# Presentation: 943680

## The Huygens Atmosphere Structure Instrument (HASI) results at Titan

**The Huygens Atmosphere Structure Instrument (HASI) results at Titan**

**M. Fulchignoni****1,2**** and the HASI team**
- 1LESIA Obs. Paris-Meudon, France, 2Université Paris 7

## Study of Titan’s atmosphere and surface

- by measuring
**acceleration** (ACC)
**pressure** (PPI)
**temperature** (TEM)
**electrical properties** (PWA, RAU)
***Heritage: ******Pioneer Venus, Venera, Galileo, and Viking probes***

_**Huygens Atmospheric Structure Instrument (HASI)**_

- Principal Investigator: M. Fulchignoni

## HASI operations

- HASI was the first instrument to be operating
- ACC measurements started at ~2800 km
- After parachute deployment, direct p & T, and electrical measurements
- HASI data represent the unique contribute to the Huygens probe   trajectory and attitude reconstruction

## 	T & p directly measured by sensors having access to the unperturbed field outside the probe boundary layer. PWA booms deployed: direct measurements of electrical properties and acoustic recording

**Entry phase**

*From ~ 160km down to surface**descent under parachute *
**	****T** & **p** directly measured by sensors having access to the unperturbed field outside the probe boundary layer. PWA booms deployed: direct measurements of **electrical properties** and **acoustic recording**

_**HASI measurements at Titan**_

*From **~ 1500 to 160 km*
- 	**atmospheric physical properties** from **accelerometer data**

**Descent phase**

## HASI switched on before atmospheric entry
HASI ACC measurements starting from ~ 2800 km
Most accurate accelerometer ever flown in a planetary probe
Sensitivity threshold (0.3g3E-06 m/s2) allows to measure Probe coning motion.

Atmosphere detected at ~ 1600 km

During entry, atmospheric physical properties from accelerometer data 

- HASI ACC measurements starting from ~ 2800 km
- Most accurate accelerometer ever flown in a planetary probe
- Sensitivity threshold (0.3g3E-06 m/s2) allows to measure Probe coning motion.
- Atmosphere detected at ~ 1600 km
- During entry, atmospheric physical properties from accelerometer data

_**HASI operational report**_

***ACC provided by UKC-Open University******CoIs: J.C. Zarnecki, J.A.M. Mc Donnell***

**Spin **derived from ACC: ~**7 rpm**

## HASI ACC during entry

***Credit: ESA / ASI / UPD / OU / ***

## Entry phase

- probe mass (kg)

- aerodynamic drag coefficient

- probe cross-sectional area (m2)

- velocity relative to atmosphere in the direction of descent (ms-1)

- acceleration component in the direction of descent (ms-2)

**density profile** from the top of atmosphere (1500 km) to parachute deployment at approx 160 km

- From **acceleration measurements**

**Entry phase**

_**HASI atmospheric structure**_

## HASI density profile

_**HASI density profile**_

***Credit: ESA / ASI / UPD / OU / ***

## Upper atmosphere

**Entry phase**

- _Indirect T & p measurements_
- Hydrostatic equilibrium + perfect gas		gravity	dp=-gdz=-(pg/RT)dz			g(z)=g0(RTitan/z)2
**p(z)** integrating (1) with measured ******(z)** (initial condition to be assumed)**T(z)** from (2) 	T= p/R
_**Density, pressure and temperature profiles**_

- Hydrostatic equilibrium 	dp=-gdz		(1)
- Equation of state of perfect gas	= p/RT		(2)
******(z)**=-2(m/CDA)(a/Vr2)**	****V****r**  and **z** from measured acceleration & initial conditions

## HASI descent phase

_**HASI descent phase**_

## HASI temperature profile

**thermosphere**

**mesosphere**

**stratosphere**

**troposphere**

**exobase**

_**HASI temperature profile**_

**stratopause**

**inversion layers**

**tropopause**

***Credit: ESA / ASI / UPD / OU / FMI ***

## Titan’s atmospheric structure

**Titan’s atmospheric structure**

- In the **upper atmosphere** density & temperature higher than expected. Wave-like nature of thermal profile => atmosphere is highly stratified and variable in time.Stratopause ~187 K at 250 km
**Lower stratosphere** & **tropopause:** very good agreement with Voyager 1 temperature.Tropopause ~ (70.43±0.25)K at 44 km 			  (113±1 mbar)
- At **surface**: 			Temperature (93.65±0.25) K  	Pressure        (1467±1) mbar

## HASI temperature gradients

***Credit: ESA / ASI / UPD / OU / FMI ***

**parachute**

**dry****adiabatic lapse rate**

**IL3**

**IL6**

**IL5**

**IL4**

**IL2**

**IL1**

**1020 km****980 km**
**800 km****680 km****600 km**
**510 km**

## HASI vs Nov2003 stellar occultations

***Credit: ESA / ASI / UPD / OU / FMI / ******            B. Sicardy et Titan’s occultation team ***

**Inversion****layer**

**dry****adiabatic lapse rate**

## Stratopause ~187K at 250 km (0.3 mbar)
CIRS stratopause at higher levels (~ 350 km) for similar T (186 K) at 15°S

**CIRS stratopause at higher levels ****(~ 350 km) for similar T (186 K) at 15°S**

_**Comparison with Voyager and Cassini results**_

**Very good agreement(within error bars with Voyager RSS profiles (pure N****2****)**
**At tropopause HASI T ~ 1K colder, but  assuming 98.5% N****2****+1.5% CH****4 ****-> 70.5 K **[Lellouch *el al. *1989]
**Temperature variations in lower stratosphere coherent with linear, free propagating gravity waves as derived from Voyager RSS **[Friedson, 1994]

