Survivor of an Ancient "Moonpocalypse"

 Sole survivor of an ancient "Moonpocalypse", A strange moon orbiting Neptune

NASA’s James Webb Space Telescope captured Neptune and its rings and inner moons in 2022. These moons could be made of the pulverized remains of Neptune’s original moons. Neptune’s moon Nereid may be the sole survivor of an ancient moonpocalypse. Neptune is definitely the odd one out of the gas giants. It’s tilted at a strange angle, and its moons are completely different from any other gas giant we know of. A new paper, published in Science Advances from researchers at CalTech, posits that might be because Triton, by far Neptune’s largest moon, absolutely obliterated the regular moon system it previously had, except for one particular exception, Nereid. 

A new study suggests that the strange satellite was born in a steady, circular orbit around Neptune, then tossed into its current elongated orbit during a chaotic encounter with a Pluto-sized body which ejected or pulverized all its sibling moons. This idea counters the assumption that Nereid formed in the Kuiper Belt, the cold reservoir of space rocks in the outer solar system, and was pitched into its present orbit later, researchers argue. Let's start with a little background on Neptune's moon system. Triton is weird. It rotates the opposite direction to Neptune itself, which means it did not form naturally as part of the planetary system. More likely, it was part of a Kuiper Belt Object (KBO) binary, similar to Pluto and Charon, which was captured by Neptune's gravitational well. Nereid is in itself an outlier.

“Maybe it got perturbed outward, rather than kicked inward,” says planetary scientist Matthew Belyakov of Caltech. “Nereid is that last remaining signature of the original satellite system.” Neptune’s largest moon, Triton, orbits backward and makes up more than 99% of the mass of all the planet’s moons combined. Most of Neptune’s other moons orbit the planet from a shorter distance and are small and rubbly, suggesting they’ve been through a lot of collisions. Planetary scientists think Triton came from the Kuiper Belt and wreaked havoc on the rest of the moons when Neptune captured it billions of years ago. Originally discovered in 1949 by Gerard Kuiper (after whom the Kuiper Belt is named), over 100 years after Triton was discovered, it remained Neptune's only other known moon until the Voyager 2 flyby in 1989. But its orbit is eccentric to say the least. It's highly elliptical and lasts 360 days, making astronomers believe for years that it was another captured KBO. This new study pretty clearly shows that it is not.

Nereid stands alone. It orbits in a wide ellipse far from Neptune. That puts it in a family of moons from across the solar system called irregular satellites, many of which are also thought to be captured Kuiper Belt objects. But it’s brighter, larger, more eccentric and closer to its host planet than other irregular satellites in the solar system. “Nereid always is an outlier,” Belyakov says. Maybe its origin story was an outlier, too. To prove that, the authors turned JWST's high-resolution infrared camera toward Nereid for the first time. They found that it looks much more like an icy native moon of Uranus or Saturn than a dark, dusty captured KBO. Compared with Phoebe, a known captured KBO, Nereid's water-rich craters appear completely different in infrared light. As the authors note , "Nereid's unique spectrum among outer Solar System bodies is not consistent with a scenario where Nereid is captured during the early Solar System's dynamic instability." So that pretty much rules out the possibility of Nereid as a KBO, which leaves the only other option as a naturally occurring moon of Neptune.

Belyakov and colleagues compared James Webb Space Telescope observations of Nereid’s makeup with those of other Kuiper Belt objects. Nereid wasn’t a good match for any of them. That left the possibility that Nereid formed locally. Belyakov and colleagues ran computer simulations of Triton’s known chaotic arrival at hundreds of different masses and orbits for Neptune’s original moons, including the destroyed ones. A computer rendering of a blue planet surrounded by moons, with two more distant moons labeled Triton and Nereid. The moons' orbits are depicted in grey. Nereid’s out-there, elongated orbit sets it apart from the other moons. Its distance, ellipticality and other factors are different from most other moons in the Solar System. To answer this question, the authors turned to simulations. They used a dynamic simulator called REBOUND to map Neptune as a series of normal, circular moons. Then they hit that nice, neat model with Triton. As the captured KBO entered a highly eccentric, backward orbit, it wreaked absolute havoc with Neptune's existing moon systems.

