Geography Reference
In-Depth Information
For modeling a shaded gravity anomaly with the ENDDB tools,
Global
marine gravity
data (of models V16.1 and V18.1 [72]) are embedded into the
system. These models, which are arrays of gravity pixel values, are of the
same size (resolution), but V16.1 gives the maximum resolution only for the
marine global map, while V18.1 includes also the land and can update data for
coastal areas by interpolation. The resulting resolution of the V18.1 model
becomes uneven only latitudinally because the original (Mercator) projection
distorts the cells, and we transform it into a rectangular (i.e. transverse
cylindrical conformal) projection. As a result, the resolution increases from the
equator to the poles, being 30 arc-seconds per point on average, which is the
same as in the more recent V21.1 model.
The gravity data sources for all these models are from the ERS-1 and
Geosat/GM missions, as well as the recently published EGM-2008 global
gravity model [72].
Identifying impact craters by ENDDB begins with selecting the optimum
base colors of the image, the parameters of illumination and shadow depth
[62] for shaded modeling on a regular grid. This procedure allows obtaining
precise 3D images of the terrain and gravity patterns, and, moreover, furnishes
data for recognizing standard morphological elements diagnostic of impact
structures.
Typical Elements of Impact Craters Identified in GIS Digital
Models
In addition to the elements reported in [7, 62], the EISC catalog [1]
includes other new morphological elements typical of impact structures, which
are expressed in the shaded elevation and gravity models and identified using
the
ENDDB
visualization
tools:
tail-shaped
asymmetry,
heart-shaped
geometry of craters, and tail-shaped gravity lows
.
The tail-shaped crater asymmetry is an elongate topographic low
accompanying the ring depression of the main crater (of a similar or even
lesser expression) (Figure 31a-b).
This asymmetry was observed in eight reliably proven and four probable
impact craters, as well as in seven potential and two questionable structures.
To assess the diagnostic validity of this element, one has to take into account
the astrobleme relaxation associated with the sedimentation rate and duration,
besides the destructive effects of erosion, tectonism, volcanism, and later
meteoritic activity [62]. For example, for the 455 Ma Lockne crater [1, 75],
