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Rosetta’s Comet: Getting To Know You!

Posted on November 24, 2019 in Uncategorized

On November 14, 2014, the European Space Agency’s (ESA) Rosetta mission’s Philae Probe made the first-ever, successful soft-landing on a comet when it touched down on 67/P Churyumov-Gerasimenko at close range. In the January 22. 2015 special issue of the journal Science, the initial results of the mission are presented from seven of Rosetta’s 11 science instruments based on measurements made during its approach to, and soon following after, its arrival at Comet 67/P Churyumov-Gerasimenko in August 2014. Rosetta is successfully uncovering many of the well-hidden secrets of its comet, which shows a remarkable array of surface features, as well as numerous processes contributing to its activity–unveiling to the curious eyes of observers a complex history and evolution.

The interestingly shaped dual-lobed “Rosetta’s Comet” has finally had a large number of its features measured by planetary scientists as a result of this historic mission: the small lobe measures 2.6 X 2.3 X 1.8 kilometers and the large lobe 4.1 X 3.3 X 1.8 kilometers. The Radio Science Instrument has successfully measured its mass to be 10 billion tons. Clearly, “Rosetta’s Comet” is the most closely studied comet in history, and this first major treasure trove of research reveals that it has a rich and varied landscape. Images of 67P/Churyumov-Gerasimenko were first presented at the December 2014 meeting of the American Geophysical Union (AGU) held in San Francisco, California.

Using important information gleaned from the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) and the Radio Science Investigation Instrument, the Rosetta mission scientists were able to calculate the comet’s gravitational field, also taking into account a pull resulting from the comet’s rotation. The gravitational force that results is greatest on top of the remarkable duo of lobes, but it is approximately six times weaker in the slender neck region, where dust can flutter off much more easily into interplanetary space. The team of planetary scientists also used the data to calculate the comet’s density, finding it to be a relatively porous and rather fluffy object. In fact, 67P/Churyumov-Gerasimenko possesses a density around 50% that of water, which provides some very important clues in regard to its strength and structure.

By assuming a general composition that is dominated by water ice and dust, the Rosetta team show that the highly porous comet likely harbors an interior comprising weakly bonded ice-dust blobs with small empty spaces between them.

The OSIRIS scientific camera has imaged about 70% of the surface of the comet as of January 2015. The 30% that remains to be observed is situated in the southern hemisphere that has not yet been fully illuminated since Rosetta’s arrival.

Strange, Icy Visitors From Afar

Comets are visitors from afar, originating in the distant deep freeze and murky, perpetual twilight of the outer regions of our Solar System, where our Sun’s feeble glow can only weakly invade an endless night. Small and icy, comets sport extremely eccentric orbits that shoot them periodically into the brightly lit, warmer inner Solar System, where they form thin, tenuous atmospheres, and weird, brilliant tails that thrash and flash as they approach the heat and magnificent glare of our fiery Star.

Fragile, ephemeral objects, comets are the mysterious left overs from our Solar System’s formation about 4.56 billion years ago. They are believed to be what is left of an extremely abundant population that built up the quartet of giant outer planets long ago. Often dismissively referred to as “icy mud-balls” or “dirty snowballs”, depending on the particular scientist’s point of view, comets streak into the inner Solar System from their distant, dark, and frozen home far beyond the ice-giant Neptune–the furthest planet from our Sun. Comets are thought to harbor in their frozen hearts the most pristine relics of ancient ingredients that formed our newborn Solar System. These primordial ingredients were kept hidden in the “deep freeze” of our Solar System’s outer limits, where it is both frigidly cold and dimly lit. Here our brilliant, golden Sun appears as only a particularly large star, casting its weak, silvery flames into a hovering black sky. Determining the precious ingredients that frozen comets hold in their secretive hearts is important because this identifies which elements contributed to the formation of our Sun and its entire family.

Comets are really icy planetesimals. That is, they are what is left of the ancient building blocks that created the four giant, gaseous planets inhabiting our Solar System’s outer regions: Jupiter, Saturn, Uranus, and Neptune. The asteroids that circle our Star, primarily within the Main Asteroid Belt between Mars and Jupiter, are really the relic rocky planetesimals that built up the quartet of rocky, terrestrial planets: Mercury, Venus, Earth, and Mars. Planetesimals, both rocky and icy, slammed into each other in the “cosmic shooting gallery” that was our ancient Solar System, merging together into ever larger and larger bodies, until at last they grew to become major planets.

