Tag Archives: #atmosphere

How Much Longer Will the Oxygen-rich Atmosphere be Sustained On Earth? (Earth Science)

Earth’s surface environments are highly oxygenated – from the atmosphere to the deepest reaches of the oceans, representing a hallmark of active photosynthetic biosphere. However, the fundamental timescale of the oxygen-rich atmosphere on Earth remains uncertain, particularly for the distant future. Solving this question has great ramifications not only for the future of Earth’s biosphere but for the search for life on Earth-like planets beyond the solar system.

A new study published in Nature Geoscience this week tackles this problem using a numerical model of biogeochemistry and climate and reveals that the future lifespan of Earth’s oxygen-rich atmosphere is approximately one billion years.

“For many years, the lifespan of Earth’s biosphere has been discussed based on scientific knowledge about the steadily brightening of the sun and global carbonate-silicate geochemical cycle. One of the corollaries of such a theoretical framework is a continuous decline in atmospheric CO2 levels and global warming on geological timescales. Indeed, it is generally thought that Earth’s biosphere will come to an end in the next 2 billion years due to the combination of overheating and CO2 scarcity for photosynthesis. If true, one can expect that atmospheric O2 levels will also eventually decreases in the distant future. However, it remains unclear exactly when and how this will occur,” says Kazumi Ozaki, Assistant Professor at Toho University.

To examine how Earth’s atmosphere will evolve in the future, Ozaki and Christopher Reinhard, Associate Professor at Georgia Institute of Technology, constructed an Earth system model which simulates climate and biogeochemical processes. Because modelling future Earth evolution intrinsically has uncertainties in geological and biological evolutions, a stochastic approach was adopted, enabling the researchers to obtain a probabilistic assessment of the lifespan of an oxygenated atmosphere. Ozaki ran the model more than 400 thousand times, varying model parameter, and found that Earth’s oxygen-rich atmosphere will probably persist for another one billion years (1.08±0.14 (1σ) billion years) before rapid deoxygenation renders the atmosphere reminiscent of early Earth before the Great Oxidation Event around 2.5 billion years ago.

“The atmosphere after the great deoxygenation is characterized by an elevated methane, low-levels of CO2, and no ozone layer. The Earth system will probably be a world of anaerobic life forms,” says Ozaki.

Earth’s oxygen-rich atmosphere represents an important sign of life that can be remotely detectable. However, this study suggests that Earth’s oxygenated atmosphere would not be a permanent feature, and that the oxygen-rich atmosphere might only be possible for 20-30% of the Earth’s entire history as an inhabited planet. Oxygen (and photochemical byproduct, ozone) is most accepted biosignature for the search for life on the exoplanets, but if we can generalize this insight to Earth-like planets, then scientists need to consider additional biosignatures applicable to weakly-oxygenated and anoxic worlds in the search for life beyond our solar system.

The research was supported by the Japan Society for the Promotion of Science (grant number JP20K04066) and the NASA Nexus for Exoplanet System Science (NExSS) (grant number 80NSSC19KO461).

Featured image: The authors of this study: Dr. Christopher Reinhard (left) and Dr. Kazumi Ozaki (right). © Kazumi Ozaki


Reference: Ozaki, K., Reinhard, C.T. The future lifespan of Earth’s oxygenated atmosphere. Nat. Geosci. (2021). https://www.nature.com/articles/s41561-021-00693-5 https://doi.org/10.1038/s41561-021-00693-5


Provided by Toho University

HKU Planetary Scientists Discover Evidence For a Reduced Atmosphere on Ancient Mars (Planetary Science)

Both Earth and Mars currently have oxidising atmospheres, which is why iron-rich materials in daily life develop rust (a common name for iron oxide) during the oxidation reaction of iron and oxygen. The Earth has had an oxidising atmosphere for approximately two and a half billion years, but before that, the atmosphere of this planet was reducing – there was no rust. 

The transition from a reduced planet to an oxidised planet is referred to as the Great Oxidation Event or GOE. This transition was a central part of our planet’s evolution, and fundamentally linked to the evolution of life here – specifically to the prevalence of photosynthesis that produced oxygen. Planetary geologists at HKU have discovered that Mars underwent a great oxygenation event of its own – billions of years ago, the red planet was not so red.  

