Comets are frozen, alien visitors from a distant, dimly lit region of ice in the outer realm of our Solar System, where our Star’s ferocious, roiling fires and dazzling brilliance can barely reach. Here, in this strange place, our Sun appears to be only an especially large star suspended in a dark sky of perpetual twilight, and it can do little to light up the darkness of this faraway place. In August 2014, the European Space Agency’s (ESA’s) Rosetta spacecraft arrived at comet 67P/Churyumov-Gerasimenko (67P), and entered into orbit around this strange object that is shaped like a “rubber ducky”. Since then, Rosetta mission scientists have been surveying the surface and the environment of this curiously shaped body, and in August 2015, a year after the Rosetta spacecraft reached its target comet, the astronomers announced that they have worked out how this dusty, icy, wandering “oddball” got its unusual shape, saying that it likely formed from a doomed duo of different pieces, rather than through the erosion of a single object into a “ducky” shape–and in October 2015, the scientists also got their first peek at this bizarre comet’s dark side!
Comets are ephemeral, fragile, frozen relics of our Solar System’s formation about 4.56 billion years ago. Astronomers think that they are the leftovers of what was an abundant population of a multitude of icy chunks that built up the four outer gaseous, giant planets that inhabit the cold regions of the outer Solar System: Jupiter, Saturn, Uranus, and Neptune. In the past, astronomers frequently referred to comets as “icy dirt-balls” or “dirty ice-balls”, depending on their differing world views. Comets come screaming into the warm, golden, inner Solar System from their remote, frigid, and dimly lit domain far beyond the ice-giant Neptune–which is the furthest major planet from our Sun. Comets are generally believed to hold, deep in their frozen hearts, the mysterious pristine relics of the primordial ingredients that went into the ancient formation of our Star and its family of planets, moons, comets, and asteroids. These pristine, ancient ingredients were preserved in the “deep freeze” of our Solar System’s outer realm, where it is both dark and cold. Determining which precious ingredients dusty, icy comets harbor in their hearts is of great importance. This is because it would identify exactly which elements contributed to our Solar System’s birth.
Comets are really icy planetesimals. This means they are what is left over from the ancient, abundant population of building blocks that went into the construction of the outer quartet of giant planets. The asteroids that orbit our Sun, primarily within what is termed the Main Asteroid Belt, situated between the orbits of Mars and Jupiter, are really the leftovers of the rocky planetesimals that contributed to the construction of the four rocky and metallic inner terrestrial planets: Mercury, Venus, Earth, and Mars. Planetesimals of both the icy and rocky kind blasted into one another in the “cosmic shooting gallery” that was our ancient Solar System, merging together into progressively larger and larger bodies, until they finally grew to become major planets.
Dusty, dirty, icy comets shriek like rampaging banshees into the inner regions of our Solar System, closer to the melting heat and brilliant, dazzling light of our fiery Star. Comets come from three distant, dim, and frozen regions of our Solar System: the Kuiper Belt, Scattered Disk, and Oort Cloud.
The core, or nucleus, of a comet is composed mostly of water ice–with a large amount of dust added into the frozen concoction to make things more interesting. The comet’s nucleus is covered with a dark organic goo, and even though most of its ice is frozen water, there are probably additional frozen ingredients, as well, such as methane, carbon monoxide, carbon dioxide, and ammonia. The nucleus could also contain a hard heart of ice.
As the comet invades the warm, inner regions of our Solar System, closer to our Sun, the ice on its surface turns to gas, forming the dusty, brilliant, extensive tail that comets are so famous for. The nucleus usually is 10 miles–or less. However, some comets display comas that can be as much as 1 million miles wide. The coma of a comet is a nebulous envelope surrounding its nucleus. The coma forms when a comet travels toward the melting heat of our Star on its ellipical orbit. As the comet basks in the warmth of our Sun, parts of it sublimate.
ESA’s Rosetta is a robotic space probe that, along with the Philae lander module, is successfully performing a detailed examination of comet 67P. It also performed a flyby of Mars and the asteroids 21 Lutetia and 2867 Steins. On November 12, 2014, the mission succeeded in making the first landing on a comet, and it currently continues to dispatch data back to Earth from both orbit and the comet’s surface.
