The story of our solar system’s origin is pretty well known. It goes like this: the Sun began as a protostar in its “solar nebula” over 4.5 billion years ago. Over the course of several million years, the planets emerged from this nebula and it dissipated away. Of course, the devil is in the details. For example, exactly how long did the protoplanetary disk that gave birth to the planets last? A recent paper submitted to the Journal of Geophysical Research takes a closer look at the planetary birth crèche. In particular, it shows how the magnetism of meteorites helps tell the story.
bout That Solar Nebula
Some 5 billion years ago, our neighborhood of the galaxy was a nebula made of hydrogen gas and some dust. That provided the seeds of what became our solar system. Somehow, a part of this molecular cloud began to clump on itself. Maybe a passing star sent shock waves and ripples through the dust and caused it to compress. Or, maybe a nearby supernova did the deed. Whatever happened, it started the birth process of the protostar which eventually became the Sun.
Artist’s impression of the solar nebula. Astronomers study the leftovers of solar system formation that once existed in this cloud to understand conditions at that time. They want to know how long it lasted after the formation of the solar system. Image credit: NASA
During its birth process, the infant Sun in its birth crêche went through what’s called the T Tauri phase. It blew extremely hot winds filled with protons and neutral helium atoms out to space. At the same time, some of the material was still falling onto the star.
While all that was happening, the cloud was in motion and flattening out like a pancake. Think of it like an accretion disk feeding material into the center where the star was forming. Not only was it filled with the seeds of planets, but it was also threaded with a magnetic field. This active disk is where the planets formed. They started out as clumps of dust, which stuck to each other to become pebble-sized rocks. Those rocks crashed together to form larger and larger conglomerations called planetesimals. Those, in turn, collide and form planets. That’s the executive summary of solar system formation. But, to get more details, scientists have to dig a bit more.
Studying Rocks from the Solar Nebula
Once the planets were born, what happened to the rest of the nebula? In 2017, planetary scientist Huapei Wang and collaborators reported on their studies of meteorites dating back to that time. They found that the solar nebula had cleared by about four million years after solar system formation.
A team of scientists, led by Cauê S. Borlina of Johns Hopkins University and MIT, wondered if the system cleared out all at once. Or, did it happen over two separate timescales? To answer that, the team turned to a characteristic called “solar nebula paleomagnetism”. That’s a fancy way of saying that there was a magnetic field in the nebula. Meteoroids formed in the nebula at that time (called carbonaceous chondrites) contain imprints of that field. Borlina and the team speculated that there was one timetable for the inner solar system and one for the outer regions. But, how to find out for sure what that timetable was? Those magnetic field imprints held some clues.
The rocks that formed in the nebula should show a magnetic imprint reflecting the magnetic fields at the time. Those formed after the nebula cleared wouldn’t show much (or any) magnetic fingerprint. They would record the magnetism (or lack of it) of that time and place.
Magnetism in Primordial Rocks
Borlina’s team studied meteorites found in Antarctica in late 1977/78 and 2008. Those rocks are made of a primordial material called “carbonaceous chondrite” that formed early in solar system history. The team focused on magnetite (an iron oxide mineral) found in each sample. Magnetite “records” what’s called “remanent magnetization” imposed by the presence of the local field. Then, they compared to other paleomagnetic studies of certain rocks called “angrites” that were not magnetized. Presumably, these formed after the solar nebula (and its intrinsic magnetic fields) had dissipated.
The further analysis gave a time frame for clearing the inner and outer solar system. For the inner region—1-3
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Is it Life, or is it Volcanoes?
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
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
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
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 “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
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
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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