© 2005 Nature Publishing Group
In situ measurements of the physical
characteristics of Titan’s environment
M. Fulchignoni1,2, F. Ferri3, F. Angrilli3, A. J. Ball4, A. Bar-Nun5, M. A. Barucci1, C. Bettanini3, G. Bianchini3,
W. Borucki6, G. Colombatti3, M. Coradini7, A. Coustenis1, S. Debei3, P. Falkner8, G. Fanti3, E. Flamini9, V. Gaborit1,
R. Grard8, M. Hamelin10,11, A. M. Harri12, B. Hathi4, I. Jernej13, M. R. Leese4, A. Lehto12, P. F. Lion Stoppato3,
J. J. Lo ´pez-Moreno14, T. Ma ¨kinen12, J. A. M. McDonnell4, C. P. McKay6, G. Molina-Cuberos15, F. M. Neubauer16,
V. Pirronello17, R. Rodrigo14, B. Saggin18, K. Schwingenschuh13, A. Seiff‡, F. Simo ˜es10, H. Svedhem8, T. Tokano16,
M. C. Towner4, R. Trautner8, P. Withers4,19& J. C. Zarnecki4
On the basis of previous ground-based and fly-by information, we knew that Titan’s atmosphere was mainly nitrogen,
with some methane, but its temperature and pressure profiles were poorly constrained because of uncertainties in the
detailed composition. The extent of atmospheric electricity (‘lightning’) was also hitherto unknown. Here we report the
temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an
altitude of 1,400km down to the surface. In the upper part of the atmosphere, the temperature and density were both
higher than expected. There is a lower ionospheric layer between 140km and 40km, with electrical conductivity peaking
near 60km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 ^ 0.25K, and
the pressure was 1,467 ^ 1hPa.
that Titan’s atmosphere is composed of N2with small amounts of
CH4. The surface pressure was determined to be approximately
1,400hPa, with a surface temperature of about 95K decreasing to a
temperature minimum of about 70K at 40km altitude before
increasing again to about 170K in the stratosphere1–3. The atmos-
pheric structure at high elevations (1,000–1,500km) was inferred
from the solar occultation measurements by the Voyager ultraviolet
well determined, although telescopic observations indicated a com-
plex vertical structure5–10and models have been used to predict the
atmospheric structurein this region11–13. Very little was known about
the surface of Titan because it is hidden by a thick haze and is almost
undetectable, exceptbyradarsounding14andafew infraredwindows
that have been observed from telescopes15,16. Initial speculation was
that the surface was covered by a deep hydrocarbon ocean, but
possibly consistent with lakes, but not with a global ocean. Recently,
frequencies provided new results on the nature of the surface of the
Earlierobservations showed that the surface pressure on Titanwas
comparable to that on the Earth, and that CH4formed a plausible
counterpart to terrestrial H2O for cloud and rain formation. There
was also speculation on the possibility of lightning occurring in
Titan’s atmosphere20–22which could affect the chemical composition
of the atmosphere.
In this Article, we report results from the HASI instrument on
the Huygens probe23. By monitoring the probe deceleration, the
HASI instrument directly determined the density of the upper
atmosphere and derived the temperature from the density scale
height. In the lower atmosphere and on the surface of Titan, the
HASI instrument directly measured the pressure and temperature.
During the probe descent, electrical activity was monitored to search
for evidence of lightning activity. A search for acoustic signals
produced by any thunder or other shock waves was also conducted.
In the upper atmosphere, the density profile is used to infer the
temperature profile. Above 500km, the temperature structure shows
strong wave-like variations of 10–20K about a mean of about 170K.
Below 500km, the temperature increases to a relative maximum of
about 200km, the temperature and pressure profile measured by
HASI agrees with the results of the Voyager radio occultation data2.
The surface temperature is determined to be 93.65 ^ 0.25K, and the
surface pressure is 1,467 ^ 1hPa. The values are within the range
allowed by the uncertainties in the Voyager data13owing to previous
uncertainties in the mixing ratio of CH4and argon. Electrical
conductivity measurements indicate the presence of charged particle
1LESIA, Observatoire de Paris, 5 Place Janssen, 92195 Meudon, France.2Universite ´ Denis Diderot – Paris 7, UFR de Physique, 2 Place Jussieu, 75006 Paris, France.3CISAS “G.
