On Christmas day 2004 the Huygens probe detached from the Cassini mothercraft to begin its journey to the surface of Saturn’s most mysterious moon, Titan. On board is the Open University’s payload – designed to examine Titan's surface.
In this virtual mission you’ll discover the key-hole view through which Huygens scientists will be examining this alien landscape. But our journey will be a little closer to home.
In this website you are going to learn all about how scientists use the tiniest glimpse of data to work out a picture of a planet - and how easy it is to get it wrong. Here you have a chance to get a "probe-eye view" of the Huygens probe as it sits on a planetary surface.
Find out all about the instrumentation and science behind the recent Titan expedition, then have a go at your very own scientific mission! All photographs and illustrations are copyright ESA unless otherwise stated.
The Cassini-Huygens spacecraft was launched in 1997 on a mission to Saturn and Titan. The mission is made up of two separate elements: the mothercraft, the Cassini orbiter which will orbit Saturn for four years, and the Huygens probe, which detached from the main craft on the 25th December 2004 to begin its descent to the mysterious moon, Titan.
Titan is Saturn's largest moon and is bigger than the planet Mercury. This strange world fascinates scientists because it is the only moon in our solar system to have a dense atmosphere. Little is known about this moon, which is why scientists are so keen to have a look.
Scientists at the Open University have been part of the team developing the Huygens probe. They've developed a number of ingenious instruments that will hopefully give us a better sense of what makes up the surface of Titan.
Here at OpenLearn we wanted to give you an experience of how to interpret the data from these instruments. However, early on we discovered the obvious: we don't have the 250 million pounds to send our own probe to Titan. So we're going to send three virtual probes over somewhere a little easier to get to - Brighton!
On our Mission to Brighton you can be the scientist. Readings from the instruments will help you find out about the surface properties, topography and movement of the probe once it's landed. But we've got to warn you, the instrumentation is now seven years old and we've got our fingers crossed that nothing breaks down and the mission is a success.
Titan – a strange place
Titan is the largest of Saturn's 33 known satellites. It is the only satellite in our Solar System to have a dense atmosphere, which is mostly made up of nitrogen and methane. These gases were probably captured by Titan long ago from the gas cloud from which the Solar System formed. Part of the fascination in studying Titan is that the Earth's original atmosphere may once have been like this.
A smog of 'organic' molecules, made by solar ultraviolet radiation causing methane molecules to link together into chains, hides the surface from view except using special techniques. It also means that the surface may contain lakes of liquid ethane and methane and/or patches of tarry scum. Don't believe anyone who tells you that Titan would smell terrible, because without a space suit the cold temperature (-180 degrees Centigrade) and lack of oxygen would kill you before you could take a sniff.
Another misconception is that all the methane and ethane makes Titan's atmosphere 'inflammable', but this too is wrong because these gases won't burn in the absence of oxygen.
Titan facts and figures
Titan compared to other bodies
Titan: Radius 2575 km; Surface Temperature -180 ºC; Mass (relative to Earth) 0.022 Density (g per cubic cm) 1.88.
Earth: Radius 6378 km; Surface Temperature 15 (ºC); Mass (relative to Earth) 1; Density (g per cubic cm) 5.52.
The Moon: Radius 1738 km Surface Temperature 1 (ºC); Mass (relative to Earth) 0.0123 Density (g per cubic cm): 3.34.
Ganymede: (largest satellite of Jupiter): Radius 2631 km Surface Temperature -160 (ºC); Mass (relative to Earth) 0.0181; Density (g per cubic cm): 1.83.
Mercury: (largest satellite of Jupiter): Radius 4878 km Surface 170 (ºC); Mass (relative to Earth) 0.0181; Density (g per cubic cm): 5.43.
Core: rocky, maybe with an iron inner core. Mantle and crust - ice (mostly frozen water but mixed with frozen methane and other ices).
Surface: possible lakes and rivers of liquid ethane, solid icy surfaces may be covered in scum or 'goo' made of complex, tarry, organic molecules. Thus, the Huygens lander had to be designed to survive landing on either a solid or a liquid surface.
Atmosphere: surface pressure 1.5 bar, composition nitrogen 94%, methane 6%, plus traces of ethane, acetylene, diacetylene, methylacetylene, cyanoacetylene, propane, carbon dioxide, cyanogens, hydrogen cyanide and helium.
Titan was discovered in 1655 by the Dutch astronomer Christiaan Huygens, after whom the Titan landing probe is named. To Huygens, Titan was simply 'the moon of Saturn', and it did not get its present name, proposed by John Herschel, until 1847.