Launched on March 2, 2004 from the Guiana Space Center in French Guiana on an Ariane 5 rocket, it reached its comet on August 6, 2014, becoming the first spacecraft to go into orbit around a comet. Rosetta is one of ESA’s Horizon 2000 cornerstone missions, and it is composed of the Rosetta orbiter and the Philae lander. The Rosetta orbiter carries 12 instruments, and the Philae lander features an additional nine instruments. The Rosetta mission will orbit its comet for 17 months in all, and it is designed to complete the most detailed study of a comet ever attempted.
The comet 67P likely formed from two separate chunks–a small head and a large body–rather than through the erosion of a single object into its unusual duck shape, according to a research paper published in Astronomy & Astrophysics. Exactly how the duo of chunks managed to meet up and then merge together is still a matter of some debate. However, the Astronomy & Astrophysics paper proposes that the chunks are likely fragments from what was once a larger comet that was blasted apart by a devastating collision with another object. But there is an alternative hypothesis that proposes a somewhat gentler explanation: two small proto-comets, that formed only a few million years after our Solar System came to be, merged together.
“We have a lot of data showing that the comet is likely to be made of two bodies. The big question is how these two bodies came together, and we don’t know yet,” commented Dr. Jean-Baptiste Vincent in an August 7, 2015 report published in Nature.com News. Dr. Vincent, who was not involved in the study, is a planetary scientist at the Max Planck Institute for Solar System Research in Gottingen, Germany.
The strange saga of how Rosetta’s comet got its weird shape could carry important implications for the evolution of our Solar System, according to Dr. Simone Marchi, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who is one of the study’s co-authors. In their research paper, Dr. Marchi and his colleagues explain that–given what astronomers assume about the ancient Solar System–it would have been a violent place, characterized by numerous devastating collisions between primordial objects. The ancient environment around Rosetta’s comet would have been no exception. Therefore, if 67/P is the result of a merger of a duo of small proto-comets without having experienced such a catastrophic collision, this could suggest that such assumptions about our early Solar System’s “cosmic shooting gallery” conditions are not exactly on target.
“The scenarios are two radically different pathways that would imply a radically different structure of the Solar System,” Dr. Marchi commented in the August 7, 2015 Nature.com News.
It’s apparent that Rosetta’s comet is the weird construction of two separate chunks. Terrace-like features observed on each of the two cometary lobes–as imaged by the Rosetta orbiter–line up within the head or the body. However, they do not match up in orientation across the two pieces, as they would have to if the comet had been eroded from a single larger object. Furthermore, a set of almost-parallel fissures on one region of the comet’s head are best explained by the meeting and merging of the duo of comet chunks, the authors of the paper explain.
An earlier study found it improbable that one large comet could have been eroded into a duck-like shape by sunlight burning dust and gas away.
Comet 67/P’s two lobes are somewhat similar in composition. This suggests that they probably formed in the same environment in our ancient Solar System. This explanation fits both of the two-body scenarios, but given the likelihood of collisions, Dr. Marchi’s team favors the hypothesis that the two chunks are fragments from a once-larger comet. However, additional evidence derived from the comet itself could still potentially demonstrate that the scenario of a duo of proto-comets meeting up and merging together is the correct one, Dr. Vincent added.
A comet whose larger parent body was once devastated by a catastrophic collision, for example, should be compacted and show fewer voids in its interior than a pristine comet would. “We’re looking at our data now and trying to see which model is supported,” Dr. Vincent continued to explain in the August 7. 2015 Nature.com News.
Rosetta’s First Peek At Its Comet’s Dark Side
For a long time, a region of 67P’s nucleus–the frigid, dark portion around the comet’s south pole–remained beyond the observational reach of almost all instruments aboard the Rosetta spacecraft.
Because of the combination of its two-lobed shape and the inclination of its rotation axis, Rosetta’s comet experiences bizarre seasonal changes over its 6.5-year-long orbit. Indeed, this strange comet’s seasons are distributed very unevenly between the two hemispheres. Each hemisphere contains regions of both comet lobes and the “neck.”
For most of 67P’s orbit, the northern hemisphere basks in a very long summer that lasts for 5.5-years, while the southern hemisphere suffers a dark, long and very frigid winter. However, a few months before the comet approaches its perihelion–which is the closest point to our Star along its orbit–the situation changes, and the southern hemisphere transitions to experience a short, and very toasty, summer.
