Anthonares

This week we will focus on a set of published findings from the European Space Agency’s (ESA) Huygens probe. The Huygens probe descended into Titan’s atmosphere on 14 January 2005 after separating from its Cassini mothership on 25 December 2004. You may recall that a whole host of information was released to the public last January, including this spectacular photograph showing what looked to be river systems on Saturn’s largest moon. Finally this week , almost a year later, Nature publishes a whole series of articles presenting the peer-reviewed findings of the Huygens instrument teams. (Image: ESA)

I’ve selected this series of articles to examine this week for a few reasons: 1) These findings are very important to our understanding of the solar system, 2) This set allows me to introduce the topic of “publication by press release” and contrast it to publication in peer-reviewed journals, 3) The articles are free for public access, so everyone can check out the results, format, and figures that are presented in these articles.

Citations (see full citations and author lists at Nature’s website):

• Lebreton, JP and others, (2005). An overview of the descent and landing of the Huygens probe on Titan. Nature 438, 758–764
• Fulchignoni, M and others, (2005). In situ measurements of the physical characteristics of Titan’s environment. Nature 438, 785–791
• Bird, MK and others, (2005). The vertical profile of winds on Titan. Nature 438, 800–802
• Israël, G and others, (2005). Complex organic matter in Titan’s atmospheric aerosols from in situ pyrolysis and analysis. Nature 438, 796–799
• Niemann, HB and others, (2005). The abundances of constituents of Titan’s atmosphere from the GCMS instrument on the Huygens probe. Nature 438, 779–784
• Tomasko, MG and others, (2005). Rain, winds and haze during the Huygens probe’s descent to Titan’s surface. Nature 438, 765–778
• Zarnecki, JC and others, (2005). A soft solid surface on Titan as revealed by the Huygens Surface Science Package. Nature 438, 792–795

Very Brief Synopsis:
There are seven articles in this set of publications, one (Lebreton and others) describing the operation of the probe itself after separation from Cassini through descent and touchdown on Titan’s surface. The other six correspond to each of the Huygens probe’s instruments.

Lebreton and others report the detailed mission timeline of the Huygens probe and describe its instruments and their science objectives. At the left is a diagram showing two views of the Huygens probe along with labels of the instruments and their locations on the craft. On board are the Huygens Atmospheric Structure Instrument (HASI), the Doppler Wind Experiment (DWE), the Aerosol Collector and Pyrolyzer (ACP), the Gas Chromatograph and Mass Spectrometer (GCMS), the Surface Science Package (SSP), and the Descent Imager and Spectral Radiometer (DISR).

You may recall that, in 2000 ESA and NASA scientists revealed that a potentially fatal flaw had been discovered in the Huygens probe communications equipment aboard the Cassini spacecraft. The problem, they discovered, was that the frequency that the probe would use to beam back data to Cassini would be doppler-shifted beyond the range of sensitivity of Cassini’s receivers during the probe’s descent. So, though the probe would be sending back telemetry and data from its six instruments, Cassini might not be able to hear it. To compensate, mission planners adjusted the distance by which Cassini would pass Titan during Huygens’ descent. This reduced the doppler shift of Huygens signal to within the range of sensitivity of Cassini’s receivers. The figure at the right is a cartoon of this mission plan adjustment.

Fulchignoni and others detail the atmospheric temperature, pressure, and electricity profiles of Titan based on observations from the HASI. They present detailed temperature and pressure profiles as the probe descended into Titan’s atmosphere. Additionally, they report the presence of an ionosphere between 140 and 40km of altitude, as well as the signature of electrical activity (lightning). The figure on the right plots the measured temperature of the atmosphere (solid line), versus model atmosphere temperatures. There are a number of similarities between Titan’s atmosphere and that of the Earth, including the presence of a “cold trap” that peaks somewhere around 60km on Titan. Earth’s cold trap serves to condense water vapor into clouds, thus powering our weather. Titan may experience a very similar phenomenon, except the condensation would be of gaseous methane into liquid, strongly suggesting that Titan has methane rain showers.

Bird and others use the results from the DWE to present a vertical profile of windspeeds in Titan’s atmosphere to a remarkable precision. Their findings include the fact that most of the high-level winds on Titan are super-rotational (as had been measured by telescopes on Earth), meaning they rotate faster than the planet itself. The graph on the right plots the wind speed versus height (solid line), along with the expected wind speeds from a model created prior to the Huygens descent (dashed-line). The most dramatic difference between the model and the measured wind speeds is the very low dip seen at approximately 75-80 km. The authors do not suggest a reason for this dip, but note that some of general circulation models (GCMs, discussed in a previous research synopsis) exhibit this feature albeit much more weakly. Their probe obtained measurements only from the upper atmosphere, wind speeds below 15 km are plotted in Tomaski and others.

Israël and others use the ACP attached to the GCMS to provide evidence of ammonia and hydrogen cyanide (both combinations of Nitrogen and Hydrogen) in Titan’s atmosphere at two sampling locations. The function of the ACP was to intake atmospheric gases and particulates, burn (pyrolize them), and feed the resulting gases into the GCMS. The GCMS would then provide the composition of the gases from the ACP as data like that shown on the left. This plot shows the difference between the local atmosphere (green bars) and the products from the ACP (red bars). At each location,m/z (corresponding to the mass of an ion divided by its charge), a pair of red and green bars indicates that something was detected. If a red bar is present where a green was not, this means that an aerosol particle burned to produce something not present in the gaseous background atmosphere. Plots such as this one were used to infer the presence of ammonia and hydrogen cyanide at both sampling altitudes.

