Heavens

A team at the Natural History Museum (NHM), London is paving the way for future rovers to search for meteorites on Mars. The scientists are using the NHM's extensive meteorite collection to test the spectral instruments destined for the ExoMars rover Rosalind Franklin, and develop tools to identify meteorites on the surface of the red planet. The project is being presented today (23 July) at the virtual National Astronomy Meeting 2021.

The cratered surface of our nearest planetary neighbour has a long and complex history, and searching for rocks amidst more rocks may seem like a futile activity. Despite this, Martian rovers statistically have a significantly higher 'find per mile' success rate than dedicated meteorite hunts on Earth: for every kilometre travelled by a Mars rover, approximately one meteorite is found, even though the rovers have not been specifically looking for them up till now.

However, as part of the European Space Agency's upcoming ExoMars mission, the next rover - named Rosalind Franklin, after the chemist best known for her pioneering work on DNA - will drill down into the Martian surface to sample the soil, analyse its composition and search for evidence of past or present life buried underground.

Meteorites are important pieces of evidence that can help us understand this story; once a meteorite lands on a planet, it is subjected to the same atmospheric conditions as the rest of the surface. Chemical and physical weathering can provide information on climate weathering rates and water-rock interactions, meteorite sizes and distribution can help to infer information about the density of the atmosphere, and stony meteorites could be a potential delivery mechanism for organic materials to Mars.

"Meteorites act as a witness plate across geological time," said Sara Motaghian, the PhD student at the NHM and Imperial College London who is carrying out the work. "Generally, the surfaces of Mars we are exploring are incredibly ancient, meaning there have been billions of years for the surface to accumulate these meteorites and lock in information from across Mars' past."

The team are looking in particular at the use of multispectral imaging with the PanCam instrument, hoping to be able to highlight features in images that could be associated with meteorites as the rover moves across the surface. They are also investigating the possibility of using pattern recognition techniques to distinguish features such as Widmanstätten patterns, which can be revealed by extreme weathering.

The launch of the ExoMars rover was originally scheduled for 2020, however was delayed until 2022 due to technical issues and growing concerns over the coronavirus pandemic. Once the rover reaches Mars in 2023, the team hope that their work will allow meteorites on the surface to be studied for longer by the Rosalind Franklin rover before it drives on, helping to build a more complete understanding of the Martian surface and its history, if any, of life.

Using information obtained from around a dozen earthquakes detected on Mars by the Very Broad Band SEIS seismometer, developed in France, the international team of NASA's InSight mission has unveiled the internal structure of Mars. The three papers published on July 23, 2021 in the journal Science, involving numerous co-authors from French institutions and laboratories, including the CNRS, the Institut de Physique du Globe de Paris, and Université de Paris, and supported in particular by the French space agency CNES and the French National Research Agency ANR, provide, for the first time, an estimate of the size of the planet's core, the thickness of its crust and the structure of its mantle, based on the analysis of seismic waves reflected and modified by interfaces in its interior. It makes this the first ever seismic exploration of the internal structure of a terrestrial planet other than Earth, and an important step towards understanding the formation and thermal evolution of Mars.

Before NASA's InSight mission, the internal structure of Mars was still poorly understood. Models were based only on data collected by orbiting satellites and on the analysis of Martian meteorites that fell to Earth. On the basis of gravity and topographical data alone, the thickness of the crust was estimated to be between 30 and 100 km. Values of the planet's moment of inertia and density suggested a core with a radius of 1 400 to 2 000 km. The detailed internal structure of Mars and the depth of the boundaries between the crust, mantle and core were, however, completely unknown.

With the successful deployment of the SEIS experiment on the surface of Mars in early 2019, the mission scientists, including the 18 French co-authors involved and affiliated to a wide range of French institutions and laboratories , together with their colleagues from ETH in Zurich, the University of Cologne and the Jet Propulsion Laboratory in Pasadena, collected and analysed seismic data over one Martian year (almost two Earth years).

It should be pointed out that to simultaneously determine a structural model, the (arrival) time of an earthquake, and its distance, more than one station is usually required. However, on Mars the scientists only have one station, InSight. It was therefore necessary to search the seismic records for the characteristic features of waves that had interacted in various ways with the internal structures of Mars, and identify and validate them. These new measurements, coupled with mineralogical and thermal modelling of the planet's internal structure, have made it possible to overcome the limitation of having a single station. This method ushers in a new era for planetary seismology.

