A Science Strategy for the Exploration of Europa (1999) / Chapter Skim
Currently Skimming:
2 Current State of Knowledge About Europa
2 Current State of Knowledge About Europa
Pages 13-30
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 13... ...
Early indications of Europa's geologic characteristics were derived from Earth-based telescopic observations indicating that the surface is predominantly water ice.3-5 Determination of its density suggested that Europa is a rocky object, but with a substantial volume of ice. Theoretical studies have suggested that Europa, like its neighbor lo, experiences interior heating from tidal stresses.
|
From page 14... ...
, National Optical atmospheric motions and Imaging with an image sensor, filter wheel, Astronomy structures of small-scale clouds camera focal plane shutter, and electronics Observatories NIMS A 22.8-cm diameter (f/3.5) , 80-cm Robert Carlson, Surface/atmospheric Near-Infrared focal length, Ritchey-Chretien Jet Propulsion reflection/emission Mapping telescope with a spatial scanning Laboratory Spectrometer secondary mirror and diffraction grating spectrometer PPR A Cassegrainian Dall-Kirkham James Hansen, Atmospheric particles, Photo- telescope with a 10-cm aperture, Goddard Institute thermal/reflected radiation polarimeter a 50-cm focal length, and a for Space Studies Radiometer 2.5-milliradian instantaneous field of view UVS/EUV The UVS is a Cassegrainian Charles Hord, Atmospheric gases, aerosols, (spinning)
|
From page 15... ...
have become detached from adjacent "ice sheets" and have been translated and rotated to new positions (Figure 2.2~. These high-resolution images provide the best evidence that the ice crust of Europa was thin at the time that the disruption took place.7 New varieties of pits, domes, and spots observed in the mottled terrain are consistent with the occurrence of solid-state convection near the surface and are interpreted as the surface expression of upwelling masses of warm ice or diapirs.8 The age and orientation of varying large-scale linear features seen in global images of Europa change systematically, as would be expected if the surface ice were decoupled from the interior by a liquid or ductile layer.
|
From page 16... ...
16 A SCIENCE STRATEGY FOR THE EXPLORATION OF EUROPA Such decoupling leads to non-synchronous rotation of the surface ice shell with respect to the interior and appears to be expressed in stress fractures of the surface as the shell changes shape.9 The non-synchronous rotation may arise because the balance between the torque exerted by Jupiter, tending to speed up Europa, and a resisting torque, associated with a slight departure of Europa's minimum-moment-of-inertia axis from the Jupiter line, cannot be permanently maintained if the material of the satellite can adjust in some way to bring the minimum-moment-ofinertia axis back into alignment with Jupiter. This may be the case on Europa if the ice shell is decoupled from the underlying rock.
|
From page 17... ...
However, this model only works if the amplitude of the diurnal tides is some tens of meters, the value predicted for a relatively thin ice shell decoupled from the satellite' s interior by a liquid layer. The higher-resolution Galileo images have revealed many additional small impact craters.
|
From page 18... ...
The resolution of the close-up image is about 250 m/pixel. Images courtesy of the Jet Propulsion Laboratory.
|
From page 19... ...
Image courtesy of the Jet Propulsion Laboratory.
|
From page 20... ...
This is true even in the brightest regions that were inferred to be nearly free of "contaminants." While this has commonly been interpreted as implying a contribution from hydrated minerals such as evaporite salts or clays in all regions of the satellite, it has been shown that light reflected from fairly clear ice with entrained bubbles or defects also exhibits band shifts similar to those observed on Europa. Many of Europa' s dark regions exhibit spectral characteristics that appear distinct from those of water ice, and these have been interpreted as being consistent with the spectral characteristics of water-bearing salts such as hexahedrite (MgSO4.
|
From page 21... ...
Image courtesy of the Jet Propulsion Laboratory.
|
From page 22... ...
Early models, which considered only the conductive cooling and freezing with time of the outer layer of water,33 resulted in the presence today of liquid water beneath the ice shell. However, other models showed that the outer layer of ice would become unstable to convection with sufficient thickening, thereby promoting heat transfer through the ice and the cooling and solidification of the underlying water.34 These models predicted the complete freezing of the outer layer of water in a small fraction of geologic time.
|
From page 23... ...
. A thermally insulating layer at the surface of Europa would promote stabilization of a liquid-water ocean.39 The occurrence of minor constituents in the ice and ocean such as salts and ammonia would affect both the rheology of the ice and the freezing temperature of the ocean.40 Tidal heating along major faults in Europa's ice shell may be important,4i and tidal heating due to forced circulation in a thin liquid-water ocean sandwiched between the rock interior and the overlying ice may prevent complete solidification of the ocean.42 Tidal heating is too dependent on many unknown or poorly known properties of Europa's ice shell, therefore, to settle the debate on the existence of a liquid-water ocean beneath the ice of Europa theoretically, without the benefit of direct observations.
|
From page 24... ...
The energetic magnetospheric plasma that sputters the surface materials to form the neutral atmosphere also is able to ionize the gases once they are in the atmosphere. While energetic ions are probably the dominant sputtering agent, it is the energetic electron population incident upon O2 that can lead to the formation of plasma.
|
From page 25... ...
Galileo collected data in Europa's vicinity four times during its primary mission and, ultimately, an additional seven times during the subsequent Galileo Europa Mission (see Box 2.1~. Plasma measurements from Galileo show that Europa acts in a manner similar to a cometary source of plasma; while it both absorbs and emits charged particles, it is a net source of plasma emitted into the magnetosphere.
|
From page 26... ...
Measurements from Galileo's E-12 orbit showed that the field indeed increased by more than 400 nT upstream of Europa. The expected induction response was of the order of 40 nT during the E- 12 flyby and could not be separated from the very large comet-like response.52 POTENTIAL FOR BIOLOGICAL ACTIVITY If liquid water exists beneath the surface ice layer on Europa, then one of the environmental requirements for life will have been met.
|
From page 27... ...
Though Europa' s gravity field may be known to spherical-harmonic-degree three or better through analysis of Galileo-tracking data, knowledge of the properties of this field will not tell us if there is liquid water beneath the surface. The two most intriguing arguments in favor of a subsurface liquid-water ocean on Europa at present or in the recent geologic past are the geologic evidence for rafting of blocks of ice floating on an underlying fluid and the detection by the Galileo magnetometer of an electromagnetic induction response in Europa that appears to be
|
From page 28... ...
A summary, then, of the major outstanding scientific issues and questions for Europa includes the following: · Is there liquid water on Europa and, if so, what is its spatial distribution? If there is a globally distributed "ocean" of water, how thick is the layer of ice that covers it, and what are the properties of the liquid?
|
From page 29... ...
Cassen, "On the Internal Structure of the Major Satellites of the Outer Planets," Geophysical Research Letters 6: 121, 1979.
|
From page 30... ...
D.J. Stevenson, "Heterogeneous Tidal Deformation and Geysers on Europa," abstract, Europa Ocean Conference, San Juan Capistrano Research Institute, San Juan Capistrano, California, November 12-14, 1996.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.