Now, researchers have a new explanation for the two-faced Moon — one that relates to a giant impact billions of years ago near the Moon’s south pole.

A new study published in the journal Science Advances shows that the impact that formed the Moon’s giant South Pole-Aitken (SPA) basin would have created a massive plume of heat that propagated through the lunar interior. That plume would have carried certain materials — a suite of rare-Earth and heat-producing elements — to the Moon’s nearside. That concentration of elements would have contributed to the volcanism that created the nearside volcanic plains.

“We know that big impacts like the one that formed SPA would create a lot of heat,” said Matt Jones, a Ph.D. candidate at Brown University and the study’s lead author. “The question is how that heat affects the Moon’s interior dynamics. What we show is that under any plausible conditions at the time that SPA formed, it ends up concentrating these heat-producing elements on the nearside. We expect that this contributed to the mantle melting that produced the lava flows we see on the surface.”

The study was a collaboration between Jones and his advisor Alexander Evans, an assistant professor at Brown, along with researchers from Purdue University, the Lunar and Planetary Science Laboratory in Arizona, Stanford University and NASA’s Jet Propulsion Laboratory.

The differences between the near and far sides of the Moon were first revealed in the 1960s by the Soviet Luna missions and the U.S. Apollo program. While the differences in volcanic deposits are plain to see, future missions would reveal differences in the geochemical composition as well. The nearside is home to a compositional anomaly known as the Procellarum KREEP terrane (PKT) — a concentration of potassium (K), rare earth elements (REE), phosphorus (P), along with heat-producing elements like thorium. KREEP seems to be concentrated in and around Oceanus Procellarum, the largest of the nearside volcanic plains, but is sparse elsewhere on the Moon.

Some scientists have suspected a connection between the PKT and the nearside lava flows, but the question of why that suite of elements was concentrated on the nearside remained. This new study provides an explanation that is connected to the South Pole-Aitken basin, the second largest known impact crater in the solar system.

For the study, the researchers conducted computer simulations of how heat generated by a giant impact would alter patterns of convection in the Moon’s interior, and how that might redistribute KREEP material in the lunar mantle. KREEP is thought to represent the last part of the mantle to solidify after the Moon’s formation. As such, it likely formed the outermost layer of mantle, just beneath the lunar crust. Models of the lunar interior suggest that it should have been more or less evenly distributed beneath the surface. But this new model shows that the uniform distribution would be disrupted by the heat plume from the SPA impact.

According to the model, the KREEP material would have ridden the wave of heat emanating from the SPA impact zone like a surfer. As the heat plume spread beneath the Moon’s crust, that material was eventually delivered en masse to the nearside. The team ran simulations for a number of different impact scenarios, from dead-on hit to a glancing blow. While each produced differing heat patterns and mobilized KREEP to varying degrees, all created KREEP concentrations on the nearside, consistent with the PKT anomaly.

The researchers say the work provides a credible explanation for one of the Moon’s most enduring mysteries.

“How the PKT formed is arguably the most significant open question in lunar science,” Jones said. “And the South Pole-Aitken impact is one of the most significant events in lunar history. This work brings those two things together, and I think our results are really exciting.”

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“The gut microbiome is the ecosystem of microbial life that populates an animal’s digestive system — and this microbiome plays a major role in an animal’s well-being,” says Erin McKenney, co-author of a paper on the work and an assistant professor of applied ecology at North Carolina State University. “Everything we learn about these microbiomes helps us make more informed decisions to support the health of wildlife species.”

“The Alaska wilderness, important habitat for brown bears, is changing,” says Grant Hilderbrand, co-author of the study and associate regional director for resources for the National Park Service in Alaska. “The gut microbiome serves as a new diagnostic tool for understanding the health of wildlife populations. It can also help us predict how animal health will change as the environment changes. The study we’ve done here lays the foundation for advancing our understanding of gut microbiomes in Alaska’s iconic brown bears.”

The researchers launched the project with the goal of characterizing what the gut microbiome of Alaskan brown bears looks like.

“Bears are particularly interesting in this context, because they’re omnivores,” says Diana Lafferty, co-author of the paper and an assistant professor of biology at Northern Michigan University. “Bears function as predators and as scavengers, and they have a relatively simple gut system — which means their microbiome may play a particularly important role in helping them extract nutrients from their diet. With this project, we wanted to establish a baseline of what ‘healthy’ microbiomes look like for these animals, which are relatively undisturbed.”

The researchers also benefited from piggybacking on other research efforts that were already underway. The National Park Service was already engaged in research projects that involved tracking and observing wild brown bears at three national parks and preserves: Katmai; Lake Clark; and Gates of the Arctic.

“By collaborating with ongoing projects, we were able to collect all of the data and samples at no additional cost,” Hilderbrand says.

For this study, the researchers analyzed 66 fecal samples from 51 brown bears that were sampled during previous research operations. Because these samples were collected as part of ongoing research initiatives, the researchers also had data on each bear’s age, sex, size and body condition.

The researchers extracted microbial DNA from the fecal samples, and assessed those results to determine what kinds of microbes were in each sample.

