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.”