Impact is an ubiguitous physical process in the solar system. It occurs on all solid bodies and operates over a spectrum of scales. Understanding impact phenomena is therefore paramount in constraining and underpinning a large number of research efforts into fundamental problems in planetary geology. Gravity is an important parameter in impact processes. The advantages of microgravity experimentation are discussed

Grieve, Richard A. F.

English

NASA Technical Reports Server

IKPACT EXPERIKENTATIOM AND THE KICROGRAVITY EWIRONMENT: AN OVERVIEW 
Richard A.F. Grieve, Earth Physics Branch, EMR, Ottawa, Canada K1A OY3 for the 
Kicrogravity Cratering Working Group. 
Impact is an ubiquitous physical process in the solar system. It occurs 
on all solid bodies and operates over a spectrum of scales, influencing- 
geologic processes ranging from accretion, the early evolution of planetary 
bodies, the petrogenetic and spatial relations of-lunar samples, the surface 
characteristics and interpretation of spectral data of asteroidal bodies, to 
the nature of some meteorites. Understanding impact phenomena is therefore 
paramount in constraining and underpinning a large number of research efforts 
into fundamental problems in planetary geology. Gravity is an important 
parameter in impact processes. For example, in cratering it affects the size 
of crater excavation, the post-excavation modification of the cavity by 
gravitational collapse, the spatial distribition of ejected materials, and the 
effectiveness of this ejecta in producing secondary cratering events. 
few exceptions (Gault and Wedekind, 1977) previous experimental studies of 
cratering processes have been undertaken at gravitational accelerations of lg 
or higher. 
bodies in the solar system. 
Space Station represents an unique opportunity to extend the experimental 
aspect of impact studies into the microgravity (<lg) regime. 
Uith 
These are not the gravity conditions occurring on most solid 
The physical environment offered ultimately by 
Previous and current experimental studies of impact phenomena address a 
The bulk of impact experimentation, however, has been variety of problems. 
concerned with crater growth and scalin . Experimental data have established 
that at impact energies above -1018-101s ergs (equivalent to the impact 
of an iron meteorite in the meter-size range impacting at 20 km s-1 in a lg 
environment), crater excavation occurs in the so-called "gravity regime**, 
where target strength effects are unimportant (Schmidt, 1980). This condition 
is simulated experimentally by using low strength materisls, such as sand or 
water, and by the use of elevated gravitational accelerations. 
elevated gravity is to displace the onset of the gravity regime to lower 
energies. 
relations, where cratering efficiency is related to a dimensionless parameter 
which includes the effects of projectile velocity and size, and gravitational 
acceleration.' 
The effect of 
Such experimentation has led to the development of scaling 
It has been sugggested recently that additional parameters such as the 
shape of the experimental projectile (Schultz and Gault, 1985) and variable 
energy losses due to waste heat in the target (Cintala and Grieve, 1984) m e  
not fully accounted for in the current dimensionless parameters. Thus it is 
important to continue work in this area. 
experiments at gravities directly applicable to that of planetary bodies will 
contribute to determining and refining the relevant scaling relations for 
large craters. 
cratering mechanics, such relations are required to correctly relate crater 
densities on different solar system bodies to absolute surface ages (Basaltic 
Volcanism Study Project, 1981). 
The opportunity to conduct new 
Apart from their importance in problems concerned with 
A-40 
https://ntrs.nasa.gov/search.jsp?R=19890005645 2020-03-20T05:05:03+00:00Z
A further advantage of .the microgravity environment is that, for a given 
impact event, reduced gravity increases the crater growth time. 
possible, therefore, through high-speed photography to observe the crater 
growth and ejecta dynamics in considerably more detail than in previous lg 
experiments. This will lead to a better understanding of the relative 
importance of rebound and collapse phenomena in crater formation and the 
nature of the ejecta plume as an erosional and depositional agent. 
It will be 
In the low strength materials used in impact experiments under terrestrial 
conditions, gravitational forces dominate other bonding forces, such as 
surface tension, electrostatic effects etc. This may not be the case under 
highly reduced gravity conditions, which prevail on small asteroidal bodies. 
Even if current dimensionless scaling relations are shown to be substantially 
correct for large planetary craters, they can not be applied to the energy 
regime associated with small cratering events. 
highly reduced gravities, corresponding to asteroidal bodies, will therefore 
provide basic and currently unavailable information on cratering and regolith 
development or lack of it on these bodies. 
Cratering experiments at 
Previous experimentation provides little or no information on the spatial 
distribution, source region and physical state of ejecta under differenct 
gravity conditions. The few experiments designed to address these fundamental 
questions all have been undertaken at lg (Stoffler et al., 1975). Similarly, 
the only direct observational data on these questions are from terrestrial 
craters. It is well-established that for a specific impactor size and 
velocity and target materials, crater size will increase with decreasing 
gravity. However, peak shock pressures and the spatial distribution of shock 
isobars in the target are not a function of gravity and will remain constant. 
They are a function of impact velocity, pulse length and target 
characteristics. The reduced gravity environment afforded in near-earth orbit 
provides an opportunity to consider the questions of ejecta source and shock 
state and its final distribution under varying gravitational accelerations. 
These questions are highly germane to problems such as the physical and 
thermal state of ejecta blankets and regolith development on both planetary 
and smaller bodies. These relate directly to questions in lunar sample and 
meteorite analyses and the interpretation of remotely-sensed spectral and 
geochemical data. 
The microgravity environment also provides a new and potentially rewarding 
area of impact experimentation not previously possible. 
free-floating targets, it may be possible to explore in detail phenomena 
associated with the collision of bodies. Such experiments can address 
questions regarding early and late accretional processes, catastrophic 
disruption and asteroidal evolution, as well as the effects of large impacts 
on the momentum and spin of the target bodies. 
considerable topical interest with respect to the hypothesized origin of the 
moon by a Kars-sized impact on the early Earth. 
Through the use of 
The last question is of 
REFERENCES: Basaltic Volcanism Study Project, chap. 8, Pergamon, 1981. 
K.J. Cintala and R . A . F .  Grieve, Lunar Planet. Sci. X V ,  156-157, 1984. 
D.E. Gault and J . A .  Wedekind, Impact and Explosion Cratering, 1231-1244, 
Pergamon, 1977. R . H .  Schmidt, Proc. Lunar Planet. Sci. Conf. 11th. 1099-2128, 
1980. P.K. Schultz and D.E. Gault, Lunar Planet. Sci. XV1, 742-743, 1985. 
D. Stoffler et al., JGR, 80, 4062-4077, 1975. 
A-41 