“We found significant differences in the diversity of the bears’ microbiomes at each of the three parks,” says Sarah Trujillo, corresponding author of the paper who worked on the study while a graduate student at NMU. “Katmai had the most diversity, and also had the most diverse array of food sources available. Gates of the Arctic, which had the most limited array of food resources, also had bears with the least diverse gut microbiomes. In other words, we found what we expected: the more diverse the diet, the more diverse the gut microbiome.

“However, while we found clear distinctions in microbiome diversity at each park, those differences could not be fully explained by diet alone. There appears to be something else at play that we don’t fully understand yet. That’s an area for future research.”

The work was done with support from Northern Michigan University and the National Park Service.

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Published today in Science Advances, the study reveals rapid degradation in air quality and increases in urban exposure to air pollutants hazardous to health. Across all the cities, the authors found significant annual increases in pollutants directly hazardous to health of up to 14% for nitrogen dioxide (NO₂) and up to 8% for fine particles (PM_2.5), as well as increases in precursors of PM_2.5 of up to 12% for ammonia and up to 11% for reactive volatile organic compounds.

The researchers attributed this rapid degradation in air quality to emerging industries and residential sources like road traffic, waste burning, and widespread use of charcoal and fuelwood.

Lead author Dr Karn Vohra (UCL Geography), who completed the study as a PhD student at the University of Birmingham, said: “Open burning of biomass for land clearance and agricultural waste disposal has in the past overwhelmingly dominated air pollution in the tropics. Our analysis suggests we’re entering a new era of air pollution in these cities, with some experiencing rates of degradation in a year that other cities experience in a decade.”

The scientists also found 1.5- to 4- fold increases in urban population exposure to air pollution over the study period in 40 of the 46 cities for NO₂ and 33 of the 46 cities for PM_2.5., caused by a combination of population growth and rapid deterioration in air quality.

According to the study, the increase in the number of people dying prematurely from exposure to air pollution was highest in cities in South Asia, in particular Dhaka, Bangladesh (totalling 24,000 people), and the Indian cities of Mumbai, Bangalore, Kolkata, Hyderabad, Chennai, Surat, Pune and Ahmedabad (totalling 100,000 people).

The researchers say that while the number of deaths in tropical cities in Africa are currently lower due to recent improvements in healthcare across the continent resulting in a decline in overall premature mortality, the worst effects of air pollution on health will likely occur in the coming decades.

Study co-author Dr Eloise Marais (UCL Geography) said: “We continue to shift air pollution from one region to the next, rather than learning from errors of the past and ensuring rapid industrialisation and economic development don’t harm public health. We hope our results will incentivise preventative action in the tropics.”

The study was funded by a University of Birmingham Global Challenges PhD Studentship awarded to Dr Vohra and a NERC/EPSRC grant awarded to Dr Marais.

*Cities analysed in the study:

• Africa — Abidjan, Abuja, Addis Ababa, Antananarivo, Bamako, Blantyre, Conakry, Dakar, Dar es Salaam, Ibadan, Kaduna, Kampala, Kano, Khartoum, Kigali, Kinshasa, Lagos, Lilongwe, Luanda, Lubumbashi, Lusaka, Mombasa, N’Djamena, Nairobi, Niamey, Ouagadougou.
• South Asia — Ahmedabad, Bangalore, Chennai, Chittagong, Dhaka, Hyderabad, Karachi, Kolkata, Mumbai, Pune, Surat.
• Southeast Asia — Bangkok, Hanoi, Ho Chi Minh City, Jakarta, Manila, Phnom Penh, Yangon.
• Middle East — Riyadh, Sana’a.

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The study was published online April 6 in Nature.

“Previously, we knew that Gram-negative bacteria construct their outer membrane with two main, non-protein components — lipids and sugars — which together form an impermeable barrier. The missing link was how this lipopolysaccharide component comes together,” says Filippo Mancia, PhD, a co-leader of the study and professor of physiology & cellular biophysics at Columbia University Vagelos College of Physicians and Surgeons.

Using cutting-edge single-particle cryo-electron microscopy, Mancia and colleagues were able to determine the structures of the enzyme that links together the lipids and sugars (called an O-antigen ligase), in two different functional configurations. Then, combining genetic, biochemical, and molecular dynamics experiments, the team learned how the enzyme positions the lipids and sugars so that they can combine to form the protective membrane.

The lipopolysaccharide component of the outer membrane is critical to the survival of Gram-negative bacteria. “If you could block its assembly, then you would make the bacteria more sensitive to antibiotics and more vulnerable to the immune system,” Mancia says.

Construction of this membrane is an ongoing process, beginning when Gram-negative bacteria are first formed and continuing as the membrane naturally degrades and requires repair. “This means that we would have many opportunities to disrupt the membrane, not just at one stage of the bacteria’s life cycle,” Mancia says.

After revealing the structure of the enzyme that performs the last and critical step in assembling lipopolysaccharide barriers in drug-resistant bacteria, the researchers may begin to custom-design drugs that inhibit the biosynthesis of this protective membrane.

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