Most of the original moons were smashed to pieces or ejected from the system altogether as part of this process. Their debris eventually settled down to create Neptune's current ring system, and some of the tiny "ring-moons" like Proteus. None reproduced Nereid’s exact present-day orbit. And some ended with Triton leaving the system or crashing into Neptune. But about 20% produced a moon on a Nereid-like orbit, without destroying Triton. That’s enough to make the story believable, Belyakov says. But the simulations also showed another feature. In about 20% of all simulation runs, Triton kicked one of the native inner moons that was there before its arrival into a stable, highly elongated, tilted orbit. Just like Nereid's. So, in simulation at least, Nereid appears to be an original moon of Neptune which was kicked to its current wacky orbit by the capture of the planetary system's biggest current resident.

 If that's the case, it could offer pristine insight into the formation of the Neptunian system, since its distant orbit would have kept it relatively well preserved compared to other gas giant moons. We likely won't be able to confirm that theory until we send another probe that way, though. Planetary scientists have been clamoring for one for over a decade, to no avail as of yet. But until we do, we can shift our thinking of this specific gas giant moon from that of a captured ice ball to a battle-scarred survivor of one of the most violent moonpocalypses the Solar System has ever witnessed. Nereid itself is still largely mysterious. The best picture we have of it is about five pixels across, from the Voyager 2 mission in 1989. Belyakov is holding out for a spacecraft flyby someday. “That’s the next frontier, missions to the ice giants,” he says.

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Alaska’s glaciers and rising temperatures

 Reaction of Alaska's glaciers to rising temperatures 

An international team of researchers led by Albin Wells from Carnegie Mellon University in the US found that an increase in the average summer temperature of just 1 degree leads to an increase in the melting period of Alaska's glaciers by about three weeks. This was reported by Science Daily (SD) magazine. Alaska’s glaciers are proving to be highly sensitive to warming temperatures. Using radar satellites to monitor more than 3,000 glaciers, researchers found that every 1°C (1.8°F) increase in average summer temperature extends glacier melting by about three weeks. The study also revealed that intense heat waves can strip away up to 28% more protective snow cover, exposing ice much earlier than normal and accelerating ice loss.

The authors analyzed data from the Sentinel-1 radar satellites from mid-2016 to 2024. The observations covered almost every glacier in the region with an area of more than 1.3 square km's. Scientists have found that the use of synthetic aperture radar (SAR) allows you to track the state of ice through clouds and in the dark, which makes this method more reliable than traditional optical measurements. Alaska's glaciers are highly sensitive to rising temperatures. A single degree Celsius equals 1.8 degrees Fahrenheit. The study also demonstrates that synthetic aperture radar (SAR) can automatically and consistently monitor glaciers and their snowlines throughout the year. Traditionally, snowlines are usually measured only near the end of the melt season using optical instruments. Researchers found that SAR provides more dependable data than conventional surface-based optical methods.

The study was led by Albin Wells, a recent Ph.D. graduate from Carnegie Mellon University. Co-authors include Carnegie Mellon assistant professor David Rounce and Mark Fahnestock of the University of Alaska Fairbanks Geophysical Institute. Rounce previously worked at the Geophysical Institute as a postdoctoral fellow and research associate. Our ability to quantify these changes is really important as per research team. Melting volumes and snow lines are indicators of glacier mass balance. Researchers have found that short-term heat waves cause significant damage to protective snow cover. During abnormally warm periods, glaciers lost more snow than in normal years. A striking example was the heat wave in Alaska in 2019, which lasted from June 23 to July 10. During this period, the temperature in some areas exceeded the norm by 11-16 degrees. The research team used radar observations to measure glacier "melt days." A melt day may represent a full 24-hour period when an entire glacier is melting, or it can consist of several days during which melting occurs across different portions of the glacier until the total affected area equals the glacier's full surface. An increase in melt days indicates that the melt season is becoming longer, which contributes to greater overall ice loss.

