Ten years ago, ESA’s Huygens probe entered the history books by descending to the surface of Titan, the largest moon of Saturn. Humanity’s first successful attempt to land on an alien world in the outer Solar System took place on January 14, 2005.

Huygens not only survived the descent and landing, but continued to transmit data for 72 minutes on the frigid surface of Titan, until its batteries were drained.

Since that historic moment, scientists from around the world have pored over volumes of data about Titan, sent to Earth by Huygens and its mothership, NASA’s Cassini spacecraft.

“A mission of this ambitious scale represents a triumph in international collaboration,” said Dr Earl Maize, Cassini project manager at NASA’s Jet Propulsion Laboratory in Pasadena.

“From the mission’s formal beginning in 1982, to Huygens’ spectacular landing 23 years later, to the present day, Cassini-Huygens owes much of its success to the tremendous synergy and cooperation between more than a dozen countries.”

“This teamwork is still a major strength of the project as the Cassini orbiter continues to explore the Saturn system.”

To mark the 10th anniversary of Titan landing, Cassini-Huygens scientists have selected ten important results from the pioneering mission.

1. Profiling the atmosphere of Titan:

Huygens Atmospheric Structure Instrument (HASI) made the first in situ measurements of Titan’s atmosphere. HASI determined the atmospheric temperature, pressure, and density from an altitude of 1,400 km down to the surface.

Long before Huygens arrived at the largest moon of Saturn, scientists knew that its dense atmosphere was mainly composed of nitrogen, with some methane, but the atmosphere’s structure was poorly understood.

By monitoring the probe’s rate of deceleration as it plunged into the atmosphere, the HASI instrument directly determined the density of the upper atmosphere.

The temperature was derived from models of how it should change with density and altitude. In the lower atmosphere and on the surface of Titan, HASI directly measured the pressure and temperature, as well as electrical properties such as permittivity and the distribution of ions.

HASI data showed that the upper atmosphere was generally warmer and more dense than expected. Titan’s atmosphere was also found to be highly stratified.

Above 500 km, the average temperature was about minus 100 degrees Celsius but strong variations of 10-20 degrees Celsius were detected due to inversion layers and other phenomena, such as, gravity waves and tides. The mesosphere was virtually absent, in contrast with theoretical predictions.

Below 500 km, the temperature increased quite rapidly, reaching a maximum of minus 87 degrees Celsius at the top of the stratosphere, at an altitude of 250 km.

The temperature then decreased steadily throughout the stratosphere, reaching a minimum of minus 203 degrees Celsius at an altitude of 44 km. This marked the boundary between the stratosphere and the troposphere.

The temperature increased again as the probe neared the surface, rising to a chilly minus 180 degrees Celsius at the landing site. The surface pressure was 1.47 times that on Earth.

2. Superrotating winds:

Although spacecraft observations had indicated that strong zonal winds may exist in Titan’s atmosphere, the first direct measurements were made by Huygens’ Doppler Wind Experiment.

By measuring the Doppler shift of the radio signal from Huygens and studying panoramic mosaics from the onboard imager to work out the descent trajectory, it was possible to create a high resolution vertical profile of Titan’s winds, with an estimated accuracy of better than 1 m/s.

Huygens found that the zonal winds were prograde – the same direction as the Moon’s rotation – during most of the atmospheric descent.

The probe generally drifted east, driven by remarkably strong westerly winds which peaked at roughly 120 m/s at an altitude of about 120 km.

Down to a height of 60 km, large variations in the Doppler measurements were observed – evidence that Huygens endured a rough ride as the result of significant vertical wind shear.

Wind speeds then decreased toward the surface, dropping from 30 m/s at an altitude of 55 km to 10 m/s at a height of 30 km, eventually slowing to 4 m/s at 20 km. The winds dropped to zero and then reversed direction at around 7 km.

The large prograde wind speeds measured between 45 and 70 km altitude and above 85 km were much faster than Titan’s equatorial rotation speed. It was the first in situ confirmation of the predicted superrotation of the moon’s atmosphere, even though the speed observed was slightly lower than expected.

A layer with surprisingly slow wind, where the sideways velocity decreased to near zero, was detected at altitudes between 60 and 100 km. During the last 15 minutes of the descent, Huygens headed west-northwest at a speed of about 1 m/s. The wind speed on the surface was between 0.3 and 1 m/s. Over the duration of the descent, the probe drifted eastward a distance of 165.8 km with respect to the surface of Titan.

