Planetsystemets fysik MN1

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Planetsystemets fysik MN1

• General inventory list
• The Sun, heliosphere
• 8 major planets
• Natural satellites, ring systems, magnetospheres
• Minor planets (asteroids, Trojans, NEOs, TNOs,
  scattered disk)
• Comets
• Meteorides
• Dust

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Relative sizes and colors of the planets
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Planetary physical properties
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Properties of solar system objects

• Quantities derivable from observations (Earth
  based or in situ)
• Orbit
• Mass
• Size
• Rotation state and direction
• Shape
• Temperature
• Magnetic field
• Surface composition
• Surface structure
• Atmospheric structure and composition
• Observations are used to constrain planetary
  properties such as bulk composition and interior
  structure

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Orbits

• Determined from
• Observations of astrometric positions
• 3 observations required
• In the 2-body problem, the orbit is described by
• Keplers and Newtons laws
• Six orbital elements
• a, semimajor axis
• i, inclination
• e, eccentricity
• ?, longitude of ascending node
• ?, argument of perihelion
• T, time of perihelion passage

Q a(1e)
Q a(1-e)
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Keplers laws

• All planets move along elliptical paths with the
  Sun at one focus
• The heliocentric distance rs is given by
• rs a(1-e2)/ (1e cos f)
• where a semimajor axis, e eccentricity, f true
  anomaly (angle between perihelion and
  instantaneous position),
• e (1 - bm2/a2)1/2,
  where 2bm b
• A line segment connecting any given planet and
  the Sun sweeps out an area at a constant rate due
  to conservation of angular momentum,
• dA/dt constant
• The square of a planets orbital period about the
  Sun is proportional to the cube of the semimajor
  axis
• P2 4p2 a3 / G (m1 m2)
• where P period of the orbit, m1 mass of central
  object, m2 mass of planet, G gravitational
  constant

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Mass

• The mass of an object can be derived from
  the gravitational force it exerts on other bodies
• Orbits of satellites (Kepler III)
• P2 4p2a3/G(m1m2),
• m1m2 is solved for from observed orbit (P and
  a), assuming m1gtgtm2
• Perturbations (for objects without satellites)
• short-term (close encounters)
• long-term (objects locked in stable orbital
  resonances)
• Spacecraft tracking data
• From Doppler shift of transmitted radio signal
• Resonant action on rings (amplitude of spiral
  density waves or wakes)
• Non-gravitational forces (comets)

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Rotation

• Movement of markings on resolved disk
• Useful for planets with albedo features
• Atmospheric winds may cause period to vary with
  time and latitude
• Radio emission of core or magnetosphere
• Particles trapped in magnetic fields are
  accelerated and emit radio waves
• Periodicity observed is that of the core
• Light curve observations
• Total brightness as a function of time
• Due to variations in albedo or projected area
• Doppler shifts derived from radar or reflectance
  spectra
• Useful for resolved featureless bodies
• Gives period and direction of rotation axis

Great Red Spot longitude drift chart
Asteroid light curve
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Size

• Measurement of angular diameter (for resolved
  objects)
• knowledge of the orbit provides the distance
• Occultations (for unresolved objects)
• timing of a number of observed chords within
  occultation zone
• Radar echoes
• only for nearby objects radar echo signal
  strength drops as 1/r4
• Lander and orbiter triangulation
• Radiometric method
• thermal emission spectrum observed
• size and albedo derived F(?)
  F(Ap, ?, ?, Teff)
• (size/albedo ambiguity in visible)

Occultation coords of asteroid Laetitia
Radar echo image of asteroid Toutatis
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Shape

• Direct imaging
• From ground or spacecraft
• Stellar occultation chords
• Radar echoes
• Light curve models
• Visible observations shape/albedo ambiguity
• Shape of central flash
• Observed when the center of a body with an
  atmosphere passes in front of an occulted star
  caused by focusing of light rays refracted by the
  atmosphere

Asteroid shape model based on visible light curve
observations
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Temperature