The discovery was published recently in Nature Astronomy in a paper led by research postgraduate student Jiacheng LIU and his advisor Associate Professor Dr Joe MICHALSKI, both affiliated with the Research Division for Earth and Planetary Science and Laboratory for Space Research. The researchers used infrared remote sensing and spectroscopy to measure the molecular vibration of the material on the Martian surface from orbit, in order to reveal the mineralogy and geochemistry of ancient rocks on Mars. Through detailed comparisons of infrared remote sensing data and data collected in the laboratory here on Earth, the team showed that ancient rocks on Mars exposed at the surface had been weathered under reducing conditions, indicating a reduced atmosphere did exist. 

Many people are aware that Mars is cold and dry now, but ~ 3.5 billion years ago, it was warmer and wetter. It was warm enough to allow the formation of river channels, lakes and minerals that formed by interaction with water. Scientists who have used mathematical models to constrain the conditions of an early Martian atmosphere, have concluded that greenhouse warming occurred, but they also concluded from their models that the greenhouse must have included reduced gases rather than carbon dioxide, implied that a reducing atmosphere might have existed. Yet until now, there has not been any evidence that the reduced atmosphere of early Mars actually occurred. This work indicates that it did exist.

A 3-dimensional view of weathered bedrock shows the exposure of Fe-rich red rocks beneath Fe-depleted blue-toned rocks in a crater wall. © University of Hong Kong

This project involved detailed infrared remote sensing of Mars, using infrared spectroscopy to map minerals in exposed, weathered rock units. The work was built on detailed analysis of weathered volcanic rocks in Hainan Island in southwestern China, where thick sequences of basalt, similar to volcanic rocks on Mars occur. Jiacheng Liu analysed the altered rocks systematically using infrared spectroscopy in the laboratory and produced a paper on that research published recently in Applied Clay Science

“Jiacheng has carried out a truly excellent PhD project, built on careful analysis in the laboratory and application of those laboratory results to remote sensing of Mars,” Dr Michalski commented, “Jiacheng has built on his detailed work on samples from Hainan Island to show that similar mineralogical trends occurred in rocks on Mars.”

Assistant Professor Dr Ryan MCKENZIE from Research Division for Earth and Planetary Science is also impressed by these findings. “This is a rather remarkable study with findings that will significantly impact how we understand the early evolution of terrestrial planets and their surface environments. The transition from a reducing to oxidising atmosphere on Earth ~2.5 billion years ago was only possible because the existence of life, as oxygen is a waste product of metabolic processes like photosynthesis. Without microbes producing oxygen, it would not accumulate in our atmosphere, and we could not be here. While there are certainly differences in the local conditions Mars and Earth have been subjected to during their evolutionary histories, my mind can’t help but start thinking about what Jiancheng’s results may mean for a potential early Martian biosphere,” Dr McKenzie remarked.

As China’s first mission to Mars Tianwen-1 is underway – has successfully arrived in Mars orbit on February 10 and set to land on Mars in May 2021, scientists are preparing for an exciting year of Mars exploration and discovery. This work demonstrates how spectroscopy and remote sensing lead to fundamental discoveries of significant importance for understanding Mars’ history. As we begin to understand the most ancient history of Mars, researchers are ready to directly search of any signatures that life might have once existed on ancient Mars, and HKU plans to be at the centre of this great scientific adventure. 

Featured image: The blue-toned rocks in the upper-left of the image are depleted in iron because it was removed during weathering on ancient Mars. This is geological evidence that iron was lost from the rocks in reduced conditions. © University of Hong Kong


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Provided by University of Hong Kong

Pluto’s Atmosphere Just Prior to New Horizon’s Arrival: Consistent Pressure and Hazy (Planetary Science)

The study of an occultation of Pluto by a bright star showed that the planet’s atmospheric pressure has remained consistent even as it has moved away from the Sun in its orbit, and found a haze component in Pluto’s atmosphere, according to a new paper on which PSI Senior Scientist Amanda Sickafoose is an author.

