Black holes are powerful gravitational engines. So you might imagine that there must be a way to extract energy from them given the chance‚ and you’d be right. Certainly‚ we could tap into all the heat and kinetic energy of a black hole’s accretion disk and jets‚ but even if all you had was a black hole in empty space‚ you could still extract energy from a trick known as the Penrose process. First proposed by Roger Penrose in 1971‚ it is a way to extract rotational energy from a black hole. It uses an effect known as frame dragging‚ where a rotating body twists nearby space in such a way that an object falling toward the body is dragged slightly along the path of rotation. We’ve observed the effect near Earth‚ though it is tiny. Near a rotating black hole‚ the effect can be huge. So strong that within a region known as the ergosphere objects can be dragged around the black hole at speeds greater than light in free space. Trajectories of bodies in a Penrose process. Credit: Aleksandr Berdnikov‚ CC BY-SA 4.0 Roughly‚ the Penrose process is to fly into the ergosphere of a swiftly spinning black hole‚ and then release a bit of mass or radiation into the black hole. The resulting rotational kick sends you away from the black hole faster than you approached it. The extra energy you get is balanced by slowing down the black hole’s rotation. This process can in theory extract up to 20% of the black hole’s mass energy‚ which is huge. In comparison‚ fusing hydrogen into helium only yields about 1% of the mass energy. Of course‚ theoretical physicists are never satisfied. If you can extract 20% of the mass energy from a black hole‚ why not more? This is the focus of a recent paper‚ though it should be noted that it focuses on a more abstract idea of a black hole than we see in the Universe. Simple black holes can be characterized by three things: mass‚ rotation‚ and electric charge. The black holes we observe have the first two‚ but since matter is electrically neutral‚ not the third. This paper focuses on charged black holes. Our Universe is also expanding and can be roughly described by a solution to Einstein’s equations known as de Sitter space. It describes an empty universe with a positive cosmological constant. Anti-de Sitter space (AdS) would be a universe with a negative cosmological constant. Although AdS doesn’t describe our Universe‚ it allows for a few mathematical tricks theorists love‚ so it is often used to explore the limits of general relativity. This paper specifically looks at a charged black hole in anti-de Sitter space. A visualization of anti-de Sitter space. Credit: Alex Dunkel Although this study is entirely hypothetical‚ it’s interesting as a “what-if” scenario. Rather than extracting energy from a black hole’s rotation‚ the authors look at how to extract energy through particle decay using the Bañados-Silk-West (BSW) effect. Using some kind of electromagnetic or physical confinement mirrors‚ particles can be reflected back and forth near the event horizon‚ gaining energy from the black hole until they decay as usable energy. The problem with this idea‚ as the authors show‚ is that this can lead to a runaway effect where particle energy amplifies particle energy in a feedback look‚ leading to what’s known as a black hole bomb. So if you find yourself building a power plant near a charged black hole in an anti-de Sitter universe‚ tread carefully. But more interesting is that the authors also looked at the case of a charged black hole in an otherwise empty anti-de Sitter universe. In this case‚ energy would also be extracted from the black hole. Instead of mirrors‚ the structure of spacetime itself would act as a kind of confinement chamber. So the charged black hole would release energy on its own. It would be similar to Hawking radiation‚ but in this case‚ it doesn’t rely upon quantum gravity. The authors also found that this case doesn’t lead to a black hole bomb. As mentioned earlier‚ none of this applies to real black holes in our Universe. As far as we know‚ the Penrose process is the best we could really do. But studies like this are useful because of what they reveal about the fundamental nature of space and time. And now we know that even in a strange anti-universe we can only imagine‚ black holes can release energy over time. Reference: Penrose‚ Roger‚ and R. M. Floyd. “Extraction of rotational energy from a black hole.” Nature Physical Science 229.6 (1971): 177-179. Reference: Feiteira‚ Duarte‚ José PS Lemos‚ and Oleg B. Zaslavskii. “Penrose process in Reissner-Nordström-AdS black hole spacetimes: Black hole energy factories and black hole bombs.” arXiv preprint arXiv:2401.13039 (2024). Reference: De Sitter‚ Willem. “On the relativity of inertia. Remarks concerning Einstein’s latest hypothesis.” Proc. Kon. Ned. Acad. Wet 19.2 (1917): 1217-1225. The post It's a Fine Line Between a Black Hole Energy Factory and a Black Hole Bomb appeared first on Universe Today.

