-TIME TEMPERATURE EVOLUTION OF THE IMPACT FLASH AND BEYOND. C. M. Ernst
EARLY-TIME TEMPERATURE EVOLUTION OF THE IMPACT FLASH AND BEYOND. C. M. Ernst
and P. H. Schultz, Brown University, Department of Geological Sciences, Box 1846, Providence, RI 02912
(Carolyn_Ernst@brown.edu).
Introduction: When a hypervelocity projectile
impacts a target, a light flash is produced at the mo-
ment of first contact. Time-resolved light intensity
curves can be used to determine the starting conditions
of the observed impacts [1,2]. For impacts into metal
targets, the peak intensity of the flash occurs during the
initial projectile penetration, with the signal quickly
decaying [3,4]. Impacts into particulate targets pro-
duce a source of blackbody radiators in addition to the
initial flash. This source extends the resulting light
intensity profile well beyond the time of initial contact.
A time-resolved temperature profile is an addi-
tional useful tool for analyzing the early-time evolution
of an impact based solely on its emitted light. Ob-
served thermal signatures indicate that the increase in
light intensity to a peak value is dependent on an ex-
panding radiating source area, not on an increasing
temperature. Impact temperature profiles evolve dif-
ferently for various angles and projectile diameters.
Experiments: Hypervelocity impact experiments
were performed at the NASA Ames Vertical Gun
Range (AVGR) in order to study the evolution of the
impact flash and the resulting thermal plume. Two
high-speed photodiode systems recorded the time-
resolved light intensity. The first system, also used in
previous studies [1,2], consists of a single photodiode
with a spectral range of 350-1100 nm. The second
system consists of six similar photodiodes that can be
used in conjunction with bandpass filters (5080 nm
FWHM). This multi-channel photodiode/filter system
was calibrated using a carbon arc blackbody ther-
mogauge.
The systems viewed the impacts through windows
in the ceiling of the target chamber. The single photo-
diode system was used as an unfiltered channel. Fil-
ters used with the multi-channel system were centered
at 500, 550, 600, 650, 700, and 880 nm. The recorded
intensity data have a time resolution of 100 ns.
Spherical Pyrex projectiles (0.3180.476 cm in di-
ameter (=a) ) were launched at velocities between
4.506.10 km/s and at angles of 30º and 90º (measured
from the horizontal). The impact targets were pumice
dust, which was used to simulate loose, particulate
regolith material. All experiments were performed in
near-vacuum conditions (< 0.5 Torr). At levels below
~10 Torr, ambient pressure does not change the ob-
served light intensity for macroscopic impacts [5].
This has been confirmed with these experiments.
Data: The seven channels of raw data collected for
a 30º impact are depicted in Figure 1. For all of the
recorded impacts, the characteristics of the unfiltered
channel include an intensity peak that occurs 1050 µs
after impact (time = t
0
) and lasts from 50100 µs, and a
long-duration decay signal (timing depends on initial a,
, and v). Prior to this broad feature is a brief intensity
spike lasting less than 23 µs. These features have
been reported previously [1,2].
0.000
0.050
0.100
0.150
0.200
0
25
50
75
100
TIME (µs)
RELATIVE INTENSITY
Unfiltered
500
550
600
650
700
880
FIGURE 1. Seven channel raw intensity data from an impact of a
0.318 cm Pyrex sphere into a pumice dust target at 30º and 6.04
km/s. The unfiltered channel has the highest intensity and is shown
in black. The filtered channels are represented by different colors.
Results: The filtered signals have the same overall
shape as the unfiltered light curve but vary in relative
intensity. Analysis yielded a thermal evolution curve
for each impact based on ratios of the filtered channels.
The calculated temperature profiles for several of the
experiments are plotted in Figure 2. Curve fitting has
been used to reduce the noise and this smoothes the
appearance of the signals. The peak intensity times are
indicated for each curve by diamonds.
Experiments with similar starting conditions (a, )
yielded similarly shaped temperature profiles. The
highest temperatures were observed at the time of first
contact. Higher temperature levels occurred for the
30º impacts. This contrasts with other results [6,7] but
most likely indicates a difference in the emitting
source (blackbody versus vapor). Temperature then
decreased with time for all cases, though more sharply
for 90º impacts than for 30º impacts. The 30º profiles
exhibit a gradual decrease in temperature out to a time
of ~50 µs, where they begin to level off. The 90º pro-
files all exhibit very shallow slopes throughout the
entire profile.
