NASA’s Juno mission, led by an SwRI scientist, recently provided the first resolved subsurface measurements of the ice-encased Jovian moon Europa. This cutaway illustration shows an 18-mile-thick shell with a shallow layer containing small imperfections — cracks, pores and voids. The icy moon is thought to harbor a vast ocean beneath its icy exterior that could contain the ingredients for life.
Image Credit: NASA/JPL-Caltech/SwRI/K. Kuramura
Scientific Frontline: "At a Glance" Summary
- Main Discovery: Data from NASA’s Juno spacecraft reveals that the rigid, conductive outer ice shell of Jupiter’s moon Europa is approximately 29 kilometers thick.
- Methodology: Researchers utilized the Microwave Radiometer (MWR) instrument aboard Juno to measure thermal emissions and probe the ice shell at varying depths during a close flyby in September 2022.
- Key Data: The estimated thickness of the conductive ice layer is 29 ± 10 kilometers, though this figure could be reduced by approximately 5 kilometers if the ice contains significant salt levels.
- Significance: A shell of this thickness creates a substantial barrier to the transport of oxidants and nutrients from the surface to the subsurface ocean, potentially limiting the moon's habitability.
- Future Application: These findings characterize the ice shell properties to refine observation strategies for the upcoming Europa Clipper mission, particularly for calibrating its ice-penetrating radar.
- Branch of Science: Planetary Science and Astrobiology.
- Additional Detail: The MWR instrument detected shallow structural irregularities such as cracks and voids within the top hundreds of meters, but these features likely do not extend deep enough to serve as conduits for material exchange.Scientific Frontline: "At a Glance" Summary
Data from NASA’s Juno mission has provided new insights into the thickness and subsurface structure of the icy shell encasing a subsurface liquid ocean within Jupiter’s moon Europa. Using the spacecraft’s Microwave Radiometer (MWR), mission scientists determined that the shell averages about 18 miles (29 kilometers) thick in the region observed during Juno’s 2022 flyby of Europa.
The thick shell suggested by the MWR data implies a longer route that oxygen and nutrients would have to travel to connect Europa’s surface with its subsurface ocean. Understanding this process may be relevant to future studies into Europa’s habitability.
"The thickness of Europa’s icy shell and the existence of cracks or pores within the ice shell are two crucial pieces of the puzzle for understanding Europa’s potential habitability,” said Scott Bolton, principal investigator of Juno from Southwest Research Institute. “They provide critical new information relevant to the further study of Europa by NASA’s Europa Clipper and the European Space Agency’s Juice (Jupiter Icy Moons Explorer) — both on their way to the Jovian system.”
The new constraints on the ice thickness in the near-surface icy crust were published in the journal Nature Astronomy. Previous models have suggested the ice shell could be less than half a mile to as many as tens of miles thick.
Slightly smaller than Earth’s moon, Europa is one of the solar system’s highest-priority science targets for investigating habitability. Evidence suggests that the ingredients for life may exist in a vast saltwater ocean believed to exist beneath its ice shell. Juno’s determination of the ice shell thickness and the subsurface characteristics provides fundamental new constraints for understanding the moon’s structure, and internal processes and potential habitability.
Catching waves
The MWR instrument was invented and designed by SwRI’s Bolton to investigate Jupiter’s atmosphere below the cloud tops. The novel instrument has also proven invaluable for studying Jupiter’s ice-covered and volcanic moons as well.
On Sept. 29, 2022, Juno came within about 220 miles (360 kilometers) of Europa’s frozen surface. During the flyby, MWR collected data on about half the moon’s surface, peering beneath the ice to measure its temperatures at various depths.
“The 18-mile estimate relates to the cold, rigid, conductive outer-layer of a pure water ice shell,” said Steve Levin, Juno project scientist and co-investigator from NASA’s Jet Propulsion Laboratory in Southern California, which manages the mission. “If an inner, slightly warmer (convective) layer also exists, which is possible, the total ice shell thickness would be even greater. If the ice shell contains a modest amount of dissolved salt, as suggested by some models, then our estimate of the shell thickness would be reduced by about three miles.”
Cracks, pores
The MWR data also provides new insights into the makeup of the ice just below Europa’s surface. The instrument revealed the presence of irregularities in the near-surface ice. These cracks, pores and voids scatter the instrument’s microwaves reflecting off the ice, similar to the way bubbles in ice cubes scatter visible light. These imperfections are estimated to be no bigger than a few inches in diameter and appear to extend to depths of hundreds of feet below Europa’s surface.
The small size and shallow depth of these features, as modeled in this study, suggest they are unlikely to be a significant pathway for oxygen and nutrients to travel from Europa’s surface to its salty ocean.
Scott Bolton of Southwest Research Institute in San Antonio is the Juno principal investigator leading the mission and is responsible to NASA for all aspects of mission success. The Juno mission is part of NASA’s New Frontiers Program, which is managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate in Washington. The Jet Propulsion Laboratory of Caltech provides management support to the Juno principal investigator.
Published in journal: Nature Astronomy
Title: Europa’s ice thickness and subsurface structure characterized by the Juno microwave radiometer
Authors: S. M. Levin, Z. Zhang, S. J. Bolton, S. Brown, A. I. Ermakov, J. Feng, K. Hand, S. Misra, M. Siegler, D. Stevenson, W. McKinnon, and R. Akiba
Source/Credit: Southwest Research Institute
Reference Number: ps012826_01
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