• Energy balance between solar insolation and
  thermal reradiation Fabsorbed Femitted
• Complicated by
• Internal heat sources (e.g., Io, Jupiter)
• Diurnal, latitudinal and seasonal temperature
  variations
• Greenhouse effect, a temperature increase due to
  an atmosphere or surface layer more transparent
  at optical than thermal wavelengths
• In-situ thermometer on lander
• Shape of thermal infrared spectrum
• Most solid and liquid planetary materials are
  nearly perfect blackbody radiators with emission
  peak at near to mid infrared wavelengths
• Derived temperature may be a function of
  wavelength due to
• variation of temperatures on surface
  (pole-equator variation, volcanic hot spots,
  albedo variations, surface material variation of
  emissivity)
• opacity variations of an atmosphere, allowing
  different altitudes to be probed

Infrared spectra and blackbody temperatures of
Mars, quartz, and the Earth
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Surface composition

• Reflectance spectrum
• Optical, IR windows from Earths surface
• UV from spacecraft
• Thermal (difficult to interpret)
• infrared spectra
• radio data
• Microwave radar reflectivity (from surface or
  orbiting spacecraft)
• electrical and thermal conductivity
• X-ray and ?-ray fluorescence (from orbiter or
  lander)
• Chemical analysis (in situ or on returned
  samples)
• Mass spectroscopy (in situ or on returned samples)

Near-infrared spectra of the Moon
Isotope abundance ratios on Mars
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Surface structure

• Large scales (gtcm)
• Imaging (at optical/IR/radio wavelengths)
• Single camera imaging
• Stereoscopic imaging
• Two angle geometry data resolves slope-albedo
  ambiguity
• Radar imaging
• Laser altimetry (spacecraft)
• Small scales (ltcm)
• Radar echo brightness
• High reflectance for size scales near radar
  wavelength
• Phase angle intensity observations
• Opposition effect
• Large phase angle brightness

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Atmospheric structure and composition

• Composition and structure (temperature-pressure
  profile) may be derived from
• spectral reflectance data at visible wavelengths
• thermal spectra and photometry and infrared and
  radio wavelengths
• Doppler shift of satellite signal during
  occultation
• phase shift due to atmospheric refraction
• density and temperature profile derived from
  refractivity profile
• attenuation of radio signals from surface landers
  and atmospheric probes
• in situ mass spectrometers

Mars temperature profile in S winter
Pressure profiles of the giant planets
atmospheres
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Magnetic fields

• The magnetic fields of the planets may be
  approximated by dipoles, with higher order
  perturbations to account for irregularities
• Types of magnetic field
• generated by internal dynamo operating in a fluid
  region (giant planets, Earth, Mercury)
• induced by interaction between solar wind and
  charged particles in ionosphere/atmosphere
  (Venus, comets)
• localized crustal magnetic fields due to remnant
  ferromagnetism (Mars, Moon)
• very small fields caused by interaction between
  solar wind and conducting regions within the body
  (e.g., asteroids)
• Magnetic fields may be detected
• directly using an in situ magnetometer
• indirectly via
• effects of accelerating charges producing radio
  emissions
• presence of aurorae, caused by charged particle
  precipitation in in a plöanets upper atmosphere

Jupiters synchrotron radiation at ? 20 cm
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Planetary interiors

• Interior not directly accessible to study, but
  properties may be inferred from observable
  parameters, indirect clues and constraints
• Bulk composition
• meteorites studied in the laboratory
• observed size and mass, surface composition,
  heliocentric distance, cosmogonic abundance
  assumptions
• material properties from laboratory data and
  quantum mechanical calculations
• Internal structure
• gravitational field and rotation rate degree of
  concentration of mass
• velocity and attenuation of seismic waves
  measured by in situ seismometric network
  density, rigidity and other physical properties,
  which in turn depend on composition
• surface structure (volcanism, plate tectonics),
  energy output
• magnetic field structure (dipole fields generated
  near core, higher-order fields closer to surface)
• free oscillation periods of gaseous planets
  (similar to helioseismology)