Colombo”, Universita ` di Padova, Via Venezia 15, 35131 Padova, Italy.4PSSRI, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK.5Department of Geophysics and
Planetary Sciences, University of Tel Aviv, 69978 Tel Aviv, Israel.6NASA/AMES Research Center, MS 244-30, Moffett Field, California 94035, USA.7ESA Headquarters,
Science Directorate, 8-10 rue Mario-Nikis, 75015 Paris, France.8ESA-ESTEC, European Space Agency, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands.9Agenzia Spaziale
Italiana, Viale Liegi 26, 00198 Roma, Italy.10CETP-IPSL, 4 Avenue de Neptune, 94107 Saint Maur, France.11LPCE-CNRS, 3A, Avenue de la Recherche Scientifique, 45071 Orle ´ans
cedex 2, France.12Finnish Meteorological Institute (FMI), Vuorikatu 15 A 00100 Helsinki, Finland.13Space Research Institute, Austrian Academy of Sciences (IWF),
Schmiedlstrasse 6, 8042 Graz, Austria.14Instituto de Astrofisica de Andalucia (IAA-CSIC), PO Box 3004, 18080 Granada, Spain.15Applied Electromagnetic Group, Department
of Physics, University of Murcia, Murcia 30100, Spain.16Institut fu ¨r Geophysik und Meteorologie, Universita ¨t zu Ko ¨ln, Albertus-Magnus-Platz, 50923 Ko ¨ln, Germany.17DMFCI,
Universita ` di Catania, Viale A. Doria 6, 95125 Catania, Italy.18Politecnico di Milano, Dipartimento di Meccanica, Piazza Leonardo da Vinci 32, 20133 Milano, Italy.19Center for
Space Physics, Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215, USA.
© 2005 Nature Publishing Group
species in an ionized layer, presumably induced by cosmic rays, and
the detection of some electrical discharges.
We inferred the atmospheric structure of Titan on the basis of
measurements taken during entry phase and while the probe was
descending under the parachutes. The atmosphere was first detected
at an altitude of ,1,500km, when it exceeded the sensitivity
threshold of the accelerometer25. Broadly speaking, the temperature
and density of the upper atmosphere exceeded predictions. Titan’s
atmosphere is apparently highly stratified. The density of the upper
atmosphere was derived from the probe deceleration due to aero-
planetary atmospheres such as Venus, Mars and Jupiter. The velocity
as a function of time was determined by integrating the measured
probe deceleration. Altitude was determined by integrating the
vertical component of the velocity using the state vector of the
probe provided by the Cassini navigation team. The entry altitude
has a 1j uncertainty of about 30km; we adjusted the nominal entry
the entry phase and descent phase measurements26.
the engineering model13obtained from the reanalysis2–4,11,12of
Voyager data (radio occultations, infrared interferometry (IRIS) and
UVS spectrometers). In the upper part of the atmosphere down to an
altitude of about 500km, the HASI measurements show density
values systematically higher than those expected. Pressures were
obtained from the density profile under the assumption of hydro-
static equilibrium and the knowledge of planetary gravity
(1.354ms22at surface level), mass (1.35 £ 1023kg) and radius
(2,575km). Temperatures were derived from the pressures, the
inferred densities and the equation of state of a perfect gas using
the atmospheric mean molecular weight, as a function of altitude
given by the engineering model. The pressure versus temperature
profile of Titan’s atmosphere is shown in Fig. 2. The thermosphere is
characterized by the presence of temperature variations due to
inversion layers or other dynamic phenomena (such as gravity
waves and gravitational tides) between 500km and 1,020km.
Temperatures in this region are generally higher than those
predicted by the engineering model, with a minimum value of
152K at an altitude of ,490km (2 £ 1023hPa, which could mark
the mesopause) and then increase down to the stratopause (,186K
at 250km, 0.3hPa). In the region between the lower part of the
mesosphere and the upper part of the stratosphere, the temperatures
are 5–10K higher than those predicted by the model12.
The temperature gradient profile, shown in Fig. 3, exhibits in
general a cut-off at the dry adiabatic lapse rate, implying that
fluctuations lead to marginallyconvective instabilities. The inversion
layers in the upper atmosphere are clearly visible, with strong peaks
towards positive values. The peak at 510km corresponds to the
inversion layer already observed from the ground on 14 November
2003 when Titan occulted two bright Tycho stars10. These lines of
evidence all indicate that Titan’s atmosphere is highly stratified.
