On the original schedule, the Master Timing Unit would have powered up Huygens about 45 minutes before the predicted moment of contact with Titan's atmosphere, but the schedule had been revised to do this with 4 hours remaining, to enable the radio system to warm up and further reduce the Doppler problem. With 90 minutes to go, Cassini started the command sequence by which it was to turn to aim its high-gain antenna towards the probe, record the transmission, and finally turn back to Earth to report its success. The probe penetrated the outermost fringe of the moon's atmosphere at an altitude of 1,270 kilometres and a velocity of 21,000 kilometres per hour on a trajectory dipping at an angle of 65 degrees below the local horizontal. The temperature of the forward heat shield rapidly rose to 1,800°C. In terms of real time, entry occurred on 14 January 2005 at 09:06 GMT. If the probe had attempted to send a signal at that moment, this would have taken 1 hour 7 minutes to reach Earth and arrived at 10:13 GMT,218 but because the probe was enveloped by plasma the transmitter was not yet active. The deceleration reached its peak after 3 minutes, by which time the speed had been slowed to 1,800 kilometres per hour. The major optical observatories with a view of Titan watched for any sign of a fireball trail. Although nothing was detected, Fred Chaffee, director of the Keck Observatory, pointed out, ''It was worth getting up in the middle of the night for this historic moment.'' Antonin Bouchez, a member of the team observing in the near-infrared in search of thermal radiation from the probe's passage, pointed out, ''the observations provided the best images that characterise the satellite at the moment of the probe's entry". About 5 minutes after first contact - by which time Huygens was at an altitude of 180 kilometres and had slowed to 1,500 kilometres per hour, corresponding to Mach 1.5 in the local environment - a mortar deployed the 2.6-metre-diameter pilot parachute. This pulled off the back cover of the heat shield and drew out the 8.3-metre-diameter main parachute, which rapidly slowed the rate of descent to 320 kilometres per hour. At an altitude of 160 kilometres the heavy forward heat shield was released to fall away and the scientific instruments started to report data.219
In addition to receiving and storing the data transmitted by the probe, Cassini was to facilitate the Doppler Wind Experiment by measuring the Doppler on the carrier signal due to the relative motions of the spacecraft and the probe. Although the signal from the probe was not designed to be detected on Earth, while the mission was being developed and flown the capabilities of radio telescopes had improved so dramatically that it was now possible to 'listen in' to the transmission. In order to reconstruct the trajectory of the parachute descent, the European Space Agency had teamed up with the Joint Institute for VLBI in Europe (JIVE) to monitor the probe's transmission. The signal would be too weak to extract the data, but Very-Long-Baseline Interferometry processing of the carrier signal received by radio telescopes across the globe would enable the probe's position to be monitored with an accuracy of about 1 kilometre. In addition, JPL had equipped several radio telescopes with apparatus to enable them to measure the radial velocity relative to Earth. In combination, these observations would enable the probe's parachute descent to be plotted in three dimensions, and its landing site located. Cassini would be best positioned to measure the zonal component of the wind, while the telescopes coordinated by JPL determined the meridional component.
At about 10:23 GMT, some 10 minutes after entry, the Robert C. Byrd Green Bank Telescope in West Virginia, operated by the National Radio Astronomy Observatory, detected the carrier signal, which served to confirm that Huygens had survived the deceleration. The Doppler was as predicted for the nominal descent, indicating that the main parachute had successfully deployed. Since Titan was soon to set below the local horizon, Green Bank would not be able to follow the entire descent, but the similarly equipped Parkes Observatory in Australia would be able to monitor the final portion of the descent, as well as any transmission from the surface. Of course, the fact that the probe was transmitting did not in itself mean that its instruments were functioning. Furthermore, since Cassini was not in communication with Earth, it was not possible to confirm that it was listening to the probe and recording the data. The scientists would have to wait until Cassini swung around to point its high-gain antenna to Earth and replayed the transmission.220 Even so, given the audacity of the mission, the omens were good.