A single station, multiple findings

Another difficulty on Mars is its low seismicity and the seismic noise generated by its atmosphere. On Earth, earthquakes are much stronger, while seismometers are more effectively located in vaults or underground, making it possible to obtain an accurate image of the planet's interior. As a result, special attention had to be paid to the data. "But although Martian earthquakes have a relatively low magnitude, less than 3.5, the very high sensitivity of the VBB sensor combined with the very low noise at nightfall enabled us to make discoveries that, two years ago, we thought were only possible with earthquakes with a magnitude greater than 4," explains Philippe Lognonné, a Professor at the University of Paris and the Principal Investigator for the SEIS instrument at IPGP.

Every day, the data, processed by CNES, IPGP and CNRS, and transferred to the scientists, was carefully cleaned of ambient noise (wind and deformation related to rapid temperature changes). The international Mars Quake Service team (MQS) recorded the seismic events on a daily basis: more than 600 have now been catalogued, of which over 60 were caused by relatively distant earthquakes.

Around ten of the latter contain information about the planet's deep structure: "The direct seismic waves from an earthquake are a bit like the sound of our voices in the mountains: they produce echoes. And it was these echoes, reflected off the core, or at the crust-mantle interface or even the surface of Mars, that we looked for in the signals, thanks to their similarity to the direct waves," Lognonné explains.

An altered crust, a mantle revealed, and a large liquid core

By comparing the behaviour of seismic waves as they travelled through the crust before reaching the InSight station, several discontinuities in the crust were identified: the first, observed at a depth of about 10 km, marks the boundary between a highly altered structure, resulting from circulation of fluid a very long time ago, and crust that is only slightly altered. A second discontinuity around 20 km down, and a third, less pronounced one at around 35 km, shed light on the stratification of the crust beneath InSight: "To identify these discontinuities, we used all the most recent analytical methods, both with earthquakes of tectonic origin and with vibrations caused by the environment (seismic noise)," says Benoit Tauzin, Senior Lecturer at the University of Lyon and a researcher at LGL-TPE.

In the mantle, the scientists analysed the differences between the travel time of the waves produced directly during the earthquake, and that of the waves generated when these direct waves were reflected off the surface. These differences made it possible, using only a single station, to determine the structure of the upper mantle, and in particular the variation in seismic velocities with depth. However, such variations in velocity are related to temperature. "That means we can estimate the heat flow of Mars, which is probably three to five times lower than the Earth's, and place constraints on the composition of the Martian crust, which is thought to contain over half the heat-producing radioactive elements present in the planet," adds Henri Samuel, a CNRS researcher at IPGP.

Finally, in the third study, the scientists looked for waves reflected off the surface of the Martian core, the measurement of whose radius was one of the main achievements of the InSight mission. "To do this," explains Mélanie Drilleau, a research engineer at ISAE-SUPAERO, "we tested several thousand mantle and core models against the phases and signals observed." Despite the low amplitudes of the signals associated with the reflected waves (known as ScS waves), an excess of energy was observed for cores with a radius between 1 790 km and 1 870 km. Such a large size implies the presence of light elements in the liquid core and has major consequences for the mineralogy of the mantle at the mantle / core interface.

Goals achieved, new questions emerge

More than two years of seismic monitoring has resulted in the very first model of the internal structure of Mars, right down to the core. Mars thus joins the Earth and the Moon in the select club of terrestrial planets and moons whose deep structures have been explored by seismologists. And, as often happens in planetary exploration, fresh questions emerge: is the alteration of the top 10 km of crust general, or is it limited to the InSight landing zone? What impact will these first models have on theories of the formation and thermal evolution of Mars, in particular for the first 500 million years when Mars had liquid water on its surface and intense volcanic activity?

With the two-year extension of the InSight mission and the additional electrical power obtained following the successful cleaning of its solar panels carried out by JPL, new data should consolidate and further improve these models.

Using the Atacama Large Millimetre/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, astronomers have unambiguously detected the presence of a disc around a planet outside our Solar System for the first time. The observations will shed new light on how moons and planets form in young stellar systems.

"Our work presents a clear detection of a disc in which satellites could be forming," says Myriam Benisty, a researcher at the University of Grenoble, France, and at the University of Chile, who led the new research published today in The Astrophysical Journal Letters. "Our ALMA observations were obtained at such exquisite resolution that we could clearly identify that the disc is associated with the planet and we are able to constrain its size for the first time," she adds.

The disc in question, called a circumplanetary disc, surrounds the exoplanet PDS 70c, one of two giant, Jupiter-like planets orbiting a star nearly 400 light-years away. Astronomers had found hints of a "moon-forming" disc around this exoplanet before but, since they could not clearly tell the disc apart from its surrounding environment, they could not confirm its detection -- until now.