“For the past 30 years, Pluto has been moving away from the Sun in its orbit. The resulting decrease in solar insolation has been predicted to cause a decrease in Pluto’s atmospheric pressure, or even a complete collapse. There was some question as to whether or not Pluto would have a detectable atmosphere by the time the New Horizons spacecraft arrived,” Sickafoose said. “These observations showed that 15 days before the New Horizons flyby in 2015, Pluto’s atmosphere was at essentially the same size and pressure as it had been in previous observations in 2013 and 2011 — there was no significant expansion or contraction since that time. Furthermore, the multi-wavelength occultation data suggested that Pluto’s atmosphere contained a low-altitude haze composed of submicron-sized particles.

The occultation was observed from the Stratospheric Observatory for Infrared Astronomy (SOFIA), an airborne observatory, and several ground based stations in New Zealand and Australia. Sickafoose traveled to New Zealand to take data from the Stardome Observatory in Auckland, where she worked with local astronomers to use the 0.5-meter Edith Winstone Blackwell Telescope and a 0.4-meter Meade telescope. Sickafoose generated light curves from multiple telescope datasets and led the work on the flux versus wavelength dependence, which indicated the presence of haze particles less than 0.1 micron in radius.

Stellar occultations, when a star is hidden by a foreground object that passes between it and the observer, are the best way to study Pluto’s tenuous atmosphere. Recording the starlight as it passes through the atmosphere, gets blocked by Pluto’s surface, and then reappears through the atmosphere on the other side provides an extremely sensitive method of measuring atmospheric density, temperature, and pressure. This work allows comparison between nearly coincident spacecraft and ground-based occultation data – the results of which are in general agreement – providing an important connection and verification between the two methods.

Amanda Sickafoose worked at the Stardome Observatory in Auckland, New Zealand, where she and her colleagues used a 0.5-meter Edith Winstone Blackwell Telescope and a 0.4-meter Meade telescope. Credit: Stardome Observatory

Sickafoose is a co-author on “Haze in Pluto’s atmosphere: Results from SOFIA and ground-based observations of the 2015 June 29 Pluto occultation.” Michael J. Person of the MIT Department of Earth, Atmospheric, and Planetary Sciences is lead author.

Featured image: Image of Pluto from New Horizons, looking back toward the Sun 15 minutes after close approach. The backlighting highlights multiple layers of haze in Pluto’s tenuous atmosphere. (More information is at https://www.nasa.gov/feature/pluto-wows-in-spectacular-new-backlit-panorama, Credits: NASA/JHUAPL/SwRI.)


Reference: Wiedemann, Manuel et al. (2021) Haze in Pluto’s atmosphere: Results from SOFIA and ground-based observations of the 2015 June 29 Pluto occultation. Icarus, 356 . Art. No. 113572. ISSN 0019-1035.


Provided by Planetary Science Institute

How Aerosols Are Formed? (Chemistry)

ETH Zurich researchers conducted an experiment to investigate the initial steps in the formation of aerosols. Their findings are now aiding efforts to better understand and model that process – for example, the formation of clouds in the atmosphere.

Clouds are water droplets dispersed in the air and thus an aerosol. (Photograph: Colourbox)

Aerosols are suspensions of fine solid particles or liquid droplets in a gas. Clouds, for example, are aerosols because they consist of water droplets dispersed in the air. Such droplets are produced in a two-​step process: first, a condensation nucleus forms, and then volatile molecules condense onto this nucleus, producing a droplet. Nuclei frequently consist of molecules different to those that condense onto them. In the case of clouds, the nuclei often contain sulphuric acids and organic substances. Water vapour from the atmosphere subsequently condenses onto these nuclei.

Scientists led by Ruth Signorell, Professor at the Department of Chemistry and Applied Biosciences, have now gained new insights into the first step of aerosol formation, nucleation. “Observations have shown that the volatile components can also influence the nucleation process,” Signorell says, “but what was unclear was how this was happening at the molecular level.” Previously it was impossible to observe the volatile components during nucleation in an experimental setting. Even in a famous CERN experiment on cloud formation, the “Cloud” experiment, certain volatile components could not be directly detected.

The experimental setup in an ETH Zurich laboratory. (Photograph: ETH Zurich / Ruth Signorell)

Volatile components detected for the first time

The ETH researchers developed an experiment aimed at the first microseconds of the nucleation process. In the experiment, the particles formed remain intact during this time and can be detected using mass spectrometry. The scientists looked at nucleation in various gas mixtures containing CO2 and for the first time, they were able to detect the volatile components as well – in this case, the CO2. The researchers could show that the volatile components were essential for the formation of nuclei and also accelerated this process.

