Interplanetary dust, dust that has come from comets, asteroids, and leftover debris from the birth of the solar system, continually rains down on the Earth and other star system bodies. These particles are bombarded by solar wind, predominately hydrogen ions. This ion bombardment knocks the atoms out of order in the silicate mineral crystal and leaves behind oxygen that is more available to react with hydrogen, for example, to create water molecules.
"It is a thrilling possibility that this influx of dust has acted as a continuous rainfall of little reaction vessels containing both the water and organics needed for the eventual origin of life on Earth and possibly Mars," said Hope Ishii, new Associate Researcher in the Hawaii Institute of Geophysics and Planetology (HIGP) at UHM SOEST and co-author of the study. This mechanism of delivering both water and organics simultaneously would also work for exoplanets, worlds that orbit other stars. These raw ingredients of dust and hydrogen ions from their parent star would allow the process to happen in almost any planetary system.
Implications of this work are potentially huge: Airless bodies in space such as asteroids and the Moon, with ubiquitous silicate minerals, are constantly being exposed to solar wind irradiation that can generate water. In fact, this mechanism of water formation would help explain remotely sensed data of the Moon, which discovered OH and preliminary water, and possibly explains the source of water ice in permanently shadowed regions of the Moon.
"Perhaps more exciting," said Ishii, "interplanetary dust, especially dust from primitive asteroids and comets, has long been known to carry organic carbon species that survive entering the Earth's atmosphere, and we have now demonstrated that it also carries solar-wind-generated water. So we have shown for the first time that water and organics can be delivered together."
It has been known since the Apollo-era, when astronauts brought back rocks and soil from the Moon, that solar wind causes the chemical makeup of the dust's surface layer to change. Hence, the idea that solar wind irradiation might produce water-species has been around since then, but whether it actually does produce water has been debated. The reasons for the uncertainty are that the amount of water produced is small and it is localized in very thin rims on the surfaces of silicate minerals so that older analytical techniques were unable to confirm the presence of water.
Using a state-of-the-art transmission electron microscope, the scientists have now actually detected water produced by solar-wind irradiation in the space-weathered rims on silicate minerals in interplanetary dust particles. Futher, on the bases of laboratory-irradiated minerals that have similar amorphous rims, they were able to conclude that the water forms from the interaction of solar wind hydrogen ions (H+) with oxygen in the silicate mineral grains.
This recent work does not suggest how much water may have been delivered to Earth in this manner from IDPs.
"In no way do we suggest that it was sufficient to form oceans, for example," said Ishii. "However, the relevance of our work is not the origin of the Earth's oceans but that we have shown continuous, co-delivery of water and organics intimately intermixed."
In future work, the scientists will attempt to estimate water abundances delivered to Earth by IDPs. Further, they will explore in more detail what other organic (carbon-based) and inorganic species are present in the water in the vesicles in interplanetary dust rims.
Recent discoveries in vast interstellar dust clouds permeating the universe and in nebula have revealed hints of organic matter that could be created naturally by stars, according to research completed at the University of Hong Kong in 2011. The discovery team observed stars at different evolutionary phases and found that they are able to produce complex organic compounds and eject them into space, filling the voids between stars.
The compounds are so complex that their chemical structures resemble the makeup of coal and petroleum, the study's lead author, Sun Kwok of the University of Hong Kong, said. Kwok and his colleague Yong Zhang, also of the University of Hong Kong, studied a set of well-known but mysterious infrared emissions found in stars, interstellar space and galaxies. These phenomena, which are collectively called Unidentified Infrared Emission (UIE) features, have been known for 30 years, but the exact source of the emissions has not been identified, and remains a broad assumption.
Such chemical complexity was thought to arise only from living organisms, but the results of the new study show that these organic compounds can be created in space even when no life forms are present. In fact, such complex organics could be produced naturally by stars, and at an extremely rapid pace.
"What impressed me most is that complex organics are easily formed by stars, they are everywhere in our own galaxy and in other galaxies," Kwok told Space.com. "Nature is much more clever than we had imagined."
"In the astronomy community, it has been commonly assumed that the UIE features are emitted by (polycyclic aromatic hydrocarbon, or PAH) molecules, which are simple, purely aromatic, molecules made of carbon and hydrogen," Kwok said. Their findings have overturned this assumption.
Kwok and Zhang analyzed data from the European Space Agency's Infrared Space Observatory and NASA's Spitzer Space Telescope to show that the Unidentified Infrared Emission features are not emitted by PAH molecules because the emissions have chemical structures that are far more complex.
"I have been suspecting this for many years," Kwok said. "Now we think we have the evidence."
The researchers observed stars at different phases of stellar evolution — first low- to medium-mass stars, then stars in the protoplanetary nebula phase, which is a short-lived episode during a star's rapid evolution, and finally stars in the planetary nebula phase, which is characterized by an expanding shell of ionized gas that is ejected by certain types of stars late in their life.
Kwok and his colleague found that characteristics of the Unidentified Infrared Emission features could not be detected in low- to medium-mass stars. But, the astronomers found that the emissions began to appear in stars in the protoplanetary nebula stage and grew stronger as the stars matured into the planetary nebula phase.
"Since we know their dynamical and evolutionary ages of these objects (dynamical age is how fast the nebula will disperse, and evolutionary age is how fast the star is evolving), we can put constraints on the chemical time scales," Kwok said. "Since the dynamical/evolution ages are of the order of thousands of years, the appearance of the spectral features suggests that the organic compounds are made on time scales shorter than thousands of years."
"Their spectra changed from a pure gas spectrum to a dust spectrum on a matter of days or weeks," Kwok added. "The sudden appearance of the features suggests that organic dust can be made extremely quickly."
Detection of solar wind-produced water in irradiated rims on silicate minerals, John Bradley, Hope Ishii, Jeffrey Gillis-Davis, James Ciston, Michael Nielsen, Hans Bechtel, Michael Martin. Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1320115111The Daily Galaxy via nature.com and space.com
The Daily Galaxy via University of Hawaii – Manoa and nature.com
Image credit: NASA, NGC-2359 known as Thor's Helmet is located in Canis Major. It is a bright bubble nebula surrounding a Wolf-Rayet Star which is an extremely hot star near the center of the helmet.
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