In addition, with the help of ALMA, Benisty and her team found that the disc has about the same diameter as the distance from our Sun to the Earth and enough mass to form up to three satellites the size of the Moon.

But the results are not only key to finding out how moons arise. "These new observations are also extremely important to prove theories of planet formation that could not be tested until now," says Jaehan Bae, a researcher from the Earth and Planets Laboratory of the Carnegie Institution for Science, USA, and author on the study.

Planets form in dusty discs around young stars, carving out cavities as they gobble up material from this circumstellar disc to grow. In this process, a planet can acquire its own circumplanetary disc, which contributes to the growth of the planet by regulating the amount of material falling onto it. At the same time, the gas and dust in the circumplanetary disc can come together into progressively larger bodies through multiple collisions, ultimately leading to the birth of moons.

But astronomers do not yet fully understand the details of these processes. "In short, it is still unclear when, where, and how planets and moons form," explains ESO Research Fellow Stefano Facchini, also involved in the research.

"More than 4000 exoplanets have been found until now, but all of them were detected in mature systems. PDS 70b and PDS 70c, which form a system reminiscent of the Jupiter-Saturn pair, are the only two exoplanets detected so far that are still in the process of being formed," explains Miriam Keppler, researcher at the Max Planck Institute for Astronomy in Germany and one of the co-authors of the study [1].

"This system therefore offers us a unique opportunity to observe and study the processes of planet and satellite formation," Facchini adds.

PDS 70b and PDS 70c, the two planets making up the system, were first discovered using ESO's Very Large Telescope (VLT) in 2018 and 2019 respectively, and their unique nature means they have been observed with other telescopes and instruments many times since [2].

The latest high resolution ALMA observations have now allowed astronomers to gain further insights into the system. In addition to confirming the detection of the circumplanetary disc around PDS 70c and studying its size and mass, they found that PDS 70b does not show clear evidence of such a disc, indicating that it was starved of dust material from its birth environment by PDS 70c.

An even deeper understanding of the planetary system will be achieved with ESO's Extremely Large Telescope (ELT), currently under construction on Cerro Armazones in the Chilean Atacama desert. "The ELT will be key for this research since, with its much higher resolution, we will be able to map the system in great detail," says co-author Richard Teague, a researcher at the Center for Astrophysics | Harvard & Smithsonian, USA. In particular, by using the ELT's Mid-infrared ELT Imager and Spectrograph (METIS - https://elt.eso.org/instrument/METIS/), the team will be able to look at the gas motions surrounding PDS 70c to get a full 3D picture of the system.

The arrival of plants on land about 400 million years ago may have changed the way the Earth naturally regulates its own climate, according to a new study led by researchers at UCL and Yale.

The carbon cycle, the process through which carbon moves between rocks, oceans, living organisms and the atmosphere, acts as Earth's natural thermostat, regulating its temperature over long time periods.

In a new study, published in the journal Nature, researchers looked at samples from rocks spanning the last three billion years and found evidence of a dramatic change in how this cycle functioned about 400 million years ago, when plants started to colonise land.

Specifically, the researchers noted a change in the chemistry of seawater recorded in the rock that indicates a major shift in the global formation of clay - the "clay mineral factory" - from the oceans to the land.

Since clay forming in the ocean (reverse weathering) leads to carbon dioxide being released into the atmosphere, while clay on land is a byproduct of chemical weathering that removes carbon dioxide from the air, this reduced the amount of carbon in the atmosphere, leading to a cooler planet and a seesawing climate, with alternating ice ages and warmer periods.

The researchers suggested the switch was caused by the spread of land plants keeping soils and clays on land, stopping carbon from being washed into the ocean, and by the growth in marine life using silicon for their skeletons and cell walls, such as sponges, single-celled algae and radiolarians (a group of protozoa), leading to a drop in silicon in the seawater required for clay formation.

Senior author Dr Philip Pogge von Strandmann (UCL Earth Sciences) said: "Our study suggests that the carbon cycle operated in a fundamentally different way for most of Earth's history compared to the present day.

"The shift, which occurred gradually between 400 to 500 million years ago, appears to be linked to two major biological innovations at the time: the spread of plants on land and the growth of marine organisms that extract silicon from water to create their skeletons and cells walls.

