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


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 astronomy 


Press release from the Friedrich Schiller University Jena

Heat stress: the climate is putting European forests under sustained pressure

No year since weather records began was as hot and dry as 2018. A first comprehensive analysis of the consequences of this drought and heat event shows that central European forests sustained long-term damage. Even tree species considered drought-resistant, such as beech, pine and silver fir, suffered. The international study was directed by the University of Basel, which is conducting a forest experiment unique in Europe.

In a forest near Basel researchers study the effects of climate change on the most important and sensitive part of the trees – the canopy. A total of 450 trees between 50 and 120 years old grow on the 1.6 hectare research area. Credits: University of Basel

Until now, 2003 has been the driest and hottest year since regular weather records began. That record has now been broken. A comparison of climate data from Germany, Austria and Switzerland shows that 2018 was significantly warmer. The average temperature during the vegetation period was 1.2°C above the 2003 value and as high as 3.3°C above the average of the years from 1961 to 1990.

Part of the analysis, which has now been published, includes measurements taken at the Swiss Canopy Crane II research site in Basel, where extensive physiological investigations were carried out in tree canopies. The goal of these investigations is to better understand how and when trees are affected by a lack of water in order to counter the consequences of climate change through targeted management measures.

When trees die of thirst

Trees lose a lot of water through their surfaces. If the soil also dries out, the tree cannot replace this water, which is shown by the negative suction tension in the wood’s vascular tissue. It’s true that trees can reduce their water consumption, but if the soil water reservoir is used up, it’s ultimately only a matter of time until cell dehydration causes the death of a tree.

Physiological measurements at the Basel research site have shown the researchers that the negative suction tension and water shortage in trees occurred earlier than usual. In particular, this shortage was more severe throughout all of Germany, Austria and Switzerland than ever measured before. Over the course of the summer, severe drought-related stress symptoms therefore appeared in many tree species important to forestry. Leaves wilted, aged and were shed prematurely.

Death of a beech tree in a forest near Basel: during the 2018 heatwave the leaves died prematurely, the following year the tree stopped forming new shoots. Credits: Urs Weber, University of Basel

Spruce, pine and beech most heavily affected

The true extent of the summer heatwave became evident in 2019: many trees no longer formed new shoots – they were partially or wholly dead. Others had survived the stress of the drought and heat of the previous year, but were increasingly vulnerable to bark beetle infestation or fungus. Trees with partially dead canopies, which reduced the ability to recover from the damage, were particularly affected.

“Spruce was most heavily affected. But it was a surprise for us that beech, silver fir and pine were also damaged to this extent,” says lead researcher Professor Ansgar Kahmen. Beech in particular had until then been classified as the “tree of the future”, although its supposed drought resistance has been subject to contentious discussion since the 2003 heatwave.

heat European forests
Death of a beech tree in a forest near Basel: during the 2018 heatwave the leaves died prematurely, the following year the tree stopped foring new shoots. Credits: Urs Weber, University of Basel

Future scenarios to combat heat and drought

According to the latest projections, precipitation in Europe will decline by up to a fifth by 2085, and drought and heat events will become more frequent. Redesigning forests is therefore essential. “Mixed woodland is often propagated,” explains plant ecologist Kahmen, “and it certainly has many ecological and economic advantages. But whether mixed woodland is also more drought-resistant has not yet been clearly proven. We still need to study which tree species are good in which combinations, including from a forestry perspective. That will take a long time.”

Another finding of the study is that it is only possible to record the impacts of extreme climate events on European forests to a limited extent using conventional methods, and thus new analytical approaches are needed.“The damage is obvious. More difficult is precisely quantifying it and drawing the right conclusions for the future,” says Kahmen. Earth observation data from satellites could help track tree mortality on a smaller scale. Spatial patterns that contain important ecological and forestry-related information can be derived from such data: which tree species were heavily impacted, when and at which locations, and which survived without damage? “A system like this already exists in some regions in the US, but central Europe still lacks one.”

Original source

Schuldt, Bernhard & Buras, Allan & Arend, Matthias & Vitasse, Yann & Beierkuhnlein, Carl & Damm, Alexander & Gharun, Mana & Grams, Thorsten & Hauck, Markus & Hajek, Peter & Hartmann, Henrik & Hilbrunner, Erika & Hoch, Günter & Holloway-Phillips, Meisha & Körner, Christian & Larysch, Elena & Luebbe, Torben & Nelson, Daniel & Rammig, Anja & Kahmen, Ansgar.

A first assessment of the impact of the extreme 2018 summer drought on Central European forests.
Basic and Applied Ecology (April 2020); doi: 10.1016/j.baae.2020.04.003


Press release on the heat stress upon European forests from the University of Basel.

