Research activities on the icy satellites in the outer Solar System
Regional context image of the Galileo SSI camera with spatial resolution of 950 m/pxl in Voyager context. The mosaic shows Ganymedes older, dark terrain, including the western border of Galileo Regio (located at the right), which was tectonically resurfaced, creating lanes of younger, bright grooved and smooth terrain. Detailed geologic mapping and crater counts for age dating is a major scientific topic in our research group. North in the mosaic is approximately pointing upwards. Image Credit: NASA
Regional context image of the Galileo SSI camera with spatial resolution of 950 m/pxl in Voyager context. The mosaic shows Ganymedes older, dark terrain, including the western border of Galileo Regio (located at the right), which was tectonically resurfaced, creating lanes of younger, bright grooved and smooth terrain. Detailed geologic mapping and crater counts for age dating is a major scientific topic in our research group.
Small, mid-sized and planet-sized moons in orbit around the four giant planets Jupiter, Saturn, Uranus and Neptune are abundant in the outer Solar System. These bodies are different from the terrestrial planets and are therefore grouped into a class of planetary objects termed icy satellites because their surfaces are dominated by the presence of water ice, indicated by H2O absorptions in the near infrared. Further, their average densities are much lower than those of the terrestrial planets, on the order of 2 gcm-3 or less.
Mainly, we use imaging data from the SSI camera aboard the Galileo Jupiter orbiter (1995 – 2003) and from the ISS cameras aboard the Cassini Saturn orbiter (since 2004), as well as spectral data at visual and near-infrared wavelengths returned by the mapping spectrometers Galileo NIMS and Cassini VIMS. Icy satellites show similar landforms in the camera images as terrestrial planets, however, specific morphologies occur which are restricted to these surfaces due to the fact that ice and its unique physical properties replaces silicate rocks as a geologic host material. Surface processes on airless icy satellites include impact cratering, erosion, degradation and deposition by thermal and/or gravitational effects (e.g., mass wasting), and space weathering from the outside, and tectonism and cryovolcanism from the inside. The term cryovolcanism is used to distinguish volcanism on the terrestrial planets, where molten silicates and volatiles erupt, from volcanic processes on icy satellites, involving the ejection and emplacement of icy materials at low surface temperatures, plume-like exhalations of dust and gas, and extrusion of viscous cryomagmatic materials.
One of our main scientific topics is the investigation of impact craters, both in terms of their morphologies and of their frequency distributions in order to date icy satellite surfaces. The two largest Galilean satellites of Jupiter, Ganymede and Callisto, show the widest range in impact crater morphology known from any satellite or planet. Impacts into a weak substrate at the subsurface produces special crater forms such as dome craters, palimpsests, and multi-ring basins with a much larger number of rings than those on, e.g., the Moon.
The size-frequency distribution of impact craters superimposed on geologic units, including other craters, is used in our research group as an important tool to date these units, supporting stratigraphic relationships based on their mutual superposition documented in geologic maps. We use crater counts on the surfaces of the satellites of Jupiter and Saturn, and, for comparative analyses, on the moons of Uranus and Neptune imaged by Voyager 2, to extract relative ages. Specific scientific questions addressed by using crater counts are the spatial distribution of crater forms, and of tectonic features and their stratigraphic sequences. Absolute ages can be derived also but are model-dependent since cratering rates in the outer Solar System are highly uncertain. All impact cratering chronology models used in our group agree that the most densely cratered units are on the order of 4 Ga old but diverge on the order of a factor of ten for the youngest units.
Geologic mapping of icy satellite surfaces and crater counts are used in tandem in our research group for studies of satellite stratigraphy and geological history. In collaboration with colleagues from the Planetary Geodesy Department (PLD), we examine the topography of landforms and their evolution with time by analyzing stereo images. Most of the satellites show little geologic evolution since the time when their densely cratered plains formed, but others, such as Ganymede or Dione, feature landforms indicating widespread as well as local tectonism, inferring these bodies have undergone past periods of intense tectonic deformation. Europa and Enceladus are characterized by young surfaces with low crater frequencies and are cryovolcanically active at present (Enceladus), or are assumed to have been active in a recent past (Europa). Enceladus shows the widest range in surface ages, from densely cratered old units to regions with active cryovolcanism which are devoid of impact craters. Tectonism and cryvolcanism and their evolution through time are a topic in which we colleborate with colleagues from the Planetary Physics Department (PLP) working on the thermal evolution of icy satellites.
One major topic of research in or group is surface spectroscopy of the icy satellites. We use Cassini and Galileo mapping spectrometer data to map the spatial distribution of ice and non-ice materials, the spatial distribution of water ice particle diameters across the surface, and to relate these distributions with underlying geologic units. The stratigraphic position of craters on the satellites of Jupiter and Saturn correlates well with their water ice abundance. Bright ray craters, such as Osiris on Ganymede, Creusa on Dione, or Inktomi on Rhea, are characterized by deep water ice absorptions inferring they are, supported by their morphological freshness, stratigraphically young. Deep water ice absorptions are also found in tectonically resurfaced regions, e.g., along graben scarps on Dione and Rhea.
Further, we use Cassini VIMS data to map the global distribution of ice and non-ice material across icy satellite surfaces and its correlation with sputtering by particles from the Jovian and Saturnian magnetosphere. Sputtering from Saturn’s magnetosphere causes a concentration of dark, non-ice material on the trailing hemispheres of Dione and Rhea. Similarily, we investigate the spatial distribution of water ice particle diameters across a surface in order to identify relationships between the age of geologic units or surface activity and the action of mechanical weathering of surface materials by micrometeorites. On Enceladus, the largest ice particles are found in the cryovolcanically active zones in the South Polar Terrain, and the smallest particles in the oldest densely cratered plains.
Tectonic features on Dione. The global view in the left panel shows various sets of fractures, troughs and graben, crosscutting or truncating each other which infers several tectonic episodes. Outlined areas show the location of the detailed views in the middle and right panel. The middle panel shows two sets of graben in detail, Aurunca Chasmata (West-East), truncated by the younger graben Padua Chasmata (North-South). North is pointing up approximately. The largest crater in this scene is Ascanius which is superimposed on the graben and therefore younger than the tectonic structures. The right panel shows a highly oblique view of the Aurunca Chasmata (up-down) and Padua Chasmata (left-right) with landslides in some graben in Aurunca Chasmata. Images were taken by the ISS narrow angle camera (NAC) aboard the Cassini orbiter.
For further information see also:
NASA – Saturn http://saturn.jpl.nasa.gov/CICLOPS http://ciclops.org/NASA – Galileo http://galileo.jpl.nasa.govNASA - Voyager http://voyager.jpl.nasa.gov/