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Extrasolar planets are being discovered at a rapid rate, with 4,531 planets in 3,363 systems (with another 7,798 candidates awaiting confirmation). Of these, 166 have been identified as rocky planets (aka. “Earth-like”) while another 1,389 have been rocky planets that are several times the size of Earth (“Super-Earths). As more and more discoveries are made, the focus is shifting from the discovery process towards characterization.

In order to place tighter constraints on whether any of these exoplanets are habitable, astronomers and astrobiologists are looking for ways to detect biomarkers and other signs of biological processes. According to a new study, astronomers and astrobiologists should be on the lookout for indications of a carbon-silicate cycle. On Earth, this cycle ensures that our climate remains stable over the course of eons and could be the key to finding life on other planets.

The study, titled “Carbon cycling and habitability of massive Earth-like exoplanets,” was conducted by Amanda Kruijver, Dennis Honing, and Wim van Westrenen – three Earth scientists with the Vrije Universiteit Amsterdam. Honing is also a fellow with the Origins Center, a Netherlands-based national science institute committed to researching the origins and evolution of life in our Universe. Their study is currently being reviewed for publication in The Planetary Science Journal.

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Diagram of the fast carbon cycle, showing the movement of carbon between land, atmosphere, and oceans. Credit: U.S. DOE/BERIS

On Earth, this two-step cycle ensures that carbon dioxide (CO2) levels in our atmosphere remain relatively consistent over time. This first step consists of carbon dioxide being removed from our atmosphere by reacting with water vapor to form carbonic acid, which weathers and dissolves silicate rock. The products of this weathering are washed into the oceans (forming carbonate rock), which sink to the seafloor and become part of the Earth’s mantle.

This is where the second step comes into play. Once in the mantle, carbonate rocks are melted down to create silicate magma and CO2 gas, the latter of which is released back into the atmosphere through volcanic eruptions. As Dr. Honing explained to Universe Today via email, the process is also affected by changes in surface conditions:

“Importantly, the speed of this process depends on the surface temperature: If the surface gets hotter, weathering reactions speed up, and more CO2 can be removed from the atmosphere. Since CO2 is a greenhouse gas, this cools mechanism down the surface, so we have a stabilizing feedback. We have to point out that this stabilizing feedback needs a long time to be efficient, in the order of hundreds of thousand years or even millions of years.”

A key consideration here is how the Sun has been getting hotter with time, Dr. Honing added. Compared to Earth’s early history, our planet now receives roughly 30% more energy from the Sun, which is why atmospheric CO2 levels were higher in the distant past. Therefore, it is safe to say that weathering becomes more pronounced as a planet gets older and that atmospheric CO2 levels will drop at an increasing rate at this point in their evolution.

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The terrestrial planets of our Solar System at approximately relative sizes (left to right): Mercury, Venus, Earth, and Mars. Credit: LPI

Since this is a simple chemical process, there is no reason to think that a carbon-silicate cycle couldn’t function on other planets – provided they have liquid water on their surfaces. For exoplanet researchers and astrobiologists, the presence of liquid water has been a key biosignature in the ongoing search for extraterrestrial life. The issue of plate tectonics has also been raised since this plays a significant role in maintaining Earth’s habitability over time. Said Dr. Honing:

“In our

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Is it Life, or is it Volcanoes?

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September 29, 2023

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Molecules space FEA

Astronomers are working hard to understand biosignatures and how they indicate life’s presence on an exoplanet. But each planet we encounter is a unique puzzle. When it comes to planetary atmospheres, carbon is a big piece of the puzzle because it has a powerful effect on climate and biogeochemistry. If scientists can figure out how and where a planet’s carbon comes from and how it behaves in the atmosphere, they’ve made progress in solving the puzzle.

But one of the problems with carbon in exoplanet atmospheres is that it can send mixed signals.

Carbon, in this context, means all of the major species of carbon, things like carbon dioxide, carbon monoxide, and methane (CO2, CO, and CH4.) A new study investigates the diversity of these chemicals in the atmospheres of exoplanets similar to Earth orbiting stars similar to the Sun.

The study is “Relative abundances of CO2, CO, and CH4 in atmospheres of Earth-like lifeless planets.” It’s been submitted to The Astrophysical Journal and is available on the pre-press site arxiv.org. The authors are Yasuto Watanabe and Kazumi Ozaki. Watanabe is affiliated with the Department of Earth and Planetary Science at the University of Tokyo, and Ozaki is affiliated with the Department of Earth and Planetary Sciences at the Tokyo Institute of Technology.

