{"title":"Tidal Forcing in Icy-Satellite Oceans Drives Mean Circulation and Ice-Shell Torques","authors":"Hamish C. F. C. Hay, Ian Hewitt, Richard F. Katz","doi":"10.1029/2024JE008408","DOIUrl":null,"url":null,"abstract":"<p>Tidal forces generate time-varying currents in bodies with fluid layers, such as the icy ocean moons of the outer solar system. The expectation has been that tidal currents are periodic—they average to zero over a forcing period—so that they are not associated with a mean flow. This expectation arises from the assumption of linearity. Here, we relax this assumption and develop a theory that predicts the emergence of mean currents driven by any periodic forcing. The theory, derived in the context of a global, uniform, shallow ocean, constitutes a set of mean flow equations forced by non-linear eddy fluctuations. The latter are the canonical, periodic tidal currents predicted by the Laplace Tidal equations. We show that the degree-2 tide-raising potential due to obliquity and/or orbital eccentricity can drive time-averaged currents with zonal wavenumbers from 0 to 4. The most prominent of these is a retrograde zonal jet driven by the obliquity-forcing potential. Assuming Cassini state obliquities, this jet has speeds ranging from 0.01 to 1 mm s<sup>−1</sup>, which can exert torques up to roughly 10<sup>15</sup> N m at the ice–ocean interfaces of Europa, Callisto, Titan, and Triton. Depending on the viscosity of the ice shell, these torques could drive ice shell drift rates of tens to potentially hundreds of meters a year. Thinner or stably stratified global oceans can experience much faster mean currents.</p>","PeriodicalId":16101,"journal":{"name":"Journal of Geophysical Research: Planets","volume":null,"pages":null},"PeriodicalIF":3.9000,"publicationDate":"2024-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1029/2024JE008408","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of Geophysical Research: Planets","FirstCategoryId":"89","ListUrlMain":"https://onlinelibrary.wiley.com/doi/10.1029/2024JE008408","RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
引用次数: 0
Abstract
Tidal forces generate time-varying currents in bodies with fluid layers, such as the icy ocean moons of the outer solar system. The expectation has been that tidal currents are periodic—they average to zero over a forcing period—so that they are not associated with a mean flow. This expectation arises from the assumption of linearity. Here, we relax this assumption and develop a theory that predicts the emergence of mean currents driven by any periodic forcing. The theory, derived in the context of a global, uniform, shallow ocean, constitutes a set of mean flow equations forced by non-linear eddy fluctuations. The latter are the canonical, periodic tidal currents predicted by the Laplace Tidal equations. We show that the degree-2 tide-raising potential due to obliquity and/or orbital eccentricity can drive time-averaged currents with zonal wavenumbers from 0 to 4. The most prominent of these is a retrograde zonal jet driven by the obliquity-forcing potential. Assuming Cassini state obliquities, this jet has speeds ranging from 0.01 to 1 mm s−1, which can exert torques up to roughly 1015 N m at the ice–ocean interfaces of Europa, Callisto, Titan, and Triton. Depending on the viscosity of the ice shell, these torques could drive ice shell drift rates of tens to potentially hundreds of meters a year. Thinner or stably stratified global oceans can experience much faster mean currents.
潮汐力会在具有流体层的天体(如外太阳系的冰洋卫星)中产生随时间变化的水流。人们一直认为潮汐流是周期性的--它们在一个强迫周期内的平均值为零,因此它们与平均流无关。这种预期源于线性假设。在此,我们放宽了这一假设,并提出了一种理论,预测了由任何周期性作用力驱动的平均潮流的出现。该理论是在全球均匀浅海的背景下推导出来的,由一组受非线性涡波动影响的平均流方程组构成。后者就是拉普拉斯潮汐方程所预测的典型周期性潮汐流。我们的研究表明,由斜度和/或轨道偏心率引起的2度潮汐上升势能可以驱动带状波数为0到4的时间平均潮流,其中最突出的是由斜度强迫势能驱动的逆行带状喷流。假设卡西尼状态为倾斜,这种喷流的速度为 0.01 到 1 mm s-1,可以在欧罗巴、卡利斯托、土卫六和海卫一的冰洋界面产生大约 1015 N m 的扭矩。根据冰壳粘度的不同,这些力矩可以驱动冰壳以每年几十米到可能几百米的速度漂移。较薄或稳定分层的全球海洋的平均流速要快得多。
期刊介绍:
The Journal of Geophysical Research Planets is dedicated to the publication of new and original research in the broad field of planetary science. Manuscripts concerning planetary geology, geophysics, geochemistry, atmospheres, and dynamics are appropriate for the journal when they increase knowledge about the processes that affect Solar System objects. Manuscripts concerning other planetary systems, exoplanets or Earth are welcome when presented in a comparative planetology perspective. Studies in the field of astrobiology will be considered when they have immediate consequences for the interpretation of planetary data. JGR: Planets does not publish manuscripts that deal with future missions and instrumentation, nor those that are primarily of an engineering interest. Instrument, calibration or data processing papers may be appropriate for the journal, but only when accompanied by scientific analysis and interpretation that increases understanding of the studied object. A manuscript that describes a new method or technique would be acceptable for JGR: Planets if it contained new and relevant scientific results obtained using the method. Review articles are generally not appropriate for JGR: Planets, but they may be considered if they form an integral part of a special issue.