HST Observations of Planetary Atmospheres

Transcription

1 HST Observations of Planetary Atmospheres John T. Clarke Boston University Hubble Science Legacy 3 April 2002

2 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

3 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

4 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

5 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

6 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

7 Future UV/Optical Planetary Science from Space: - What are the BIG scientific questions? - What capabilities are needed to answer these BIG questions?

8 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

9 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

10 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!)

11 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.

12 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.

13 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.

14 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)