Using speed of sound measurements to constrain the Huygens Probe descent profile

1. Using speed of sound measurements to constrain the Huygens Probe descent profile H. Svedhem, J-P. Lebreton ESA/RSSD, NL J. Zarnecki, B. Hati Open University, UK Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

2. John Tyndall’s atmospheric experiment 1875 Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

3. About 100 years later Jean-Pierre Lebreton proposes to fly an acoustic sensor to Titan • We now talk about miniaturisation….. Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

4. The Acoustic Properties Investigation (API) of the Surface Science Package on Huygens • API has two sets of sensors and one card of electronics incorporated in to the SSP Top Hat and electronics box • API-S, (sounder) is a monostatic SODAR for detection of atmospheric precipitation during the descent, surface characterisation during the last phase of the descent and detection of sea depth in case of landing in a liquid. • API-V, (velocity) will measure the speed of sound across a 15 cm long path during the descent from an altitude of about 50 km down to the surface, and in the liquid in case of landing in a liquid. Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

5. API-S • The API-S in principle works as a conventional SODAR. • The return signal is proportional to the number density of the scattering particles in the scattering volume and to the particle diameter to the 6th power. (Rayleigh scattering) • For both volume scattering and surface scattering the signal is inversely proportional to the square of the distance. • The similarities to Radars are striking. By coincidence the wavelength of the API-S and the probe altimeter are both about 2 cm. Comparisons will be useful. Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

6. API-S Modes 1/2 • Atmospheric sounding mode, >7km. Search for hydrometeors and turbulence. Pulse length 10 ms. Binned samples for the closes 50 m are stored each 2 seconds. • Surface proximity mode,7km>h>1km. Pulse length 10 ms. Search for surface return AND hydrometeors. Binned samples at higher resolution around the surface bin each 3 seconds. • Near surface mode. h<1km. Pulse length 2 ms. Search for surface structure and topography. Binned samples at highest resolution around the surface bin each second. Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

7. API-S Modes 2/2 • Surface mode. After impact, search for depth of liquid. Pulse length 10 ms. Send one pulse, listen for 10 s. Binned data around the maximum return. Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

8. API-V mode • One mode only at h<60 km. Sensor A transmits a pulse and start a 4 Mhz counter, sensor B receives the pulse and stops the counter. Immediately afterwards the sequence is repeated in the reverse direction. Both data are stored. Frequency of measurement is 1 s. Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

9. API-S reflectivity factors, Z Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

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11. Performance API-S • Reflectivity factors on earth are known, (previous graph). Titan situation is hard to estimate. • Garry (1996)estimated, based on data from Toon et al (1988) that the reflectivity factors at Titan are too low to be detected by API-S. The method for these calculations was however unconventional and seem to give too pessimistic results. Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

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13. Error analysis • M=  ·R · T · c-2 • M/M=((T/T)2+ (2c/c)2)1/2 • For the gasses at the temperatures we have c is about 200 m/s, we get with 250 ns resolution and 15 cm path, 2c/c  7 ·10-4. T  0.1 K which at 100 K gives T/T = 10-3. The contributions are thus of the same order of magnitude. • M/M  1.2 ·10-3 . Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

14. Empirical approach, gas • In stead of calculating the mean molecular weigh one may go directly to find mixing ratios from calibrated measurements. • The mixing ratio will be accurate to better than 1 % for binary gasses. The number is dependent on which species are involved. • This will work well for binary mixtures but is difficult for mixtures of three or more gasses (or liquids) • For mixtures of three or more components a test of the expected sound speed can give useful constraints to measurements by other instruments. Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

15. Empirical approach, liquid • For the liquids the sound speeds are typically ten times higher while the resolution remains 250 ns. Hence the c/c is the dominating error. • The spread in sound speed is larger and therefore a precision in the mixing ratio similar to that of gasses will be achieved, i.e. about 1% for binary mixtures. Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

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21. What about the miniaturisation, did it work out? Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003

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