An analysis of the experimental data revealed that this acceleration is the result of the volatile components catalysing the nucleation of other, less volatile components. They do this by forming short-​lived, heterogeneous molecular aggregates, known as chaperon complexes. “Because temperature determines the volatility of gas components, it also plays a decisive role in these processes,” Signorell explains.

One reason the new research results are interesting is that they improve the understanding of nucleation, its molecular mechanisms and speed, in order to properly account for it in models for, say, cloud formation in the atmosphere. In addition, the results should help to improve the efficiency of technical processes for producing aerosols – such as the use of rapid cooling to capture CO2 from natural gas.

Reference:

Li C, Krohn J, Lippe M, Signorell R: How volatile components catalyze vapor nucleation, Science Advances, 13 January 2021, doi: 10.1126/sciadv.abd9954 https://advances.sciencemag.org/content/7/3/eabd9954

Provided by ETH Zurich

Anthropogenic Heat Flux Increases Frequency of Extreme Heat Events (Earth Science)

Anthropogenic, or human-made, heat flux in the near-surface atmosphere has changed urban thermal environments.

Meanwhile, the number of extreme temperature events in the first decade of the 21st century grew faster than in the last 10 years of the 20th century. During this period, urban extreme heat events have become more frequent, breaking temperature records more often.

“We found the relationships between anthropogenic heat flux and extreme temperature events, including both extreme cold and heat events, based on seven extreme temperature indices by conducting the advanced model,” said Prof. XIE Zhenghui, a scientist with the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences.

Anthropogenic heat flux reduces extreme cold events and increases the extreme heat events. (Image by LIU Bin)

Many researchers have studied urban extreme temperature events, including the heat effect of the anthropogenic heat flux from different time scales, urban heat island effect, and the synergistic interactions between urban heat island and heat waves. However, the relationships between anthropogenic heat flux and extreme temperature events have been less studied.

“Anthropogenic heat increased the frequency and trend of the extreme heat events, while the extreme cold events were opposite,” said Prof. XIE. Along with Dr. LIU Bin, XIE developed a case study of Beijing, China, analyzing anthropogenic heat data based on energy consumption. Using the Advanced Research (ARW) version of the Weather Research and Forecasting (WRF) model, they implemented a dynamic representation scheme of urban anthropogenic heat release.

Their study was published in Advances in Atmospheric Sciences on Jan. 8.

By analyzing the dynamic process of atmosphere’s boundary layer, the team also found differences in seasonal heating efficiency. This research might help to mitigate the impact of extreme temperature events in different seasons.

Reference: Liu, B., Xie, Z., Qin, P. et al. Increases in Anthropogenic Heat Release from Energy Consumption Lead to More Frequent Extreme Heat Events in Urban Cities. Adv. Atmos. Sci. (2021). https://doi.org/10.1007/s00376-020-0139-y https://link.springer.com/article/10.1007/s00376-020-0139-y

Provided by Chinese Academy of Sciences

Terrestrial Exoplanet-exomoon Coupled Magnetospheres Work Together To Protect Their Early Atmospheres (Planetary Science)

Habitable terrestrial planets are those occupying orbits around a star that can maintain surface liquid water under a supporting atmosphere. The study of the evolution of planetary atmospheres has been an important research topic for the last several years, in an effort to determine the most important factors in creating a habitable environment for an exoplanet. A planet found in the habitable zone (HZ) around a star does not necessarily mean that the planet is habitable since stellar activity cannot be neglected. For example, it is well known that stellar emissions in the X-ray and extreme ultraviolet (EUV) leads to enhanced ionization and inflation of planetary ionospheres and atmospheres leading to atmospheric loss. Magnetically active stars produce very intense stellar flares that are often, but not always, accompanied by a coronal mass ejection (CME) and stellar winds leading to more atmospheric loss as observed at Mars. When the Martian dynamo shut down ~4.1 Ga and Mars lost its global magnetosphere, the intense solar wind and radiation ravished its atmosphere resulting in ocean evaporation and atmospheric loss transforming Mars from an early warmer and wetter world to a cold and dry planet with an average surface pressure of only 6 mbar. Simply stated, preservation of an atmosphere is one of the chief ingredients for surface habitability.

