Results and analysis of in situ
measurements of Titan’s atmosphere
Darsa Donelan
October 3, 2013
AST 6112
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The shape of things to come
• Atmospheres
– Planetary atmospheres
– Titan’s atmosphere
• Cassini-Huygens
– Mission
– Huygens results
• Gravity Waves
– Definition
– Wavelet analysis (if time)
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Planetary atmospheres
• An atmosphere is a gaseous envelope
surrounding a planetary body.
• Planetary atmospheres are nonlinear
systems that resist easy analysis or
prediction.
• Atmospheric waves are of dynamical
interest because they transport
energy and momentum.
• There is a relationship between the
observed structure of Titan’s
atmosphere and the generation and
propagation of gravity waves.
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Murky meteorology
Titan
Titan has extended atmosphere
due to scale height:
Earth
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Shall I compare thee to a terrestrial
planet?
Titan’s surface-atmospheric interactions give many similarities with
terrestrial planets. Titan is an outstanding laboratory for comparative
planetology.
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Optically thick atmosphere (~95% N2, ~5% CH4)
Superrotating atmosphere (like Venus)
Pole inclined 26.7° (seasons: Earth-23.5 °, Mars-25.2 °)
Hydrological cycle (with CH4)
An active hydrology like Earth’s, atmospheric dynamics like Venus and
Mars, seasons like Earth and Mars, etc.
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Tedious seasons
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Cassini-Huygens: tools for Titan
Spacecraft specs:
• Height: 6.8 m (22 ft)
• Diameter: 4m (13 ft)
• Mass: 2500 kg (2.8 tons)
(fueled) 5600 kg (6 tons)
• Power: 700 Watts at SOI
• Huygens Probe: 320 kg (~700 lbs)
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More tools for Titan
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GCMS – gas chromatograph/mass spectrometer
DISR – descent imager/spectral radiometer
DWE – Doppler wind experiment
ACP – aerosol collector/pyrolyzer
HASI – Huygens atmospheric structure instrument
SSP – surface science package
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Blast off!
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Cover me – I am going in
Entry, descent, and landing of Huygens (January 14, 2005)
156 km
1st parachute
155 km
(2sec)
2nd parachute
(15 min)
105 km
3rd parachute
2 hr 25 minutes of descent
72 min of data from the surface (via Cassini)
3 hr 13 min of radio signal from the surface
(from Earth-based radio astronomy telescopes)
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Recovering the Doppler wind
experiment
First in situ measurement of superrotation!
Uncertainty: 1 m/s
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What are you, dense?
Uncertainty (entry): 10%
Due to probe velocity
and drag coefficient
Entry
Descent
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Digesting the data - HASI
• Acceleration data -> density profile ρ(z)
• Hydrostatic equilibrium eqn -> pressure profile p(z)
• Ideal gas law -> temperature profile T(z)
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Titan under pressure
Uncertainty (entry): 10%
Uncertainty (descent): 1%
Entry
Descent
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Taking Titan’s temperature
Uncertainty (entry): 10%
Uncertainty (descent):
±0.25 K (60-100 K)
±1 K (>110 K)
Entry
Descent
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Gravity (Buoyancy) waves
• Phenomena in planetary atmospheres that transport energy and
momentum from the lower regions into the upper atmosphere
• Transverse waves
• Maintained by gravitational and buoyancy forces
• Amplitude increases exponentially with height (density decreases)
in order to conserve energy (if no wind shear is present)
• As wave dissipates, energy and momentum absorbed into the
upper atmosphere
• Sources: wind shear, convection, air flow over topography
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Temperature gradient suggests GWs
on Titan
Adiabatic lapse rate:
≈ 1.4 K/km
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Wavelet analysis
• Continuous wavelet transform
• Morlet wavelet basis for discrete series measurements xn (n=0…N1) with equal spacing δt
•
- dimensionless ‘time’ parameter
• ω0 =6 - dimensionless frequency (enough vanishing moments)
• S - scale of variability
• Preferred method over Fourier analysis – show how amplitude of
variations changes within the length (altitude) of the data set
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Wavelet analysis – zonal winds
Re{Wn(S)}
|Wn(S)|
Edge
effects
Contours:
White: +2 m/s
Green: +1 m/s
Red: -1 m/s
Black: -2 m/s
Black: 2 m/s
White: 1 m/s
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Wavelet analysis - temperature
Re{Wn(S)}
|Wn(S)|
Contours: intervals of ΔT = 1 K
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Wavelet analysis - results
• Layers with the same vertical scale resembling a wavetrain at a fixed wavelength – consistent with GW
– 4 m/s amp in winds and 2 K amp in temp at S=17 km
– 10 m/s amp in winds and 4 K amp in temp at S=35 km
• Smaller scales have smaller amp whereas larger scales
reach larger amp – consistent with GW
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