HST Observations of
Planetary Atmospheres
John T. Clarke
Boston University
Hubble Science Legacy
3 April 2002
Venus
- Near-UV images reveal
cloud motions and winds
- UV spectra track SO2
composition, atomic
abundances, and
isotopic ratios
- Line profiles indicate
superthermal atoms and
atmospheric escape
Mars
- Images reveal
seasonal cycles
in polar caps and
in atmosphere
- Images and spectra reveal water and ozone abundances
- UV spectra measure atmospheric D/H ratio, with
implications for escape of H20 over Martian lifetime
Jupiter/Saturn
- UV images reveal complex
auroral structures and
time variations
- UV spectra measure Io’s
plasma torus composition
and temperature
- Campaign of images and spectra tracked the comet
Shoemaker/Levy 9 impacts and effects on the
atmosphere and magnetosphere
Uranus/Neptune
- Visible and IR images
permit tracking of cloud
features and motions,
atmospheric winds
- Seasonal cycles in
atmosphere are revealed
in series of images
spanning many years
- Planetary rings and satellites revealed in near-IR images
Pluto and Triton
- UV spectra indicate
compositional
differences between
Pluto and Triton
- Stellar occultation first
revealed altitude structure of Pluto’s atmosphere
- Pluto’s atmosphere believed to be in hydrodynamic
escape, flowing out into space
Future UV/Optical Planetary Science
from Space:
- What are the BIG scientific questions?
- What capabilities are needed to answer these
BIG questions?
Perspective: the Changing Nature of
Planetary Science
- The discovery of extra-solar planets is historic, opening
new areas of research that overlap with classical
astronomy, offering new targets for study, and changing
the way planetary scientists think
- An increasing fraction of planetary science is being done
by remote sensing observations
- We need to characterize the planets in our solar system
to be able to understand extra-solar planets
Two BIG Scientific Questions in
Planetary Science:
1. How and why do solar systems form, and what do
they look like when they do?
2. What makes planets evolve into habitable worlds?
- To address these questions, we will need to
understand the physical processes in planetary
atmospheres in general, for both terrestrial and
giant planets
Specific Techniques for Future
Observations of Planetary Atmospheres
- High sensitivity UV spectroscopy (increased
aperture compared with HST)
- UV and visible stellar occultations (increased
aperture compared with HST)
- Scale height measurements in planetary
atmospheres (increased angular resolution
compared with HST)
- Cause and effect measurements of variable
phenomena (more observing time!)
The Nature of Distant Planetary
Atmospheres and Surfaces
- Solar reflection spectra of planets and satellites reveal
composition of atmospheres and surfaces.
- Present HST sensitivity limits spectra of distant and/or
small objects (e.g. Galilean and more distant satellites,
Neptune, Pluto) to near-UV wavelengths λ > 1800 Å.
- An increase in effective area would extend UV spectra to
more distant and fainter objects, including asteroids and
possibly Kuiper belt objects, and extend wavelengths down
to far-UV range where simple atoms and molecules have
the strongest absorptions.
Detailed Studies of Atmospheres
by Stellar Occultations
- Visible/UV stellar occultations provide altitude profiles
of planetary and satellite atmospheres, with altitude
resolution proportional to the time resolution or S/N.
- UV occultations provide the highest sensitivity to small
columns of gas (e.g. Io and Ganymede, Triton etc.)
- The present rate of suitable candidate events is 1 per
several years with HST.
- An increased effective area compared with HST
will greatly increase the number of occultations available
and the signal to noise of each event.
Thermal Profiles and Escape Fluxes
of Planetary Atmospheres
- Sufficiently high angular resolution can measure
atmospheric emission scale heights, yielding
temperatures and # of superthermal atoms.
- ex.’s:
Mars, 0.1 arc sec = 40 km
Jupiter, 0.1 arc sec = 300 km
- Typical scale heights are on the order of tens of km,
and up to hundreds or thousands of km for lightest
atoms and highest temperatures.
- Higher angular resolutions & sensitivities are needed.
Observations of Planetary Atmospheres:
Improvements
- High sensitivity UV spectroscopy: increase
Aeff by X times (X = 2-10)
- UV and visible stellar occultations: increase
Aeff by X times (X = 2-10)
- Scale height measurements in planetary
atmospheres: increase angular resolution
by X times (X = 2-10)