Traffic density, wind and air stratification influence concentrations of air pollutant NO2

Leipzig researchers use a calculation method to remove weather influences from air pollution data

traffic air pollutant nitrogen dioxide COVID-19
Traffic density, wind and air stratification influence the pollution with the air pollutant nitrogen dioxide, according to the conclusion of a TROPOS study commissioned by the LfULG. Credits: Burkhard Lehmann, LfULG

Leipzig/Dresden. In connection with the effects of the COVID-19 pandemic, satellite measurements made headlines showing how much the air pollutant nitrogen dioxide (NO2) had decreased in China and northern Italy.  In Germany, traffic density is the most important factor. However, weather also has an influence on NO2 concentrations, according to a study by the Leibniz Institute for Tropospheric Research (TROPOS), which evaluated the influence of weather conditions on nitrogen dioxide concentrations in Saxony 2015 to 2018 on behalf of the Saxon State Office for Environment, Agriculture and Geology (LfULG). It was shown that wind speed and the height of the lowest air layer are the most important factors that determine how much pollutants can accumulate locally.

In order to determine the influence of various weather factors on air quality, the team used a statistical method that allows meteorological fluctuations to be mathematically removed from long-term measurements. The air quality fluctuates, in some cases very strongly, due to different emissions and the influence of the weather. Until now, however, it has been difficult to estimate, what share legal measures such as low emission zones or diesel driving bans have and what share the weather influences have in the actual air quality? With the method used, this will be easier in the future.

Nitrogen dioxide (NO2) is an irritant gas which attacks the mucous membrane of the respiratory tract, causes inflammatory reactions as an oxidant and increases the effect of other air pollutants. As a precursor substance, it can also contribute to the formation of particulate matter. Limit values have been set in the EU to protect the population: For nitrogen dioxide, an annual average value of 40 micrograms per cubic metre of air applies (μg/m³). To protect the health of the population, measures must be taken if these limit values are not complied with. In 2018/2019, for example, various measures were taken in Germany, ranging from a reduction in the number of lanes (e.g. in Leipzig) to driving bans for older diesel vehicles (e.g. in Stuttgart).

To evaluate the effectiveness of such measures, it would be helpful to determine the exact influence of weather conditions. The Saxon State Office for Environment, Agriculture and Geology (LfULG) therefore commissioned TROPOS to carry out a study on the influence of weather factors on NO2 concentrations and provided its measurement data from the Saxon air quality measurement network and meteorological data for this purpose. The researchers were thus able to evaluate data from 29 stations in Saxony over a period of four years, which represent a cross-section of air pollution – from stations at traffic centres to urban and rural background stations and stations on the ridge of the Erzgebirge mountains. They also calculated the height of the lowest layer in the atmosphere and incorporated data from traffic counting stations in Leipzig and Dresden into the study. A method from the field of machine learning was used for the statistical modelling, the application of which in the field of air quality was first published by British researchers in 2009.

In this way, the study was able to demonstrate that the traffic density at all traffic stations is most significantly responsible for nitrogen oxide concentrations. However, two weather parameters also have a significant influence on nitrogen dioxide concentrations: wind speed and the height of the so-called mixing layer. The latter is a meteorological parameter that indicates the height to which the lowest layer of air, where the emissions mix, extends. “It was also shown that high humidity can also reduce the concentration of nitrogen dioxide, which could be due to the fact that the pollutants deposit more strongly on moist surfaces. However, the exact causes are still unclear,” says Dominik van Pinxteren.

The statistical analysis has also enabled the researchers to remove the influence of the weather from the time series of pollutant concentrations: Adjusted for the weather, the concentration of nitrogen oxides (NOx) decreased by a total of 10 micrograms per cubic meter between 2015 and 2018 on average over all traffic stations in Saxony. In urban and rural areas and on the ridge of the Erzgebirge, however, NOx concentrations tend to remain at the same level. Even though there have been some improvements in air quality in recent years, there are good scientific arguments for further reducing air pollution.

In a way, this also applies to premature conclusions from the corona crisis: in order to find out how strong the influence of the initial restrictions on air quality actually was, the influence of the weather would have to be statistically removed in a longer series of measurements. To this end, investigations for the Leipzig area are currently underway at TROPOS, as is a Europe-wide study of the EU research infrastructure for short-lived atmospheric constituents such as aerosol, clouds and trace gases (ACTRIS), the German contribution to which is coordinated by TROPOS.


Dominik van Pinxteren, Sebastian Düsing, Alfred Wiedensohler, Hartmut Herrmann (2020): Meteorological influences on nitrogen dioxide: Influence of weather conditions and weathering on nitrogen dioxide concentrations in outdoor air 2015 to 2018. Series of publications of the LfULG, issue 2/2020 (in German only)
This study was commissioned by the State Office for Environment, Agriculture and Geology (LfULG).


LfULG-Projekt „Meteorologische Einflüsse auf Stickstoffdioxid“:


Press release on traffic density, wind and air stratification influence concentrations of air pollutant NO2 by Tilo Arnhold from the Leibniz Institute for Tropospheric Research (TROPOS)