Life's Building Blocks Found on a Comet: Philae Lander Reveals New Surprises
Seven studies assess new data from the Philae lander, which woke up from hibernation on June 13.
Concept drawing of Philae. Credit: DLR
Last month, the Philae comet lander delighted space enthusiasts by finally awakening from its seven months of accidental slumber on Comet 67P/Churyumov–Gerasimenko (67P), which is barreling towards the Sun as you read this.
The probe had been silent since November 15, 2014, after missing its target landing site and settling in shady region lacking the sunlight exposure necessary to power its systems.
But now that the comet is hurtling into its closest approach with the Sun, Philae has enough juice to communicate with the Rosetta comet orbiter, and by extension, Earth. Today, Science published the first big payload of information from the newly awakened lander, consisting of seven research studies based on the data collected by Philae over the last few weeks.
Read on for a roundup of the latest dispatches from the resurrected lander, but here's the short version: 67P is a freezing, organics-rich world, sculpted by "splashing" and other diverse erosive forces. Philae's off-kilter landing resulted in scientists losing information in some areas, but gaining unexpected insights in others.
Philae has detected 16 different organic compounds on 67P, four of which were previously unknown to have existed on comets.
Some theories suggest that the organic compounds necessary for life on Earth were seeded on our planet by comets. That's why several instruments aboard Philae are designed to sniff out and study organic molecules with a sophisticated pair of gas analyzers.
One of these instruments—known as the Cometary Sampling and Composition experiment, or COSAC—took samples throughout Philae's descent phase. In their paper, COSAC principal investigator Fred Goesmann and his co-authors announce that the instrument was able to detect 16 distinct organic compounds on the comet.
"Twenty-five minutes after Philae's initial comet touchdown, the COSAC mass spectrometer took a spectrum in sniffing mode, which displayed a suite of 16 organic compounds, including many nitrogen-bearing species but no sulfur-bearing species, and four compounds—methyl isocyanate, acetone, propionaldehyde, and acetamide—that had not previously been reported in comets," the team announced in the paper's abstract.
Not only does this discovery expand our understanding of cometary composition, it also lends weight to the idea that comets play an active role in distributing organic molecules to planetary bodies.
"The complexity of cometary nucleus chemistry and the importance of [nitrogen]-containing organics imply that early solar system chemistry fosters the formation of prebiotic material in noticeable concentrations," the team writes.
Crash-landing on a comet is a great way to study cometary surface mechanics.
Rosetta gif of Philae bouncing off the target. Image: ESA/Rosetta/NAVCAM/M. Catania
The first of the Science studies accounts for Philae's bouncy landing, which nearly resulted in the lander being lost to deep space.
"At touchdown it was planned to activate a cold gas system, pushing the lander to the surface, as well as firing two anchoring harpoons to fix Philae to the ground," the authors, led by Philae payload manager Jens Biele, wrote in the study. "Unfortunately, both subsystems did not work, leading to a bouncing of the lander."
However, the subsystem malfunctions turned out to be something of a blessing in disguise. "An analysis of the exact bouncing dynamics, however, fortuitously allowed determinations of the comet surface properties," wrote Biele's team.
Basically, Philae discovered that its target landing site, named Agilkia, is composed of soft, granular surface material by banging into it and floating away. It also learned that its final resting place in Abydos is much harder by comparison, also with the bump-and-float method.
So not only did the lander's two rebounds off the comet yield great photos of the surface, it also provided some unintentional—but useful—data about surface mechanics.
"The landings of Philae, despite not proceeding as planned, provide direct observations of the surface mechanical properties [...] thus providing insight into the structural features of cometary matter and informing future comet missions," Biele and his colleagues concluded.
67P has a fairly uniform dust/ice ratio of roughly 0.4 to 2.6 and is 75 to 85 percent porous.
The second study was led by Wlodek Kofman, the principal investigator for the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) instrument. One CONSERT radio transponder is installed aboard Philae, while another rides along with the Rosetta orbiter.
The two modules work in tandem by shooting radio waves at each other when Rosetta is on the opposite side of the comet from Philae. This allows for radar imaging of the comet's interior properties—including those of its nucleus—which is why Kofman's team was able to calculate specifics like the ratio of dust to ice (0.4 to 2.6)and its rough porosity (75 to 85 percent).
