This Month in Astronomical History: January 2023
Michael Marotta American Astronomical Society (Amateur Affiliate)
Each month as part of this series from the AAS Historical Astronomy Division (HAD), an important discovery or memorable event in the history of astronomy will be highlighted. This month's author, Mike Marotta, writes about the European Space Agency’s Huygens Probe to Saturn’s moon, Titan. Interested in writing a short (500-word) column? Instructions along with previous history columns are available on the HAD web page.
The Huygens Probe Lands on Titan
On 14 January 2005, NASA and the European Space Agency (ESA) achieved our farthest reach to the surface of another solar system body. Partnered aboard NASA’s Cassini mission to Saturn, the ESA’s Huygens probe separated from its carrier and fell for 21 days to that planet’s largest moon, Titan. Parachuting down for 2 hours and 27 minutes, the robot continued broadcasting for another 72 minutes before its signal was lost.1
A Titan 4B/Centaur rocket lifted the Cassini and Huygens instrument packages into orbit on 15 October 1997. (The naming of the launch vehicle system was coincidental.) Similar to the space shuttle of that era, the rocket had two solid-fuel boosters in addition to its four liquid-fuel stages. The combined Cassini/Huygens payload weighed 5650 kg, near the 5800 kg limit of the launch system. Therefore, the path to Saturn was not direct and instead required gravity boosts from Venus (April 1998 and June 1999), Earth (August 1999), and Jupiter (December 2000). With periodic wellness checks by telemetry every six months, Huygens arrived at Saturn 6.7 years later, on 1 July 2004.2
The pair entered Saturn's orbit insertion after three orbits of the planet. The Huygens mission was released on 25 December 2004,3 about 60,000 km from Titan to begin its long descent. On 14 January 2005 Huygens entered the atmosphere of Titan at 6,035 meters per second (13,500 miles per hour). The probe’s special design enhanced the atmospheric drag and at 389 meters per second (870 miles per hour), parachutes opened.4 In point of fact, despite the drag fins that were intended to induce a counter-clockwise rotation, Huygens spun clockwise, though without detriment.
“Huygens was released from Cassini spinning anti-clockwise but, approximately 10 minutes after entering Titan's atmosphere, the probe's spin unexpectedly reversed to become clockwise. It kept spinning this way for the rest of the descent; luckily, the magnitude of this reversed spin was similar to that expected by the researchers, meaning that the unexpected flip affected the timing of the planned observations, but did not dramatically affect their quality."
“The recent tests now confirm the cause of this flip in spin direction. While the probe was equipped with vanes to regulate its spin, other appendages on the spacecraft produced a torque in the opposite direction; this was only exacerbated by how these vanes redirected the gas flow around the body of the probe, so that an overall 'negative', or clockwise, the spin effect was created. There are also indications that the booms of the Huygens Atmospheric Structure Instrument (HASI) might have not been fully or symmetrically deployed during descent; this effect is under further investigation.”5
During its two-hour and 27-minute descent, the probe sampled the atmosphere with a gas chromatograph and mass spectrometer and captured several hundred visible light images of the ground below. Everything about Titan that reminds us of Earth only reveals deeper contrasts and stark differences between the two worlds. At first blush, it is easy to identify features caused by weather, erosion, fluid flow, evaporation, and precipitation, creating rivers, lakebeds, and shoals. However, on Titan, it rains methane.6,7Titan does have water below its surface and that water is ejected by volcanos.8
In all, five automated scientific investigation platforms gathered and reported data.9
- Huygens Atmospheric Structure Instrument (HASI)
- The Doppler Wind Experiment (DWE)
- The Descent Imager/Spectral Radiometer (DISR)
- The Aerosol Collector and Pyrolyser (ACP)
- The Surface Science Package (SSP)
In its public relations releases, ESA identified 10 compelling “mysteries” surrounding Titan.1 Depending on the phenomena, “anomalies” or “compelling curiosities” might be more fitting labels.
- Profiling the atmosphere of Titan: Long before ESA's Huygens probe arrived at Titan, scientists knew that the moon's dense atmosphere was mainly composed of nitrogen, with some methane, but the atmosphere's structure — its temperature and pressure at different altitudes — was poorly understood.
- Super rotating winds: The large prograde wind speeds measured between 45 km and 70 km altitude and above 85 km were much faster than Titan’s equatorial rotation speed.
- Methane mystery: Sunlight destroys methane irreversibly. Therefore, its lifetime in the atmosphere is only tens of millions of years. Somehow the methane must be continually or periodically replenished.
- The origin of Titan's nitrogen atmosphere: Titan and Earth are the only two worlds abundant in atmospheric nitrogen. Unlike Earth, Titan’s nitrogen comes from the dissociation of ammonia.