Icy, dirty, delicate comets streak their wild way into the inner regions near our ferociously hot Star from two remote, dimly lit domains. The closer of the two, the Kuiper Belt, orbits our Sun beyond Neptune. The considerably more distant Oort Cloud is a huge shell of icy comet nuclei surrounding our entire Solar System. The Oort Cloud is thought to extend 10% of the way to the nearest star. Obviously, because Earth is located closer to the Kuiper Belt than to the Oort Cloud, most of the comets that we observe dazzling their way across our sky originate from there.

The core, or nucleus, of a comet is primarily made up of water ice with just a pinch of dirt added into the concoction, and this is well-coated with an organic, dark goo. Although the ice is mainly frozen water, there are likely other frozen ingredients, as well, including ammonia, carbon dioxide, methane, and carbon monoxide. The nucleus may also hide a hard heart of ice.

As the comet screams its sparkling way towards our warm, golden Star, the ice that exists on its surface changes to a gas, thus forming the dusty, luminous, extensive tail that comets are so famous for.

The comet’s nucleus is usually about 10 miles or less. Some comets, however, sport comas that can be as much as 1 million miles wide.

Some astronomers think that comets may have been responsible for delivering water to our Earth, as well as other basic, precious ingredients that made it possible for life to emerge. Life, as we know it, depends on the existence of liquid water. In fact, these strange icy objects from afar are time-capsules that carry hidden in their frozen hearts a tattle-tale record of the ancient materials that contributed to the formation of our Solar System.

Getting To Know You!

As of January 2015, the Rosetta scientists have detected 19 regions separated by very distinct boundaries on the mysterious surface of Comet 67/P Churyumov-Gerasiminko. Maintaining the ancient Egyptian theme that has characterized the entire Rosetta mission, the team of astronomers named these 19 regions for ancient Egyptian gods and goddesses, and they are grouped according to the type of terrain dominant within their borders.

Five diverse, basic categories of terrain type have been observed: large-scale depressions; smooth terrains; dust-covered terrains; exposed more consolidated (“rock-like”) surfaces; and brittle materials sporting circular structures and pits.

The lion’s share of the northern hemisphere of Rosetta’s Comet is literally covered with dust. As the comet heats up as it approaches the glare and heat of our Star, the ice changes directly into gas, and this escapes to create the atmosphere of the comet, or its coma. Dust is carried along for the ride, along with the gas, but at lazier speeds. Particles of dust that are not soaring quickly enough to overcome the weak gravity of the comet and escape to freedom, tragically tumble right back down to the surface instead.

Some of the sources of discrete jets of activity have also been observed by the astronomers. While a large percentage of activity originates in the smooth, slender neck region, jets have also been detected rising from pits.

The gases that soar away from the comet’s surface have also been observed to take on an important role in transporting dust across the surface. This produces dune-like ripples, as well as boulders sporting “wind tails”. The boulders serve as natural obstacles to the direction that the flowing gas takes, forming streaks of material residing “downwind” of them.

The comet’s dusty coating may be several meters thick in some areas and measurements of the surface and subsurface temperature by the Microwave Instrument (MIRO) on the Rosetta Orbiter indicate that the dust plays a starring role in insulating the comet’s hidden heart, helping to protect the ices believed to exist beneath the surface.

Small patches of ice may also exist on the comet’s surface. At scales of 15-25 meters, Rosetta’s Visible, InfraRed and Thermal Imaging Spectrometer (VIRTIS), determined that the surface is compositionally extremely homogeneous and dominated by dust and carbon-rich molecules–but largely bereft of ice. However, smaller and brightly sparkling areas observed in images are probably rich in ices. Usually, they are associated with exposed surfaces or debris piles where collapse of weaker material has taken place, thus removing the obscuring veil and exposing the fresher material situated underneath.

Many of the cliff walls observed on Comet 67P/Churyumov-Gerasimenko are covered in fractures that are randomly oriented. The formation of the fractures has been linked to the rapid heating-cooling cycles that the comet is subjected to over the course of its 12.4-hour day and over its 6.5-year elliptical orbit around our Star. One especially interesting and prominent feature is a 500-meter long crack observed to be approximately parallel to the slender neck between the duo of lobes. However, it has still not been determined if this results from stresses in this region.

Some of the particularly steep regions of the exposed cliff faces appear to be textured on scales of approximately 3-meters with features scientists have playfully nicknamed “goosebumps”. The origin of the strange features remains unexplained, but their characteristic size may provide important clues concerning the unknown processes that were going on when the comet formed.

On the largest scale of all, the origin of the comet’s global double-lobe shape is still a mystery. The two components appear to be compositionally similar, potentially favoring the interpretation of erosion on what was once a larger, single body. However, the information currently available cannot rule out an alternative scenario that suggests two separate comets formed in the same part of the outer limits of our Solar System, eventually bumping into each other and then merging together later on.

