First here is a quick update on the progress of the Mars rover Curiosity. After spending its first weeks on Mars checking out its systems, Curiosity has slowly been making its way towards a location named Glenelg. During its journey Curiosity has sent back dramatic pictures of a dried up stream bed that looks just those right here on earth *1. This proves that water once flowed freely across the surface of Mars. Curiosity is currently in the process of analyzing a sample of Martian soil from an interesting spot called Rocknest. Curiosity carries sophisticated miniaturized laboratories for analyzing the mineral and chemical composition of these samples. It will be exciting to learn what they find!
Other Landings on Alien Worlds
In my first post I mentioned that robotic spacecraft have flown past all the planets in the solar system except the dwarf planet Pluto. But did you know that spacecraft have also landed on other bodies in the solar system besides the Moon and Mars? The Soviet Union, which is now Russia, successfully landed a total of 10 times on the hellishly hot surface of Venus between 1970 and 1985. Some of these probes managed to send back both pictures and data for a short time before their electronics got toasted. Other spacecraft have made contact with asteroids and comets. But for me the most dramatic of these adventures was the landing by the Huygens probe on the mysterious surface of Titan in January 2005.
Titan is the largest moon of the beautiful ringed planet Saturn *2. It is the only moon in the solar system which has an atmosphere. Little was known about Titan until the Voyager I & II spacecraft flew past Saturn in November 1980 and August 1981 respectively. Voyager I flew within 6500 km of Titan. But the surface remained a complete mystery since close up pictures showed that this moon is blanketed by a thick impenetrable haze *3. The probes found that Titan has an atmosphere that is 1.5X as dense as earth. Its main component is nitrogen, but the simple organic molecules methane and ethane are also present. Titan is extremely cold with a temperature of around -180 C.
Scientists, like the famous Carl Sagan, speculated that the smog-like haze that blankets Titan might be made of complex organic compounds called tholins which consist of nitrogen-methane chains. Such compounds are considered as possible chemical precursors for life. Scientists theorized that tholins had been raining down onto the surface for billions of years forming a thick layer. There was also speculation that Titan might have oceans of methane which is a liquid in the cold conditions on Titan. But this was all pure speculation since the surface could not be seen.
The Cassini-Huygens mission *4 & *6 to Saturn and Titan was launched in 1997 on a tortuous journey lasting 7 years that included flybys of Earth, Venus, and Jupiter. The probe consisted of two separate spacecraft joined together: the Cassini Saturn orbiter and the Huygens Titan lander. Cassini-Huygens is an international project between the US and Europe, it took nearly 10 years to complete, and is the most complex and expensive interplanetary probe ever built. The project was so complex that scientists waited until after the successful launch to plan the multiyear program of activities once the probe reached Saturn.
The basic mission plan was to enter into a large looping elliptical orbit around Saturn. Within a few days of entering orbit, the Huygens’ probe *5 & *6 would separate from the Cassini orbiter and land on Titan while using Cassini to record and then relay data back to earth. The Cassini probe would continue to orbit Saturn while sending back data and pictures of its large assembly on moons (62 known to date). But in the middle of the seven year journey to Saturn, two engineers discovered that a potentially fatal technical problem would occur in the communications link between Cassini and Huygens and that all data from the Titan landing could be lost! Scientists worked feverishly to find a solution. After the spacecraft entered into orbit around Saturn in July 2004, the landing was delayed until January 2005. With this new trajectory, the relative distance and speed between the orbiter and lander enabled communications until 90 minutes after touchdown.
Plunging into the Unknown
But these technical challenges pale in comparison with the daring plan of plunging through the opaque haze and landing blind onto the never seen surface of an alien moon! There was no clue what lay below. Would it land on a mountain range, a smooth plain or an ocean? The probe was designed to survive a wide range of conditions. It had the ability to float on an ocean of liquid methane. Even if it did not survive the landing, Huygens was designed to send back data and pictures during its long 2.5 hour descent through the deep atmosphere. It also carried a landing light to illuminate the surface in case it was dark!