"Before this change, atmospheric carbon dioxide remained high, leading to a stable, greenhouse climate. Since then, our climate has bounced back and forth between ice ages and warmer periods. This kind of change promotes evolution and during this period the evolution of complex life accelerated, with land-based animals forming for the first time.

"A less carbon-rich atmosphere is also more sensitive to change, allowing humans to influence the climate more easily through the burning of fossil fuels."

First author Boriana Kalderon-Asael, a PhD student at Yale University, said: "By measuring lithium isotopes in rocks spanning most of Earth's history, we aimed to investigate if anything had changed in the functioning of the carbon cycle over a large time scale. We found that it had, and this change appears to be linked to the growth of plant life on land and silicon-using animal life in the sea."

In the study, researchers measured lithium isotopes in 600 samples of rock taken from many different locations around the world. Lithium has two naturally occurring stable isotopes - one with three protons and three neutrons, and one with three protons and four neutrons.

When clay forms slowly on land, it strongly favours lithium-6, leaving surrounding water enriched with the other, heavier isotope, lithium-7. Analysing their samples using mass spectrometry, the researchers found a rise in the levels of lithium isotope-7 in seawater recorded in the rock occurring between 400 and 500 million years ago, suggesting a major shift in Earth's clay production coinciding with the spread of plants on land and emergence of silicon-using marine life.

Clay forms on land as a residue of chemical weathering, the primary long-term process through which carbon dioxide is removed from the atmosphere. This occurs when atmospheric carbon combines with water to form a weak acid, carbonic acid, which falls to the ground as rain and dissolves rocks, releasing ions including calcium ions that flow into the ocean. Eventually, the carbon is locked up in rocks on the ocean floor. In contrast, carbon drawdown by plant photosynthesis is negated once the plants decay, and rarely affects carbon dioxide levels on timescales longer than a few hundred years.

When clay forms in the ocean, carbon stays in the water and is eventually released into the air as part of the continual exchange of carbon that occurs when air meets water.

Rare-earth compounds have fascinated researchers for decades due to the unique quantum properties they display, which have so far remained totally out of reach of everyday compounds. One of the most remarkable and exotic properties of those materials is the emergence of exotic superconducting states, and particularly the superconducting states required to build future topological quantum computers. While these specific rare-earth compounds, known as heavy fermion superconductors, have been known for decades, making usable quantum technologies out of them has remained a critically open challenge. This is because these materials contain critically radioactive compounds, such as uranium and plutonium, rendering them of limited use in real-world quantum technologies.

New research has now revealed an alternative pathway to engineer the fundamental phenomena of these rare-earth compounds solely with graphene, which has none of the safety problems of traditional rare-earth compounds. The exciting result in the new paper shows how a quantum state known as a "heavy fermion" can be produced by combining three twisted graphene layers. A heavy fermion is a particle - in this case an electron - that behaves like it has a lot more mass than it actually does. The reason it behaves this way stems from unique quantum many-body effects that were mostly only observed in rare-earth compounds until now. This heavy fermion behavior is known to be the driving force of the phenomena required to use these materials for topological quantum computing. This new result demonstrates a new, non-radioactive way of achieving this effect using only carbon, opening up a pathway for sustainably exploiting heavy fermion physics in quantum technologies.

In the paper authored by Aline Ramires, (Paul Scherrer Institute, Switzerland) and Jose Lado (Aalto University), the researchers show how it is possible to create heavy fermions with cheap, non-radioactive materials. To do this, they used graphene, which is a one-atom thick layer of carbon. Despite being chemically identical to the material that is used in regular pencils, the sub-nanometre thickness of graphene means that it has unexpectedly unique electrical properties. By layering the thin sheets of carbon on top of one another in a specific pattern, where each sheet is rotated in relation to the other, the researchers can create the quantum properties effect that results in the electrons in the graphene behaving like heavy fermions.

"Until now, practical applications of heavy fermion superconductors for topological quantum computing has not been pursued much, partially because it required compounds containing uranium and plutonium, far from ideal for applications due to their radioactive nature", says Professor Lado, "In this work we show that one can aim to realize the exactly very same physics just with graphene. While in this work we only show the emergence of heavy fermion behavior, addressing the emergence of topological superconductivity is a natural next step, which could potentially have a groundbreaking impact for topological quantum computing."

Topological superconductivity is a topic of critical interest for quantum technologies, also tackled by alternative strategies in other papers from Aalto University Department of Applied Physics, including a previous paper by Professor Lado. "These results potentially provide a carbon-based platform for exploitation of heavy fermion phenomena in quantum technologies, without requiring rare-earth elements", concludes Professor Lado.