The extreme heat caused the snow line of the glaciers to rise almost 107 m above the usual level. It reached such levels two months ahead of schedule, as a result of which the ice remained unprotected and melted more intensively. Using data from Europe's Sentinel-1 radar satellites, the scientists monitored seasonal changes on nearly every Alaska glacier larger than about half a square mile between mid-2016 and 2024. Synthetic aperture radar operates by transmitting microwave pulses from a moving satellite or aircraft toward Earth's surface and then combining the returning signals into detailed images. Because it does not rely on sunlight, SAR can collect data through clouds and in darkness. Sentinel-1 revisits the same location every 12 days and covers more than 3,000 glaciers across Alaska.

In optical data, the snow line can be very difficult to spot. If the picture was taken a day after the fall of fresh snow, optical devices do not allow you to see the boundary between open ice and firn, granular snow turning into ice. SAR radar technology is devoid of these disadvantages and allows you to record the condition of the surface throughout the season. The researchers also discovered that short-term heat waves can dramatically reduce the snow cover that protects glaciers. During unusually warm periods, glaciers lost up to 28% more protective snow than they do in typical years. This percentage applies at the scale of individual mountain ranges and does not necessarily affect every glacier equally within those regions. Melt extents and snowlines are proxies for glacier mass balance. Glacier mass balance refers to the difference between how much snow and ice a glacier gains and how much it loses over time. "These correlations with temperature begin to give a sense for how much melt or snowline retreat we can anticipate under future, warmer climates across the region," Wells said. A snowline marks the boundary between a glacier's accumulation zone, where snow builds up and adds mass, and its ablation zone, where melting removes snow and ice.

Glaciologists generally rely on optical instruments to evaluate snowlines near the end of the melt season, usually in late summer or early autumn. "In optical data, the snowline can be really hard to observe," Fahnestock said. "If you're a day late taking your picture, it might have snowed on the entire glacier, and you can't see where the bare glacier ice is down below and where the snow and firn is above." Firn is partially compacted granular snow found near the upper portions of glaciers. Over time, it can gradually transform into glacier ice. According to Fahnestock, optical observations can be affected by changing lighting conditions, shadows, cloud cover and variations in whether firn appears clean or dirty." SAR avoids many of those limitations and can provide regular snowline measurements throughout the melt season. "What Albin has done is operationalize the tracking of surface conditions on the glaciers in a way that can be applied anywhere," Fahnestock said. The researchers closely examined an intense Alaska heat wave in 2019. The event affected every glaciated region of the state except the Brooks Range. For nearly two weeks, temperatures at many locations ran 20 to 30 degrees above average. Several all-time records were broken, including a reading of 90 degrees Fahrenheit at Ted Stevens Anchorage International Airport. Typical summer highs in Anchorage are usually in the mid-60s.

Scientists have also identified differences in the response of glaciers: objects located on the coastal slopes of mountains show more melting in summer and more accumulation of snow in winter compared to glaciers inland. The researchers note that the established correlations with temperature make it possible to predict the extent of the retreat of snow lines in conditions of further climate warming. The extreme heat pushed glacier snowlines nearly 350 feet higher in elevation. In an average year, snowlines would not reach those elevations until roughly two months later. As a result, bare ice and firn remained exposed for longer periods, increasing overall ice loss. This highlights the sensitivity of glaciers to short-term climatic variability. The study also identified consistent differences between glaciers located on the coastal side of mountain ranges and those farther inland. Wells said the number of melt days varied between the two groups, suggesting they respond differently to environmental conditions even though many are losing ice at broadly similar rates.

"This is an important finding," Wells said, "because it corroborates prior knowledge that glaciers in Alaska on the coastal side of mountains have more melt in summer and more accumulation in winter than those on the continental side of the ranges." It was reported a reduction in the area of sea ice off the coast of Antarctica. According to the research, a section of winter sea ice with an area of about 650 thousand square km's has not formed in the western part of the continent. According to scientists, the figure is comparable to the territory of France. It was clarified that the Bellingshausen Sea is usually covered with ice in June, but now it remains free of it and, as noted, the current oceanic conditions may prevent the formation of a significant volume of it in winter. An alarming state of affairs for the world.

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