̽��ֱ�� of Cambridge - Jerome Neufeld

‘Slushy’ magma ocean led to formation of the Moon’s crust

sc604 — Thu, 13 Jan 2022 14:00:00 +0000

̽��ֱ��scientists, from the ̽��ֱ�� of Cambridge and the Ecole normale supérieure de Lyon, have proposed a new model of crystallisation, where crystals remained suspended in liquid magma over hundreds of millions of years as the lunar ‘slush’ froze and solidified. ̽��ֱ��results are reported in the journal Geophysical Research Letters.

Over fifty years ago, Apollo 11 astronauts collected samples from the lunar Highlands. These large, pale regions of the Moon – visible to the naked eye – are made up of relatively light rocks called anorthosites. Anorthosites formed early in the history of the Moon, between 4.3 and 4.5 billion years ago.

Similar anorthosites, formed through the crystallisation of magma, can be found in fossilised magma chambers on Earth. Producing the large volumes of anorthosite found on the Moon, however, would have required a huge global magma ocean.

Scientists believe that the Moon formed when two protoplanets, or embryonic worlds, collided. ̽��ֱ��larger of these two protoplanets became the Earth, and the smaller became the Moon. One of the outcomes of this collision was that the Moon was very hot – so hot that its entire mantle was molten magma, or a magma ocean.

“Since the Apollo era, it has been thought that the lunar crust was formed by light anorthite crystals floating at the surface of the liquid magma ocean, with heavier crystals solidifying at the ocean floor,” said co-author Chloé Michaut from Ecole normale supérieure de Lyon. “This ‘flotation’ model explains how the lunar Highlands may have formed.”

However, since the Apollo missions, many lunar meteorites have been analysed and the surface of the Moon has been extensively studied. Lunar anorthosites appear more heterogeneous in their composition than the original Apollo samples, which contradicts a flotation scenario where the liquid ocean is the common source of all anorthosites.

̽��ֱ��range of anorthosite ages – over 200 million years – is difficult to reconcile with an ocean of essentially liquid magma whose characteristic solidification time is close to 100 million years.

“Given the range of ages and compositions of the anorthosites on the Moon, and what we know about how crystals settle in solidifying magma, the lunar crust must have formed through some other mechanism,” said co-author Professor Jerome Neufeld from Cambridge’s Department of Applied Mathematics and Theoretical Physics.

Michaut and Neufeld developed a mathematical model to identify this mechanism.

In the low lunar gravity, the settling of crystal is difficult, particularly when strongly stirred by the convecting magma ocean. If the crystals remain suspended as a crystal slurry, then when the crystal content of the slurry exceeds a critical threshold, the slurry becomes thick and sticky, and the deformation slow.

This increase of crystal content occurs most dramatically near the surface, where the slushy magma ocean is cooled, resulting in a hot, well-mixed slushy interior and a slow-moving, crystal-rich lunar ‘lid’.

“We believe it’s in this stagnant ‘lid’ that the lunar crust formed, as lightweight, anorthite-enriched melt percolated up from the convecting crystalline slurry below,” said Neufeld. “We suggest that cooling of the early magma ocean drove such vigorous convection that crystals remained suspended as a slurry, much like the crystals in a slushy machine.”

Enriched lunar surface rocks likely formed in magma chambers within the lid, which explains their diversity. ̽��ֱ��results suggest that the timescale of lunar crust formation is several hundreds of million years, which corresponds to the observed ages of the lunar anorthosites.

Serial magmatism was initially proposed as a possible mechanism for the formation of lunar anorthosites, but the slushy model ultimately reconciles this idea with that of a global lunar magma ocean.

̽��ֱ��research was supported by the European Research Council.

Jerome Neufeld is also affiliated with the Department of Earth Sciences. He is a Fellow of Trinity College.

Reference:
Chloé Michaut and Jerome A Neufeld. ‘Formation of the lunar primary crust from a long-lived slushy magma ocean.’ Geophysical Research Letters (2022). DOI: 10.1029/2021GL095408

Adapted from an ENS-Lyon press release.

Scientists have shown how the freezing of a ‘slushy’ ocean of magma may be responsible for the composition of the Moon’s crust.

Cooling of the early magma ocean drove such vigorous convection that crystals remained suspended as a slurry, like the crystals in a slushy machine.

Jerome Neufeld

NASA/Goddard Space Flight Center

Magma ocean and first rocky crust on the Moon

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

Yes

Licence type:

Public Domain

Carbon capture: universities and industry work together to tackle emissions

sc604 — Wed, 25 Oct 2017 07:12:18 +0000

̽��ֱ��world is not going carbon-free any time soon: that much is clear. Developed and developing countries alike rely on fossil fuels for transport, industry and power, all of which release CO₂ into the atmosphere. But as sea levels rise, ‘unprecedented’ weather events become commonplace and the polar ice caps melt, how can we balance our use of fossil fuels with the imperative to combat the catastrophic effects of climate change?

“Everything suggests that we won’t be able to stop burning carbon-based fuels, particularly in rapidly developing countries like India and China,” says Professor Mike Bickle of Cambridge’s Department of Earth Sciences. “Along with increasing use of renewable energy and improved energy efficiency, one way to cope with that is to use carbon capture and storage – and there is no technical reason why it can’t be deployed right now.”