After the parachute deployment and heatshield separation, the
Titan’s environment during the entire descent under parachute. The
altitude and velocity are derived from these measurements, the
hypothesis of hydrostatic equilibrium, and the equation of state for
a real gas29, given the atmospheric mean molecular weight measured
by the Gas Chromatograph-Mass Spectrometer (GCMS)30. The
measured pressure and temperature profiles shown in Figs 4 and 5
connect well with the profiles derived during the entry phase. From
very good agreement (within 1–2K) with the temperature measure-
ments obtained by the Voyager 1 radio occultation assuming a pure
The temperature minimum of 70.43 ^ 0.25K is reached at the
stratosphere and the strong increase in temperature with altitude
between 80km and 60km are visible. Below 200km, the fine
measured by HASI (solid line) is shown compared to Titan’s atmospheric
gas equation; below 160km, temperature data are direct measurements
collected by the TEM sensor. The temperature profile in the upper
atmosphere (thermosphere) is characterized by several temperature
variations due to inversion layers and other dynamic phenomena (for
example, gravity waves and tides). Temperatures in this region are higher
than those predicted by the model. The virtual absence of a mesosphere (in
contrast with the theoretical models’ predictions11,12) and the wave-like
nature of the temperature profile suggest that the region in Titan’s
atmosphereabove250km maynot be dominated by radiative processes and
may be strongly influenced by wave activity. Thus the structure that we
observe may vary with time. The horizontal lines mark the mesopause
(152K at 490km), the stratopause (186K at 250km) and the tropopause
(70.43K at 44km).
Figure 1 | The atmospheric density profile of Titan as measured by
HASI. The density profile as derived from HASI measurements (solid line)
is shown in comparison with the engineering model of Titan’s atmosphere13
derived from Voyager 1 data2–4,11,12(dashed line). Density in the upper part
of the atmosphere is derived from the ACC accelerometer data. The
threshold density was 5 £ 10212kgm23. The uncertainty on the density
determination25is of the order of 10%,mainlydue to the uncertaintyon the
aerodynamic drag coefficient and on the probe velocity. Density values
relevant to the lower atmosphere, below 160km, have been inferred from
HASI direct measurements of pressure and temperature with the
assumption of hydrostatic equilibrium and real gas law29.
© 2005 Nature Publishing Group
structure seen in Fig. 5 provides evidence for a regime of gravity
waves similar to those observed in the Voyager radio occultation
data31,32. Turbulence due to shear instability (Kelvin–Helmholtz
instability) is expected wherever the vertical shear of the wind
speed is large. The wind shear measured by the Doppler Wind
Experiment33is sufficiently large that the features present between
50 and 150km are likely to be related to turbulence.
The vertical resolution of the temperature measurement was
sufficient to resolve the instantaneous structure of the planetary
boundary layer. On the basis of the nearly constant values of the
thickness of about 300m at the place and time of landing.
Models of Titan’s ionosphere predicted that galactic cosmic rays
would produce an ionospheric layer with a maximum concentration
of electrons between 70 and 90km altitude34–37. The Permittivity,
the atmosphere below 140km. We found that the electrical conduc-
tivity peaks at ,60km. We might have seen evidence for lightning.
Observations of the electron and ion conductivities were made
with two different techniques: relaxation and mutual impedance
probes. The results of the relaxation probes (shown in Fig. 7a, b)
indicate peaks in the electron/negative-ion conductivities at 60km.
This instrument gives the impedance of the medium at 45Hz and
yields a phase shift, which is sensitive to the presence of electrons
signal induced in the probe environment by the 45Hz stimulus, in
the bandwidth 0–9.22kHz (Fig. 8a, active mode).
The electric field due to natural wave emissions was investigated
during the descent, using the receiving dipole of the mutual impe-
dance probe in two frequency ranges, 0–11.5kHz and 0–100Hz
(Fig. 8b, c, passive mode). This provided a unique opportunity to
investigate in situ lightning and related phenomena (for example,
corona discharges) on Titan21that would produce electromagnetic
waves38, excite global and local resonancephenomena in the surface–
ionospheric cavity39,40and could drive a global electric circuit22.