Titan seen by Pioneer 11
The Pioneer 11 spacecraft, launched on the 5th April 1973 was the first spacecraft to visit Saturn. It made the first close-up images of Titan in 1979. The spacecraft also took the first close-up views of Saturn, and found that Titan was too cold for life.
Titan seen by Voyager
The two space probes of NASA's Voyager series flew past Saturn in November 1980 and August 1981. These provided close-up pictures of Titan, but the dense atmosphere prevented any glimpse of the surface.
Titan seen by the Hubble Space Telescope
Prior to the current Cassini-Huygens mission, the best view of Titan's surface was obtained using the Hubble Space Telescope; from its vantage point above Earth's atmosphere it recorded images at 0.85 to 1.05 micrometres wavelength (near infrared), at which Titan's atmosphere turned out to be partly transparent. Here is a montage of four such images (at ninety degrees rotation), showing bright and dark areas on the surface. The prominent continent-sized bright region has now been named Xanadu Regio.
Titan seen by Cassini
The Cassini probe, which began orbiting Saturn in July 2004, will fly past Titan many times during its nominal five-year mission. Cassini has two ways of seeing Titan's surface. One is to use its Imaging Science Subsystem (ISS) to record images at about 0.938 micrometres, at which wavelength Titan's atmosphere is fairly transparent to light. This is the same trick used by the Hubble Space Telescope but because Cassini is much closer far more detail can be seen.
The other technique is to use radar to construct an image of the surface. On radar images, rough surfaces appear bright and smooth surfaces appear dark. There is no straightforward correspondence with the bright and dark areas on ISS images.
The image obtained covers an area about 150 km across. The upper central part contains some narrow, bright, winding lines that may be channels carved by liquid ethane flowing away from the rough (radar-bright) highlands occupying the left-hand half of the picture.
Some of the channels appear to discharge into rough (radar-bright) areas that may be debris deposited as the flow fanned out and its strength weakened. The smooth (radar-dark) area in the upper right is possibly a lake. On the other hand, this interpretation could be mostly wrong!
Latest news and pictures
Dateline: 28th December 2004
The Cassini spacecraft successfully performed a "getaway" manoeuvre to keep it from following the Huygens probe to the surface of Titan. As the probe has no facility for navigation, the Cassini spacecraft had to take a deliberate collision course with Titan to ensure accurate delivery of the probe.
More information from the ESA website.
Images from the current mission
Saturnian satellite factsheet
General information about Titan
The Surface Scientific Package (SSP) is only part of the instrument suite that is on the Huygens probe. Most of the other instruments are designed to examine Titan's atmosphere in depth. Of course, on our mission we're going to Brighton, not Titan. We already know a substantial amount about the Earth's atmosphere. The SSP instruments are housed at the bottom of the probe in a small section known as the "top hat".
The SSP is designed to withstand temperatures of -198 degrees Celsius and has to cope with the possibility of landing on a liquid surface. It's also been designed to be flooded by any surface liquid
Accelerometer External (ACC-E)
The accelerometer subsystem is designed to find out what sort of surface the probe lands on. The ACC-E sticks out from the bottom of the Huygens probe and is thrust into any solid surface as the probe lands. It senses the force of impact as it does so and graphs of the readings represent the different surface properties. If the probe lands in liquid, this sensor does not send back any useful information.
The force of impact is sensed by a piezoelectric ceramic element that is mounted between the titanium alloy head and the pylon shaft. The ACC-E can distinguish between materials such as fine sands, grits and coarse gravel. When combined with images, the information from this instrument will enable scientists to get a pretty good overview of what the surface is like.
Accelerometer Internal (ACC-I) This sensor sits inside the SSP's electronics box, which is not positioned in the "top hat". It's placed elsewhere in the probe because it should not be exposed to liquid. This device provides information about vertical accelerations experienced by the entire probe, and especially how it bounces on and after impact.
If the probe strikes a solid surface, this sensor determines the compressive properties of the surface at the probe's impact site. It's designed to cater for two of the most extreme possibilities, an impact with a perfectly stiff solid, and an oblique landing in a fluid body of low density and low viscosity.
Acoustic Properties Instrument - Sonar (API-S) This instrument uses sonar to measure the topography of the surface during the final few hundred metres of the probe's parachute descent. It sends out a sound signal with a wavelength of around 13mm and "listens" for its echo. Each echo is sampled at a rate of 1 kHz and during the final part of the descent will provide information about the topography with a precision of around 10cm.
f the probe touches down into liquid, this handy instrument may also provide measurements of the depth of the liquid in which it is floating, effectively becoming a depth-sounder. It uses information from the Acoustic Properties Instrument - Velocimeter (API-V) on the speed of sound in the liquid to do this.
