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Water trapped in star dust

Astrophysicists at the University of Jena (Germany) prove that dust particles in space are mixed with ice

water star dust
Clouds of interstellar dust and gas, here in the region “Cygnus-X” in the Swan constellation. Credits: ESA/PACS/SPIRE/Martin Hennemann & Frédérique Motte, Laboratoire AIM Paris-Saclay, CEA/Irfu – CNRS/INSU – Univ. Paris Diderot, France

The matter between the stars in a galaxy – called the interstellar medium – consists not only of gas, but also of a great deal of dust. At some point in time, stars and planets originated in such an environment, because the dust particles can clump together and merge into celestial bodies. Important chemical processes also take place on these particles, from which complex organic – possibly even prebiotic – molecules emerge. However, for these processes to be possible, there has to be water. In particularly cold cosmic environments, water occurs in the form of ice. Until now, however, the connection between ice and dust in these regions of space was unclear. A research team from Friedrich Schiller University Jena and the Max Planck Institute for Astronomy has now proven that the dust particles and the ice are mixed. They report their findings in the current issue of the research journal “Nature Astronomy”.

Better modelling of physico-chemical processes in space

Until now, we didn’t know whether ice is physically separated from the dust or mixed with individual dust moieties,” explains Dr Alexey Potapov of the University of Jena. “We compared the spectra of laboratory-made silicates, water ice and their mixtures with astronomical spectra of protostellar envelopes and protoplanetary disks. We established that the spectra are congruent if silicate dust and water ice are mixed in these environments.”

Astrophysicists can gain valuable information from this data. “We need to understand different physical conditions in different astronomical environments, in order to improve the modelling of physico-chemical processes in space,” says Potapov. This result would enable researchers to better estimate the amount of material and to make more accurate statements about the temperatures in different regions of the interstellar and circumstellar media.

 

Water trapped in dust

Through experiments and comparisons, scientists at the University of Jena also observed what happens with water when the temperatures increase and the ice leaves the solid body to which it is bound and passes into the gas phase at about 180 Kelvin (-93 degrees Celsius).

Some water molecules are so strongly bound to the silicate that they remain on the surface or inside dust particles,” says Potapov. “We suspect that such ‘trapped water’ also exists on or in dust particles in space. At least that is what is suggested by the comparison between the spectra obtained from the laboratory experiments and those in what is called the diffuse interstellar medium. We found clear indications that trapped water molecules exist there.”

The existence of such solid-state water suggests that complex molecules may also be present on the dust particles in the diffuse interstellar medium. If water is present on such particles, it is not a very long way to complex organic molecules, for example. This is because the dust particles usually consist of carbon, among other things, which, in combination with water and under the influence of ultraviolet radiation such as that found in the environment, promotes the formation of methanol, for example. Organic compounds have already been observed in these regions of the interstellar medium, but until now it has not been known where they originated.

The presence of solid-state water can also answer questions about another element: although we know the amount of oxygen in the interstellar medium, we previously had no information about where exactly around a third of it is located. The new research results suggest that the solid-state water in silicates is a hidden reservoir of oxygen.

Does solid-state water help in the formation of planets?

In addition, the “trapped water” can help in understanding how the dust accumulates, as it could promote the sticking together of smaller particles to form larger particles. This effect may even work in planet formation. “If we succeed in proving that ‘trapped water’ existed – or could exist – in building blocks of the Earth, there might possibly even be new answers to the question of how water came to Earth,” says Alexey Potapov. But as yet, these are only suppositions that the Jena researchers want to pursue in the future.

[1] ESA/PACS/SPIRE/Martin Hennemann & Frédérique Motte, Laboratoire AIM Paris-Saclay, CEA/Irfu – CNRS/INSU – Univ. Paris Diderot, France

INFORMATION

Original publication:
A. Potapov, J. Bouwman, C. Jäger, Th. Henning (2020): Dust/ice mixing in cold regions and solid-state water in the diffuse interstellar medium, Nature astronomyhttps://doi.org/10.1038/s41550-020-01214-x 

 

Press release from the Friedrich Schiller University Jena

What started out as a hunt for ice lurking in polar lunar craters turned into an unexpected finding that could help clear some muddy history about the Moon’s formation.

Team members of the Miniature Radio Frequency (Mini-RF) instrument on NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft found new evidence that the Moon’s subsurface might be richer in metals, like iron and titanium, than researchers thought. That finding, published July 1 in Earth and Planetary Science Letters, could aid in drawing a clearer connection between Earth and the Moon.