Meanwhile, having descended to an altitude of 120 kilometres, the probe replaced its main parachute with a smaller 3-metre-diameter canopy designed to increase the rate of descent through the tropopause, the coldest part of the atmosphere. For most of the descent, the winds were west-to-east (with the rotation of the moon), and the maximum speed of 120 metres per second occurred at an altitude of 125 kilometres. In a study of imagery from previous fly-bys, the motions of tropospheric clouds near the south pole had confirmed that the atmosphere circulated at a rate exceeding that of Titan's 16-day axial rotation - a phenomenon known as super-rotation.221 This was predicted a decade earlier by a computer model made by Anthony DelGenio of the Goddard Institute for Space Studies in New York. As his colleague John Barbara put it, ''To discriminate clouds from the surface features, I used imagery of the same region taken at different times and subtracted them from each other. When I did this, time-variable clouds stood out as regions of changing brightness." Severe wind gusts and vertical shear in the altitude range 100 to 80 kilometres gave Huygens a wild ride, tilting it by as much as 60 degrees and threatening to collapse its parachute. The atmosphere was almost stagnant in the altitude range 80-70 kilometres, then the winds picked up again, reaching a steady 40 metres per second at 60 kilometres, after which they gradually tailed off.222
The only instrument to be activated for the deceleration phase was the Atmospheric Structure Instrument, which was to indirectly infer the pressure and temperature of the detached haze layers; when the forward heat shield was released, it switched to direct sampling.223 The temperature and density of the upper atmosphere were greater than predicted. There was an ionospheric layer between 140 kilometres and 40 kilometres, with the maximum electrical conductivity at about 60 kilometres. A sensor might have detected a radio burst from lightning, but there was no acoustic evidence of thunder.224 The Gas Chromatograph Mass Spectrometer took four samples, at 140, 85, 55 and 20 kilometres.225,226 In addition, the mass spectrometer subsystem analysed the samples taken by the Aerosol Collector and Pyrolyser in the stratosphere and upper troposphere.227 It had been expected that the probe would emerge from the base of the optically thick haze at an altitude of about 70 kilometres and provide the Descent Imager and Spectral Radiometer with a view of the surface, but this instrument did not get its first glimpse until 45 kilometres, and the scene remained murky until 30 kilometres.228 In fact, the number density of haze particles increased from an altitude of 150 kilometres all the way down to the surface. Once the imaging 'triplets' had been received on Earth, they were to be mosaicked into a series of panoramas taken at successively lower altitudes and improving spatial resolutions. At 25 kilometres, the upward-viewing spectrometer indicated that methane was becoming a major constituent of the troposphere. Direct sampling showed a constant ratio of methane to nitrogen in the stratosphere and upper troposphere, but in the lower troposphere the ratio began to increase. At 8 kilometres the methane reached saturation, and remained so down to the surface. The fact that the densities of the haze aerosols decreased as their sizes increased implied that they were being coated by liquid. Evidently the motes in the lower troposphere were 'seeding' the rain drops. The 12C/13C isotope ratio indicated a continuous (or perhaps episodic) replenishment of the methane in the atmosphere. A biological source was ruled out. On Earth, living organisms have a preference for 12C, and methane made biologically is enriched with this isotope. This serves as a biomarker in the chemical study of rock. There was no enrichment of 12C in Titan's atmosphere. The 14N/15N isotope ratio was consistent with the measurement at the fringe of the atmosphere by Cassini's Ion and Neutral Mass Spectrometer, and supported the hypothesis that the lighter isotope was leaking to space. Argon-40 was confirmed. Argon-36 was also detected, but only at a trace level. Argon-38, xenon and krypton were all absent. The haze aerosols proved to contain ammonia and hydrogen cyanide, which was significant because ammonia was not present in gaseous form. This indicated that the aerosols must include the results of chemical reactions that gave rise to complex organic molecules - they were not simply condensates.
At 10 kilometres, the downward-looking infrared spectrometer was to start to take reflectance spectra to determine the composition of the surface in the vicinity of the landing point. At about 7 kilometres, the wind speed decreased to a mild 1-2 metres per second and the direction became variable, possibly indicating that the probe had descended into a convective region. At an altitude of 700 metres the probe switched on its 20-watt lamp in order to 'fill in' the wavelengths of sunlight that were filtered out when passing through the atmosphere, to improve the spectroscopy. To the imager, the result resembled an automobile shining its headlamps into fog, suggesting there was a drizzle of fine droplets of liquid methane.