One of the most dramatic landings in the history of space exploration was successfully executed on January 14, 2005 when Huygens touched down on Titan *7 & *8. The Huygens probe had coasted silently through space for three weeks after separation from the Cassini orbiter on Dec 25, 2004. To save precious battery power, an on board timer woke up the spacecraft only 15 minutes before it was to enter Titan’s atmosphere. Due to a software programming error only one of two radio links between the Cassini orbiter and the Huygens lander were activated. As a result half the pictures and some data were lost. None the less enough pictures were sent back to make a remarkable movie of the bold descent to this alien world
During the first part of the landing *8 nothing can be seen except a dense haze. Then some fuzzy surface features are visible as the spacecraft breaks through the clouds. This video was created by using a computer to combine the series of images from the descent camera as the spacecraft swung back and forth on its parachute. The “flashes” in the video occur whenever a new photo is added. The result is a kind of “fish eye lens” panorama of the surface getting closer and closer.
A remarkable photograph taken during the descent shows a coastline bordering a large lake or sea, with a network of rivers and streams running into it *9. At the time of the landing this lake was “dry”. This data, plus other pictures of Titan taken by the radar on the Cassini orbiter, shows that Titan has many lakes *10 - some of which do contain liquid methane. It also rains methane on Titan. The image of Titan that has emerged is of an alien world that has a striking resemblance to earth. But on Titan, with its frigid temperature of -180 C, methane plays the same role as water on earth.
After years of analysis, NASA has recently released new details of the landing on Titan *11. This data indicates that Huygens bounced and then wobbled before settling down. The spot where Huygens landed had the consistency of soft damp sand. But the very top layer is like snow with a frozen crust. Huygens broke through this crust to sit in the soft damp soil beneath. There is also evidence that Huygens kicked up some fluffy dusty material – most likely the organic tholins mentioned earlier. The dust indicates that the surface crust was dry and that it had not rained there for some time.
The Cassini Saturn orbiter continues to function, sending back a wealth of data about Saturn and wide assortment of moons. To date Cassini has executed 86 flybys of Titan, enabling scientists to map new areas and also to track seasonal surface changes such fluctuating lake levels. Could the organic tholins on Titan have once given rise to life? Among its many other discoveries, Cassini has found geysers of water erupting into space from the moon Enceladus, indicating that this body may harbor a subsurface ocean and possibly life! But Cassini-Huygens has only scratched the surface raising many more questions than answers. Even more exciting mysteries have now been discovered to be investigated by you and future generations of scientists.
View other Lorne's Lens on Space articles
Links to Images and Videos
1. Dried up stream bed on Mars: http://www.nasa.gov/mission_pages/msl/multimedia/pia16156.html
2. Saturn and its rings: http://en.wikipedia.org/wiki/File:Saturn-cassini-March-27-2004.jpg
3. Titan haze Voyageur 1 http://en.wikipedia.org/wiki/File:Titan%27s_thick_haze_layer-picture_from_voyager1.jpg
4. Wikipedia article on the Cassini-Huygens probe: http://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens
5. Wikipedia article on the Huygens Titan lander: http://en.wikipedia.org/wiki/Huygens_%28spacecraft%29
6. NASA Cassini-Huygens web site: http://www.nasa.gov/mission_pages/cassini/main/index.html
7. NASA Titan landing video with commentary: http://www.nasa.gov/mov/148112main_pia08118-320-cc.mov
8. Huygens descent camera video: http://www.youtube.com/watch?v=xUlr8fKI2bc
9. Titan shoreline image: http://www.nasa.gov/mission_pages/cassini/multimedia/gallery-indexTitan.html Image 24
10. Titan lakes: http://en.wikipedia.org/wiki/File:Liquid_lakes_on_titan.jpg
11. Image from Titan surface: http://en.wikipedia.org/wiki/File:Huygens_surface_color_sr.jpg