“The LRO mission and its radar instrument continue to surprise us with new insights about the origins and complexity of our nearest neighbor,” said Wes Patterson, Mini-RF principal investigator from the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, and a study coauthor.

Moon metallic
This image based on data from NASA’s Lunar Reconnaissance Orbiter spacecraft shows the face of the Moon we see from Earth. The more we learn about our nearest neighbor, the more we begin to understand the Moon as a dynamic place with useful resources that could one day even support human presence. Credits: NASA / GSFC / Arizona State University

Substantial evidence points to the Moon as the product of a collision between a Mars-sized protoplanet and young Earth, forming from the gravitational collapse of the remaining cloud of debris. Consequently, the Moon’s bulk chemical composition closely resembles that of Earth.

Look in detail at the Moon’s chemical composition, however, and that story turns murky. For example, in the bright plains of the Moon’s surface, called the lunar highlands, rocks contain smaller amounts of metal-bearing minerals relative to Earth. That finding might be explained if Earth had fully differentiated into a core, mantle and crust before the impact, leaving the Moon largely metal-poor. But turn to the Moon’s maria — the large, darker plains — and the metal abundance becomes richer than that of many rocks on Earth.

This discrepancy has puzzled scientists, leading to numerous questions and hypotheses regarding how much the impacting protoplanet may have contributed to the differences. The Mini-RF team found a curious pattern that could lead to an answer.

Using Mini-RF, the researchers sought to measure an electrical property within lunar soil piled on crater floors in the Moon’s northern hemisphere. This electrical property is known as the dielectric constant, a number that compares the relative abilities of a material and the vacuum of space to transmit electric fields, and could help locate ice lurking in the crater shadows. The team, however, noticed this property increasing with crater size.

For craters approximately 1 to 3 miles (2 to 5 kilometers) wide, the dielectric constant of the material steadily increased as the craters grew larger, but for craters 3 to 12 miles (5 to 20 kilometers) wide, the property remained constant.

“It was a surprising relationship that we had no reason to believe would exist,” said Essam Heggy, coinvestigator of the Mini-RF experiments from the University of Southern California in Los Angeles and lead author of the published paper.

Discovery of this pattern opened a door to a new possibility. Because meteors that form larger craters also dig deeper into the Moon’s subsurface, the team reasoned that the increasing dielectric constant of the dust in larger craters could be the result of meteors excavating iron and titanium oxides that lie below the surface. Dielectric properties are directly linked to the concentration of these metal minerals.

If their hypothesis were true, it would mean only the first few hundred meters of the Moon’s surface is scant in iron and titanium oxides, but below the surface, there’s a steady increase to a rich and unexpected bonanza.

Comparing crater floor radar images from Mini-RF with metal oxide maps from the LRO Wide-Angle Camera, Japan’s Kaguya mission and NASA’s Lunar Prospector spacecraft, the team found exactly what it had suspected. The larger craters, with their increased dielectric material, were also richer in metals, suggesting that more iron and titanium oxides had been excavated from the depths of 0.3 to 1 mile (0.5 to 2 kilometers) than from the upper 0.1 to 0.3 miles (0.2 to 0.5 kilometers) of the lunar subsurface.

“This exciting result from Mini-RF shows that even after 11 years in operation at the Moon, we are still making new discoveries about the ancient history of our nearest neighbor,” said Noah Petro, the LRO project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “The MINI-RF data is incredibly valuable for telling us about the properties of the lunar surface, but we use that data to infer what was happening over 4.5 billion years ago!”

These results follow recent evidence from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission that suggests a significant mass of dense material exists just a few tens to hundreds of kilometers beneath the Moon’s enormous South Pole-Aitken basin, indicating that dense materials aren’t uniformly distributed in the Moon’s subsurface.

The team emphasizes that the new study can’t directly answer the outstanding questions about the Moon’s formation, but it does reduce the uncertainty in the distribution of iron and titanium oxides in the lunar subsurface and provide critical evidence needed to better understand the Moon’s formation and its connection to Earth.

“It really raises the question of what this means for our previous formation hypotheses,” Heggy said.

Anxious to uncover more, the researchers have already started examining crater floors in the Moon’s southern hemisphere to see if the same trends exist there.

LRO is managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland for the Science Mission Directorate at NASA Headquarters in Washington. Mini-RF was designed, built and tested by a team led by APL, Naval Air Warfare Center, Sandia National Laboratories, Raytheon and Northrop Grumman.

For more information on LRO, visit:

https://www.nasa.gov/lro

 

Press release from NASA/Space Goddard Flight Center, by Jeremy Rehm