The Mighty Weather of Saturn Andrew Ingersoll ([e-mail])

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The Mighty Weather of SaturnAndrew Ingersoll
(api_at_gps.caltech.edu)

• Winds and temperatures - no longer the windiest
  planet, or is it just a variation with altitude?
• Rotation rate - kilometric radio emissions and
  the magnetic field the ground is shifting
• Storms - cloud activity correlated with radio
  discharges - evidence for lightning
• Composition - CH4, NH3, PH3, C2H2, C2H6, tracers
  of the large-scale stratospheric circulation,
  aurora, H2/He (progress and plans)

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CIRS Temperatures and the thermal wind equation
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VIMS Conclusions

• VIMS can measure Saturn winds using backlit
  features in 5 microns
• Early results consistent with Voyager but
  inconsistent with Hubble observations 390 90
  m/s (8 deg S lat), 465 90 m/s (2 deg N lat)
• Our technique is more sensitive to cloud base
  not cloud top

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RPWS Saturn kilometric radiation
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MAG SOI FGM Data
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Line Voyager Yellow HST Red Green
ISS Cassini continuum Blue ISS Cassini methane
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ISS Merging of spots in an anticyclonic shear
zone
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RPWS ISS Electrostatic discharges storms
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UVIS H2 band data on Saturn
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Saturns H2 Dayside S. Aurora UVIS lab
spectrum (differ due to CH4, etc.)
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Saturn in atomic and molecular hydrogen emission
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UVIS acetylene
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CIRS spectrum (a small portion of it) CH4, PH3,
CH3D
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Thats all for now Expect monthly updates
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Commentary on Ingersolls Saturn slides

• 1. Title slide The Mighty Weather of Saturn
• 2. Earth Absorbed sunlight greatest at equator.
  Atmosphere and oceans transport heat poleward but
  transport does not eliminate T(eq) - T(pole),
  which is 30 C. One eastward jet stream in each
  hemisphere, speeds 40 m/s (90 mph).
• 3. Winds on the giant planets (Voyager) Several
  jet streams in each hemisphere, speeds 450 m/s
  Saturn the windiest.
• 4 5. Tracking spots to measure winds (ISS).
  Choose a uniformly rotating reference frame (not
  obvious for a fluid planet), wait one rotation
  (10 hours), measure the new positions of the
  spots. Radio emissions define the reference
  frame, but there are problems and surprises.
• 6. Peering below the clouds (IRTF, Orton) High
  thermal emission at infrared wavelengths reveals
  hole in the clouds at the South Pole.
• 7. Variable winds of Saturn. Solid line is
  Voyager in 1981. Yellow is Hubble in 1990s
  (Sanchez-Lavega et al). Red and green are Cassini
  ISS visible (peers deep). Blue is ISS methane
  filter (sees high clouds only). Wind speed
  decreases with altitude. Is that the whole story,
  or did it vary with time? The kinetic energy is
  huge. Turning it into heat would have warmed the
  atmosphere for years.

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• 8. Atmospheric temperatures (Cassini CIRS) T(eq)
  - T(pole) lt 3 C at 500 mbar (cloud tops).
  Increase with latitude (by 10 C) within 10 deg of
  equator above 100 mbar implies winds decrease
  with altitude (thermal wind equation), consistent
  with ISS methane band results.
• 9. Viewing the planet at several levels (Cassini
  VIMS) 2.12 microns sees only the high cloud and
  rings in reflected sunlight. 1.59 microns sees
  deeper in reflected sunlight. 5.10 microns sees
  the deepest - thermal radiation emerging through
  holes in the clouds.
• 10 11. Tracking features at 5.10 microns
  (VIMS) Follow the holes in the deep clouds.
  Speeds agree with Voyager, implying speed greater
  at depth, consistent with ISS.
• 12. Radio waves from the magnetosphere are used
  to infer the planetary rotation rate (Cassini
  RPWS) Assume the source is tied to the magnetic
  field assume the field rotates with the
  planetary interior. SKR Saturn kilometric
  radiation.
• 13. Change in SKR period between Voyager and
  Cassini (RPWS) Either the planet is spinning
  more slowly (unlikely) or SKR is not measuring
  planetary rotation.
• 14. Magnetic field of Saturn (Cassini MAG)
  Symmetric about the pole - offers no information
  about rotation rate.
• 15. Discrete storms in Saturns atmosphere (ISS)
  Similar iin size but much longer-lived than
  terrestrial storms. The Dragon Storm was active
  for at least 9 months.

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• 16. Jet structure (ISS) Anticyclonic
  counterclockwise in Southern Hemisphere high
  pressure south side of the westward jets
  where all the storms are. Storm alley is the
  band from 35 S - 40 S.
• 17. Spots merging in storm alley (ISS) One week
  sequence. Merging destroys spots. How do they
  form?
• 18. Formation of spots in the Dragon Storm (ISS)
  Time increases down the page, then over. Last two
  frames show new spots going off to the left.
• 19. Saturn electrostatic discharges (SEDs) and
  the Dragon Storm (RPWS and ISS) Short radio
  bursts (AM static), probably from lightning. Up
  to 50 bursts per hour on some days. Highly
  periodic, correlated with the Dragon Storm. The
  asterisks are when the DS was crossing the
  central meridian as seen from Cassini. Inset
  shows all the data together The burst activity
  is 1-3 hours before the DS crossing, while the DS
  was still on the dark side (the planet was half
  illuminated as seen from Cassini). Perhaps the
  radio waves are blocked by the dayside
  ionosphere.
• 20. Aurora over the South Pole (Cassini UVIS)
  H, O, e- in the magnetosphere cause the
  atmosphere to glow. Ice in the rings is the
  probable source of H and O. The rings absorb
  charged particles, so the plasma density is low.
• 21. Auroral emission is from H2 in Saturns
  atmosphere.
• 22. H and H2 permeate the Saturn system (UVIS)
  Auroras and escaping neutrals.

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• 23. Acetylene (C2H2), a disequilibrium species
  (UVIS) Produced from methane (CH4) by solar UV
  and charged particles from magnetosphere.
• 24. Thermal spectrum of Saturns atmosphere
  (CIRS) Wavenumber is 1/(wavelength in cm).
  Wavenumber 1000 is 10 microns wavelength. The
  big questions hinge around the enrichment of each
  element (C/H, O/H, N/H, S/H, P/H) on Saturn
  relative to the Sun.
• 25. Methane, phosphine, and ammonia lines in the
  far IR (CIRS) The shapes of the lines reveal
  composition, temperature, and pressure of the
  constituents.
• 26. Thats all for now, stay tuned.