Exoplanet K2-18b is garnering a lot of attention. James Webb Space Telescope spectroscopy shows it has carbon and methane in its atmosphere. Those results‚ along with other observations‚ suggest the planet could be a long-hypothesized ‘Hycean World.’ But new research counters that. Instead‚ the planet could be a gaseous mini-Neptune. K2-18b is in the habitable zone of a red dwarf star about 134 light-years away. It’s about 2.6 Earth radii and about 8.6 Earth masses. Its orbital period is only 33 days‚ so it’s close to its star. But since the star is a dim red dwarf‚ K2-18b receives about the same amount of energy from its star as Earth does from the Sun. Scientists are still puzzling over the planet’s density and composition. Its density is in between the densities of Earth and Neptune. Since it’s not predominantly rock like Earth or all gas like Neptune‚ that led to speculation that it’s a hycean (ocean) world. The only way scientists can determine what K2-18b is made of is to discover what’s in its atmosphere. That’s what the JWST did‚ and its observations found a number of chemicals‚ including CO2 and methane. It also found a lack of ammonia. Earlier this month‚ scientists presented some research (Shorttle et al. 2024) based on the JWST’s findings. By working with climate atmosphere models‚ those researchers concluded that K2-18b is most likely a magma ocean world. “The magma ocean model reproduces the present JWST spectrum of K2-18b‚” they wrote‚ “… suggesting this is as credible an explanation for current observations as the planet hosting a liquid water ocean.” But another group of researchers don’t agree with that. Those researchers don’t think the planet is a hycean world or a lava world. They’ve presented a paper titled “JWST observations of K2-18b can be explained by a gas-rich mini-Neptune with no habitable surface.” The lead author is Nicholas Wogan‚ a post-doctoral researcher in the Space Science Division at NASA’s Ames Research Center. Wogan studies the early Earth‚ as well as exoplanets and astrobiology. The JWST found methane (CH4) and carbon dioxide (CO2) in K2-18b’s atmosphere‚ and it also detected no ammonia. Those results generally indicate a hycean world with a thick Hydrogen/Helium atmosphere. But Shorttle et al.’s analysis showed otherwise‚ saying that the results could also show a planet with a magma world. The new paper from Wogan et al. comes to a different conclusion. “… we favour the mini-Neptune interpretation because of its relative simplicity and because it does not need a biosphere or other unknown source of methane to explain the data‚” they write. In their work‚ the researchers used photochemical and climate models to simulate different versions of K2-18b‚ including hycean worlds and a gas-rich mini-Neptune with no defined surface. Their work shows that the gas-rich mini-Neptune model fits the data best. There’s an extraordinary amount of complexity in planetary atmospheres‚ and figuring out what’s going on from such a great distance is an enormous task. Not only do scientists need to know what chemicals are present (thanks‚ JWST)‚ but they need to understand all the processes taking place. The temperature and pressure in an atmosphere play huge roles in what we can see and in what may remain hidden. One aspect of K2-18b’s atmosphere is supercriticality. A supercritical fluid is one that’s above its critical point in temperature and pressure. Above this critical point‚ neither gas nor liquid phases exist. But the pressure isn’t high enough to force the material into a solid. Jupiter and Saturn have supercritical fluids deep in their atmosphere‚ and they behave very differently than liquids or gases. That adds another layer of complexity. Researchers have climate models that embody the complexity as best they can‚ and the researchers compared the JWST’s findings to three modelled exoplanets: an uninhabitable hycean world‚ a habitable hycean world‚ and a gaseous mini-Neptune with no surface. This figure from the research helps explain the findings. Each panel is a separate model compared to the JWST’s NIRSpec and Single Object Slitless Spectroscopy observations. JWST data rules out the lifeless hycean world model because it doesn’t have enough methane. The inhabited hycean model and the mini-Neptune model fit the JWST data better‚ but invoking a biotic source for the planet’s methane is too much of a reach for the authors. Instead‚ they’ve settled on the mini-Neptune model as the best fit. Image Credit: Wogan et al. 2024. “Given the additional obstacles to maintaining a stable temperate climate on Hycean worlds due to H2 escape and potential supercriticality at depth‚ we favour the mini-Neptune interpretation because of its relative simplicity and because it does not need a biosphere or other unknown source of methane to explain the data‚” the authors write. The authors point out that for a hycean world to maintain its 1% atmospheric methane‚ there would need to be a biogenic source or some other unknown source. They also write that if K2-18b is a hycean world‚ it would be very difficult for it to avoid the runaway greenhouse effect and maintain a stable temperature. The authors discard the hycean hypothesis because it is full of challenges. According to them‚ a gaseous mini-Neptune scenario fits the data and models better. They point to the planet’s deep atmosphere to explain the JWST’s findings. “Deep-atmosphere thermochemical quenching” can explain the methane and carbon dioxide that JWST found‚ and deep atmosphere kinetics like upwelling can explain the lack of ammonia and carbon monoxide. This won’t be the last word on K2-18b. The data will be subjected to further analysis. As the effort to understand it continues‚ the results will also strengthen our existing atmospheric and climate models. One day in the future‚ scientists will know how to differentiate exoplanets. But for now‚ they’re still figuring it out. The post Another Explanation for K2-18b? A Gas-Rich Mini-Neptune with No Habitable Surface appeared first on Universe Today.