The study is particularly concerned with CO. “We focused on the conditions for the formation of a CO-rich atmosphere, which would be favourable for the origin of life,” the authors write.

There's no escaping carbon's importance. Earth life is carbon-based, and there's no particular reason to think it'll be different on other planets. This illustration shows carbon molecules in space. Credit: IAC; original image of the Helix Nebula (NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner, STScI, & T.A. Rector, NRAO
There’s no escaping carbon’s importance. Earth life is carbon-based, and there’s no particular reason to think it’ll be different on other planets. This illustration shows carbon molecules in space. Credit: IAC; original image of the Helix Nebula (NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner, STScI, & T.A. Rector, NRAO

In Earth’s present atmosphere, CO can’t build up because chemical reactions destroy it. But in the deep past, three billion years ago, when the oceans were teeming with simple life, CO could’ve accumulated in Earth’s atmosphere. It’s because there was very little oxygen in the atmosphere, and the Sun was dimmer.

So when we’re searching for biosignatures, an atmosphere with CO could indicate simple life. That’s because it can be an important source of both carbon and oxygen for life. But it’s not that cut and dried. This study aims to untangle some of the details of exoplanet atmospheres so we can identify which mixtures of carbon molecules, including carbon monoxide, might be a biosignature.

“Consequently, a detailed understanding of those factors that govern the relative abundances of CO2, CO, and CH4 in planetary atmospheres has far-reaching implications in the search for habitable planets beyond our solar system,” the paper states.

A key concept in this research is called CO runaway. In an atmosphere like early Earth’s, which contained very little oxygen, CO is produced by photodissociation from UV radiation. On the other side of the equation, it’s destroyed by chemical reactions stemming from the photodissociation of water. When conditions are right, more CO is produced than destroyed, and that can lead to CO runaway.

Understanding CO runaway is critical in the appearance of life because prebiotic chemicals necessary for life—especially peptides—are more readily created in a CO-rich atmosphere than in a CO2-rich atmosphere. Evidence from Mars bolsters this point.

The pair of researchers used atmospheric chemistry models to investigate the details behind CO runaway and how it might help us discern which exoplanets could shelter life.

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Colliding Moons Might Have Created Saturn’s Rings

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September 29, 2023

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1280px Saturn September 9 2007 Annotated 50316920862 1024x391 1

If we could wind the clock back billions of years, we’d see our Solar System the way it used to be. Planetesimals and other rocky bodies were constantly colliding with each other, and new objects would coalesce out of the debris. Asteroids rained down on the planets and their moons. The gas giants were migrating and contributing to the chaos by destroying gravitational relationships and creating new ones. Even moons and moonlets would’ve been part of the cascade of collisions and impacts.

When nature crams enough objects into a small enough space, it breeds collisions. A new study says that’s what happened at Saturn and created the planet’s dramatic rings.

The research is “A Recent Impact Origin of Saturn’s Rings and Mid-sized Moons,” and it’s published in The Astrophysical Journal.” The lead author is Luis Todorow, a Research Fellow at the School of Physics and Astronomy at the University of Glasgow.

Saturn’s rings are so iconic that even schoolchildren can identify them. Astronomers have puzzled over them for a long time, trying to figure out how they formed and when. We know they’re mostly made of ice, but a consensus for their formation has been hard to reach.

This study, conducted by NASA and its partners, says a collision between two icy moons is responsible, and the debris is still circling the planet.

We don’t have to wind the clock back too far to find the impact the research identifies. It occurred only a few hundred million years ago, maybe even more recently than that. The research team says that it was triggered by “resonant instabilities in a previous satellite system.”

The research is based on detailed simulations of Saturn and its system of moons (it has 146 confirmed satellites) and rings.

NASA’s Cassini mission laid the groundwork for this research. The spacecraft spent more than ten years in the Saturn system. One of its main discoveries was that the gas giant’s rings and moons are not very old in astronomical terms. The larger ones are probably old, and their cratered surfaces are a clue to their ages. But some of the planet’s smaller moons are likely much younger.