Artist impression of an exoplanet and an exomoon ©stockimage

It has been shown in previous studies that young stars, particularly one solar mass and smaller, produce extremes such as stellar flares and CMEs that lead to planetary atmospheric loss and that these extremes are closely related to the stellar rotation rate in addition to mass. Johnstone and colleagues found that almost all solar mass stars have converged to slow rotators by 500 Myr after formation producing slower stellar wind. Compared with solar-type stars, it takes a longer time for M-type stars (with lower masses) to slow down. M-type stars also keep magnetically active for a longer period of time. Therefore, the ravaging of planetary atmospheres in the young solar system due to extreme solar radiation and particle fluxes is believed to be a significant factor for our understanding of how an exoplanet will develop and maintain an atmosphere, which is a critical element of a habitable environment. Meanwhile, recent studies show that planetary magnetic fields may protect planets from atmospheric losses, indicating planetary magnetic fields play an important role in planetary habitability.

Another factor that should be considered with respect to the habitability of a terrestrial exoplanet, unrecognized until this paper, concerns the magnetic characteristics of an associated exomoon. With the existence of exoplanets well established, one of the next frontiers is the discovery of exomoons. Today there are a number of exomoon candidates waiting to be confirmed. Since it is without a doubt that they must exist around some exoplanets, it is important to examine what role, if any, they would have in creating an environment that contributes to the habitability of their host planet.

Although speculated for several decades, only recently have scientists determined that our Moon had an extensive magnetosphere for several hundred million years soon after it was formed. Recently, Green and colleagues investigated the expected magnetic topology of the early Earth-Moon magnetospheres and found that they would couple in such a way as to protect the atmosphere of both the Earth and Moon. Assuming similar formation processes for terrestrial planets and their moons, how would these two magnetospheres interact, and what protection would such a combined magnetosphere afford to the atmospheres of early exoplanets and their moons orbiting young stars is the subject of this current research done by James Green and colleagues.

In their paper, they modeled two dipole fields simulating the main field of the exoplanet and the exomoon when the exomoon was at several locations ranging from 4 to 18 Rp from the exoplanet in a stellar wind environment. They take the Earth-Moon dipole strengths presented in their previous paper as their starting conditions illustrating a basic magnetic topology that would evolve over time.

Their results demonstrated that terrestrial exoplanet-exomoon coupled magnetospheres work together to protect the early atmospheres of both the exoplanet and the exomoon. When exomoon magnetospheres are within the exoplanet’s magnetospheric cavity, the exomoon magnetosphere acts like a protective magnetic bubble providing an additional magnetopause confronting the stellar winds when the moon is on the dayside. In addition, magnetic reconnection would create a critical pathway for the atmosphere exchange between the early exoplanet and exomoon. When the exomoon’s magnetosphere is outside of the exoplanet’s magnetosphere it then becomes the first line of defense against strong stellar winds, reducing exoplanet’s atmospheric loss to space.

They have also given a brief discussion on how this type of exomoon would modify radio emissions from magnetized exoplanets.

“Based on the solar system, one of the most well-known wave phenomena for planets with magnetospheres is the release of escaping radio emissions generated by the cyclotron maser instability (CMI) mechanism that derives their named based on the frequency range of the observed emission. It is well known that this type of emission is closely related to the local gyrofrequency above a planet’s aurora and therefore provides important clues to the presence of a planet’s magnetic field and its strength. For example, the Earth’s intense auroral-related radio emission is called Auroral Kilometric Radiation (AKR) and the Jovian Decametric (DAM) emissions are CMI related emissions from Jupiter’s auroral zone.”, said James Green lead author of the study.

Some previous studies believed that the intense auroral related radio emission is the best indicator of planetary magnetospheres. The CMI generated radio emissions produce intense radiation perpendicular to the local magnetic field but the resulting emission cone can be filled-in by refraction or hollow. For instance, the emission cone of AKR has been observed to be relatively well filled in higher frequencies and may be hollow at the lower frequencies, while the Jovian DAM emissions produce hollow emission cones as illustrated in Figure 1A and Figure 1B, respectively. The Earth’s AKR emission cone points tailward with partially overlapping northern and southern hemisphere cones (only the northern hemisphere cone at one frequency is shown in Figure 1A) and is not dependent on Earth’s rotation or the location of the Moon.

Figure 1: A schematic view of radio emission cones, at one emission frequency, modeled after AKR at Earth (panel A) and Io-control DAM at Jupiter (panel B).

One aspect of the Jovian DAM emission is that it is strongly coupled with the moon Io which has a thin atmosphere allowing an ionospheric current to connect field-lines from Jupiter creating a constant current and therefore a constant aurora and resulting CMI related radio emissions. These Io controlled DAM emissions produce, hollow emission cones that move with Io around the planet which has an orbital period of about 42 hours. Io is in an elliptical orbit and is so close to the planet Jupiter that the energy from the very strong tidal forces is dissipated through volcanic activity on the moon that are so strong that a torus of escaping material is left in its wake that stretches around Jupiter. Alfven waves are set up in the Io torus that produce magnetospheric currents stretching all the way to the Jovian auroral regions that also trigger additional CMI emissions that produce a set of nested hollow emission cones. Near equatorial spacecraft, such as the Voyager 1 and 2 missions, observed the Io-DAM emissions as a series of arc-like structures in frequency-time spectrograms. The shape of the nested emission cones, in frequency-time spectrograms, are strongly controlled by the higher moments of the Jovian magnetic fields since the strongly right-hand polarized DAM radiation propagating from the source over these intense magnetic islands suffer significant refraction. In addition, Jupiter’s moon Ganymede also produces aurora, not only in the upper atmosphere of Jupiter, but also in the very tenuous Ganymede atmosphere since that moon is the only one in the solar system that has been observed to currently generate its own magnetosphere. The rather small Ganymede magnetosphere is anti aligned with Jupiter which is similar to the lower left panel in Figure 2. These connected field lines also facilitate the exchange of atmospheric constituents.

Figure 2: Simulation results for the exoplanet- exomoon coupled magnetospheres with the dipoles aligned (top panels) and dipoles antialigned (bottom panels) with three exomoon locations. This figure illustrates the evolution of the magnetic topology of an exoplanet-exomoon over a period of time delineated by the exomoon moving away from the exoplanet (expected to be a time period of 100’s of millions of years). © James Green et al.

In the case of both an exoplanet and exomoon with magnetospheres, Green and colleagues now observed a new situation in which the exomoon would be controlling the location of a potential CMI emission cone and producing either a hollow or filled in emission pattern. From a distant radio observer, periodicities in an observed CMI emission cone pattern along with the radio emission frequency not only could point to the existence and strength of an exoplanet’s magnetosphere but also the existence of an exomoon. The extent of the emission cone, ranging from completely hollow to completely filled-in provides additional information about the extent of the exoplanet’s ionosphere. In this manner, the detection and analysis of CMI generated radio emissions may provide additional information as to the habitability of the exoplanet.

“In order to understand the long-term evolution of exoplanetary atmospheres and their suitability for creating a habitable environment that may host life, we must understand not only the stellar environment, but also whether these planets and their associated moons have magnetic fields.”, said James Green.

Researchers concluded that, “future detection of exoplanet-exomoon magnetic fields from the detection of CMI radio emissions will provide a wealth of new information that will draw our attention to these systems having a greater chance of habitability.”

References: James Green, Scott Boardsen, Chuanfei Dong, “Magnetospheres of Terrestrial Exoplanets and Exomoons: Implications for Habitability and Detection”, ArXiv, pp. 1-13, 2020. https://arxiv.org/abs/2012.11694v1

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Muddying The Waters: Rock Breakdown May Play Less of a Role in Regulating Climate Than Previously Thought (Earth Science)

The weathering of rocks at the Earth’s surface may remove less greenhouse gases from the atmosphere than previous estimates, says new research from the University of Cambridge.

“People have spent decades looking on the continents for weathering – so maybe we now need to start expanding where we look”, – said Ed Tipper.

The findings, published in PNAS, suggest Earth’s natural mechanism for removing carbon dioxide (CO2) from the atmosphere via the weathering of rocks may in fact be weaker than scientists had thought – calling into question the exact role of rocks in alleviating warming over millions of years.

The research also suggests there may be a previously unknown sink drawing CO2 from the atmosphere and impacting climate changes over long timescales, which researchers now hope to find.

Weathering is the process by which atmospheric carbon dioxide breaks down rocks and then gets trapped in sediment. It is a major part of our planet’s carbon cycle, shuttling carbon dioxide between the land, sea and air, and influencing global temperatures.

“Weathering is like a planetary thermostat – it’s the reason why Earth is habitable. Scientists have long suggested this is why we don’t have a runaway greenhouse effect like on Venus,” said lead author Ed Tipper from Cambridge’s Department of Earth Sciences. By locking carbon dioxide away in sediments, weathering removes it from the atmosphere over long timescales, reducing the greenhouse effect and lowering global temperatures.

The team’s new calculations show that, across the globe, weathering fluxes have been overestimated by up to 28%, with the greatest impact on rivers in mountainous regions where rocks are broken down faster.

They also report that three of the largest river systems on Earth, including the neighbouring Yellow and Salween Rivers with their origins on the Tibetan Plateau and the Yukon River of North America, do not absorb carbon dioxide over long timescales – as had been thought.

For decades, the Tibetan plateau has been invoked as a long-term sink for carbon and mediator of climate. Some 25% of the sediment in the world’s oceans originate from the plateau.

“One of the best places to study the carbon cycle are rivers, they are the arteries of the continents. Rivers are the link between the solid Earth and oceans – hauling sediments weathered from the land down to the oceans where their carbon is locked up in rocks,” said Tipper.

“Scientists have been measuring the chemistry of river waters to estimate weathering rates for decades,” said co-author Victoria Alcock “Dissolved sodium is one of the most commonly measured products of weathering – but we’ve shown that it’s not that simple, and in fact sodium often comes from elsewhere.”

Sodium is released when silicate minerals, the basic building blocks of most of Earth’s rocks, dissolve in carbonic acid – a mix of carbon dioxide in the atmosphere and rainwater.

However, the team found not all sodium comes from this weathering process. “We’ve found an additional source of sodium in river waters across the globe,” said co-author Emily Stevenson. “That extra sodium is not from weathered silicate rocks as other studies assume, but in fact from very old clays which are being eroded in river catchments.”

Tipper and his research group studied eight of the largest river systems on Earth, a mission involving 16 field seasons and thousands of lab analyses in search of where that extra sodium was coming from.

They found the answer in a soupy ‘gel’ of clay and water – known as the cation exchange pool – which is carried along by muddy river sediment.

The exchange pool is a reactive hive of cations – positively charged ions like sodium – which are weakly bonded to clay particles. The cations can easily swap out of the gel for other elements like calcium in river water, a process that can take just a few hours.

Although it has been described in soils since the 1950s, the role the exchange pool plays in supplying sodium to rivers has been largely neglected.

“The chemical and isotopic makeup of the clays in the exchange pool tell us what they are made of and where they’ve come from,” said co-author Alasdair Knight. “We know that many of the clays carried by these rivers come from ancient sediments, and we suggest that some of the sodium in the river must come from these clays.”

The clays were originally formed from continental erosion millions of years ago. On their journey downstream they harvested cations from the surrounding water – their exchange pool picking up sodium on reaching the sea. Today, after being uplifted from the seafloor, these ancient clays – together with their sodium – are now being eroded by modern rivers.

This old sodium, which can switch out of the clays in the exchange pool and into river water, has previously been mistaken as the dissolved remnants of modern weathering.

“Generating just one data point took a huge amount of work in the lab and we also had to do a lot of maths,” said Stevenson. “It’s like unmixing a cake, using a forensic approach to isolate key ingredients in the sediments, leaving behind the exchange pool and the clays. People have used the same methods for a really long time – and they work – but we’ve been able to find an extra ingredient that provides the sodium and we need to account for this.”

“It’s thanks to the hard work of many collaborators and students over many years that our samples had the scope to get to grips with this complex chemical process at a global scale,” said Tipper.

Scientists are now left to puzzle over what else could be absorbing Earth’s carbon dioxide over geological time. There are no certain candidates – but one controversial possibility is that life is removing carbon from the atmosphere. Another theory is that silicate dissolution on the ocean floor or volcanic arcs may be important. “People have spent decades looking on the continents for weathering – so maybe we now need to start expanding where we look,” said Tipper.