Impact experimentation and the microgravity environment: An overview

Impact is an ubiquitous physical process in the solar system. It occurs on all solid bodies and operates over a spectrum of scales, influencing geologic processes ranging from accretion, the early evolution of planetary bodies, the petrogenetic and spatial relations of lunar samples, the surface characteristics and interpretation of spectral data of asteroidal bodies, to the nature of some meteorites. Understanding impact phenomena is therefore paramount in constraining and underpinning a large number of research efforts into fundamental planetary geology. Gravity is an important parameter in impact processes. The physical environment offered by the Space Station represents an unique opportunity to extend the experimental aspect of impact studies into the microgravity (less than 1 g) regime. Through the use of free floating targets, it may be possible to explore in detail phenomena associated with the collision of bodies. Such experiments can address questions regarding early and late accretional processes, catastrophic disruption and asteroidal evolution, as well as the effects of large impacts on the momentum and spin of the target bodies. The last question is of considerable topical interest with respect to the hypothesized origin of the moon by a Mars-sized impact on Earth

Grieve, R. A. F.

IMPACT EXPERIMENTATION AND THE MICROGRAVITY ENVIRONMENT: AN OVERVIEW
Richard A.F. Grieve, Earth Physics Branch, EMR, Ottawa, Canada KIA OY3 for the
Microgravity Cratering Working Group.
Impact is an ubiquitous physical process in the solar system. It occurs
on all solid bodies and operates over a spectrum of scales, influencing
geologic processes ranging from accretion, the early evolution of planetary
bodies, the petrogenetic and spatial relations oflunar samples, the surface
characteristics and interpretation of spectral data of asteroidal bodies, to
the nature of some meteorites. Understanding impact phenomena is therefore
paramount in constraining and underpinning a large number of research efforts
into fundamental problems in planetary geology. Gravity is an important
parameter in impact processes. For example, in cratering it affects the size
of crater excavation, the post-excavation modification of the cavity by
gravitational collapse, the spatial distribition of ejected materials, and the
effectiveness of this ejecta in producing secondary cratering events. With
few exceptions (Gault and Wedekind, 1977) previous experimental studies of
cratering processes have been undertaken at gravitational accelerations of Ig
or higher. These are not the gravity conditions occurring on most solid
bodies in the solar system. The physical environment offered ultimately by
Space Station represents an unique opportunity to extend the experimental
aspect of impact studies into the microgravity (<ig) regime.
Previous and current experimental studies of impact phenomena address a
variety of problems. The bulk of impact experimentation, however, has been
concerned with crater growth and scaling. Experimental data have established
that at impact energies above -1018-1019 ergs (equivalent to the impact
of an iron meteorite in the meter-size range impacting at 20 km s-I in a Ig
environment), crater excavation occurs in the so-called "gravity regime",
where target strength effects are unimportant (Schmidt, 1980). This condition
is simulated experimentally by using low strength materials, such as sand or
water, and by the use of elevated gravitational accelerations. The effect of
elevated gravity is to displace the onset of the gravity regime to lower
energies. Such experimentation has led to the development of scaling
relations, where cratering efficiency is related to a dimensionless parameter
which includes the effects of projectile velocity and size, and gravitational
acceleration.
It has been sugggested recently that additional parameters such as the
shape of the experimental projectile (Schultz and Gault, 1985) and variable
energy losses due to waste heat in the target (Cintala and Grieve, 1984) are
not fully accounted for in the current dimensionless parameters. Thus it is