An annotated picture of Saturn's many moons captured by the Cassini spacecraft. Image Credit: By Kevin Gill from Los Angeles, CA, United States - Saturn - September 9 2007 - Annotated, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=131463918
An annotated picture of Saturn’s many moons captured by the Cassini spacecraft. Image Credit: By Kevin Gill from Los Angeles, CA, United States – Saturn – September 9, 2007 – Annotated, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=131463918

A moon’s distance from its planet plays a role in this. The gravitational struggle between a planet and its moon tends to drive moons away. Earth’s Moon is receding a tiny yet measurable amount each year. Some research shows that if the moons nearest to Saturn’s rings were old, they would’ve been pushed away by now. Since they’re still there, they must be young.

But it’s not that cut and dry because the smaller inner moons also have cratered surfaces.

Saturn's moon Mimas is covered in craters, including the dramatic Herschel crater that gives the moon its
Saturn’s moon Mimas is covered in craters, including the dramatic Herschel crater that gives the moon its “Death Star” nickname. But it’s close to Saturn. What’s going on? Image credit: NASA/JPL/SSI

So Saturn is still mysterious.

Adding to the intrigue is our fascination with icy moons. Saturn’s moon Enceladus, as well as other moons like Jupiter’s Europa, contain vast oceans underneath icy shells. They’re prime targets in the
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Chinese Astronauts May Build a Base Inside a Lunar Lava Tube

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September 28, 2023

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1080px Lava tube Big island Hawaii 44459923190 1024x683 1

Caves were some of humanity’s first shelters. Who knows what our distant ancestors were thinking as they sought refuge there, huddling and cooking meat over a fire, maybe drawing animals on the walls. Caves protected our ancient ancestors from the elements, and from predators and rivals, back when sticks, stones, furs and fire were our only technologies.

So there’s a poetic parallel between early humans and us. We’re visiting the Moon again, and lunar caves could shelter us the way caves sheltered our ancestors on Earth.

On the Moon, astronauts will need protection from a different set of hazards. They’ll have to contend with cosmic and solar radiation, meteorites, wild temperature swings, and even impact ejecta. The Lunar Reconnaissance Orbiter (LRO) has found hundreds of lunar ‘skylights,’ locations where a lava tube’s ceiling has collapsed, making a natural opening into the tube. It’s hard to tell without exploring, but lava tubes several hundred meters in diameter could exist on the Moon. That’s a lot of room to work with, and they could provide the shelter astronauts will need. The idea is to build a base inside a lunar lava tube, where astronauts gain additional protection from the thick rock ceiling overhead.

China is considering the idea now, just like others before them. Lunar lava caves might be a resource too valuable to ignore.

Lava tubes are also called pyroducts. They formed when lava flowing across the surface of the Moon began to cool. The top of the flowing lava formed a hardened crust, but the molten lava kept flowing underneath it and eventually drained, leaving an empty tube. They’re here on Earth as well.

This is the entrance to a lava tube on Hawaii's Big Island. Image Credit: By dronepicr - Lava tube Big island Hawaii, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=75616740
This is the entrance to a lava tube on Hawaii’s Big Island. Image Credit: By dronepicr – Lava tube Big island Hawaii, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=75616740

Scientists aren’t sure when lunar volcanism ended. It may have been as far back as one billion years ago, though some evidence shows there was small-scale volcanism in the last 50 million years. In either case, these lava tubes are ancient and untouched.

On the Moon, astronauts will have to contend with the temperature swings. Earth’s natural satellite is a world of temperature extremes. One side of the Moon is in direct sunlight for half of the time, and surface temperatures reach as high as 127 Celsius (260 F.) The side that’s shrouded in darkness sinks as low as -173 C (-280 F.) That wild temperature swing makes it challenging to work on the lunar surface, and to engineer and build equipment that can be effective in such a large range. Lava tubes provide a natural steady-temperature environment that can’t be found elsewhere on the Moon.

Radiation is also hazardous on the lunar surface. It can be as much as 150 times more powerful than on Earth’s surface. That’s deadly, but in lunar caves astronauts would be sheltered by several metres of overhead rock. That’s a thick enough barrier to provide effective protection.

The risk of impacts and impact debris is much smaller, but it has to be accounted for. Obviously, lava tubes provide shelter from small impacts.

Different teams of scientists from different countries and agencies have studied the idea of using lava tubes as shelter. At a recent conference in China, Zhang Chongfeng from the Shanghai Academy of Spaceflight Technology presented a study into the underground world of lava tubes. Chinese researchers did fieldwork in Chinese lava tubes to understand how to use them on the Moon.

According to Zhang, there’s enough similarity between lunar and Earthly lava tubes for one to be an analogue of the other. It starts with their two types of entrances, vertical and sloped. Both worlds have both types.

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