component of radiation pressure, the Poynting-
Robertson drag gives rise in signi cant change of torus geometry. Furthermore, the action of radiation pressure
on rotating non-spherical dust particles results in stochastic dispersion of initially con ned ensemble of particles,
which causes decrease of particle number densities and corresponding optical depth of the torus.
Second, we investigate the dust dynamics in the vicinity of Saturnian moon Enceladus. During three ybys of
the Cassini spacecraft with Enceladus, the on-board dust detector registered a micron-sized dust population around
the moon. Surprisingly, the peak of the measured impact rate occurred 1 minute before the closest approach of
the spacecraft to the moon. This asymmetry of the measured rate can be associated with locally enhanced dust
production near Enceladus south pole. Other Cassini instruments also detected evidence of geophysical activity
in the south polar region of the moon: high surface temperature and extended plumes of gas and dust leaving the
surface. Comparison of our results with this in situ measurements reveals that the south polar ejecta may provide
the dominant source of particles sustaining the Saturn’s E ring.Contents
1 Introduction 1
2 Dynamical Life of a Dust Grain 7
2.1 Dust Creation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.1 Exogenic Processes - impact ejecta Scenario . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.2 Endogenic - (Cryo)Volcanism . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2 Dust Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.1 Deterministics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.1.1 Gravity of Oblate Planet (G+J2) . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1.2 Radiative Effects (RP + PR + SH) . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.1.3 Lorentz Force (L) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.1.4 Plasma Drag (PD + CD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.2 Stochastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2.2.1 Sources of Stochasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2.2.2 Stochasticity Induced by Particle Non-sphericity . . . . . . . . . . . . . . . . . 17
2.2.2.3 Analytical Solution of the Stochastic RP . . . . . . . . . . . . . . . . . . . . . 20
2.3 Grain Lifetimes and Sinks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4 Brief Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3 Applications 26
3.1 impact ejecta Dust Production at Mars and Saturn . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2 Martian Dust Complex: The Deimos Torus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2.1 Orbit-averaged Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2.2 Deterministic Solution of the Photo-Gravitational Problem (J2 + RP + PR) . . . . . . . . 30
3.2.2.1 Radiation Pressure and Planetary Oblateness . . . . . . . . . . . . . . . . . . . 30
3.2.2.2 Poynting-Robertson Drag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.2.2.3 Impact of PR Drag on the Deimos Torus Geometry . . . . . . . . . . . . . . . 33
3.2.2.4 Particle Lifetimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.2.3 Stochastic In uence of Radiation Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.3 Enceladus Dust Plumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.3.1 The Cassini Observation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.3.2 Dust Ejecta Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.3.2.1 Isotropic impact ejecta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.3.2.2 Localised South Pole Source . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.3.3 Comparison of Theory and Observation . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4 Summary and Conclusions 51
4.1 Model of Particle Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.3 Limitations & Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Acknowledgments 54
vvi CONTENTS
Bibliography 55
A Long-term Dynamical Evolution of Dusty Ejecta from Deimos 63
B Stochastic Circumplanetary Dynamics of Rotating Non-spherical Dust Particles 79
C E Ring Dust Sources: Implication from Cassini’s Dust Measurements 97
D Cassini Dust Measurements at Enceladus and Implications for the Origin of the E Ring 109
E Supporting Online Material for Paper D 115Chapter 1
Introduction
Our Solar system is not composed by the Sun and the eight planets only. Additionally besides asteroidal and
cometary objects it contains a signi cant amount of dust. In the past, the cosmic dust was being overlooked or
even considered as nonexistent. Some well known phenomena as meteors, zodiacal light or cometary tails, which
were for long time being considered to be of atmospheric origin, have been related to cosmic dust for the rst
time in the 17th century. The observation of a spectacular Leonid meteor shower (left panel of Fig. 1.1) and the
fact that the meteors appeared to emerge from a stationary point in the constellation Leo led many scientists to
the conclusion that these meteors were of extraterrestrial origin. The idea that the observed meteors or so-called
shooting stars are caused by dust particles entering the Earth’s atmosphere was put forward for the rst time
by Ernst Chladni, the father of acoustics. Similarly, Giovanni Cassini proposed that the phenomena as zodiacal
light (right panel of Fig. 1.1) or gegenschein are results of light being scattered at dust complexes in the ecliptic.
Slowly, with the overall headway in astronomy, empty space between planets and stars got lled by gas clouds
and dust particles. Even in the beginning of the 20th century astronomers considered dust in space merely as an
annoying obstacle, that blocks the light coming from astronomical objects.
Figure 1.1: left: An all-sky image taken during the maximum o