Several impulsive events have been observed during the descent,
for example at 2,800s. The narrow-band wave emission seen near
36Hz is reminiscent of a possible resonance generated by lightning
the inner boundary of its ionosphere, but should be interpreted with
caution. A comparison of the records presented in Fig. 8a and b
shows that the first spectrogram (active mode) not only displays the
temperature sensors, TEM27(expanded from Fig. 2). Temperature
The temperature minimum of 70.43K is reached at the tropopause (about
44km; 115 ^ 1hPa). HASI temperatures are in very good agreement
(within the error bars) with data obtained by Voyager radio occultation2
(ingress, circles; egress, crosses) assuming a pure nitrogen atmosphere. The
error bars for Voyager data are reported: ^15K (egress) ^10K (ingress)
near the 200-km level, ^0.5K at the tropopause. At the tropopause, HASI
measured temperature values ,1K colder than Voyager2, but reanalysis of
these data3suggested a similar temperature value (70.5K) assuming a
stratospheric composition of 98.5% N2plus 1.5% CH4.
of the HASI measurements is of the order of 20km from the top of the
atmosphere down to the 400-km altitude level, decreasing down to 1km at
the 160-km level24. The profile shows ingeneral a cut-off atthe dry adiabatic
lapse rate (dotted line), implying that fluctuations may lead to convective
the black curve. Six inversion layers in the upper atmosphere (at about 510,
600, 680, 800, 980 and 1,020km) could be detected by strong peaks towards
positive values. The strong lower inversion layer (4Kkm21at ,510km)
corresponds to the feature already observed from the ground during Titan’s
stellar occultations10. The strong peaks between the 160- and 110-km levels
correspond to the parachute deployment sequence.
Figure 4 | Pressure profile of the lower atmosphere as measured by the
Pressure Profile Instrument (PPI)28. Measurements (solid line) corrected
for dynamic effects are shown together with values obtained by Voyager 1
determined with an uncertainty of 1% along the entire descent.
© 2005 Nature Publishing Group
signals seen in the second spectrogram (passive mode), but also
includes a broadband emission in the altitude range 110–80km, and
to lesser extent at altitudes lower than 25km. It is believed that the
energy injected in the medium at 45Hz is partly dissipated in
nonlinear effects, which seems to strengthen the evidence for the
presence of free charges in the upper atmosphere.
Before the probe landed, the nature of the surface was unknown.
From the abundance of methane in its atmosphere, there was
speculation that Titan might be covered by a methane ocean41, but
recent observations14have restricted the fraction of the surface
covered with liquid to be just a few per cent. The probe touched
down on a solid surface, which has properties something like wet
sand42. The instruments continued to monitor the meteorological
conditions for almost half an hour after impact.
The natureofTitan’s surfaceatthe landing site was investigated by
spectral analysis of the Huygens radar return signal, the recording of
the impact signature, in situ measurements of the ground electrical
properties, and the surface environmental conditions.
The piezoresistive accelerometers of HASI recorded the impact
instant at T0þ 2h27min49.840s (where T0is the time of the
parachute deployment device firing and corresponds to the
beginning of the descent phase), when the event exceeded the
threshold of ,40ms22. A complete trace of the impact in the three
orthogonal reference axes is shown in Fig. 9. The initial small peak in
be related to a touch down on uneven topography, or the possible
initial contact of a portion of the probe foredome, given the likely
probe tilt at landing42. A sharp drop in acceleration is seen briefly in
all three sensors at 8,869.86s. The peak probe deceleration measured
is141ms22,inreasonable agreementwiththevaluemeasuredby the
of the full data set, two possible events are seen in all three axes, at
impact (,8,869.86s) and ,3s later at ,8,872.2s. These correspond
respectively to the initial impact event, and then to some short-term
settling that may be surface related, or probe related (parachute
system dynamics or structural relaxation of foredome). Further
Figure 6 | The temperature lapse rate for the low atmosphere (expanded
fromFig.3). Anumberofinversionlayers inthelowerstratosphereandthe
strong temperature increase with altitude between 80 and 60km are visible.
Features present between 50 and 150km could be related to turbulence due
to Kelvin–Helmholtz instability induced by the large vertical shear of the
windspeed, measuredby theDoppler Wind Experiment33. The temperature
gradient in this part of the atmosphere has been derived from direct
temperature measurements with vertical spatial resolution of the order of
200–150m above 60-km altitude, and decreasing from 70m down to 11m
until the last kilometre.
Figure 7 | A synopsis of PWA data: the signature of the ionosphere. The
with the atmosphere is indicated by a thick black line along the top axis.
a, b, Relaxation carpets for Fo¼ þ5Vand 25V, respectively. The
relaxation probe, initially biased at a potential Fowith respectto the vehicle
body, subsequently returns to its equilibrium potential, F1, with a time
constant that yields the d.c. conductivity of the charges with polarity
opposite to that of Fo2 F1. The measurements taken during each
relaxation cycle form a string of pixels aligned with the ordinate axis;
voltages are given by the colour scales shown on the right-hand side. The
electrode potential is measured every 20ms during the first second, then
every 2s for the reminder of each 1min cycle. These panels give a visual
impression of the speed at which the potential of a conductive body (colour
coded) returns from ^5V to zero (‘relaxes’), owing to the collection of
ambient charges with opposite polarities. In the lower altitude range, for
example, the colour of the carpet is uniform (brown for þ5Vand blue for
25V),whichshowsthatthe ambientchargedensities arelow. Above40km,
on the contrary, the distinctive carpet patterns tell us that the probe voltage
is strongly affected by the ionized environment. c, Mutual impedance phase
shift,Df ¼ fo2 f(non-calibrated).Thea.c.conductivityismeasuredwith
a quadrupolar array. A current I with frequency 45Hz and amplitude
,10210A is injectedbetween two transmittingelectrodes, and the voltage V
of V/I at 45Hz is foin a vacuum and f in a collisional medium, then the
conductivity of the medium is proportional to tan(fo2 f).
© 2005 Nature Publishing Group
modelling of the probe structure behaviour is required to quantify
these effects. Additionally, the area of stable data points immediately
following the initial impact (8,870.1–8,870.3s) maybedue to asmall
bounceof the probe or to some structurevibrations. Integration of Y
and Z axes after further processing, in combination with other
sensors, will indicate any possible probe lateral movement. The
integration of the accelerometer data gives a probe impact velocity
of 4.33ms21, in reasonable agreement with the values obtained by
SSP42and from the velocity profile during the last kilometre of the
descent as derived from pressure measurements. For the final
rest position of the probe, the X servo accelerometer gives an
estimate of the probe tilt of about 118, in good agreement with SSP
At the surface, the HASI temperature and pressure sensors mon-
impact,measuringatemperatureof93.65 ^ 0.25Kandapressureof
1,467 ^ 1hPa. The complex permittivity of the surface material is
measured after impact with the PWA mutual impedance probe43, at
five frequencies. As a first estimation, the mean relative permittivity
reasonable agreement with the measurements performed with the
radar on board Cassini19.
In addition to providing altitude (Fig. 8), the Radar Altimeter
measures the signal backscattered within the footprint of the beam,
whose diameter is 0.14 times the altitude. This signal is strong and
smooth with small variations over the ground track, indicating a
Figure 8 | A synopsis of PWA data: electric field, acoustic pressure and
radar measurements. a, Dynamic spectrum of the voltage V measured
between two electrodes 2m apart, in the bandwidth 0–9.22kHz, when a
current stimulus I is injected between two transmitting electrodes. The
spectrumofthe signal provides information aboutits energydistributionas
a function of frequency, at a given time. Successive spectra are represented
by adjacent strings of pixels aligned with the ordinate axis, where spectral
right-hand side. b, Dynamic spectrum of the voltage V measured with two
electrodes 2m apart, in the bandwidth 0–11.5kHz, without current
stimulus. c, Same as b, but in 0–100Hz bandwidth. d, Dynamic spectrumof
acoustic differential pressure in the bandwidth 0–6.7kHz. A sound pressure
level (SPL) of 0dB corresponds to 20mPa. The variability of the acoustic
noise is caused by changes in the atmospheric density and wind velocity50.
e, The altitude represented by the red dots is measured whenever the Radar
Altimeter (RA) is locked on the surface; permanent lock is maintained from
34km down to 150m. At higher altitudes, the green dots indicate the
distances at which the signal is returned by the atmosphere. Several events
are identified with triangles along the top axis: (1) stabilizer parachute
opening, (2) mode change, (3) impulsive event in b, (4) surface touch down.
Discontinuities in time or frequency are artefacts due to mode change.
Figure 9 | The HASI signature of the impact trace, at 200 samples per
second. The complete impact trace (6s) is shown; the inset shows a
magnified view of the deceleration peak. The X sensor (blue line) is aligned
to the probe symmetry axis, corresponding to the descent direction. The Y
probe symmetry axis.
