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The clue came from ten heat images taken across 60% of Juno\u2019s 7.2-hour spin, enough to follow changing warmth.<\/p>\n
Across that rotating surface, Jian-Yang Li, Ph.D., at Planetary Science Institute (PSI<\/a>) showed that Juno\u2019s dust packed moderately but cooled too easily.<\/p>\n The same heat pattern pointed to dust that should have been much fluffier if ordinary grain contact controlled the heat flow.<\/p>\n That mismatch does not prove one hidden process, yet it narrows where scientists should look next.<\/p>\n Heat below dust<\/p>\n At 0.05 inch (1.3 millimeters), the signal carried heat from material just under Juno\u2019s surface<\/a>.<\/p>\n Regolith, loose rock and dust covering an airless world, controls how quickly sunlight warms and leaves those layers.<\/p>\n Low thermal inertia, resistance to temperature change, means heat does not travel well between nearby grains.<\/p>\n For Juno, Li\u2019s team found the surface held onto heat very poorly, far less than solid rock would.<\/p>\n Telescope catches warmth<\/p>\n High in northern Chile, the Atacama Large Millimeter\/submillimeter Array (ALMA), a network of radio antennas, recorded Juno in October 2014.<\/p>\n Those observations<\/a> resolved Juno at about 37 miles (60 kilometers) across, far finer than earlier asteroid heat surveys.<\/p>\n Because millimeter wavelengths \u2013 radio waves far longer than visible light \u2013 sense shallow heat, ALMA gave the model more than a surface snapshot.<\/p>\n Even so, the telescope watched only part of one rotation, so local differences remain hard to separate.<\/p>\n Shape rules signal<\/p>\n Juno\u2019s uneven body shaped much of the signal before any dust physics entered the model.<\/p>\n A shape reconstruction<\/a>, a three-dimensional outline built from telescope data, described Juno as about 155 miles (250 kilometers) wide.<\/p>\n As the asteroid turned, broad faces and narrower ends changed how much warm ground faced Earth.<\/p>\n Shape explained the strongest ups and downs, but it could not erase the strange thermal numbers.<\/p>\n Porosity meets heat<\/p>\n Density offered one side of the conflict because Juno\u2019s index of refraction, how strongly material bends radiation, pointed to 45% open space.<\/p>\n That level of porosity, empty volume inside a material, is loose but not extreme for asteroid dust.<\/p>\n Heat models using ordinary chondrites<\/a>, common stony meteorites, required far more void space to match the weak heat flow.<\/p>\n The numbers left researchers with a hard physical problem: moderately open dust seemed to behave like extremely open dust.<\/p>\n Grains barely touching<\/p>\n Fine grains offered another clue, since the best-fitting heat values pointed toward particles near 0.0004 inch (0.01 millimeters) across.<\/p>\n Earlier grain-size work<\/a> showed that asteroid heat behavior depends on particle size, since heat moves through the tiny contact points between grains.<\/p>\n If Juno\u2019s grains touch only lightly, less heat can move downward before the surface cools again.<\/p>\n Repulsive electric charges, forces from uneven electrical buildup, could reduce those contact points, but the model did not prove that mechanism.<\/p>\n Electric depth matters<\/p>\n Another number made Juno look unusual: its dust absorbed millimeter radiation more strongly than lunar-like powder.<\/p>\n The dielectric loss tangent, a measure of electrical absorption, was about 0.4 before correction and near 0.5 after scaling.<\/p>\n That value limits the electric skin depth, the depth radiation can escape from, to roughly 0.004 to 0.06 inch (0.1 to 1.5 millimeters).<\/p>\n \u201cThe fitted loss tangent is high compared to the model<\/a> predictions,\u201d wrote Li and co-authors.<\/p>\n Brightness creates puzzle<\/p>\n Juno also shines oddly around 0.04 inch (one millimeter), where its brightness temperature, a radio estimate of heat, rises instead of fading with longer wavelengths.<\/p>\n Similar behavior appears on Ceres, a dwarf planet in the asteroid belt, but not on most stony asteroids.<\/p>\n PSI\u2019s analysis linked that signal to stronger electrical absorption, which keeps the observed heat closer to the surface.<\/p>\n Composition may play a role, yet Juno\u2019s exception shows dust structure can also change the signal.<\/p>\n Limits guide next<\/p>\n Whole-asteroid measurements kept the result honest because they averaged all visible terrain into one changing signal. <\/p>\n A radiative transfer model \u2013 math that tracks escaping radiation \u2013 separated shallow heat from deeper warmth.<\/p>\n Even with that tool, the team could not map which exact patches had rougher, cooler, or more absorbent dust. <\/p>\n Future resolved views from ALMA could test whether Juno\u2019s surface varies by longitude or by local slope.<\/p>\n PSI\u2019s model now leaves three linked clues in shallow dust: moderate porosity, weak heat flow, and unusually strong electrical absorption. <\/p>\n Those signals point the way forward \u2013 guiding lab tests of meteorite powders and future ALMA observations \u2013 but they still stop short of identifying a single underlying cause.<\/p>\n The study is published in The Planetary Science Journal<\/a>.<\/p>\n \u2014\u2013<\/p>\n Like what you read?\u00a0Subscribe to our newsletter<\/a>\u00a0for engaging articles, exclusive content, and the latest updates.<\/p>\n Check us out on\u00a0EarthSnap<\/a>, a free app brought to you by\u00a0Eric Ralls<\/a>\u00a0and Earth.com.<\/p>\n \u2014\u2013<\/p>\n","protected":false},"excerpt":{"rendered":"Researchers have found that a roughly 155-mile-wide (250-kilometer-wide) space rock called Juno has a dusty surface that 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