When Rosetta reached its quarry, the northern hemisphere of the comet was still experiencing its long, hot summer. Meanwhile, regions on the southern hemisphere were receiving little sunlight. In addition, a large region of this hemisphere, that was near the comet’s south pole, was in the dark embrace of a polar night, and had been submerged in complete blackness for almost five years.
With no direct illumination emanating from our Star, these regions could not be imaged with Rosetta’s Optical Spectroscopic, and Infrared Remote Imaging System (OSIRIS) science camera, or its Visible, InfraRed and Thermal Imaging Spectrometer (VIRTIS). Therefore, for the first several months after Rosetta’s successful rendezvous with its target comet, only one lone instrument aboard the spacecraft was able to get a peek at and characterize the frigid, frozen, dark southern pole of this strange object–the Microwave Instrument for Rosetta Orbiter (MIRO).
In their Astronomy & Astrophysics research paper, the team of astronomers report on the information collected by MIRO over these mysteriously dark, cold, and secretive regions between August and October 2014.
“We observed the ‘dark side’ of the comet with MIRO on many occasions after Rosetta’s arrival at 67P/C-G, and these unique data are telling us something very intriguing about the material just below its surface,” commented Dr. Mathieu Choukroun in an October 1, 2015 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Choukroun is of the JPL in Pasadena, California, and is lead author of the paper.
Dr. Choukroun and his team carefully observed the comet’s southern polar regions, and discovered important differences between the data gathered with MIRO’s millimeter and sub-millimeter wavelength channels. These significant variations might indicate the hidden presence of significant amounts of ice within the first few tens of centimeters beneath the surface of these regions.
“Surprisingly, the thermal and electrical properties around the comet’s south pole are quite different than what is found elsewhere on the nucleus. It appears that either the surface material or the material that’s a few tens of centimeters below it is extremely transparent, and could consist mostly of water ice or carbon-dioxide ice,” Dr. Choukroun continued to explain in the October 1, 2015 JPL Press Release.
The intriguing difference between the surface and subsurface composition of this part of the nucleus, and that discovered elsewhere, might be the result of the comet’s rather unusual cycle of seasons. One theory suggesting this proposes that water and other gases were emitted during the comet’s previous perihelion, when the southern hemisphere was the most Sun-drenched portion of the nucleus. The water condensed again and then fell back down to the surface after the season changed and the southern hemisphere was again blanketed by the eerie darkness of a long, bitterly cold winter.
However, these are preliminary results, because the analysis depends on the detailed shape of the strange nucleus. At the time the measurements were taken, the shape of the devastatingly dark, frigid polar region was not well-known.
“We plan to revisit the MIRO data using an updated version of the shape model, to verify these early results and refine the interpretation of the measurements,” Dr. Choukroun continued to explain.
In May 2015, the seasons changed on Rosetta’s comet and the short, hot southern summer, which will last until the beginning of 2016, began. Since the previously dark southern polar regions have started to be illuminated by light emanating from our Star, it has been possible for astronomers to observe them with other instruments on Rosetta, and the combination of all data might ultimately reveal the origin of their strange composition.
The spacecraft will come closer and closer to its comet, focusing on full orbits in order to compare the northern and southern hemispheres, as well as making some slower passes in the south to acquire better observations of that region.
“First, we observed these dark regions with MIRO, the only instrument able to do so at the time, and we tried to interpret these unique data. Now, as these regions became warmer and brighter around perihileon, we can observe them with other instruments, too,” Dr. Mark Hofstadter noted in the October 1, 2015 JPL Press Release. Dr. Hofstadter is MIRO principal investigator at JPL.
“We hope that, by combining data from all these instruments, we will be able to confirm whether or not the south pole had a different composition and whether or not it is changing seasonally,” Dr. Hofstadter continued to explain.
The MIRO instrument is a small, lightweight spectrometer that can map the abundance, velocity, and temperature of cometary water vapor and other molecules that the nucleus releases.
Comets are time capsules that harbor in their frozen hearts the primordial ingredients left over from that ancient time when our Solar System was first forming. Rosetta is the first spacecraft to observe in close proximity the way a comet changes as it experiences the increasing intensity of solar radiation. Such observations will shed new light on the origin and evolution of our Solar System, as well as the role comets may have played in the mysterious, ancient birth of planets–such as our own Earth!