One ongoing mystery on Mars is the sporadic detection of atmospheric methane. Since 1999 detections have been made by Earth-based observatories‚ orbital missions‚ and on the surface by the Curiosity Rover. However‚ other missions and observatories have not detected methane at all‚ and even when detected‚ the abundances appear to fluctuate seasonally or even daily. So‚ where does this intermittent methane come from? A group of scientists have proposed an interesting theory: the methane is being sucked out of the ground by changes in pressure in the Martian atmosphere. The researchers simulated how methane moves underground on Mars through networks of underground fractures and found that seasonal changes can force the methane onto the surface for a short time. In their paper‚ published in the Journal of Geophysical Research: Planets‚ the scientists say their simulations predict short-lived methane pulses prior to sunrise for Mars’ upcoming northern summer period‚ which is a candidate time frame for Curiosity’s next atmospheric sampling campaign. “Our work suggests several key time windows for Curiosity to collect data‚” said John Ortiz‚ a graduate student at Los Alamos National Laboratory who led the research team. “We think these offer the best chance of constraining the timing of methane fluctuations‚ and (hopefully) down the line bringing us closer to understanding where it comes from on Mars.” The presence of methane (CH4) in the Martian atmosphere is of great interest to planetary scientists and exobiologists because it could indicate present or past microbial life. Or‚ it could also be related to nonbiological processes‚ such as volcanism or hydrothermal activity. The problem with detecting methane is that it doesn’t last long. Once released into the atmosphere‚ it can be quickly destroyed by natural atmospheric processes. Therefore‚ any methane detected in Mars’ atmosphere means it must have been released recently‚ which only adds to the intrigue. On Earth‚ most methane is produced by living creatures such as microorganisms in sedimentary strata‚ or in the guts of ruminants (cows‚ sheep‚ deer‚ etc.). For methane produced through abiotic or non-living processes‚ there is a high likelihood it could have been produced millions or even billions of years ago‚ lying trapped in underground rock formations. But still‚ finding methane on Mars is a big deal because of the potential for biological sources‚ such as methanogenic microbes. This graphic is the result of an analysis that gives a percentage chance of the methane originating in each grid square centered on Gale Crater. Image Credit: Giuranna et al. (2019) In 2004‚ the Mars Express Orbiter (MEO) detected methane in the Martian atmosphere. In 2013 and 2014 Curiosity detected spikes in methane in the atmosphere at Gale Crater. Interestingly‚ MEO detected a methane spike again‚ at the same location that Curiosity did‚ only one day later. Ortiz and his team wanted to better understand Mars’ methane levels‚ and used high-performance computing clusters to simulate how methane travels through networks of underground fractures‚ and then released into the atmosphere when driven by atmospheric pressure fluctuations. They also modeled how methane is adsorbed onto the pores of rocks‚ which is a temperature-dependent process that may contribute to the methane level fluctuations. The team said their simulations predicted methane pulses from the ground surface into the atmosphere just before the Martian sunrise in the planet’s northern summer season‚ which just recently ended. This corroborates previous rover data suggesting that methane levels fluctuated not only seasonally‚ but also daily. With these insights‚ the Curiosity rover team can figure out when and where to look for methane‚ which could aid in the rover’s main goal‚ searching for signs of life. “Understanding Mars’ methane variations has been highlighted by NASA’s Curiosity team as the next key step towards figuring out where it comes from‚” Ortiz said. “There are several challenges associated with meeting that goal‚ and a big one is knowing what time of a given sol (Martian day) is best for Curiosity to perform an atmospheric sampling experiment.” Paper: “Sub-diurnal methane variations on Mars driven by barometric pumping and planetary boundary layer evolution.” Journal of Geophysical Research: Planets. DOI: 10.1029/2023JE008043LANL press release The post Atmosphere Pressure Changes Could Explain Mars Methane appeared first on Universe Today.

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