The Solar System\u2019s hundreds of moons are like puzzle pieces. Together, they make a picture of all the forces that can create and modify them and the forces that shape our Solar System. One of them is Miranda, one of 28 known moons that orbit the ice giant Uranus. Miranda is its smallest major moon, at 471 km in diameter. <\/p>\n
New research shows that this relatively small, distant moon may be hiding something: a subsurface ocean.<\/p>\n
<\/p>\n Miranda stands out from the other moons for one reason: its surface is a bizarre patchwork of jumbled terrain. There are cratered areas, rough scarps, and grooved regions. It may have the tallest cliff in the Solar System, a 20 km drop named Verona Rupes. Many researchers think its surface is deformed by tidal heating from gravitational interactions with some of the Uranus\u2019 other moons. <\/p>\n New research in The Planetary Journal set out to explain Miranda\u2019s jumbled geology. It\u2019s titled \u201cConstraining Ocean and Ice Shell Thickness on Miranda from Surface Geological Structures and Stress Modeling.\u201d The lead author is Caleb Strom, a graduate student at the University of North Dakota.<\/p>\n \u201cTo find evidence of an ocean inside a small object like Miranda is incredibly surprising,\u201d<\/p>\n Tom Nordheim, co-author and planetary scientist at the Johns Hopkins Applied Physics Laboratory<\/cite><\/p><\/blockquote>\n<\/figure>\n Scientists don\u2019t have much to go on when it comes to Miranda. The only spacecraft to image it was Voyager 2 in 1986. Even then, the flyby was brief, and the spacecraft only imaged the moon\u2019s southern hemisphere. But that was enough to reveal the moon\u2019s bizarre and complex geological surface features. Miranda\u2019s strange surface coronae attracted a lot of attention. <\/p>\n When the images were first received, scientists were baffled by Miranda\u2019s complexity. Some called it a \u201cpatchwork planet,\u201d and there was much healthy speculation about what created it. Attempts to understand the moon are still limited by the amount of data that Voyager 2 provided. However, modern scientists have access to a more powerful tool than scientists did in the 80s: computer models and simulations.<\/p>\n Strom and his co-researchers used a computer model to work backward from Miranda\u2019s current surface. They started by mapping Miranda\u2019s surface features, including its cracks, ridges, and unique trapezoidal coronae, and then reverse-engineered it. They tested different models of the moon\u2019s interior to see what could account for the varied surface. <\/p>\n The model that best matched the surface was one where Miranda had a vast ocean under its surface some 100-500 million years ago. The icy crust is probably 30 km thick or less, and the ocean could be up to 100 km thick. <\/p>\n \u201cOur results show that a thin crust (?30 km) is most likely to result in sufficient stress magnitude to cause brittle failure of ice on Miranda\u2019s surface,\u201d the authors explain in their research. \u201cOur results also suggest the plausible existence of a ?100 km thick ocean on Miranda within the last 100\u2013500 million yr.\u201d<\/p>\n \u201cTo find evidence of an ocean inside a small object like Miranda is incredibly surprising,\u201d said Tom Nordheim, a planetary scientist at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, a study co-author, and the principal investigator on the project that funded the study. \u201cIt helps build on the story that some of these moons at Uranus may be really interesting \u2014 that there may be several ocean worlds around one of the most distant planets in our solar system, which is both exciting and bizarre.\u201d<\/p>\n Tidal heating is responsible for this, and it came from gravitational relationships between Miranda and Uranus\u2019 other moons. Moons tug on each other, and when they\u2019re in an orbital resonance with one another, where each moon\u2019s period around a planet is an exact integer of the others\u2019 periods, those tugs are amplified. These forces can periodically deform the moons, and as they\u2019re squeezed, they heat up, keeping subsurface oceans warm and liquid. <\/p>\n Miranda and other moons of Uranus were likely in resonance in the past, which could\u2019ve created surface fractures and related terrain. <\/p>\n However, resonances don\u2019t last forever, and the researchers think that some time ago, Miranda left orbital resonance, and its interior began to cool. They don\u2019t think it\u2019s completely cooled yet because if the ocean had completely frozen, it would\u2019ve expanded and displayed telltale surface cracks. So, the interior ocean likely still exists but is probably much thinner than in the past. \u201cBut the suggestion of an ocean inside one of the most distant moons in the solar system is remarkable,\u201d Strom said.<\/p>\n Nobody predicted that Miranda would have an ocean. As far as scientists could tell, it was a frozen ball. But they\u2019ve been wrong about moons before.<\/p>\n Researchers used to think that Saturn\u2019s moon, Enceladus, the most reflective object in the Solar System, was just a ball of ice. After all, its surface is smooth and clearly frozen solid. However, the Cassini mission showed us that it may not be totally frozen. There\u2019s a bevy of evidence that Enceladus has a warm ocean under a layer of ice. <\/p>\n \u201cFew scientists expected Enceladus to be geologically active,\u201d said co-author Alex Patthoff. \u201cHowever, it\u2019s shooting water vapour and ice out of its southern hemisphere as we speak.\u201d <\/p>\n Since both Enceladus and Miranda are roughly the same size and may have similar ice shells, it increases the chances that Miranda also has an ocean. Other moons, like Saturn\u2019s Europa, may also be icy ocean moons. Now, scientists think these moons and their warm oceans are the best targets in the search for life in our Solar System.<\/p>\n Other recent research suggests that Miranda could be more like Enceladus than thought. One 2023 study showed that the moon may be releasing material into space like Enceladus does. The ESA and NASA are both sending probes to Jupiter to study Europa and other potential ocean moons. Should we expand that search to distant Uranus and its small moon Miranda?<\/p>\n \u201cWe won\u2019t know for sure that it even has an ocean until we go back and collect more data,\u201d said study co-author Nordheim. \u201cWe\u2019re squeezing the last bit of science we can from Voyager 2\u2019s images. For now, we\u2019re excited by the possibilities and eager to return to study Uranus and its potential ocean moons in depth.\u201d<\/p>\n For now, all we have is decades-old Voyager 2 data. However, the data and the computer models the team employed shed new light on Miranda. <\/p>\n \u201cWe interpret the tidal stress model results to indicate that at some point in Miranda\u2019s geologic past, it experienced an intense heating event that resulted in a thin crust (?30 km). Such a thin crust would also have resulted in a ?100 km thick ocean to account for the molten part of the hydrosphere. This thin ice crust and thick ocean could have allowed for intense tidal stress leading to significant geologic deformation in the form of brittle deformation at Miranda\u2019s surface,\u201d the authors explain.<\/p>\n \u201cIn conclusion, our results suggest that Miranda could have had a subsurface ocean in the geologically recent past from an intense heat pulse, consistent with dynamical modelling results of previous studies,\u201d they conclude.<\/p>\n\n
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