Geography Reference
In-Depth Information
Earlier [62] we explained the formation of such crater landforms by
superposition of three impact structures of different diameters with a common
rim produced by an originally single falling body, which broke down but did
not disperse into pieces. This asymmetry, even in the absence of a tail, is a
reliable indicator of the CB trajectory (Figures 33, 34).
Both the 3D elevation model of ENDDB (Figures 31-34) and
Google
Earth
3D satellite images can highlight the morphological elements of impact
structures at the optimum image foreshortening, or if the depressions are filled
with water or thick vegetation (Figure 31a) [1].
Another diagnostic feature found in hundreds of craters from the EISC-
catalog using ENDDB is associated with gravity:
tail-shaped gravity lows
[73]
that accompany large astroblemes (Figures 31c; 32b; 33d; 34b). Assuming that
the gravity lows and the tail-shaped asymmetry in craters are of the same
origin, one may expect these features to appear in couple. Note, however, that
such comparison is possible only for relatively large craters (D >>15 km), the
resolution of the available gravity data being much inferior to that of the
elevation models (~ 30 sec per point in V 21.1 against 1 sec in ASTER
GDEM). At the same time, the estimates of CB arrival direction from different
morphological features may be ambiguous for individual structures [75].
For example, all three indicators we describe give similar azimuth
estimates of ~ 300
о
in the case of the Qinghai structure (Figure 34) but show a
30-35
о
difference for the proven Wanapitei and Popigai craters [7, 73, and 75].
The variations in CB arrival azimuths estimated from tail-shaped asymmetry
and gravity lows for the potential Ladoga and Onega structures (Figure 35) are
25
о
and 5
о
, respectively (the contours of the lakes are colored white and show
a tail-shaped asymmetry).
We have checked the diagnostic value of tail-shaped negative gravity
anomalies with craters in Russia using the Gravity maps 2010, scale
1:2500,000 and found this feature in
all
large craters produced by bodies for
which we can assume a trajectory with a relatively low angle to the Earth's
surface [73]. However, large proven structures (D > 15 km) are quite few in
Russia (only 9), and it is important to check this pattern on a global scale.
Indeed, the gravity imprints of CB trajectories show up in the new shaded
model of ―Global marine gravity‖ for hundreds of astroblemes (the data are
available at the website [1]). Furthermore, gravity as part of the GIS-ENDDB
system can be useful to prove the impact origin of many less certain structures
(Figure 35), such as submerged or small island structures (where a small island
is a part of a crater).
