Gamma rays strike Earth from all directions of the sky. Our planet is bathed in a diffuse glow of high-energy photons. It doesn’t affect us much, and we don’t really notice it, because our atmosphere is very good at absorbing gamma rays. It’s so good that we didn’t notice cosmic gamma rays until the 1960s when gamma-ray detectors were launched into space to look for signs of atomic weapons tests. Even then, what we noticed were intense flashes of gamma rays known as gamma ray bursts.
Gamma-ray bursts are bright but short-lived. They are so bright that it was first feared they were caused by nuclear blasts on Earth, but we now know they are caused by large dying stars as their core collapses into a black hole. The collapse can trigger the formation of jets of material streaming away from the star at nearly the speed of light. When the jet collides with interstellar gas, it creates a beam of gamma rays. If the jet of a dying star happens to be pointed in our direction, we detect a gamma-ray burst.
As our gamma-ray telescopes became more sensitive, we also detected a galactic gamma-ray glow. Most of these gamma rays come to us from the plane of the Milky Way and are caused by high-energy particles that collide with interstellar gas and dust in our galaxy. There are gamma rays that come from the active galactic nuclei of distant galaxies. They are created when supermassive black holes consume matter near them. But if you exclude gamma rays from all those known sources, there is still a faint, diffuse glow of gamma rays. They come to us from all directions, even from regions that seem to be empty space. We’ve never been able to figure out the source of this faint background, but a new paper in Nature seems to have solved this mystery.
How cosmic ray particles create gamma rays. Credit: Max Planck Intitut/ESO VLT
The team looked at how gamma rays can be produced by cosmic rays. Cosmic rays are extremely energetic particles, typically protons moving at nearly light speed. These cosmic ray particles sometimes strike our atmosphere to create a cascade of particles we can detect on the surface. But cosmic ray collisions can also create gamma rays. The team thought the faint gamma-ray background might be caused by cosmic rays striking gas and dust in distant galaxies. Since most of the gas and dust in a galaxy is found in star-forming regions, the team compared the gamma-ray background to the distribution of galaxies actively forming stars. They found that star-forming regions in galaxies could be the source of diffuse gamma rays.
When high-resolution gamma-ray telescopes such as the Cherenkov Telescope Array are built, maps of the gamma-ray background could further confirm this model. It could also let us study star-forming regions in a new way, and might help us better understand the origin of cosmic rays.
Reference: Roth, Matt A., et al. “The diffuse gamma-ray background is dominated by star-forming galaxies.” Nature 597.7876 (2021): 341-344.
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The Milky Way’s Mass is Much Lower Than We Thought
How massive is the Milky Way? It’s an easy question to ask, but a difficult one to answer. Imagine a single cell in your body trying to determine your total mass, and you get an idea of how difficult it can be. Despite the challenges, a new study has calculated an accurate mass of our galaxy, and it’s smaller than we thought.
One way to determine a galaxy’s mass is by looking at what’s known as its rotation curve. Measure the speed of stars in a galaxy versus their distance from the galactic center. The speed at which a star orbits is proportional to the amount of mass within its orbit, so from a galaxy’s rotation curve you can map the function of mass per radius and get a good idea of its total mass. We’ve measured the rotation curves for several nearby galaxies such as Andromeda, so we know the masses of many galaxies quite accurately.
But since we are in the Milky Way itself, we don’t have a great view of stars throughout the galaxy. Toward the center of the galaxy, there is so much gas and dust we can’t even see stars on the far side. So instead we measure the rotation curve using neutral hydrogen, which emits faint light with a wavelength of about 21 centimeters. This isn’t as accurate as stellar measurements, but it has given us a rough idea of our galaxy’s mass. We’ve also looked at the motions of the globular clusters that orbit in the halo of the Milky Way. From these observations, our best estimate of the mass of the Milky Way is about a trillion solar masses, give or take.
The distribution of stars seen by the Gaia surveys. Credit: Data: ESA/Gaia/DPAC, A. Khalatyan(AIP) & StarHorse team; Galaxy map: NASA/JPL-Caltech/R. Hurt
This new study is based on the third data release of the Gaia spacecraft. It contains the positions of more than 1.8 billion stars and the motions of more than 1.5 billion stars. While this is only a fraction of the estimated 100-400 billion stars in our galaxy, it is a large enough number to calculate an accurate rotation curve. Which is exactly what the team did. Their resulting rotation curve is so precise, that the team could identify what’s known as the Keplerian decline. This is the outer region of the Milky Way where stellar speeds start to drop off roughly in accordance with Kepler’s laws since almost all of the galaxy’s mass is closer to the galactic center.
The Keplerian decline allows the team to place a clear upper limit on the mass of the Milky Way. What they found was surprising. The best fit to their data placed the mass at about 200 billion solar masses, which is a fifth of previous estimates. The absolute upper mass limit for the Milky Way is 540 billion, meaning that the Milky Way is at least half as massive as we thought. Given the amount of known regular matter in the galaxy, this means the Milky Way has significantly less dark matter than we thought.
Reference: Jiao, Yongjun, et al. “Detection of the Keplerian decline in the Milky Way rotation curve.” arXiv preprint arXiv:2309.00048 (2023).
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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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