important to continue work in this area. The opportunity to conduct new
experiments at gravities directly applicable to that of planetary bodies will
contribute to determining and refining the relevant scaling relations for
large craters. Apart from their importance in problems concerned with
cratering mechanics, such relations are required to correctly relate crater
densities on different solar system bodies to absolute surface ages (Basaltic
Volcanism Study Project, 1981).
6-26
https://ntrs.nasa.gov/search.jsp?R=19860017679 2020-03-20T14:26:47+00:00Z
A further advantage of the microgravity environment is that, for a given
impact event, reduced gravity increases the crater growth time. It will be
possible, therefore, through high-speed photography to observe the crater
growth and ejecta dynamics in considerably more detail than in previous ig
experiments. This will lead to a better understanding of the relative
importance of rebound and collapse phenomena in crater formation and the
nature of the ejecta plume as an erosional and depositional agent.
In the low strength materials used in impact experiments under terrestrial
conditions, gravitational forces dominate other bonding forces, such as
surface tension, electrostatic effects etc. This may not be the case under
highly reduced gravity conditions, which prevail on small asteroidal bodies.
Even if current dimensionless scaling relations are shown to be substantially
correct for large planetary craters, they can not be applied to the energy
regime associated with small cratering events. Cratering experiments at
highly reduced gravities, corresponding to asteroidal bodies, will therefore
provide basic and currently unavailable information on cratering and regolith
development or lack of it on these bodies.
Previous experimentation provides little or no information on the spatial
distribution, source region and physical state of ejecta under differenct
gravity conditions. The few experiments designed to address these fundamental
questions all have been undertaken at ig (Stoffler et al., 1975). Similarly,
the only direct observational data on these questions are from terrestrial
craters. It is well-established that for a specific impactor size and
velocity and target materials, crater size will increase with decreasing
gravity. However, peak shock pressures and the spatial distribution of shock
isobars in the target are not a function of gravity and will remain constant.
They are a function of impact velocity, pulse length and target
characteristics. The reduced gravity environment afforded in near-earth orbit
provides an opportunity to consider the questions of ejecta source and shock
state and its final distribution under varying gravitational accelerations.
These questions are highly germane to problems such as the physical and
thermal state of ejecta blankets and regolith development on both planetary
and smaller bodies. These relate directly to questions in lunar sample and
meteorite analyses and the interpretation of remotely-sensed spectral and
geochemical data.
The microgravity environment also provides a new and potentially rewarding
area of impact experimentation not previously possible. Through the use of
free-floating targets, it may be possible to explore in detail phenomena
associated with the collision of bodies. Such experiments can address
questions regarding early and late accretional processes, catastrophic
disruption and asteroidal evolution, as well as the effects of large impacts
on the momentum and spin of the target bodies. The last question is of
considerable topical interest with respect to the hypothesized origin of the
moon by a Mars-sized impact on the early Earth.
REFERENCES: Basaltic Volcanism Study Project, chap. 8, Pergamon, 1981.
H.J. Cintala and R.A.F. Grieve, Lunar Planet. Sci. XV, 156-157, 1984.
D.E. Gault and J.A. Wedekind, i__qnImpact and Explosion Cratering, 1231-1244,
Pergamon, 1977. R.M. Schmidt, Proc. Lunar Planet. Sci. Conf. llth, 1099-2128,
1980. P.M. Schultz and D.E. Gault, Lunar Planet. Sci. XVI, 742-743, 1985.
D. Stoffler e__tal_____.,JGR, 80, 4062-4077, 1975.
6-27


Impact experimentation and the microgravity environment: An overview

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