the mighty weather of saturn andrew ingersoll (api@gpsltech)

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The Mighty Weather of SaturnAndrew Ingersoll (api@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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CIRSTemperatures and the thermal wind equation

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• 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

VIMS: Conclusions

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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: ISSCassini 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

Saturn’s 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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That’s all for now Expect monthly updates

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Commentary on Ingersoll’s 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 1990’s (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) < 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 Saturn’s 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 (SED’s) 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 Saturn’s 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 Saturn’s 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. That’s all for now, stay tuned.