Carbon capture and storage (CCS) is a promising and practical solution to drastically reducing carbon emissions, but it has had a stilted development pathway to date. In 2015, the UK government cancelled a £1 billion competition for CCS technology six months before it was due to be awarded, citing high costs. Just one year later, a high-level advisory group appointed by ministers recommended that establishing a CCS industry in the UK now could save the government and consumers billions per year from the cost of meeting climate change targets.

CCS is the only way of mitigating the 20% of CO₂ emissions from industrial processes – such as cement manufacturing and steel making, for which there is no obvious alternative – to help meet the world’s commitments to limit warming to below 2^oC. It works by trapping the CO₂ emitted from burning fossil fuels, which is then cooled, liquefied and pumped deep underground into geological formations, saline aquifers or disused oil and gas fields. Results from lab-based tests, and from working CCS sites such as Sleipner in the North Sea, suggest that carbon can be safely stored underground in this way for 10,000 years or more.

“ ̽��ֱ��big companies understand the science of climate change, and they understand that we’ve got to invest in technologies like CCS now, before it’s too late,” says Dr Jerome Neufeld of Cambridge’s Department of Applied Mathematics and Theoretical Physics, and Department of Earth Sciences. “But it’s a tricky business running an industry where nobody is charging for carbon.”

“Everyone always wants the cheapest option, so without some form of carbon tax, it’s going to be difficult to get CCS off the ground at the scale that’s needed,” says Bickle. “But if you look at the cost of electricity produced from gas or coal with CCS added, it’s very similar to the cost of electricity from solar or wind. So if governments put a proper carbon charge in place, renewables and CCS would compete with each other on a relatively even playing field, and companies would have the economic incentive to invest in CCS.”

Bickle and Neufeld are following discussions about CCS closely because, along with collaborators from Stanford and Melbourne Universities, they have recently started a new CCS project with the support of BHP, one of the world’s largest mining and materials companies.

̽��ֱ��three-year project will develop and improve methods for the long-term storage of CO₂, and will test them at Otway in southern Australia, one of the largest CCS test sites in the world. Using a mix of theoretical modelling and small-, medium- and large-scale experiments, the researchers hope to significantly increase the types of sites where CCS is possible, including in China and developing economies.

In most current CCS schemes, CO₂ is stored in porous underground rock formations with a thick layer of non-porous rock, such as shale, on top. ̽��ֱ��top layer provides extra insurance that the relatively light CO₂ will not escape.

̽��ֱ��new research, which will support future large-scale CO₂ storage, will consider whether CO₂ could be effectively trapped without the top seal of impermeable rock, meaning that CCS could be deployed in a wider range of environments. Their research findings will be made publicly available to accelerate the broader deployment of CCS.

“We are seeing a growing acknowledgement from industry, governments and society that to meet emissions reductions targets we are going to need to accelerate the use of this technology – we simply can’t do it quickly enough without CCS across both power generation and industry,” says BHP Vice President of Sustainability and Climate Change, Dr Fiona Wild. “We know CCS technology works and is proven. Our focus at BHP is on how we can help make sure the world has access to the information required to make it work at scale in a cost effective and timely way.”

During the project, Stanford researchers will measure the rate at which porous rock can trap CO₂ using small-scale experiments on rock samples at reservoir conditions, while the Cambridge researchers will be using larger analogue models, in the order of metres or tens of metres. ̽��ֱ��Melbourne-based researchers will use large-scale numerical simulations of complex geological settings.

“One of the things this collaboration will really open up is the ability to deploy CCS almost anywhere,” says Neufeld, who is also affiliated with Cambridge’s Department of Earth Sciences and the BP Institute. “We know that CO₂ can be safely trapped in porous rock with a seal of shale on top, but the early results from Otway have shown that even without the impenetrable seal, CO₂ can be trapped just as effectively.”

When CO₂ is pumped into underground saline aquifers, it is in a ‘super-critical’ phase: not quite a liquid and not quite a gas. ̽��ֱ��super-critical CO₂ is less dense than the salt water, and so has a tendency to run uphill, but it’s been found that surface tension between the salt water and the rock is quite effective at pinning the CO₂ in place so that it can’t escape. This phenomenon, known as capillary trapping, is also observed when water is held in a sponge.

“ ̽��ֱ��results from Otway show that if you inject CO₂ into a heterogeneous reservoir, it will mix with the salt water and capillary trapping will pin it there quite effectively, so it opens up a much broader range of potential carbon storage sites,” says Bickle.

“However, we need to start deploying CCS now, and the biggest challenges we face are economics and policy. If these prevent us from doing anything until it’s too late, and we’re at a stage when we’d have to start capturing carbon directly from the atmosphere, it will be far more expensive. By not starting CCS now, we’re building false economies.”

An international collaboration between universities and industry will further develop carbon capture and storage technology – one of the best hopes for drastically reducing carbon emissions – so that it can be deployed in a wider range of sites around the world.

We need to start deploying CCS now, and the biggest challenges we face are economics and policy. If we’re at a stage when we’d have to start capturing carbon directly from the atmosphere, it will be far more expensive.

Mike Bickle

Jerome Neufeld

Modelling CCS

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes