Scientists are discovering that more and more Solar System objects have warm oceans under icy shells. The moons Enceladus and Europa are the two most well-known‚ and others like Ganymede and Callisto probably have them too. Even the dwarf planet Ceres might have an ocean. But can any of them support life? That partly depends on the water temperature‚ which strongly influences the chemistry. We’re likely to visit Europa in the coming years and find out for ourselves how warm its ocean is. Others on the list we may never visit. But we may not have to. Researchers at Cornell University are figuring out how to determine the temperature of an icy world’s ocean by measuring the thickness of its ice shell and associated properties. They published their results in a research article in the journal JGR Planets. It’s titled “Ice-Ocean Interactions on Ocean Worlds Influence Ice Shell Topography‚” and the lead author is Justin Lawrence‚ a visiting scholar at the Cornell Center for Astrophysics and Planetary Science. Lawrence is also a program manager at Honeybee Robotics‚ a subsidiary of Blue Origin that builds technologies for space exploration. Their research is based on what’s called ice-pumping‚ a phenomenon observed under the ice in Antarctica. “When ice is submerged‚ a melting and freezing exchange process termed the “ice pump” can affect ice composition‚ texture‚ and thickness‚” the researchers write. “We find that ice pumping is likely beneath the ice shells of several ocean worlds in our solar system.” Ice pumping is more commonly called thermohaline ice pumping‚ where thermo means heat and haline means basically the same thing as saline: salty. But whereas saline refers to fresh water‚ haline refers to ocean water. On Earth‚ large-scale thermohaline ice pumping supplies heated water to the north and south polar regions. On a smaller scale‚ affects how much ice forms on the underside of an ice sheet since ice is formed from water containing no salt or very little salt. So‚ the salt from the ice-forming water is concentrated in the water under the ice. Since that salt-concentrated water is so close to the ice‚ the water under the ice is both higher in salt and colder because it’s close to the ice. That’s why the term thermohaline is used. The high-salinity shelf water (HSSW) that forms under the ice is denser than the surrounding water and sinks. As it sinks‚ it then becomes warmer than the freezing point there since the water pressure lowers the freezing point. So now the HSSW is warmer and triggers melting on the underside of the ice shelf. Then‚ the HSSW mixes with lower-salinity meltwater to create colder‚ buoyant ice-shelf water (ISF.) The ISF upwells and forms soft ice called frazil ice on the underside of the ice shelf. The process can create ice layers hundreds of meters thick. The critical part is where the ocean and the ice interact. The researchers say that if they can determine the ice thickness‚ they can constrain the water temperature from afar. The press release presenting the results calls this “conducting oceanography from space.” This schematic from the study shows how thermohaline ice pump circulation works below a generalized ice shelf. (1) High salinity shelf water (HSSW) forms at the surface freezing point (Tf = ?1.9°C) as the brine rejected from sea ice growth mixes into the water column. (2) HSSW is dense relative to the surrounding seawater‚ so it sinks‚ and a portion circulates beneath the ice shelf to the grounding zone‚ where it is now warm compared to the pressure-depressed freezing point (positive thermal driving) and drives melting. (3) Fresh meltwater generated at the colder‚ in situ freezing point mixes with HSSW‚ generating fresher‚ colder‚ and relatively buoyant Ice Shelf Water (ISW). (4) ISW upwells‚ the freezing point increases and thermal driving commensurately decreases. With a sufficient pressure decrease‚ supercooling occurs and frazil ice forms‚ which can accumulate into hundreds of meters thick layers of marine ice at the ice shelf base. Credit: Journal of Geophysical Research: Planets (2024). DOI: 10.1029/2023JE008036 “Anywhere you have those dynamics‚ you would expect to have ice pumping‚” Lawrence said. “You can predict what’s going on at the ice-ocean interface based on the topography—where the ice is thick or thin‚ and where it is freezing or melting.” There’s uncertainty around which Solar System bodies have ice pumping and how close ice pumping on Earth is similar to other bodies. For example‚ if Europa’s ice shell is thicker than about 35 km (22 miles) and has low salt content‚ then there may be no ice pumping. “However‚ the majority of predictions for Europa’s ice shell thickness suggest that the interface falls in the marine regime‚ such that Earth’s ice shelves can serve as system analogs to inform European ice-ocean interactions‚” the authors write in their research. Ice pumping is probable on Ganymede and Titan‚ according to the authors‚ as long as bulk ocean salinity isn’t too low. On the other hand‚ Enceladus almost certainly has ice pumping. But the ice pumping on Enceladus is expected to be weaker‚ while at Europa‚ it’s expected to be much stronger. Jupiter’s icy moon Europa likely has strong ice pumping very similar to the Ross Ice Shelf in Antarctica. Credits: NASA/JPL-Caltech/SETI Institute What does it all add up to? “If we can measure the thickness variation across these ice shells‚ then we’re able to get temperature constraints on the oceans‚ which there’s really no other way yet to do without drilling into them‚” said Britney Schmidt‚ associate professor of astronomy and of Earth and atmospheric sciences in the College of Arts and Sciences and Cornell Engineering. “This gives us another tool for trying to figure out how these oceans work. And the big question is‚ are things living there‚ or could they?” Schmidt asks. We can only answer that question incrementally right now. To do that‚ we need to understand the ice shell‚ the temperature‚ and how they’re connected to make progress. “There’s a connection between the shape of the ice shell and the temperature in the ocean‚” Schmidt said. “This is a new way to get more insight from ice shell measurements that we hope to be able to get for Europa and other worlds.” Right now‚ estimates for Europa’s ice shell thickness range from 10 to 30 km (6 to 20 mi). For Enceladus‚ estimates range from 20 to 25 km (12 to 16 miles)‚ though the south pole region’s ice is much thinner‚ only 1 to 5 km thick (1/2 mile to 3 miles.) Oddly enough‚ the icy shells and underlying oceans on the Solar System’s icy worlds may be more similar to Earth than any other planets or moons. The interactions between ice and the ocean on Europa are very similar to what researchers see under Antarctica’s Ross Ice Shelf. In 2019‚ Schmidt and other researchers observed the underside of the shelf with the Icefin robot and observed ice pumping. Another factor at play here is gravity. “Ice pumping scales with gravity and so may prove important to dynamics at the ice shell-ocean interfaces of other similarly massive ocean worlds such as Ganymede or Titan‚” the authors explain. That’s one of the reasons that Enceladus is expected to have weaker ice pumping: its gravity is ten times weaker than Europa’s. This study is important because it shows how ice pumping can occur on different ocean worlds in the Solar System‚ and that has implications for life. Enceladus likely has ice pumping‚ but it’s expected to be weaker than on Europa because Enceladus’ gravity is much weaker. Image Credit: NASA/JPL/Space Science Institute “We show that ice pumping can occur for a range of ocean salinity and ice thicknesses relevant to ocean worlds and that ice pumping is an important process linking ice shell dynamics‚ ocean circulation‚ and basal ice shell topography‚” the authors write. “We show that the relationship between ice-ocean interactions and ice topography establishes a link between variability in ocean temperature and ice shell thickness that potentially makes constraining ocean temperatures possible in the absence of in situ ocean observations.” That’s a big step. The more we can learn about these worlds without visiting them‚ the better. Missions to the Solar Systems icy moons are expensive‚ though one is already planned: NASA’s Europa Clipper. It’s scheduled for launch later this year and should arrive at Jupiter in 2030. A combination of methods will help the Clipper measure Europa’s ice thickness more accurately. “The concepts described here will enable the thermal state of Europa’s upper ocean to be constrained from ice shell thickness‚” the authors conclude. The post How Warm Are the Oceans on the Icy Moons? The Ice Thickness Provides a Clue. appeared first on Universe Today.

Before the end of this decade‚ NASA plans to return astronauts to the Moon for the first time since the Apollo Era. But this time‚ through the Artemis Program‚ it won’t be a “footprints and flags” affair. With other space agencies and commercial partners‚ the long-term aim is to create the infrastructure that will allow for a “sustained program of lunar exploration and development.” If all goes according to plan‚ multiple space agencies will have established bases around the South Pole-Aitken Basin‚ which will pave the way for lunar industries and tourism. For humans to live‚ work‚ and conduct various activities on the Moon‚ strategies are needed to deal with all the hazards – not the least of which is lunar regolith (or “moondust”). As the Apollo astronauts learned‚ moondust is jagged‚ sticks to everything‚ and can cause significant wear on astronaut suits‚ equipment‚ vehicles‚ and health. In a new study by a team of Texas A&;M engineers‚ regolith also poses a collision hazard when kicked up by rocket plumes. Given the many spacecraft and landers that will be delivering crews and cargo to the Moon in the near future‚ this is one hazard that merits close attention! The study was conducted by Shah Akib Sarwar and Zohaib Hasnain‚ a Ph.D. Student and an Assistant Professor (respectively) with the J. Mike Walker ’66 Department of Mechanical Engineering at Texas A&;M University. For their study‚ Sarwar and Hasnain investigated particle-particle collisions for lunar regolith using the “soft sphere” method‚ where Newton’s equations of motion and a contact force model are integrated to study how particles will collide and overlap. This sets it apart from the “hard sphere” method‚ which models particles in the context of fluids and solids. Apollo 15 astronaut salutes next to the American flag in 1971. The Moon’s regolith or soil appears in various shades of gray. Credit: NASA While lunar regolith ranges from tiny particles to large rocks‚ the main component of “Moondust” is fine‚ silicate minerals with an average size of 70 microns. These were created over billions of years as the airless Moon’s airless surface was struck by meteors and asteroids that pounded much of the lunar crust into a fine powder. The absence of an atmosphere also meant that erosion by wind and water (common here on Earth) was absent. Lastly‚ constant exposure to solar wind has left lunar regolith electrostatically charged‚ which means it adheres to anything it touches. When the Apollo astronauts ventured to the Moon‚ they reported having problems with regolith that would stick to their suits and get tracked back into their lunar modules. Once inside their vehicles‚ it would adhere to everything and became a health hazard‚ causing eye irritation and respiratory difficulties. But with the Artemis missions on the horizon and the planned infrastructure it will entail‚ there’s the issue of how spacecraft (during take-off- and landing) will cause regolith to get kicked up in large quantities and accelerated to high speeds. As Sarwar related to Universe Today via email‚ this is one of the key ways lunar regolith will be a major challenge for regular human activities on the Moon: “During a retro-propulsive soft landing on the Moon‚ supersonic/hypersonic rocket exhaust plumes can eject a large quantity (108 – 1015 particles/m3 seen in Apollo missions) of loose regolith from the upper soil layer. Due to plume-generated forces – drag‚ lift‚ etc. – the ejecta can travel at very high speeds (up to 2 km/s). The spray can harm the spacecraft and nearby equipment. It can also block the view of the landing area‚ disrupt sensors‚ clog mechanical elements‚ and degrade optical surfaces or solar panels through contamination.” Data acquired from the Apollo missions served as a touchstone for Sarwar and Hasnain‚ which included how ejecta from the exhaust plume from the Apollo 12 Lunar Module (LM) damaged the Surveyor 3 spacecraft‚ located 160 meters (525 ft) away. This uncrewed vehicle had been sent to explore the Mare Cognitum region in 1967 and characterize lunar soil in advance of crewed missions. Surveyor 3 was also used as a landing target site for Apollo 12 and was visited by astronauts Pete Conrad and Alan Bean in November 1969. A look at the Apollo 12 landing site. Astronaut Alan Bean is shown working near the Modular Equipment Stowage Assembly (MESA) on the Apollo 12 Lunar Module (LM) during the mission’s first extravehicular activity (EVA) on Nov. 19‚ 1969. Credit: NASA. The damage was mitigated by the fact that Surveyor 3 was sitting in a crater below the landing site of the Apollo 12 LM. Another example is the Apollo 15 mission that landed in the Hadley–Apennine region in 1971. During the LM’s descent‚ astronauts David R. Scott and James B. Irwin could not see the landing site because their exhaust plume had created a thick cloud of regolith above it. This forced the crew to select a new landing site on the rim of Béla‚ an elongated crater to the east of the region. The LM could not achieve a balanced footing at this site and tilted backward 11 degrees before stabilizing itself. Research conducted since these missions took place led to the conclusion that the scattering was likely caused by collisions between regolith particles. As Sarwar indicated‚ these examples illustrate how disturbed regolith can become a hazard‚ especially where other spacecraft and facilities are positioned nearby: “The above two examples from the Apollo-era were not severe enough to jeopardize mission success. But future Artemis (and CLPS) missions will take place on the lunar south pole‚ where the soil is assumed to be significantly more porous/weak than the equatorial and mid-latitude Apollo landing regions. Also‚ Artemis landers are expected to deliver much larger payloads than Apollo‚ and therefore require more thrust to slow down. As a result‚ deep cratering can happen (not seen in Apollo) due to rocket exhaust plumes and blow the regolith at much higher angles than those seen previously (~1-3 degrees above ground).” In accordance with the long-term goals of the Artemis Program‚ NASA plans to build infrastructure around the southern polar region to allow for a “sustained program of lunar exploration and development.” This includes the Artemis Base Camp‚ consisting of a foundation surface habitat‚ a habitable mobility platform‚ a lunar terrain vehicle (LTV)‚ and the Lunar Gateway in orbit. “As such‚ protecting humans‚ structures‚ or nearby spacecraft from the hazards of lunar regolith particles is of paramount concern‚” said Sarwar. Illustration of NASA astronauts on the lunar South Pole. Mission ideas we see today have at least some heritage from the early days of the Space Age. Credit: NASA Similar research has shown how clouds of regolith caused by landing and take-off could also pose a hazard to the safe operation of the Lunar Gateway and lunar orbiters. These threats have driven considerable research into how lunar dust can be mitigated during future missions. As noted‚ Sarwar and Hasnain used the soft sphere method to evaluate the risks posed by particle-particle collisions: “In this method‚ adjacent particles are allowed to overlap each other by a tiny amount‚ which is taken as an indirect measure of the deformation expected in a real particle-particle collision. This overlap value‚ along with relevant material properties of lunar regolith‚ are then used in a spring-dashpot-friction slider representation to calculate forces in each collision event. The inelasticity involved in a collision is varied from completely inelastic to highly elastic. “Our results reveal that highly elastic collisions between relatively large regolith grains (~100 microns) cause a significant portion of them to eject at large angles (some can fly out at ~90 degrees). The rest of the grains are‚ however‚ contained in a small-angle region (<;3 degrees) along the ground – which is in line with the visible regolith sheet observed during the Apollo missions.” In terms of safeguards‚ Sarwar and Hasnain suggest that berms or fences around a landing zone are a way to mitigate ejecta sprays. However‚ as their research suggests‚ a certain percentage of regolith particles may scatter at large angles due to collisions‚ making berns or fencing insufficient. “A better solution for future Artemis missions would be to build a landing pad‚” said Sarwar. “In this regard‚ a multi-organization team with personnel from both academia (including Dr. Hasnain) and industry is working on developing the in-Flight Alumina Spray Technique‚ or FAST landing pads.” The FAST method envisions lunar landers equipped with alumina particles that are ejected during landing maneuvers. They are then liquefied by engine plumes to create molten aluminum on the lunar surface‚ which cools and solidifies to create a stable landing surface. NASA has also investigated how landing pads could be built using sintering technology‚ where regolith is blasted with microwaves to create molten ceramics that harden on contact with space. Another idea is to build landing pads with blast walls to contain ejected regolith‚ which the Texas-based construction company ICON included in their Lunar Lantern habitat concept. Illustration of the in-Flight Alumina Spray Technique (FAST). Credit: Masten Space Systems Alas‚ experimental investigations concerning lunar regolith are very difficult because lunar conditions are vastly different than those on Earth. This includes the lower gravity (roughly 16.5% of Earth’s)‚ the vacuum environment‚ and the extreme temperature variations. Hence why researchers are forced to rely heavily on numerical modeling‚ which typically focuses on plume forces and largely ignores the role of particle collisions. But as Sanwar noted‚ their research offers valuable insight and illustrates why it is important to consider this often-overlooked phenomenon when planning future lunar missions: “[However‚] our research on particle collisions has shown that this is a very important phenomenon to consider for accurate regolith trajectory prediction and‚ therefore‚ must be included. There are still a lot of challenges remaining in this area‚ such as a lack of knowledge on regolith particle restitution coefficient (which determines energy loss in a collision)‚ effects of regolith size distribution‚ implications of turbulent plumes etc. We hope to elucidate some of these uncertainties in the future and contribute towards a more comprehensive lunar PSI model for safer Artemis lunar landings.” Further Reading: Acta Astronautica The post New Study Addresses how Lunar Missions will Kick up Moondust. appeared first on Universe Today.

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