© 2005 Nature Publishing Group
surface with little relief. The atmosphere was scanned and return
signal from droplets was searched for, but no significant signature of
rain could be found.
Although the HASI data have now provided a great wealth of
information on the conditions in the atmosphere and at the surface
of Titan, many questions and challenges remain.
Atmospheric structure. The HASI temperature profile in the lower
atmosphere was compared to the separate egress and ingress profiles
based on the Voyager occultation experiment 25years earlier. This
comparison suggests that the atmosphere of Titan in mid-latitudes is
uniform and slowly changing in accordance with model predictions.
The open question is the poleward extent of this non-variability,
given the latitudinal temperature gradient in the stratosphere
inferred from infrared data44,45. One interpretation of the south
polar clouds is that they are due to heating associated with polar
summer warming46,47. If this is true, then the temperature profile in
the polar summer should be different from the mid-latitude profiles
sampled here, and could be revealed by Cassini infrared mapping45
and radio science48. In the middle and upper atmosphere (above
300km), the prominent wave-like structure reported here requires
The observed vertical variation would suggest that large-scale tem-
perature gradient in this region is also time variable. Unfortunately,
the necessary observations of the time and spatial evolution of these
structures must await future missions.
in lock), but no significant signature of rain was found. The
instrument sensitivity to mass loadings of methane or other hydro-
carbon droplets needs to be determined so that an upper limit to
droplet mass loadings can be estimated.
Atmospheric electricity. The maximum in the conductivity due to
positive ions, 20km above the peak electron conductivity at 60km,
order to preserve charge neutrality. The altitude of the maximum
conductivity due to electrons lies below that predicted by theoretical
models35–37. Several pulses similar to terrestrial sferics (natural
electromagnetic waves) have been observed during the descent.
Large convective clouds were observed near the south pole during
easily propagate from the south pole to the Huygens location.
Lightning activity would also be consistent with the observations
of waves in the Schumann frequency range.
Nature of the surface. The lack of any rhythmic motion during the
half hour of operation on the surface indicated that the probe had
landed on a solid surface rather than a liquid, which agrees with the
image taken after the landing49. The measured relative permittivity
(of the order of 2) constrains the soil composition. No evidence for
the presence of liquid phase on the surface was returned by the signal
of the radar altimeter.
The HASI measurements of the atmospheric structure, electrical
state and surface properties provide a unique insight into Titan’s
characteristics, unequalled in any planetary atmosphere except the
Earth’s. The many discoveries and puzzles will require synergetic
analysis with the Cassini orbiterobservations and years of laboratory
and modelling efforts to solve.
Received 28 May; accepted 11 October 2005.
Published online 30 November 2005.
1. McKay, C. P., Pollack, J. B. & Courtin, R. The thermal structure of Titan’s
atmosphere. Icarus 80, 23– -53 (1989).
Lindal, G. F. et al. The atmosphere of Titan—an analysis of the Voyager 1 radio
occultation measurements. Icarus 53, 348– -363 (1983).
Lellouch, E. et al. Titan’s atmosphere and hypothesized ocean: a reanalysis of
the Voyager 1 radio-occultation and IRIS 7.7mm data. Icarus 79, 328– -349
Vervack, R. J., Sandel, B. R. & Strobel, D. F. New perspectives on Titan’s upper
atmosphere from a reanalysis of the Voyager 1 UVS solar occultations. Icarus
170, 91– -112 (2004).
Coustenis, A. et al. Titan’s atmosphere from ISO mid-infrared spectroscopy.
Icarus 161, 383– -403 (2003).
Hubbard, W. B. et al. Results for Titan’s atmosphere from its occultation of 28
Sagittarii. Nature 343, 353– -355 (1990).
Sicardy, B. et al. The structure of Titan’s stratosphere from the 28Sgr
occultation. Icarus 142, 357– -390 (1999).
Tracadas, P. W., Hammel, H. B., Thomas-Osip, J. E. & Elliot, J. L. Probing Titan’s
atmosphere with the 1995 August stellar occultation. Icarus 153, 285– -294
Bouchez, A. H. et al. Adaptive optics imaging of a double stellar occultation by
Titan. Bull. Am. Astron. Soc. 34, 881 (2002).