This very important question will be studied in more detail over the coming year as Rosetta travels along with its comet as it makes its incredible journey around our Star.

The closest approach of the two objects to our Sun will occur on August 13, 2015 at a distance of 186 million kilometers, between the orbits of Earth and Mars. As the comet continues its wanderings ever closer to our Star, an important focus for Rosetta’s instruments is to monitor the evolution of the comet’s activity, primarily in terms of the quantity and composition of gas and dust shot off by the nucleus to create the coma.

Images derived from the scientific and navigation cameras have revealed a rise in the quantity of dust soaring away from the comet, and MIRO showed a general increase in the comet’s global water vapor production rate. MIRO also revealed that a large amount of the water observed originated from the comet’s slender neck.

The water is accompanied by other outgassing species, including carbon dioxide and carbon monoxide. The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), is spotting large fluctuations in the chemical composition of the coma. Water is usually the dominant outgassing molecule–but not always.

The Rosetta scientists, combining measurements obtained between July and September 2014 from MIRO, ROSINA, and GIADA (Rosetta’s Grain Impact Analyzer and Dust Accumulator), estimated that there is around four times as much mass in dust being emitted than in gas, averaged over the sunlit surface of the nucleus. However, when the comet warms up as it approaches the heat of our Sun, this value is expected to change. This is because grains of ice–rather than grains of pure dust–will be shot off from the comet’s surface.

Two separate and distinct populations of dust grains have also been identified. One population is outflowing and has been detected close to the spacecraft, while the other population orbits the comet no closer than 130 kilometers from the spacecraft.

As the gas and dust coma continues to grow, interactions with charged particles composing the solar wind, as well as with our Star’s ultraviolet light will result in the formation of the comet’s ionosphere and, eventually, its magnetosphere.

“Rosetta is essentially living with the comet as it moves towards the Sun along its orbit, learning how its behavior changes on a daily basis and, over longer timescales, how it interacts with the solar wind,” Dr. Matt Taylor explained in a January 23, 2015 Press Release. Dr. Taylor is ESA’s Rosetta project scientist.

Philae which landed on its side in a shadowy region of the comet, still has not been found. Knowing where Philae landed will help its planetary science team assess their data and enable engineers to evaluate how precarious the lander’s situation is.

Dr. Taylor continued to explain to the press on January 23, 2015 that “We have already learned a lot in the few months we have been alongside the comet, but as more and more data are collected and analyzed from this close study of the comet we hope to answer many key questions about its origin and evolution.”

A Moon For A Small World

Posted on November 23, 2019 in Uncategorized

In our Solar System’s outer limits, far from the melting heat and warm, welcoming fires of our Sun, there is a mysterious swath of space situated in the frigid darkness of a perpetual twilight. This is the Kuiper Belt, a remote region where a multitude of icy comet nuclei and other frozen objects orbit our distant Star. Here, in the deep freeze of our Solar System’s far suburbs, the ice dwarf planet Pluto and its quintet of moons do their strange dance along with a treasure trove of others of their frozen kind. Indeed, the Kuiper Belt is so far away that astronomers are only now beginning to explore this shadowy region–unveiling, at last, many of its well-kept secrets. In May 2017, astronomers announced that they had uncovered yet another of this dimly lit domain’s many mysteries–there is a little frozen moon in orbit around 2007 OR10, which is the third-largest known dwarf planet in the Kuiper Belt.

Moons orbiting distant planets can play some very successful games of hide-and-seek. For this reason, they are notoriously elusive. Indeed, the dwarf planet Pluto’s large moon Charon wasn’t discovered until the mid 1970s. However, in May 2017, astronomers made their announcement that they had detected yet another small, elusive moon circling a different dwarf planet. The astronomers used the combined power of three space observatories, including archival images obtained from the Hubble Space Telescope (HST), to find the little moon where it has been hiding. Because of the discovery of this moon, many astronomers think that most of the known dwarf planets inhabiting the Kuiper Belt–that are over 600 miles across– have companion moon-worlds. These frozen little moons could provide astronomers with valuable new insight into how moons were born in the distant, dim outer regions of our Solar System. Indeed, there is an emerging new viewpoint that collisions between planetary bodies can trigger the birth of moons. Based on lunar rock samples collected by NASA’s Apollo mission astronauts, many astronomers now propose that Earth’s solitary, lovely, large Moon was born as the result of a horrific collision between our still-forming planet and an ill-fated Mars-sized protoplanet named Theia about 4.4 billion years ago.