3. Methane mystery:

Huygens probe made the first direct measurements of the composition of Titan’s lower atmosphere. Data returned by the Gas Chromatograph Mass Spectrometer (GCMS) on Huygens included altitude profiles of the gaseous constituents, isotopic ratios and trace gases (including organic compounds).

Two of the key questions about Titan are the origin of the nitrogen and methane in its atmosphere, and the mechanisms by which methane levels are maintained. Since sunlight destroys methane irreversibly on Titan, its lifetime in the atmosphere is only tens of millions of years. Somehow the methane must be continually or periodically replenished.

The primary constituents of Titan’s atmosphere were confirmed to be nitrogen and methane. In the stratosphere, levels of methane were found to be fairly low and the gas was uniformly mixed. Then, at an altitude of 40 km, in the upper troposphere, the relative amount of methane began to increase gradually until approximately 7 km, when it reached 100% relative humidity (saturation level).

For the last part of the descent, methane amounts remained relatively constant until the probe touched down on the surface. A sudden, 40% increase in the methane signal after landing, while the nitrogen count rate remained constant, suggested the presence of liquid methane on the surface. This may have been due to the spacecraft heating the surface material. This increased value for methane remained nearly constant for about one hour, with a hint of a very slight decrease in the level toward the end of this period.

Measurements of the carbon isotopes in the methane provide no support for suggestions that it is generated by active micro-organisms on Titan. The methane was probably accreted by Titan during the moon’s formation, and large quantities of liquid methane are now trapped in ices beneath the surface, possibly reaching the surface through some form of cryovolcanism. This activity would replace the methane that is lost as a result of photochemistry in the atmosphere.

The spectra taken on the surface also showed signatures characteristic of more complex hydrocarbons, such as ethane, cyanogen and benzene.

4. The origin of Titan’s nitrogen atmosphere:

Titan and Earth are the only worlds in our Solar System that have thick nitrogen atmospheres. Although data from the Voyager mission had implied that nitrogen was the main atmospheric gas, GCMS on Huygens made the first direct identification of bulk atmospheric nitrogen and its abundance. Other GCMS atmospheric measurements provided clues about where this atmosphere came from.

During its descent to the surface, GCMS measured isotopic ratios and trace species in the atmosphere. One of the objectives for the instrument was to search for heavy, noble gases such as argon-36, argon-38, krypton, and xenon.

These primordial gases have been detected and measured in meteorites, in the atmospheres of Earth, Mars, Venus, and Jupiter. Differing patterns of relative abundances and isotopic ratios of the gases provide insights into the origin and evolution of these objects. As a result, their measurements in the atmosphere of Titan were eagerly anticipated.

Scientists had theorized that these noble gases were present throughout the Solar Nebula, and should therefore have been incorporated into both Saturn and Titan during the early stages of planet formation. In the context of the origin of nitrogen, argon-36 is of particular importance, and GCMS found that the ratio of argon-36 to nitrogen was about one million times less than is found in the Sun.

Direct condensation of gases in the young Titan would have resulted in the capture of argon-36, as well as nitrogen, in solar proportions. However, the depleted ratio detected by the GCMS on Huygens implies that the nitrogen was captured as ammonia or in other nitrogen-bearing compounds.

The rarity of noble gases on Earth has long been viewed as strong support for the atmosphere having been formed by the impacts of gas-rich planetesimals, and the near absence of noble gases from Titan provides more support for this hypothesis.

5. Radioactive decay and cryovolcanism:

One of the trace gases detected by Huygens’ GCMS was radiogenic argon-40. This isotope offers a window to the interior of the giant moon.

Radiogenic argon was detected by the GCMS below 18 km. This detection was important because argon-40 originates solely from the decay of potassium-40, a radioactive isotope of potassium found in rocks. The only possible source of this argon-40 is rocks which exist deep in Titan’s interior, below the satellite’s mantle of hydrocarbon and water ice.

Since the radioactive half-life of potassium-40 is about 1.3 billion years, much shorter than the lifetime of Titan, the small amount of argon-40 in the atmosphere provides an important indicator of how much outgassing has occurred from the deep interior.

