Finding the Hidden Oceans of the Moons with Induced Magnetic Fields


Source: Earth and Space Sciences

In the 21st century, planetary scientists have become increasingly aware that subterranean oceans made up of liquid water exist in solar system objects. Because water is a universal requirement for life on Earth, these bodies – mostly moons – are attractive targets in the search for extraterrestrial life.

One of the main ways to infer the existence of an invisible ocean is to use an induced magnetic field. These fields come from a unique application of Faraday’s law of induction, which states that a time-varying magnetic field creates an electric current when applied to a circuit. Water that is salty enough to remain liquid in cold space environments is highly conductive; at the same time, a moon’s orbit through a planet’s rotating magnetic field exposes the moon to a field strength that varies with time. These effects combine to induce an electric current in the ocean, which, in turn, generates an induced magnetic field emanating from the moon.

However, observing the magnetic field that results from this process is a difficult task. The induced field is much weaker than the planetary field that permeates local environments. If a moon has even a tenuous atmosphere, its ionosphere can generate yet another induced magnetic field, which can lead to a false positive detection of the ocean. And for flyby missions that have spacecraft equipped with a magnetometer, the available data will be quite limited or even non-existent if the spacecraft does not pass close enough to detect an induced field.

Cochrane et al. present a new method to deal with these difficulties based on predictive modeling and principal component analysis. They selected a single close flyby of Neptune’s largest moon, Triton, developed for the Trident mission concept proposed as part of NASA’s Discovery program. This event would produce only 12 minutes of usable data from which to extrapolate the existence of a subterranean ocean.

NASA’s proposed Trident mission would explore Neptune’s largest moon, Triton, which potentially hosts an ocean with liquid water beneath its icy shell. Credit: Courtesy of NASA/JPL-Caltech

The technique begins by using a computer model to speculatively simulate magnetometer measurements taken during a flyby of the Triton system with various potential physical properties, such as ocean depth, thickness and conductivity. To cover the available parameter space, they build more than 13,000 models that include magnetic fields induced either by the moon’s ionosphere alone or by the ionosphere plus an underground ocean.

A principal component analysis of this model data then identifies the features that best explain the variability of the modeled magnetometer observations. This creates a simple transformation of incoming spacecraft data into a representation that most clearly distinguishes between the ionosphere-only and ionosphere-ocean scenarios.

The authors demonstrate that this new approach is both more sensitive and more robust to noise than more traditional approaches. Importantly for exploration of the outer solar system, they show that a single flyby may be enough to identify the existence of an ocean below the surface of a moon like Triton. And if multiple flybys are available from an orbiter, identification becomes more assured and allows characterization that provides insight into habitability. (Earth and Space Sciences, 2022)

—Morgan Rehnberg, Science Editor

Quote: Rehnberg, M. (2022), Finding the Hidden Oceans of the Moons with Induced Magnetic Fields, Eos, 103, Posted March 9, 2022.
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