"The Philae lander provides a unique opportunity to investigate the internal structure of a comet nucleus, providing information about its formation and evolution in the early solar system," wrote the researchers.
Philae took several panoramic pictures during its landing, which reveal that the comet has a very fractured, diverse surface shaped by forces like wind, heat, and impact erosion.
Another fortunate byproduct of Philae's rough descent was the photoset the lander took as it bounced around. The images were taken by a suite of seven cameras collectively called CIVA-P, and reveal the lander's environment just after its first touchdown at Agilkia, and during its final landing at Abydos.
This meandering trajectory allowed CIVA-P to capture many different landscapes on the comet, rife with boulders, complex erosion patterns, and a variety of grain-scale objects.
"The structure and composition of cometary constituents, down to their microscopic scale, are critical witnesses of the processes and ingredients that drove the formation and evolution of planetary bodies toward their present diversity," wrote the CIVA-P team, led by lead lander scientist Jean-Pierre Bibring, in their analysis of the photoset.
"Philae's landing site exhibits a variety of materials translating the cometary diversity and likely preserving their pristine properties," the authors added.
67P is shaped by a phenomenon known as "splashing," in which small sand-sized particles create miniature impact events when they collide with the surface.
As Philae floated down to its Agilkia target site, its downward facing descent camera ROLIS (Rosetta Lander Imaging System) took high resolution images of the surface. A team led by ROLIS principal investigator Stefano Mottola analyzed the Agilkia images, revealing new details the microscale topography of the site.
ROLIS footage of Philae descent. Credit: ESA/Rosetta/Philae/ROLIS/DLR
One of the most interesting findings showed that free-floating dust particles in deep space impact with the cometary surface, producing a "splashing" effect—defined as "the ejection of one or more soil particles by the impact of an incoming projectile." On Earth, these granular particles burn up in the atmosphere before even getting close to the surface, but 67P is completely exposed to them.
This steady bombardment of tiny particles might explain some of the weirder erosive features on the comet, such as wind tails and moats. "In this scenario, aeolian-like features such as wind tails result from abrasion of the surface by impinging particles, except for regions that are shielded by obstacles," the team concluded.
Surface temperatures on 67P range from 90 to 130 Kelvin (or roughly minus 300 degrees Fahrenheit).
Philae is decked out with an interdisciplinary package of instruments known as the Multipurpose Sensors for Surface and Sub-Surface Science (MUPUS). True to its name, MUPUS conducts a wide range of experiments, but most of them are involved with energy and heat observations of the cometary environment over time, as it heads towards the Sun.
Unfortunately, MUPUS can't do a lot of subsurface research right now because Philae's landing site is fairly dense. But it has been monitoring surface temperatures, revealing the weather report on 67P to be consistently 90 to 130 Kelvin in the daytime, or roughly minus 300 degrees Fahrenheit, according to research led by MUPUS principal investigator Tilman Spohn.
The surface of 67P may contain polymers created by radiation.
The last of the studies in the Science mega-package was led by Ian Wright, the principal investigator for the lander's Ptolemy instrument. Like COSAC, Ptolemy is built to detect organic compounds on 67P, and it corroborates many of COSAC's findings. In addition, Ptolemy detected polymers on the surface that may be similar to radiation-induced compounds discovered in the trail of Halley's Comet.
"The mass spectra obtained during the first contact with the comet 67P indicate that the surface contains a complicated mixture of organics," Wright's team said. "However, after somewhat straightforward initial interpretations, more detailed enquiries have uncovered issues that remain unresolved."
This might be the last big load of new data transmitted from Rosetta for a while, given that Philae has been troublingly silent since July 9. But mission leads are confident that we haven't heard the last of the little lander that could.
"Philae is obviously still functional, because it sends us data, even if it does so at irregular intervals and at surprising times," said Philae's project manager Stephan Ulamec in a recent statement.
"Several times we were afraid that the lander would remain off," he pointed out, "but it has repeatedly taught us otherwise."
So until next time, Philae, sleep tight.
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