- Radioactive decay and cryovolcanism: The apparent evidence for cryovolcanism observed by the Cassini orbiter — involving water or a mixture of water and ammonia — provides one possible process for the release of both gases from the interior.
- Hazy Titan: Until Huygens, we did not know whether or not the haze which obscures the surface was limited to the upper atmosphere. The Descent Imager/Spectral Radiometer showed that these particles extend all the way down to the surface.
- Titan's tiny aerosols: Although aerosols had long been suspected, until the Huygens mission, no direct measurements had been made of the chemical composition of these particles.
- Dry river beds and lakes: Several sets of stereo image pairs taken by onboard cameras enabled scientists to construct digital terrain models.
- Schumann-like resonances lead to hints of the subsurface ocean: Permittivity, Wave and Altimetry (PWA) instruments detected an unusual ELF signal at 36 Hertz. Huygens also discovered a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km, all suggesting that Titan's atmosphere exhibits a giant electrical circuit, including a dynamo effect. Moreover, an electromagnetic “cavity” suggests that Titan may have an ocean of water and ammonia 55-80 km below its surface.
- Elusive dunes: The area around the Huygens landing site turned out to be a huge plain of dirty water ice over which lay blankets of organic (carbon-bearing) deposits. These mantles of aerosol were invisible to radar waves. So, images from Cassini’s synthetic aperture radar only revealed the underlying water ice.
Aerospatial was the prime contractor for the Huygens mission. NASA, the ESA, the Italian Space Agency, and JPL all collaborated. The Descent Imager/Spectral team was based at the University of Arizona.10 The Cassini probe continued to orbit Saturn for 13 years. It made 22 deep dives into the planet’s upper atmosphere before its final plunge.11
Fig. 1: “The European Space Agency's Huygens Probe was 2.7 meters wide and weighed approximately 318 kg. A hard shell protected its delicate interior from high temperatures during the two-hour and 27-minute descent through the atmosphere of Saturn's giant moon Titan. The probe had two parts: the Entry Assembly Module and the Descent Module. The Entry Assembly Module carried the equipment to control Huygens after separation from Cassini, and a heat shield that acted as a brake and as thermal protection. The Descent Module contained the scientific instruments and three different parachutes that were deployed in sequence to control Huygens' descent to the surface of Titan.” Credit: NASA.)
Fig. 2: Of all the bodies in the solar system, Titan is perhaps most like Earth, even more than Mars, because Titan has abundant, accessible water. Credit: NASA.
Fig. 3: “This is a perspective view of the surface of Saturn's moon Titan near the Huygens probe landing site that includes the bright-dark boundary between the bright highlands and lower dark plains. This provides stereo coverage with a resolution of about 50 feet per pixel (roughly 15 meters) and a convergence angle of approximately 15 degrees. The perspective image is color-coded in altitude with blue lowest and red highest. The total relief is approximately 500 feet (roughly 150 meters) and the area covered is about 0.6 by 2 miles (1 by 3 kilometers). The valleys exhibiting dark drainages in the brighter higher, terrains have steep sides ranging up to approximately 30 degrees.” “A stereo pair of images (insert) was acquired from the Huygens descent imager/spectral radiometer. The left image was acquired from 9 miles (14.8 kilometers) above the surface with the high-resolution imager; the right from 4 miles (6.7 kilometers) altitude with the medium resolution imager.” Credit: ESA/NASA/JPL/University of Arizona/USGS.
Fig. 4: First Color View of Titan’s Surface This processed color-view image was created from one returned on 14 January 2005 by the European Space Agency's Huygens probe during its successful descent to land on Titan. The added reflection spectra data yields an approximation of the actual color of the surface. At first, these were thought to be rocks or ice blocks. However, they are more pebble-sized. The later analysis measured the two rock-like objects just below the middle of the image at about 15 centimeters (about 6 inches) (left) and 4 centimeters (about 1.5 inches) (center) respectively. They were about 85 centimeters (about 33 inches) from Huygens. The color of the surface was inferred to be evidence of a mixture of water and hydrocarbon ice. The image was taken with the Descent Imager/Spectral Radiometer, one of two NASA instruments on the probe. Credit: NASA.
- Coustenis, Athena and Hirtzig, Mathieu. (2009.) “Cassini-Huygens Results on Titan’s Surface,” Research in Astronomy and Astrophysics, Vol. 9 No. 3, pages 249–268. Note that the date of release has also been recorded as 8 December 2004 in https://www.sciencedaily.com/releases/2004/12/041208083929.htm
- https://sci.esa.int/web/cassini-huygens/-/visualisation-of-airflow-across-esa-s-huygens-probe (This study was carried out under an ESA contract with LPC2E/CNRS-University of Orléans.)