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 SO 2 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 H 2 0 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 A eff by X times (X = 2-10) - UV and visible stellar occultations: increase A eff by X times (X = 2-10) - Scale height measurements in planetary atmospheres: increase angular resolution by X times (X = 2-10)