10. Sicardy, B. et al. The two stellar occultations of November 14, 2003: revealing
Titan’s stratosphere at sub-km resolution. Bull. Am. Astron. Soc. 36, 1119
11.Lellouch, E., Hunten, D., Kockarts, G. & Coustenis, A. Titan’s thermosphere
profile. Icarus 83, 308– -324 (1990).
12. Yelle, R. V. Non-LTE models of Titan’s upper atmosphere. Astrophys. J. 383,
380– -400 (1991).
13. Yelle, R. V., Strobel, D. F., Lellouch, E. & Gautier, D. Engineering Models for
Titan’s Atmosphere 243– -256 (ESA SP-1177, European Space Agency,
14. Campbell, D. B., Black, G. J., Carter, L. M. & Ostro, S. J. Radar evidence for
liquid surfaces on Titan. Science 302, 431– -434 (2003).
15. Coustenis, A. et al. Maps of Titan’s surface from 1 to 2.5mm. Icarus 177,
89– -105 (2005).
16. Meier, R., Smith, B. A., Owen, T. C. & Terrile, R. J. The surface of Titan from
NICMOS observations with the Hubble Space Telescope. Icarus 145, 462– -473
17. Porco, C. C. et al. Imaging of Titan from the Cassini spacecraft. Nature 434,
159– -168 (2005).
18. Brown, R. H. et al. Cassini Visual and Infrared Mapping Spectrometer (VIMS):
Results for the SOI- and near-SOI period of the Cassini orbital tour. Astron.
19. Elachi, C. et al. Cassini radar views the surface of Titan. Science 308, 970– -974
20. Grard, R. et al. An experimental investigation of atmospheric electricity and
lightning activity to be performed during the descent of the Huygens probe
onto Titan. J. Atmos. Terr. Phys. 57, 575– -578 (1995).
21. Desch, S. J., Borucki, W. J., Russell, C. T. & Bar-Nun, A. Progress in planetary
lightning. Rep. Prog. Phys. 65, 955– -997 (2002).
22. Tokano, T., Molina-Cuberos, G. J., Lammer, H. & Stumptner, W. Modelling of
thunderclouds and lightning generation on Titan. Planet. Space Sci. 49,
539– -560 (2001).
23. Lebreton, J.-P. & Matson, D. L. The Huygens probe: science, payload and
mission overview. Space Sci. Rev. 104, 59– -100 (2002).
24. Fulchignoni, M. et al. The characterization of Titan’s atmospheric physical
properties by the Huygens Atmospheric Structure Instrument (HASI). Space
Sci. Rev. 104, 395– -431 (2002).
25. Zarnecki, J. C. et al. In-Flight Performances of the Servo Accelerometer and
Implication for Results at Titan 71– -76 (ESA SP-544, European Space Agency,
26. Lebreton, J. P. et al. An overview of the descent and landing of the Huygens
probe on Titan. Nature doi:10.1038/nature04347 (this issue).
27. Ruffino, G. et al. The temperature sensor on the Huygens probe for the Cassini
mission: Design, manufacture, calibration and tests of the laboratory
prototype. Planet. Space Sci. 44– -10, 1149– -1162 (1996).
28. Harri, A.-M. et al. Scientific objectives and implementation of the Pressure
Profile Instrument (PPI/HASI) for the Huygens spacecraft. Planet. Space Sci.
46, 1383– -1392 (1998).
29. Ma ¨kinen, T. Processing the HASI measurements. Adv. Space Res. 17, 217– -222
30. Niemann, H. B. et al. The abundances of constituents of Titan’s atmosphere
from the GCMS instrument on the Huygens probe. Nature doi:10.1038/
nature04122 (this issue).
31. Hinson, D. P. & Tyler, G. L. Internal gravity waves in Titan’s atmosphere
observed by Voyager radio occultation. Icarus 54, 337– -352 (1983).
32. Friedson, A. J. Gravity waves in Titan’s atmosphere. Icarus 109, 40– -57 (1994).
33. Bird, M. K. et al. The vertical profile of winds on Titan. Nature doi:10.1038/
nature04060 (this issue).
34. Molina-Cuberos, G. J., Lo ´pez-Moreno, J. J., Rodrigo, R. & Lara, L. M. Chemistry
of the galactic cosmic ray induced ionosphere of Titan. J. Geophys. Res. 104,
21997– -22024 (1999).
35. Borucki, W. J. et al. Predictions of the electrical conductivity and charging of
the aerosols in Titan’s atmosphere. Icarus 72, 604– -622 (1987).