“The discovery of satellites around all of the known large dwarf planets–except for Sedna–means that at the time these bodies formed billions of years ago, collisions must have been more frequent, and that’s a constraint on the formation models. If there were frequent collisions, then it was quite easy to form these satellites,” explained Dr. Csaba Kiss in a May 18, 2017 Hubblesite Press Release. Dr. Kiss is of the Konkoly Observatory in Budapest, Hungary, and the lead author of a research paper announcing the little moon’s discovery.

The primordial planetary bodies, called planetesimals–the building blocks of planets–likely blasted into each other frequently. This is because they lived together in close quarters, due to the crowded conditions of their frozen distant home, located in our Solar System’s outer limits. “There must have been a fairly high density of objects, and some of them were massive bodies that were perturbing the orbits of smaller bodies,” explained team member Dr. John Stansberry in the same Hubblesite Press Release. Dr. Stansberry is of the Space Telescope Science Institute (STSI) in Baltimore, Maryland. He added that “This gravitational stirring may have nudged the bodies out of their orbits and increased their relative velocities, which may have resulted in collisions.”

However, the speed of the colliding planetesimals could not have been too fast or too slow, according to the astronomers. If the impact speed was too fast, the crash would have produced a large quantity of debris that could have escaped–screaming its way out of the Solar System altogether. Conversely, if the impact speed was too slow, the smash-up of primordial objects would have created only an impact crater left behind to tell the ancient tragic story.

For example, smash-ups in the Main Asteroid Belt, situated between Mars and Jupiter, are destructive events because asteroids are zipping around at a fast pace when they crash into each other. The Main Asteroid Belt is a region inhabited by rocky chunks where Jupiter’s powerful gravity can speed up the orbits of asteroids–producing some very violent impacts. The asteroids are lingering relics of the era of planet formation–the left over planetesimals that long ago built up the quartet of small, rocky inner planets: Mercury, Venus, Earth, and Mars. The icy comets of the distant Kuiper Belt–and the even more remote Oort Cloud –are the remains of the icy planetesimals that went into the construction of the four giant gaseous outer planets: Jupiter, Saturn, Uranus, and Neptune.

The Distant Denizens Of The Kuiper Belt

The remote Kuiper Belt is located beyond the orbit of the blue, banded, and beautiful ice giant Neptune–the outermost of the eight major planets of our Solar System. Pluto, the little world with a big heart, is a relatively large constituent of this region, and it was–originally–classified as the ninth major planet from our Sun, after its discovery by the American astronomer Clyde Tombaugh (1906-1997) in 1930. However, the eventual realization among astronomers that this distant little “oddball” ice ball is really only one of a host of other similar icy bodies dwelling in this frozen place, forced the International Astronomical Union (IAU) to formally define the term “planet” is in 2006. As a result, poor Pluto was booted out of the pantheon of major planets, only to be re-classified as a mere dwarf planet. This situation is still a topic of considerable debate among astronomers, because many of them are not ready to demote the distant small world.

Comets are really frozen invaders from far away, and they hold deep within their secret, frozen hearts the most pristine of primordial elements that contributed to the formation of our Solar System about 4.6 billion years ago. This ancient cold blend of icy material has been preserved in the “deep freeze” of our Solar System’s darkest, most distant domains. These frigid invaders come screeching into the inner Solar System, closer to the melting-heat and brilliant light of our Sun, from three domains: the Kuiper Belt, Scattered Disc, and Oort Cloud. Many planetary scientists think that by gaining an understanding of the composition of the well-preserved, frozen ingredients that make up these fragile, ephemeral visitors, a scientific comprehension of the precious recipe that formed our Solar System can be obtained.

Comets are migrating relic icy planetesimals. This means that they are the lingering left-overs of what was once an immense population of ancient dirty ice balls that went into the construction of the quartet of majestic giant gaseous planets. Comets come in two basic types: short-period and long-period. Short-period comets are evicted refugees from the Kuiper Belt that rampage into the inner Solar System more often than every 200 years. By comparison, the long-period comets invade the inner Solar System every 200 years–or more. Long-period comets fly towards our Sun from the extremely remote Oort Cloud, that is believed to form a remote shell around our entire Solar System, and is much farther away than the Kuiper Belt.

2007 OR10 (225088) is a trans-Neptunian object (TNO), circling our Sun in the scattered disc. At almost 1500 kilometers in diameter, it is the third-largest known object in the Solar System dwelling past the orbit of Neptune–as well as the largest body in our Solar System that is still unnamed.

A TNO is any minor planet in our Solar System that orbits our Star at a greater average distance (semi-major axis) than Neptune, 30 AU. A dozen minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are currently known. These objects are termed extreme trans-Neptunian objects (ETNOs).

