Impact Feature Types
In general, three different types of impacts can be distinguished for
experiment surfaces from A0201 MOS detectors and
Germanium witness plates, A0187-1 Gold surfaces,
and S1003 Aluminum surfaces:
These features have elliptic impact craters with raised lips, sometimes
accompanied with a small spall zone near the lips. Spallation was seen on
on the gold surfaces from A0187-1, due
to the manufacturing process which utilized a thin (approximately 2
micrometer) beat to cover the base Au substrate. This resulted in a
layered structure.
Central craters were usually near-circular shaped, although some features
on the S1003 Aluminum surfaces are clearly elongated
Diameter measurements
were taken as described in the M & D
preliminary report, page 8:
"(...)In practice, the microscope was focused on the top of the feature's
rim, and the center of the ridge which was in focus was used to make the
estimated diameter measurements(...)."
Be aware that diameter measurements reported in some hypervelocity impact related
literature might have been taken as diameter of the central crater at the
level of the undisturbed surface.
These discharge features have a central crater and a larger discharge zone.
Note that the term crater might be confusing as the central crater of the
discharge feature is related to particle parameters in a different manner
than is the case with "standard" hypervelocity impacts. All Particles < 3
micrometers in size that "triggered" the MOS sensors resulted in
approximately 10 micrometer "craters" i the senter of the discharge zone.
The formation mechanism of these 10 micrometer craters was dominated by
the sensor's spark discharge energy. For particles >3 micrometers, the
impact crater formation mechanism is dominated be the impactor's
energyand resemble "classic" impacts into brittle
materials.
(See also the
calibration paper,
in this archive).
Measurements for these sensors were taken as follows:
- Central crater of the discharge zone, center of edge of the discharge zone
either as seen under the optical microscope or on an SEM image
- Discharge or vaporization zone, the approximately 50-60 micrometer
diameter area surrounding the crater, from center of rim to center of rim.
- For larger features that caused an impact with asciated spall zone,
almost resembling a "normal" impact into a brittle
surface, the central crater had melting from the discharge effect
associated with it. Measurements were taken from the apparent place of
transition between the melt zone and the spall zone.
- Impacts with associated spall zones also have spall zone extensions
giving apparent maximum and minimum spall zone diameters as measured
from the edges of the spall zone through the apparent center of the
central crater.
These impacts feature a central crater or central zone of damage, depending
on feature size, associated with a large spallation zone.
Measurements for these fetures were taken as follows:
- Center of central crater rim to center of crater rim for impacts with
an obvious central crater
- Center of line of apparent transition from former central crater to
spall zone to center of line of apparent transition from former central
crater to spall zone for larger impacts with no circular shaped central
crater.
- Minumum and maximum spall zone diameters measured from spall zone edges
through the apparent center of the central crater.
Notes:
- For elliptical features minimum and maximum diameter values are given.
This affects:
- Central crater diameters
- Discharge zone diameters
- For some of the features no diameter value is available. This might
either be due to the fact that no records were kept, or no photograph
could be found for this particular feature during the time period
available for the archiving process. In this case, the feature diameter
may be deducted from SIMS images showing substrate material giving:
- The maxmimum extent of the crater lips for impacts on Al and Ag
surfaces,
- The diameter of the discharge zone (Al and Ca images), and of the
central crater (Si and Si2 images) for MOS sensors,
- The size of the spall zones (e.g. Ge images) for impacts into
brittle materials (e.g. Ge witness plates)
This being assumptions more than measurements this archive does
not contain size values deducted from SIMS images.
Klaus G. Paul, 06-20-94