Reference: Edward T. Tipper et al. ‘Global silicate weathering flux overestimated because of sediment–water cation exchange.’ PNAS (2020). DOI: 10.1073/pnas.2016430118

Provided by University of Cambridge

A Step Towards Understanding Cloud Mysteries (Geology)

In Geophysical Research Letters, the scientists from the Max Planck Institute for Dynamics and Self-Organization (MPIDS), Germany, and the Politecnico di Torino reported their new findings on how precipitating large raindrops, ice particles can favor growth of aerosols to produce new cloud condensation nuclei or ice nucleating particles.

Atmospheric clouds play a crucial role in defining the local weather and global climate. The activation of cloud aerosols, such as mineral dust, soot particles, pollutants, acid molecules and ions, impacts the life cycle of a cloud. Therefore, a detailed understanding is necessary for reliable climate prediction and weather forecasting. Out of many mysteries of clouds, we still do not understand how and why the number of ice particles inside clouds exceed the number of ice nucleating particles that could be activated. What are the major sources behind this excess (secondary) production of particles?

(Figure 1): Excess water vapor (S>0) behind a precipitating frozen hydrometeor at 0°C temperature falling through an ambient at -15°C and 90% relative humidity condition. An aerosol in black line can be seen to enter the water vapor rich environment.© Bhowmick et al.

In this letter, the scientists of MPIDS and POLITO have numerically investigated one such secondary particle production processes inside clouds resulting to new water droplets or ice particles. Out of several proposed physical processes for new droplet generation, recent experimental studies have shown that a large droplet can nucleate aerosols in the wake behind it when falling under gravity. Extending the experiments, this letter presents a detailed analysis of various physical factors that lead to an excess of water vapor behind the hydrometeors (e.g., droplets, sleet, or hail) and investigates the effectiveness of this process on activation of aerosols to create new cloud particles. This letter reports that not all aerosols, but only some “lucky aerosols” are entrained in the wake behind such precipitating hydrometeors, where they can reside in a highly humid environment for sufficiently long time. This fulfills the necessary condition for the aerosols to be activated as new cloud condensation nuclei or ice nucleating particles. This letter also reports how this activation of aerosols by hydrometeors can contribute to the life cycle of the clouds.

This study opened new potential research areas. According to TaraprasadBhowmick, PhD final year student of POLITO and scientist of MPIDS, “This letter marks a great achievement for us with new results, pointing towards the future studies relevant for cloud physics and climate sciences.

(Figure 2): A track of the excess water vapor that two aerosols experience when they entered the water vapor rich environment behind a precipitating frozen hydrometeor at 0°C temperature falling through an ambient at -15°C and 95% relative humidity condition. © Bhowmick et al.

This group of scientists from MPIDS are looking forward to extend this work with more realistic modeling of the cloud conditions, and plan to carry out a detailed growth tracking of individual aerosols that come in contact with such precipitating cloud hydrometeors.

References: Bhowmick, T., Wang, Y., Iovieno, M., Bagheri, G., & Bodenschatz, E.(2020). Supersaturation in the wake of a precipitating hydrometeor and its impact on aerosol activation. Geophysical Research Letters, 47, e2020GL091179. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL091179 https://doi.org/10.1029/2020GL091179

Provided by Politecnico Di Torino

Entering the Martian Atmosphere with the Perseverance Rover (Planetary Science)

With its heat shield facing the planet, NASA’s Perseverance rover begins its descent through the Martian atmosphere in this illustration. Hundreds of critical events must execute perfectly and exactly on time for the rover to land on Mars safely on Feb. 18, 2021.

Credit: NASA/JPL-Caltech

Entry, Descent, and Landing, or “EDL,” begins when the spacecraft reaches the top of the Martian atmosphere, travelling nearly 12,500 mph (20,000 kph).

The aeroshell, which encloses the rover and descent stage, makes the trip to the surface on its own. The vehicle fires small thrusters on the backshell to reorient itself and make sure the heat shield is facing forward as it plunges into the atmosphere.

NASA’s Jet Propulsion Laboratory in Southern California built and will manage operations of the Mars 2020 Perseverance rover for NASA.

For more information about the mission, go to: https://mars.nasa.gov/mars2020.

Provided by NASA