Pluto was the first TNO to be discovered back in 1930. However, it was not until 1992 that a second TNO (1992 QB1) was detected to keep it company. As of February 2017, more than 2,300 TNOs appear on the Minor Planet Center’s List of Transneptunian Objects. Of these TNOs, 2,000 have a perihelion that carry them farther out than Neptune. As of November 2016, 242 of these have had their orbits sufficiently well-determined to give them a permanent minor planet designation. The largest known TNO is Pluto, followed by Eris, 2007 OR10, Makemake, and Haumea.

2007 OR10 was discovered by California Institute of Technology (Caltech) astronomers as part of the doctoral thesis of Dr. Megan E. Schwamb, who was then still a graduate student studying under Dr. Michael E. Brown. Dr. David Rabinowitz was also a member of the discovery team on the hunt for distant Solar System bodies using the Samuel Oschin Telescope at the Palomar Observatory in California. Caltech is located in Pasadena, California.

Dr. Brown informally dubbed the still-not-officially named 2007 OR10 “Snow White” because of its presumed white color. This means that it would have to be either very large or very bright to be discovered by their survey. It was also the “seventh dwarf” to be detected by Dr. Brown’s team, after Quaoar in 2002, Sedna in 2003, Haumea and Orcus in 2004, and Makemake and Eris in 2005. However, Snow White turned out to be one of the reddest objects inhabiting the Kuiper Belt, comparable only to Quaoar, so the inappropriate nickname was dropped.

As of February 2016 2007 OR10 was about 87.5 AU from the Sun, and traveling at the impressive speed of 6,000 miles per hour with respect to our Star. This makes it the third-farthest, as well as the third largest, body in our Solar System. The spectrum of 2007 OR10 shows signatures of both water ice and methane, which means that it is similar in composition to Quaoar. The presence of red methane frost on the surfaces of both TNOs indicates that a thin methane atmosphere may exist on both objects, and that this atmosphere slowly evaporates into space.

Even though 2007 OR10 travels closer to our Star than Quaoar, and is therefore toasty enough for its methane atmosphere to evaporate, its larger mass makes retention of an atmosphere a possibility. In particular, the comparatively large size of 2007 OR10 indicates that it could very well hold on to even its nitrogen, which almost all TNOs lose over the course of their existence. The presence of water ice on the surface of 2007 OR10 suggests that a brief period of cryovolcanism (icy volcanism) occurred in its remote past.

A Moon For A Small World

Dr. Kiss and his international team of astronomers uncovered 2007 OR10’s moon in archival images taken of it with HST’s Wide Field Camera 3. Other observations taken of the small, frigid dwarf planet by NASA’s Kepler Space Telescope had provided the first clues that a moon might be orbiting it. Kepler showed that 2007 OR10 has a slow rate of rotation with a period of 45 hours. “Typical rotation periods for Kuiper Belt Objects are under 24 hours. We looked in the Hubble archive because the slower rotation period could have been caused by the gravitational tug of a moon. The initial investigator missed the moon in the Hubble images because it is very faint,” Dr. Kiss explained in the May 18, 2017 Hubblesite Press Release.

The astronomers detected the dim, distant little moon in two separate HST observations spaced a year apart. The two images revealed that the moon is gravitationally bound to 2007 OR10 because it travels along with the dwarf planet, as observed against a background sea of stars. However, the two observations did not provide sufficient information for the astronomers to determine an orbit.

“Ironically, because we don’t know the orbit, the link between the satellite and the slow rotation rate is unclear,” Dr. Stansberry commented in the Hubblesite Press Release.

However, the astronomers were able to calculate the diameters of both the dwarf planet and its tiny moon. Their calculations were based on observations in far-infrared light conducted by the Herschel Space Observatory, which measured the thermal emission of the two remote small icy worlds. The Herschel Space Observatory, operated by the European Space Agency (ESA), was an infrared observatory that operated from 2009 to 2013. Herschel’s observations indicated that 2007 OR10 is about 950 miles across, while its moon is estimated to be approximately 150 miles to 250 miles in diameter. The smaller dwarf planet, like Pluto, travels along an eccentric orbit, but it is currently three times farther away from our Star than Pluto.

2007 OR10 is a member of a group of only nine known dwarf planets. Of these brave new worlds, only Pluto and Eris are larger than 2007 OR10.

The team’s results appeared in the March 20, 2017 issue of The Astrophysical Journal Letters.

Rosetta’s “Rubber Ducky” Comet Has A Dark Side

Posted on November 21, 2019 in Uncategorized

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.

“Rubber Ducky”

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!

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