Just before Cassini released Huygens, Mark Leese of the Surface Science Package team at the Open University in England had pointed out, ''It is interesting that all the possible landing scenarios we envisaged still exist: a crunch down onto ice, a softer squelch into solid organics, and a splashdown in a lake of hydrocarbons.'' The rate of descent at surface contact was not expected to exceed 5 metres per second, which proved to be the case. The fact that radio telescopes continued to receive the probe's signal after the Doppler indicated it had come to rest showed that it had survived the impact.
Cassini re-established contact with Earth at 15:04 GMT and sent a brief update to indicate that it had received the probe's radio transmission. This prompted hearty applause in the European Space Agency's Huygens Project Operations Centre at Darmstadt in Germany. At 15:07 GMT Cassini started the first playback, a process that took some 3 hours. This indicated that the transmission from the probe had begun 15 seconds earlier than predicted, which meant that (in effect) the arrival and braking phase had been nominal. Cassini had recorded 2 hours 27 minutes of descent data. The probe's five batteries had been specified for a minimum duration of 153 minutes, this being the predicted maximum descent time plus 3 minutes on the surface, although it had been hoped to sustain 30 minutes of surface activity. In fact, the 64-metre radio telescope at Parkes was still receiving the signal when Titan set there at 16:00 GMT. There was initial concern that the prolonged transmission might be the result of one of more of the instruments failing to draw power, but (as the telemetry showed) the interior of the probe had been warmer than expected during the parachute descent, and this would have improved the performance of the batteries. After the probe reached the surface, Cassini recorded 1 hour 12 minutes of data until it exited the probe's transmission lobe - which occurred shortly before the spacecraft passed below the probe's horizon.
Huygens had two transmitters, known as the A/B channels. In terms of hardware, the only difference was that the A-channel included the ultrastable oscillator for the Doppler Wind Experiment. The original plan was to send all data over both channels, to provide redundancy, but the bandwidth required by the Descent Imager
Preliminary findings of the Aerosol Collector and Pyrolyser and Gas Chromatograph Mass Spectrometer on the Huygens probe.
The first image from the downward-facing Descent Imager and Spectral Radiometer of the Huygens probe to be made public, showing Titan's surface from an altitude of 16 kilometres.
and Spectral Radiometer was such that rather than repeat all of the imagery and spectral data it had been decided to send spectral data through both channels and divide up the imagery in order that, if all went well, twice as many pictures would be available. It was soon realised that the A-channel of the probe's transmission was 'missing'. Had it not been sent by Huygens? Or had Cassini not received it? The engineering telemetry would show. The loss of half of the imagery was frustrating as it would leave holes in the panoramas, but was by no means disastrous. The Doppler Wind Experiment was the real victim, because it relied on the A-channel. As Michael Bird of the University of Bonn, Germany, leading the experiment, admitted, ''I have never felt such exhilarating highs and dispiriting lows than those I experienced when Green Bank reported the signal indicating 'all is well', only to discover there was no signal on the A-channel!" However, the data from the radio telescopes would provide a basis for pursuing the intended research.
The European Space Agency began its 'first impressions' news conference at 19:45 GMT, after the first full replay of the probe's transmission. The first image to be released was taken at an altitude of 16.2 kilometres, and had a resolution of about 40 metres per pixel. As hoped, Huygens had come down over a boundary between the light and dark areas. The image seemed to show a network of dark drainage channels on a bright area leading to a shoreline, with several offshore islands on the dark area. The wind seemed to have carried the probe out over the dark area. When the image that everyone was yearning to see (the post-landing view) was presented, it showed a solid surface littered with rocks! It was not a body of liquid. Nevertheless, the overwhelming impression was of the dry bed of a lake or shallow sea. Since the camera was very close to the ground, the sense of perspective was deceptive: the rocks were not the boulders they appeared to be; they were actually no more than pebbles and cobbles ranging up to 15 centimetres in size, with the nearest no more than 1 metre away, and they were assuredly not silicate rock but water-ice. The fact that they were rounded and on top of darker fine-grained material suggested that they were erosion products washed down the drainage channels onto the plain. The imager continued to function after
The first image from the downward-facing Descent Imager and Spectral Radiometer of the Huygens probe to be made public, showing Titan's surface from an altitude of 16 kilometres.
