Evidence

Ice shells greater than 10 km in thickness on Europa are possible due to the ability of a base-layer of ice to exist as a convecting sub-layer below 10 km depth.

The viscosity of ice is highly temperature dependent, and since convection is viscosity dependent, the convecting layer is confined to the base of the shell. This means that a thicker ice shell is possible and could exist stably. While it does not rule out a thin shell, it does refute arguments that a shell thicker than 10-15 km would be unstable and initiate freezing of the entire ocean. The authors argue that the primary variables that will dictate the shell thickness are the viscosity and the tidal heating. Therefore, an ice shell thicker than 15 km can exist at Europa.

Authors

McKinnon, W. B.

Abstract

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Evidence

Models of thermal inputs for Europa at various ice shell viscosities support a two-layer ice shell with an upper brittle shell of 10-15 km and a lower, convecting layer of an additional 5-15 km.

The ocean is kept from freezing entirely through tidal dissipation in the ice shell itself. Most of the viscosity variation occurs in the upper part of the ice shell, the “stagnant lid”. Total ice shell thickness estimates range from 30 – 40 km, depending on the viscosity inputs. However, the upper 10-15 km is consistent with a rigid layer. The rest of the shell would consist of a convecting sub-layer.

Authors

Hussmann, H., Spohn, T., & Wieczerkowski, K.

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Evidence

Variation in thickness of the upper, ice shell thickness ranges from 0.2 – 2.4 km, not including a lower, ductile layer, that is likely due to spatial variability.

If a lower, convecting layer is added, then the ice shell is typically shown to be 15 km in depth or greater. Papers that utilize thermodynamic analysis tend to predict thicker ice shells, while papers that utilize purely mechanical models tend to predict thinner shells. This would be consistent with a two-layer model, as mechanical models would mostly only apply to the upper, elastic ice shell and not a lower sub-layer.

Authors

Billings, S. E., & Kattenhorn, S. A

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Evidence

The scaling relationship between depth and diameter measurements of craters on Europa is inconsistent when compared to craters on other icy satellites, implying a rheological transition within the ice shell and ice shell thickness of at least 19-25 km thick.

Measurements of the depth versus diameter of craters on icy satellites were plotted against each other. They all showed at least three trends in the relationship, correlating to types of craters. However, the slope of the line for the two largest crater types on Europa were shallower than for the same crater types on other icy satellites. This implies that ice type changes with depth. This supports previous theories for a two-layer ice shell and provides a constraint on the depth of the ice shell on Europa.

Authors

Schenk, P. M.

Abstract

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Evidence

The scaling relationship between depth and diameter measurements of craters on Europa is inconsistent when compared to craters on other icy satellites, implying a rheological transition within the ice shell and ice shell thickness of at least 19-25 km thick.

Measurements of the depth versus diameter of craters on icy satellites were plotted against each other. They all showed at least three trends in the relationship, correlating to types of craters. However, the slope of the line for the two largest crater types on Europa were shallower than for the same crater types on other icy satellites. This implies that ice type changes with depth. This supports previous theories for a two-layer ice shell and provides a constraint on the depth of the ice shell on Europa.

Authors

Schenk, P. M.

Abstract

Citation

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Evidence

The scaling relationship between depth and diameter measurements of craters on Europa is inconsistent when compared to craters on other icy satellites, implying a rheological transition within the ice shell and ice shell thickness of at least 19-25 km thick.

Measurements of the depth versus diameter of craters on icy satellites were plotted against each other. They all showed at least three trends in the relationship, correlating to types of craters. However, the slope of the line for the two largest crater types on Europa were shallower than for the same crater types on other icy satellites. This implies that ice type changes with depth. This supports previous theories for a two-layer ice shell and provides a constraint on the depth of the ice shell on Europa.

Authors

Schenk, P. M.

Abstract

Citation

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Evidence

The relationship between depth and diameter measurements of craters on Europa is inconsistent when compared to craters on other icy satellites, implying a rheological transition within the ice shell and ice shell thickness at 19-25 km thick.

Measurements of the depth versus diameter of craters on icy satellites were plotted against each other. They all showed at least three trends in the relationship, correlating to types of craters. However, the slope of the line for the two largest crater types on Europa were shallower than for the same crater types on other icy satellites. This implies that ice type changes with depth. This supports previous theories for a two-layer ice shell and provides a constraint on the depth of the ice shell on Europa.

Authors

Schenk, P. M.

Abstract

Citation

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Evidence

Models of impact craters show that for the largest craters seen on Europa, the ice shell must have been a minimum of 11.9 – 18.5 km thick to produce the observed morphologies.

For large impact craters, it would be anticipated that if Europa’s ice shell was thin, the impact would break through the shell and infill with liquid water. Models of crater excavation and deformation, compared to the largest craters seen on the surface, do not show the morphologies that would be expected if the impacts broke through the shell. Therefore, they predict that at the time the largest craters were formed, the ice shell must have been a minimum of 11.9 – 18.5 km in depth.

Authors

Turtle, E. P., & Ivanov, B. A.

Abstract

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