If the rocky component of Titan’s interior has the same composition as that of the Earth and has outgassed to the same extent, argon-40 should be about ten times more abundant than measured by Huygens, comprising approximately 0.05 percent of the atmosphere.

If the interior was warm enough in the past for a liquid water or water-ammonia mantle to have reached all the way down to the moon’s rocky core, potassium could have seeped into the liquid. The radiogenic argon-40 could then have outgassed to the surface.

Certainly, the presence of the argon-40 at the levels seen by Huygens is a strong indication of geological activity on Titan, and consistent with periodic replenishment of atmospheric methane. The apparent evidence for cryovolcanism observed by the Cassini orbiter provides one possible process for release of both gases from the interior.

6. Hazy Titan:

One of the most noticeable features of Titan is the orange blanket of haze that hides its surface. However, no one knew whether the haze extended to the surface until Huygens landed on the icy moon.

The measurements of the Descent Imager/Spectral Radiometer (DISR) provided in situ information on the optical properties, size and density of the haze particles. The observations showed that there was a significant amount of haze at all altitudes throughout the descent, extending all the way down to the surface.

With decreasing altitude, the haze particles became brighter, and the particle sizes increased, due to collisions which resulted in a snowball effect, as well as condensation of methane, ethane and hydrogen cyanide gases onto small aerosol nuclei at lower levels.

Huygens detected three distinct haze regions – region I above 80 km, region II between 80 and 30 km, and region III between 30 km and the surface, based on the density and optical properties of the atmosphere.

Before the Huygens mission, it was generally believed that the tiny haze particles slowly sink through the stratosphere, eventually acting as condensation nuclei for lower level clouds. Some scientists theorized that the haze might clear below an altitude of 50 to 70 km due to condensation of gases such as methane.

However, DISR showed that Huygens began to emerge from the haze only in the troposphere, 30 km above the surface.

Another thin layer of methane haze was detected at an altitude of 21 km, where the local temperature was minus 197 degrees Celsius and the pressure was 450 mbar. This feature may be an indication of methane condensation. Indeed, the data suggest the presence of layered methane clouds in Titan’s troposphere, at altitudes between 8 and 30 km.

When combined with ground-based measurements, the data suggest an upper methane ice cloud (or haze) between approximately 20 and 30 km and a liquid methane-nitrogen cloud layer between 8 and 16 km, perhaps with a gap in between.

7. Titan’s tiny aerosols:

Tiny particles in the atmosphere of Titan have long been suspected to play an important role in determining its thermal structure and atmospheric processes. However, until the Huygens mission, no direct measurements had been made of the chemical composition of these particles.

One set of measurements was made by GCMS and the Aerosol Collector and Pyrolyser (ACP) experiment. The collected aerosol particles were heated in the ACP oven in order to vaporize all volatile components, and the composition of the gases released by each sample was then analyzed by the GCMS.

Two atmospheric samples were obtained during the descent of Huygens. One was taken at 130-35 km (the middle stratosphere) and the other at 25-20 km (the middle troposphere). Ammonia and hydrogen cyanide were identified as the main gases released in the oven, confirming that carbon and nitrogen are major constituents of the aerosols.

No substantial difference was found between the two samples, suggesting that the aerosols’ composition was the same at both altitudes. This supports the idea that they have a common source in the upper atmosphere, where ultraviolet sunlight photochemically alters gases such as methane.

Meanwhile, DISR characterized the optical properties of the photochemical aerosols from 150 km altitude to the surface. They were found to match the properties of tholins, materials created in laboratories by sending electrical discharges into mixtures of nitrogen and methane.

The aerosols’ optical properties can be reproduced by the condensation of hydrogen cyanide close to 80 km, ethane condensation close to the tropopause (44 km), and methane condensation from the tropopause down to 8 km.

8. Dry river beds and lakes:

Hidden beneath an all-embracing blanket of haze, Titan’s surface remained a mystery until DISR sent back a series of unique, spectacular images.

DISR took several hundred visible-light images with its three cameras during its 2 hour 27 minute descent, including several sets of stereo image pairs which enabled digital terrain models to be constructed.

The cameras revealed a plateau with a large number of dark channels cut into it, forming drainage networks which bore many similarities to those on Earth. The narrow channels converged into broad rivers, which drained into a broad, dark, lowland region. The ravines cut by the rivers were approximately 100 m deep and their valley slopes were very steep, which suggested rapid erosion due to sudden, violent flows.