landing, and repeated views of the same scene raised the prospect of detecting motion. There were enticing hints of liquid trickling between the pebbles, but the way in which the data had been compressed to minimise the bandwidth requirement meant that the 'lost' information could not be regenerated, making the imagery distinctly fuzzy. In retrospect, it would have been advantageous if one in every 10 pictures had been sent at full resolution. Nevertheless, there was evidence of erosion around the pebbles that was suggestive of fluvial activity. When was the last methane rain storm? Had this liquid drained into a porous surface layer? With the sunlight being both scattered by the haze and absorbed by the methane, the uniform lighting was 1,000 times dimmer than a sunny day on Earth, and it cast an orange glow over the landscape. The infrared reflectance spectrum measured for the surface was unlike any other in the Solar System: the 'red slope' in the optical range was consistent with tholins, and there were absorption features indicating water-ice, but the 'blue slope' in the near-infrared was due to an unidentified constituent. The processes were evidently different to those on Earth, but the landscape was, as T.V. Johnson of JPL put it, ''strangely familiar''. It was apparent that by characterising the interface between the light and dark areas, Huygens would yield the 'ground truth' needed to interpret the ambiguous near-infrared remote sensing by Cassini and from Earth. The sense of perspective would certainly have been much poorer if the probe had landed on one terrain without providing a glimpse of the other.
At a press conference on 15 January it was revealed that the loss of the A-channel was due to a software error. Although the fault was on Cassini, the European Space Agency was responsible for the mistake, which was, as David Southwood's deputy, Jacques Louet, put it, ''as simple as throwing a switch''. In effect, Cassini had not been told to switch on the receiver for the A-channel. ''We made a stupid mistake,'' Louet admitted. ''It's a classic example of the most simple of things escaping review because they're so simple.'' The error had not been discovered during the 'rehearsal' in March 2004 because, with the probe mounted on Cassini, the receivers were not
operated during the test. In view of the nature of the error, it was fortunate that both channels were not disabled. Imagine if the terrestrial radio telescopes had confirmed that the probe made its transmission, and it transpired that Cassini had not been listening!
The data from the Surface Science Package indicated that the probe impacted at 4.5 metres per second, with a 15-g deceleration over an interval of 40 milliseconds. The preliminary indication from the penetrometer that projected from the base of the package was of a thin brittle crust over a material with the consistency of wet sand. To R.D. Lorenz, who designed the instrument, the signal suggested a material with the characteristics of creme brulee, a sweet food made of custard with a surface of hard-burned sugar. J.I. Lunine ventured that the sharp response interpreted as a crust was the result of the penetrometer hitting an icy pebble and nudging this aside prior to contacting the actual surface. This was supported by the full analysis.229 The penetrometry and accelerometry measurements indicated that the surface was neither hard like solid ice, nor readily compressible like a blanket of fluffy aerosols. It was a 'soft' surface, for which plausible candidates were: (1) a solid granular material with little or no cohesion; (2) a 'mud' of ice grains from impact or fluvial erosion wetted by liquid methane; and (3) a 'tar' of fine-grained ice and photochemical products. Huygens had slipped along the surface for several seconds prior to coming to a halt tilted at an angle of 10 degrees with its base dug in about 10 centimetres, and the imager viewing to the south. It then slowly settled by a few millimetres, increasing its tilt by a fraction of a degree. All sensors in the Surface Science Package operated, but the refractometer, permittivity and density sensors gave little useful data because they were present to deal with a splashdown. The speed of sound at the surface was determined to be 180 metres per second. Acoustic soundings during the final 100 metres of the descent indicated that the surface around the point of impact was fairly smooth, being an undulating topography with a vertical range of about 1 metre.
The fact that Huygens embedded itself in the soft surface and continued to operate for over an hour provided a welcome bonus. The surface temperature measured by the probe was 93.65K ( + 0.25), in excellent agreement with the value of 92K inferred from remote sensing during the Voyager 1 fly-by. The lamp for the spectrometer was still illuminated when the relay was concluded, and would have
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