- [Flasar et al 2005 *Science*]

**Presence of layers also confirmed by observations of stellar occultations and Cassini ISS.**** **
**Variability and waves observed also by INMS**
**Exopause at ****~1380 km ****similar values estimated by INMS**

- [Porco et al 2005 *Science*]

- [Waite *el al. *2005 *Science*]**	****	**

## HASI temperature gradients

**HASI temperature gradients**

***Credit: ESA / ASI / UPD /FMI ***

**dry****adiabatic lapse rate**

**wind shear ****as observed ****by DWE**

## Preliminary meteorological interpretation: correlation of HASI and DWE data

**Vertical profile of T and wind analysed as**** ****wind shear**** and ****buoyant static stability**

**Correlation of the unexpected strong vertical wind shear with ****Titan’s more buoyant stratified region.**

## Planetary Boundary Layer

_**Planetary Boundary Layer**_

**Surface layer** 0 - 5 m

**Mixed layer **up to ~300 m

**Planetary Boundary layer** up to **300 m**

- [Tokano *el al. *2006 *in press*]**	****	**

- well mixed potential temperature, strong vertical shears and a thin region of Richardson number Ri~1.

## Titan’s meteo

**Planetary Boundary layer** up to **300 m** by potential temperature.
- Correlated analysis of DWE and HASI data reveals: a significant correspondence of wind shear and buoyant stability structuresboth in Titan’s stratosphere and lower tropopause
**Lower stratosphere**: the unanticipated strong vertical wind shear region between 60 and 90 km is correlated with Titan’s most buoyantly stratified region – a layer of roughly one-scale-height where the smoothed Richardson number is small (Ri ~2–5).
**Ne****ar surface atmosphere**: correlation of HASI and DWE confirm the presence of the PBL characterized by a well mixed potential temperature, strong vertical shears and a thin region of Richardson number Ri~1 in the lowest 3 km.
- Meteorologic conditions monitored at the surface for half an hour: Temperature    93.65±0.25 K  Pressure        1467±1 hPa

## Meteo at surface:
Temperature  93.65±0.25 K
Pressure	      1467±1 hPa

**Temperature  93.65****±0.25 K**
**Pressure****	****      1467±1 hPa**

_**Surface phase**_

## Titan’s atmospheric electricity

**Titan’s atmospheric electricity**

- Presence of charged particle species (electrons and, positive and negative ions).
- Lower ionospheric layer between 140 and 40 km induced by cosmic rays with electrical conductivity peaking near 60 km.
- Detection of some events of electrical discharges (potential signature of lightning).

**Permittivity Wave Altimetry (PWA)**

**signature of the ionosphere**

## Titan’s surface

- Impact signature: instant & trace.**Solid unconsolidated** surface (e.g. gravel, wet sand).
- No evidence of liquid reflectance.The measured relative permittivity (of the order of 2) constrains the soil composition.
- Meteorological conditions monitored for half an hour after impact.

**Titan’s surface**

## Summary

- Exobase at 1380 km (n~2E07 cm-3).
- Upper atmosphere warmer than expected (Yelle et al.)>Stratopause at ~ 250 km (~ 187 K) (same value retrieved by CIRS, but at different altitude-pressure level)
- Several temperature variations observed in the thermosphere possibly related to inversion layers and other dynamic effects
- Mesopause detected at ~490 km (~152 K)
- Inversion layer at 510 km (mesopause) as observed during Titan’s stellar occultation
- Temperature structure of the lower atmosphere in very good agreement with the Voyager 1 RSS and IRIS measurements (Lindal *et al.* 1983, Lellouch *et al.* 1989)
- Tropopause (70.43±0.25)K at ~ 44 km  (115±1)hPa.
- Preliminary meteorological interpretation.
- PBL convective layer of 300 m confirmed also by correlation with  DWE data,
- At surface: ~94 K and 1467 hPa

## Conclusions

_**Conclusions**_

- The temperature and density profiles inferred by HASI are of great value since they provide
- an accurate determination of the whole atmosphere (from ground up to exobase)
- the only new and independent definition of the tropospheric thermal structure
- atmospheric parameters based on very precise characterization of the chemical structure
- Comparisons with Cassini and groundbased observations evidence some differences that should be investigated and that could imply variability of Titan’s atmospheric structure.
- Information gathered from HASI pertain to one site along the Huygens probe descent trajectory, but combined with measurements from Cassini could contribute to improve the knowledge of Titan’s atmospheric structure at all latitude and longitudes covered by Cassini.