No evidence of surface liquid was found at the time of the landing. However, it seems likely that, from time to time, the entire dark region is inundated by floods of liquid methane and ethane. If the darker region is a dry lakebed, it is too large to have been caused by the creeks and channels visible in the images. It may have been created by other larger river systems or some large-scale catastrophic event, which predates deposition by the rivers seen in the images.

Brighter regions north of the landing site displayed two different drainage patterns: (i) bright highlands with rough topography and deeply incised branching drainage networks with dark-floored valleys that indicated erosion by methane rainfall; and (ii) short, stubby channels that followed linear fault patterns, forming canyon-like features suggestive of spring sapping by liquid methane.

The topographic data showed that the bright highland terrains are extremely rugged, often with slopes of up to 30 degrees. These drain into relatively flat, dark lowland terrains. The dark material that covers the plains may have been carried along by the flows and could be made up of photochemical deposits rained down from above.

The landing site itself resembled a dried-up riverbed. Rounded cobbles, 10 to 15 cm in diameter and probably made of hydrocarbons and water ice, rested on a darker granular surface.

9. Hints of subsurface ocean:

One of the most surprising discoveries of Huygens was the detection of an unusual source of electrical excitation in Titan’s atmosphere.

Scientists had wondered whether lightning might be generated in Titan’s atmosphere, so Huygens was equipped with the Permittivity, Wave and Altimetry (PWA) experiment to detect tell-tale radio signals.

On Earth, thousands of lightning flashes take place every second, and each bolt generates a radio crackle. This means our atmosphere is continuously generating extremely low frequency (ELF) radio signals, known as Schumann resonances.

These global electromagnetic resonances, excited by lightning discharges, occur in the ‘cavity’ formed between Earth’s surface and the ionosphere – a region of electrically charged particles in Earth’s upper atmosphere.

Such a resonance is known only on Earth for being released by storm lightning and it had long been considered that its existence on other planets would make it possible to reveal the presence of both storm activity and a conductive ground.

Although no lightning or thunderstorms were detected in Titan’s atmosphere, the PWA did detect an unusual ELF signal at a frequency of around 36 Hertz.

Huygens also discovered a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km.

In order to explain the unique pattern of signals, scientists have proposed that Titan’s atmosphere behaves like a giant electrical circuit. The electrical currents are generated in the ionosphere when it interacts with Saturn’s magnetosphere. This results in a dynamo effect as plasma trapped in the magnetosphere co-rotates with the planet every 10 hours or so.

The lower boundary of Titan’s cavity, which reflects the radio signals, is thought to be a conductive ocean of water and ammonia which is buried at a depth of 55-80 km below a non-conducting, icy crust.

Huygens’ discovery of this unique Schumann resonance is seen as key supporting evidence for the existence of such a subsurface ocean, hidden far beneath the moon’s frozen surface.

10. Elusive dunes:

Scientists found that locating the Huygens landing site on images taken by the Cassini orbiter was much more difficult than expected.

Although the side-looking imager (SLI), part of Huygens’ DISR, was able to image surface features located up to 450 km from the Huygens landing site, the images it sent back were hard to match with the synthetic aperture radar (SAR) images obtained by the Cassini orbiter.

The area around the Huygens landing site turned out to be a huge plain of dirty water ice over which lay blankets of organic deposits. These mantles of aerosol were invisible to radar waves, so Cassini SAR images only revealed the underlying water ice. As a result, the boundary between the bright highlands and dark plains that Huygens drifted over simply did not show up in the radar images.

The location of the landing site was only tied down after some time by the detection of two dark, longitudinal sand dunes, about 30 km north of the landing site. The elusive landforms were visible in both the SAR and Huygens images.

Although dark, longitudinal dunes form vast sand seas throughout Titan’s optically dark equatorial regions, Huygens descended over a region of bright and dark units that was free of the pervasive dune fields found elsewhere.

The dunes on Titan are probably composed of sand-sized hydrocarbon and/or nitrile grains mixed with lesser amounts of water ice. The particles rained down from above onto the surface and were subsequently eroded and moved by surface and aeolian processes, such as liquid methane runoff and wind erosion.

In order for the sand to migrate across the surface under the influence of Titan’s weak surface winds, a process called saltation, scientists have concluded that the dune material must be between 100 and 300 microns in diameter.