A Thermal Model for Icy Satellite Hot Spots, 15-R9598

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Principal Investigators
John R. Spencer
Oleg Abramov

Inclusive Dates:  03/09/06 – 03/29/07

Background - Two of the solar system's most remarkable icy moons are Jupiter's moon Europa and Saturn's moon Enceladus. Europa likely possesses a massive ocean of liquid water beneath its icy crust, and is an object of great interest to NASA, which may send an orbiter mission there as its next outer planet flagship mission. Enceladus has recently been shown to have current geological activity – venting of gas and dust from warm fractures near its south pole. Enceladus serves as a test bed for models of still-undetected endogenic activity on Europa.

Detection of localized sources of excess infrared radiation ("hot spots") due to geological activity is a powerful technique for detecting current geological activity, as demonstrated by Enceladus. An instrument capable of mapping infrared radiation from the surface is thus a very attractive component of future Europa or Enceladus missions. However, thermal mapper instrument design is hampered by the current lack of published models of endogenic thermal anomalies on icy satellites. 

Approach - This IR&D project sought to improve our understanding of the nature, evolution and lifetime of thermal anomalies on icy satellites in two ways:

  • Adapt an existing 1-D thermal model to calculate surface temperatures caused by an underlying body of recently erupted warm ice or liquid water as it cools over a period of time. The model was then used to predict the temperatures and lifetimes of this type of thermal anomaly.
  • Construct a new model of thermal emission from fractures along which warm water vapor is escaping and warming the fracture walls – a likely explanation for the thermal anomalies detected on Enceladus, and compare the model result to Cassini observations of thermal emission from Enceladus’ active vents.

Accomplishments - The results of the 1-D model suggest that the lifetime of a thermal anomaly associated with the eruption of 100 meters of water onto the surface of Europa is several hundred years, and approximately 10 years for 10 meters of water. A thin, insulating surface layer can double thermal anomaly lifetimes, and anomalies at a latitude of 80 degrees can remain detectable for up to approximately 50 percent longer than those at equatorial latitudes. The modeled surface ice cools very rapidly to below 200 degrees K because of sublimation cooling. Assuming steady-state resurfacing, the number of detectable thermal anomalies associated with the eruption of 100 meters of water should be on the order of 10 if the typical resurfacing area is 15 km2. Results were presented at the Fall 2007 American Geophysical Union meeting in San Francisco, and a paper is under revision for publication in Icarus.

Results of the Encealdus study suggest vent temperatures as high as 225 degrees K, and these relatively high temperatures potentially strengthen the case for venting from a subsurface liquid water reservoir. The results also indicate that the vents are likely relatively narrow, and are likely the main heat source for the observed thermal anomaly. Finally, SwRI models suggest that the system must be actively maintained with continuous input of energy, and thermal surface properties near the fractures are close to those of low-porosity water ice. Results were presented at the March 2007 Lunar and Planetary Science Conference, and a paper describing the results is in preparation.

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