Next time you\u2019re visiting the seaside or a large lake, or even sipping a frosty glass of water, think about where it all originated. There are many pathways that water could have taken to the infant Earth: via comets, \u201cwet asteroids\u201d, and outgassing from early volcanism. Aster Taylor, a University of Michigan graduate student has another idea: dark comets. They\u2019re something of a cross between asteroids and comets and could have played a role in water delivery to our planet.<\/p>\n
<\/p>\n Dark comets are small Solar System bodies. They have short rotational periods thanks to non-gravitational pushes by sublimation that creates jets. These mysterious objects probably make up more than half of all near-Earth objects. <\/p>\n Planetary scientists consider dark comets as a population of active asteroids. Yet, they aren\u2019t in the same category as regular asteroids and comets. They\u2019re on near-Earth orbits, so when one passes close to the Sun, it doesn\u2019t grow a coma. That lack of a coma is why they\u2019re called \u201cdark comets.\u201d Yet, their sublimation jets appear to be a response to radiation from the Sun. They\u2019re likely rich in water ice so that raises an interesting question. Could these also<\/em> have been a source of water for Earth in the distant past?<\/p>\n \u201cWe don\u2019t know if these dark comets delivered water to Earth,\u201d said Taylor. \u201cBut we can say that there is still debate over how exactly the Earth\u2019s water got here,\u201d Taylor said. \u201cThe work we\u2019ve done has shown that this is another pathway to get ice from somewhere in the rest of the Solar System to the Earth\u2019s environment.\u201d<\/p>\n The story of how Earth got its water is still unfolding. One theory says infant Earth formed with molecular precursors to water. Another one says that water-laden asteroids and comets brought water to Earth during or just after formation. That\u2019s interesting because most asteroids exist near the so-called \u201cice line\u201d\u2014a region well beyond Earth where liquids freeze. Something propelled them to the inner solar system. When they got close to the Sun, their ice sublimated. That\u2019s actually what happens with a comet, too. So, maybe both comets and planetesimals were water-bearers during Earth\u2019s formation. Volcanic activity could have released their trapped water as vapor. <\/p>\n How about the wet asteroids, though? Where did they come from? We know that comets formed out in the cooler reaches of the protosolar nebula. Somehow they make their way (through gravitational perturbations and dynamical action) to the inner solar system. There, they have collided with Earth (just like Comet Shoemaker-Levy 9 encountered Jupiter in 1994). <\/p>\n That leaves the water ice-rich asteroids or \u201cdark comets\u201d. Most water-rich asteroids or \u201cdark comets\u201d exist in the Asteroid Belt. However, plenty of them orbit in the inner solar system, too. Those near-Earth objects probably made their way sunward due to gravitational interactions with Jupiter or other worlds. Those with some amount of water ice trapped on or below their surfaces could have been a delivery mechanism for water to early Earth.<\/p>\n The same would be true of the dark comets, according to Taylor. \u201cWe think these objects came from the inner and\/or outer main Asteroid Belt, and the implication of that is that this is another mechanism for getting some ice into the inner solar system,\u201d he said. \u201cThere may be more ice in the inner main belt than we thought. There may be more objects like this out there. This could be a significant fraction of the nearest population. We don\u2019t really know, but we have many more questions because of these findings.\u201d<\/p>\n To test their ideas about dark comets, Taylor and fellow team members created dynamical models that looked at different populations of these objects and modeled possible paths they could have taken to get to Earth. Many of these objects in the model ended up where today\u2019s dark comets exist\u2014on orbits that bring them into the inner solar system. Their model showed the team that many of these objects ended up where dark comets are today and that the main Asteroid Belt is their source.<\/p>\n The team\u2019s work also suggests that one large object may come from the Jupiter-family comets, with orbits that take them close to Jupiter. It\u2019s called 2003 RM, and follows an elliptical orbit that brings it close to Earth, then out to Jupiter and back past Earth. Its orbit is pretty typical of a Jupiter family comet that was knocked inward from its orbit.<\/p>\n Taylor\u2019s team studies focused on seven dark comets. The result of their work suggests that between 0.5% and 60% of all near-Earth objects could be dark comets that aren\u2019t accelerated by gravitational interactions. Instead, these objects experience non-gravitational accelerations\u2014that is, they are moved along by the \u201cjet action\u201d of ice as it sublimates. The researchers suggest that these dark comets likely came from the asteroid belt but that they moved due to those nongravitational accelerations. They also think that other asteroids in the Belt also contain ice.<\/p>\n The population of dark comets includes small, fast-rotating objects, especially when compared to larger asteroids. Comets are known to rotate fairly fast because they start to lose their ice to sublimation as they near the Sun. As we saw when the Rosetta spacecraft studied Comet 67P\/Churyumov-Gerasimenko, a comet nucleus sprouts little jets as part of the sublimation process. Those jets have the effect of pushing the comet nucleus along. It also accelerates it, giving the object that non-gravitational acceleration mentioned above. Sublimation can also cause the object to spin quite fast. If it rotates quickly enough, the object (comet nucleus or rubble pile asteroid) breaks apart.<\/p>\n \u201cThese pieces will also have ice on them, so they will also spin out faster and faster until they break into more pieces,\u201d Taylor said. \u201cYou can just keep doing this as you get smaller and smaller and smaller. What we suggest is that the way you get these small, fast rotating objects is you take a few bigger objects and break them into pieces.\u201d<\/p>\n As these dark objects lose their ice, they get even smaller and rotate more rapidly. Taylor\u2019s team thinks that while the larger dark comet, 2003 RM, was likely a larger object that got kicked out of the outer main belt of the Asteroid Belt, the six other objects they studied likely came from the inner main belt. They probably were part of a larger object that got knocked inward and broke apart. Further study of this and similar dark comets should help determine what contribution these objects played in the delivery of Earth\u2019s water. <\/p>\n The Origins of Dark CometsDark Comets and Asteroids<\/h3>\n
Water Delivery From Small Bodies<\/h3>\n
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Searching for Water-rich Dark Comets<\/h3>\n
One Object Implicates Many<\/h3>\n
More About Dark Comets<\/h3>\n
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For More Information<\/h4>\n
The Dynamical Origins of the Dark Comets and a Proposed Evolutionary Track<\/p>\nLike this:<\/h3>\n