Niemann and others provide measurements of the chemical composition of Titan’s atmosphere using the GCMS. A bit of explanation about the GCMS is helpful to understanding its results, and can be found under General Explanations, below. As the figure to the right shows, the GCMS on board Huygens detected nitrogen, methane (CH4), argon, hydrogen gas (H2), and carbon dioxide in the atmosphere. Near the surface, some organic compounds such as cyanogen (C2N2), ethane (C2H6), and benzene (C6H6) were detected. In addition to determining the presence of atmospheric gases, they reveal the relative composition to be 1.4% methane (though the composition varies with altitude, increasing to a maximum of about 5% near the surface), and 98.4% nitrogen, with trace amounts of argon and CO2. Niemann and others also discuss the need for a continuous replenishment of methane, as any given methane molecule can only can exist in the atmosphere for about 10-100 million years before it is broken up by sunlight and reacts with nitrogen to form other compounds.

Tomasko and others use the DISR to provide a wealth of information about Titan’s surface and near-surface atmosphere including observations of what appear to be river valley networks and solid ice surface rocks, near-surface windspeeds, near-surface methane relative humidity (50%), and measurements of the atmospheric haze starting at about 150km. Their paper has 22 figures including the image shown on the right (a “true-color” reconstruction of the surface as seen from about 8 km up) along with several others, a topographic map of Huygen’s landing site that reveals eroded valleys, and the spectral reflectance and absorbance of the surface and atmosphere (that provides elemental and molecular compositions). An enormous amount of analysis and effort went into this paper, and I would do its 40 authors (there are well over 150 authors combined on all of these papers, representing an enormous amount of work by some of the world’s most brilliant scientists) a disservice by summarizing the results in fewer than 500 words, but alas, I must continue. Suffice to say the DISR data will undergo continued analysis for years to come.

The SSP activated at approximately 90 meters above Titan’s surface; Zarnecki and others use its data to explain the physical surface characteristics. The surface is smooth but not totally flat, and has a texture not that unlike wet clay, packed snow, or sand. The two primary types of data discussed by Zarnecki and others are the surface roughness measurements from an acoustic sonar and pressure measurements taken from a probe mounted beneath the craft as it impacted the surface. Sonar roughness measurements indicate that the precise landing location of Huygens is similar to the 40 meters surrounding it, and that there is some small-scale topography (this can also be seen in images taken of the landing site itself, like the one shown below). The impact pressure measurements are shown on the figure to the right, as well as comparisons to measurements taken in the lab, back here on Earth. The authors point out some of the features in the top plot and how they compare to laboratory measurements of thin crusts, pebbles, and sand.

Context:
The view of Titan that we’ve been provided by the Huygens probe and its many scientists is virtually unparalleled in planetary science. We have many of the same types of measurements from Mars, and some from Venus, but never has a single group of papers so totally shaped our view of a world. Publishing these seven papers simultaneously only 1 year after Huygens descent is a tremendous accomplishment. Surely, teams spent months refining their analyses, and many of the papers acknowledge that their findings remain preliminary and some uncertainties are expected to shrink with further work. Still, the papers presented in this Nature focus offer a revolutionary view of a world so very alien, but strangely familiar to our own.

The fact that many of these results have been discussed widely prior to their formal publication is both worrisome and exciting for science observers. Because scientists are expected to make their findings relevant to the public while in a press conference, the desire to mis- or over-state findings can be hard to resist. The Huygens team, however, managed their press releases carefully and conservatively. They released some of the pictures shown in this synopsis, and discussed some of the findings, but refrained from damaging extrapolation.

Publishing a group of articles together in a “focus” feature such as this is a means by which the broad-interest journals like Science and Nature can help attract the most important research publications. These focus features tend to have dozens or even hundreds of authors as the number of scientist-hours involved in the project is measured in the millions. There has been some criticism about the number of authors on these papers (including the likes of Tom Bethell), however many, if not most of these scientist have spent large portions of their careers working on what may be their only major scientific discovery. To suggest that their efforts are not worthy of authorship would be to deny them one of the most important rewards that peer-reviewed science has to offer: fame and recognition.

General Explanations:
The Gas Chromatograph and Mass Spectrometer (GCMS) aboard the Huygens probe is miniaturized version of a semi-ubiquitous tool used in the branch of chemistry concerned with analyzing the composition of a substance (analytical chemistry). The GCMS works by combining the function of a gas chromatograph which separates compounds based on their properties so that they enter into the mass spectrometer at different times. The mass spectrometer then breaks these compounds into component ions and analyzes the ratio of their mass to their charge using powerful magnetic and electric fields. If this explanation has not cleared things up much, here’s another attempt: the GC takes a messy sample with all sorts of different components and lets out only one type of component at a time. This “pure” sample is then fed into the MS that ioinizes the compound by breaking up molecules and stripping off electrons and then feeds it through a small cyclotron magnet. At the output end of the MS is a detector that tells how heavy the ion are, but because the ions are charged their charge factors in the detection as well. So, the mass divided by the charge (m/z) is the final output. Luckily, this output is largely unique for most chemical elements, so the GCMS provides a wide range of detection capability.