Source Area. Light intensity is dependent on the
temperature of the radiating source as well as its sur-
face area by the relation I=AT
4
, where is the
Stephan-Boltzman constant. Since the intensity and
temperature of these impacts have been observed, the
effect of source area on the light produced can be de-
termined. The unfiltered intensity profile for the 30º
impact from Figure 1 is plotted again in Figure 3. A
theoretical intensity profile is also calculated from the
known temperature evolution using an assumption that
there was no source area growth to influence the light
output (I~T
4
).
Lunar and Planetary Science XXXV (2004)
1721.pdf 2000
3000
4000
5000
6000
0
25
50
75
100
TIME (µs)
TEMPERATURE (K)
30º, 6.04 km/s, 0.318cm
30º, 4.82 km/s, 0.476cm
30º, 5.20 km/s, 0.476cm
90º, 5.88 km/s, 0.318cm
90º, 5.46 km/s, 0.476cm
FIGURE 2. Temperature evolution profiles for impacts with differ-
ent , v, and a. Diamonds represent the time locations of the inten-
sity peaks. The peak temperatures occur at t
0
. Impacts at 30º and
90º exhibit distinct temperature trends with time.
If there were no growth of the radiating area, the
shape of the observed intensity profile would follow
that of the temperature profile. The theoretical curve,
however, is not able to reproduce the long-duration
peak in the observed intensity profile. Therefore, the
evolution of radiating surface area with time is neces-
sary to explain the recorded shape. Since the observed
peak temperature occurs at time t
0
and steadily de-
creases after that point, the measured intensity increase
to the peak can only be explained by an expanding
source area. This may occur by increasing the number
of radiating particles or by expanding the area of the
thermal plume exposed to the detectors.
0.000
0.050
0.100
0.150
0.200
0
25
50
75
100
TIME (µs)
RELATIVE INTENSITY
Observed
Theory - no area change
FIGURE 3. An observed intensity profile for an experiment at 30º,
and a theoretical intensity profile calculated assuming no change in
radiating source area. The theoretical curve cannot match the shape
of the observed intensity peak, thus the radiating surface area must
evolve over time.
The intensity and temperature data were used to
calculate the actual surface area of the radiating mate-
rial. The resulting area evolutions are plotted in Figure
4. Diamonds indicate the peak intensity time for each
of the area profiles. The radiating surface area initially
increases faster for the 90º impact than for the 30º im-
pact. By the time the 90º intensity profile reaches its
peak, the source area has already undergone most of its
growth. When the 30º data reaches its peak, the source
area has yet to undergo the majority of its growth.
This illustrates an important angular dependence of the
radiating source. After 100 µs, both the intensity and
temperature decrease as the emitting particles cool and
no new radiating material is added. At this point in the
light evolution, intensity decay rates depend on pro-
jectile and target properties that affect the exposure of
the radiating material in the transient cavity [2].
0
5
10
0
25
50
75
100
TIME (µs)
AREA (cm
2
)
90º, 5.46 km/s, 0.476cm
30º, 5.20 km/s, 0.476cm
FIGURE 4. Evolution of the radiating source area for impacts at 30º
and 90º. Diamonds represent the time locations of the intensity
peaks. The 90º source area has undergone most of its growth by the
time the intensity reaches its peak whereas the 30º source undergoes
most of its growth after the peak.
Implications: Observations of the evolution of the
impact flash and resulting thermal plume can be used
to probe the early-time impact process and may be
used to address issues such as the effect of shear heat-
ing in laboratory scale impacts. Frictional shear heat-
ing occurs at the contact between the projectile and the
target surface and appears to increase for more oblique
impacts [6]. Studies of the evolution of the impact
flash and the thermal plume for impacts of different
initial conditions (a, , v, and target) can help to ad-
dress the role of shear heating in oblique impacts.
These results can be applied to interpret both natural
(Leonid meteors hitting the Moon) and man-made
(NASAs Deep Impact mission) impacts. Results also
can be compared to computer simulations, since simu-
lations must be able to reproduce what is seen in the
laboratory and what is seen in larger-scale impacts.
Conclusions: The flash produced during macro-
scopic impacts into pumice targets is a prolonged phe-
nomenon that extends well beyond the initial penetra-
tion of the projectile into the target surface. Observa-
tions of the impact flash and the thermal plume