36. Borucki, W. J., Whitten, R. C., Bakes, E. L. O., Barth, E. & Tripathi, S. Predictions
of the electrical conductivity and charging of the aerosols in Titan’s
atmosphere. Icarus (in the press).
37. Molina-Cuberos, G. J., Lo ´pez-Moreno, J. J., Rodrigo, R. & Schwingenschuh, K.
Capability of the Cassini/Huygens PWA-HASI to measure electrical
conductivity in Titan. Adv. Space Res. 28, 1511– -1516 (2001).
© 2005 Nature Publishing Group
38. Schwingenschuh, K. et al. Propagation of electromagnetic waves in the lower
ionosphere of Titan. Adv. Space Res. 28, 1505– -1510 (2001).
39. Nickolaenko, A. P., Besser, B. P. & Schwingenschuh, K. Model computations of
Schumann resonance on Titan. Planet. Space Sci. 51, 853– -862 (2003).
40. Morente, J. A., Molina-Cuberos, G. J., Portı ´, J. A., Schwingenschuh, K. & Besser,
B. P. A study of the propagation of electromagnetic waves in Titan’s
atmosphere with the TLM numerical method. Icarus 162, 374– -384 (2003).
41. Lunine, J. I., Stevenson, D. J. & Yung, Y. L. Ethane ocean on Titan. Science 222,
1229– -1230 (1983).
42. Zarnecki, J. C. et al. A soft solid surface on Titan as revealed by the Huygens
Surface Science Package. Nature doi:10.1038/nature04211 (this issue).
43. Hamelin, M. et al. Surface and sub-surface electrical measurement of Titan
with the PWA-HASI experiment on Huygens. Adv. Space Res. 26, 1697– -1704
44. Coustenis, A. & Be ´zard, B. Titan’s atmosphere from Voyager infrared
observations. IV. Latitudinal variations of temperature and composition. Icarus
115, 126– -140 (1995).
45. Flasar, F. M. et al. Titan’s atmospheric temperatures, winds, and composition.
Science 308, 975– -978 (2005).
46. Brown, M. E., Bouchez, A. H. & Griffith, C. A. Direct detection of variable
tropospheric clouds near Titan’s south pole. Nature 420, 7995– -7997 (2002).
47. Tokano, T. Meteorological assessment of the surface temperatures on Titan:
constraints on the surface type. Icarus 173, 222– -242 (2005).
48. Kliore, A. J. et al. Cassini Radio Science. Space Sci. Rev. 115, 1– -70 (2004).
49. Tomasko, M. G. et al. Rain, winds and haze during the Huygens probe’s
descent to Titan’s surface. Nature doi:10.1038/nature04126 (this issue).
50. Ksanfomaliti, L. V. et al. Acoustic measurements of the wind velocity at the
Venera 13 and Venera 14 landing sites. Sov. Astron. Lett 8(4), 227– -229 (1982).
Acknowledgements We thank the following people for their contributions to the
realization of the HASI experiment: A. Buccheri, R. DeVidi, and M. Cosi of
Galileo Avionica, A. Aboudan, S. Bastianello and M. Fabris of CISAS,
M. Chabassie `re of LPCE, V. Brown, J.M. Jeronimo and L.M. Lara of IAA, R. Hofe
of IWF, A. Smit, L. Smit and J. Van der Hooke from RSSD-ESTEC, H. Jolly from
the UK, R. Pellinen, G. Leppelmeier, T. Siili, P. Salminen from FMI, and at the
Aerodynamics Laboratory of Helsinki University of Technology T. Siikonen and
B. Fagerstro ¨m. HASI has been realised and operated by CISAS under a contract
with the Italian Space Agency (ASI), with the participation of RSSD, FMI, IAA,
IWF, LPCE and PSSRI sponsored by the respective agencies: ESA, TEKES, CSIC,
BM:BWK, CNES and PPARC. We also acknowledge the long years of work by
some hundreds of people in the development and design of the Huygens probe.
The Huygens probe is part of the Cassini-Huygens mission, a joint endeavour of
the National Aeronautics and Space Administration (NASA), the European
Space Agency (ESA) and the Italian Space Agency (ASI).
Author Information Reprints and permissions information is available at
npg.nature.com/reprintsandpermissions. The authors declare no competing
financial interests. Correspondence and requests for materials should be
addressed to F.F. (firstname.lastname@example.org).