Acoustic Properties Instrument - Velocimeter (API-V)
This instrument is designed to help determine what gases make up the atmosphere and can also identify what (if any) liquid the probe lands in. It does this by measuring the speed of sound. It uses two sensors that transmit and then "listen for" a brief 1 MHz acoustic signal. While the probe descends towards the surface these sensors will operate once a second, giving a detailed profile of the speed of sound in the atmosphere along the probe's trajectory.
Other instruments in the probe measure temperature, so the speed of sound will help determine the molecular mass of the atmosphere. This data is an important cross-check for other instruments that measure the make up of the gases in the atmosphere. It also determines the speed of sound through any rain or liquid aerosols.
At the surface, if the probe lands in a liquid, it uses the speed of sound to find out what the liquid is. On Titan, this is presumed to be a methane/ethane ocean, on Earth, it's sea water
Density Sensor (DEN)
The "top hat" containing the Surface Scientific Package in the Huygens probe is designed to be flooded by any liquid that it lands in. This sensor is designed to measure the density of any fluid that enters this cavity.It does this using a float attached to two beams that are fitted with strain gauges.
As the float rises in the liquid, the amount of up-thrust can be measured. The DEN also measures the viscosity of the liquid as it flows into or out of the cavity by monitoring the bobbing motion of the float and the rate at which this motion decays.
Permittivity Sensor (PER)
In the event of landing in a liquid, the PER will find out the electrical qualities of the fluid, and in particular its conductivity. The instrument is made up of 22 stacked parallel plates, the capacitance of which is measured at different frequencies. By briefly pulsing the sensor with direct current voltages the conductivity of the surrounding liquid can be determined.
This instrument also has a thermometer in the form of a silicon diode. It's possible that during the descent to the surface, a build-up of residue or condensation may form on the instrument. If enough of this material collects on the PER, the sensing plates may be bridged and the conductivity of this material may also be measured
Refractive Index Sensor (REF)
The REF measures the refractivity of any liquid in the cavity of the probe. (Refraction is the change of direction of light when it enters a medium of a different density from that in which it previously travelled.)
The instrument is made up of a cylindrical prism that is lit by two light emitting diodes (LEDs), one inside and one outside the prism. When immersed in liquid, light striking the interface between the liquid and the prism will be bent. The light will either escape or be reflected or refracted into the detector.
The resulting transition from light to dark and the position of this transition is measured by a photodiode array that sits on one face of the prism. The refractive index of the liquid helps scientists to determine what the liquid is made of.
This sensor isn't of much use before landing, and then only if it lands in liquid. However, during flight, condensation may form on this sensor, and its thickness and refractive index in this case can be sensed by the REF.
Thermal Properties Sensor (THP)
The THP measures the how heat conducts and the rate at which it diffuses in the probe's cavity. The sensor measures the initial temperature of the medium, as a starting point. It can measure the thermal properties of either gases or liquids and does this by using two different sets of hot wire sensors enclosed in cylindrical shields. By applying a known current for a fixed time through the sense wires in each of the four cylindrical canisters, the wires act as regulated heat sources.
The heat generated is lost by conduction to the medium surrounding the wires at a rate that is determined by the thermal properties of the material. Measurements will be made every minute as the probe drops through the atmosphere and provides a very fine record of the thermal properties of the atmosphere.
One of the most important analyses of the probe is how it moves, both as it descends through the atmosphere on the end of the parachute and after it lands. The tiltmeter provides information about the probe's attitude with respect to its local vertical position. During the probe's descent, it's sampled once a second.
The tiltmeter will provide insights into the dynamics of the atmosphere along the descent path. It is also important in determining the probe's aerodynamic properties, which in turn can be used to reconstruct the probe's trajectory.
Once the probe has stuck the surface, the TIL outputs are measured twice a second. If the probe is floating in a liquid the TIL will be capable of measuring any waves generated by all but the gentlest of breezes.
Teach Yourself Planets (2nd edition 2003) by David Rothery (a cheap, readable and up-to-date survey of the entire Solar System)
Hodder & Stoughton ISBN 0-340-86760-4
The New Solar System (4th edition 1999) J Kelly Beatty (Editor), Carolyn Collins Petersen (Editor), Andrew Chaikin (Editor)
Cambridge University Press ISBN: 0521645875
Take your interest further
You could also try a course at the Open University. Visit the physics & astronomy course listings.