JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, E05005, doi:10.1029/2004JE002250, 2004
Effect of planetary rotation on distal tektite deposition on Mars
Kelly E. Wrobel and Peter H. Schultz
Department of Geological Sciences, Brown University, Providence, Rhode Island, USA
Received 16 February 2004; revised 31 March 2004; accepted 12 April 2004; published 25 May 2004.
[1] The rapid rotation of Mars creates a significant pseudoforce, known as the Coriolis
force, that greatly modifies the flight paths and subsequent deposition of distal ejecta
(ejecta with launch velocities of 3 km/s and greater). An accurate depiction of the effects
of the Coriolis force requires the integration of the Coriolis terms directly into a series of
spherical ballistic equations applied within a rotating reference frame. The resulting
landing positions of radially ejected particles tend to form distinct wrapping patterns that
are focused to specific areas in the hemisphere opposite from the source crater: around the
pole for high-latitude craters (45 in latitude and above) and in an equatorial band for
low-latitude craters (below 45 latitude). Consequently, particular locations on the surface
receive deposits in stages: direct delivery of ejecta followed by deposits of higher-velocity
ejecta. This staged deposition leads to regions of enhanced ejecta accumulations
where meters of material collect (millimeters would be predicted in nonrotational radial
decay models). Thus high-latitude craters could supply a considerable amount of distal
products (tektites and melts) to polar locations, perhaps contributing to the dark
INDEX TERMS: 5420 Planetology: Solid Surface Planets: Impact phenomena
circumpolar materials.
(includes cratering); 6225 Planetology: Solar System Objects: Mars; 6240 Planetology: Solar System Objects:
Meteorites and tektites; KEYWORDS: Coriolis force, impact ejecta, Mars, tektites
Citation: Wrobel, K. E., and P. H. Schultz (2004), Effect of planetary rotation on distal tektite deposition on Mars, J. Geophys. Res.,
109, E05005, doi:10.1029/2004JE002250.
1. Introduction
[2] The surface of Mars preserves a history of impact
cratering dating from about 4 Ga to the present [Tanaka,
1986]. The lunar impact record is much better preserved due
to the absence of an atmosphere. However, high-velocity
ejecta are lost from the Moon due to its low escape velocity
(2.38 km/s). Such ejecta on Mars are retained because of its
higher escape velocity (5.02 km/s) and its tenuous atmosphere, which serves to decelerate and capture fine distal
ejecta.
[3] Gravity and the presence of an atmosphere are not the
only important differences for cratering between the Moon
and Mars. The rapid rotation of Mars creates a significant
pseudoforce, known as the Coriolis force, that can greatly
modify the ballistic trajectories of ejecta in flight. This force
is most influential on the flight paths of ejecta with launch
velocities of 3 km/s and greater (distal ejecta). Similar
effects of planetary rotation on ejecta trajectories occur on
the Earth [see Wrobel and Schultz, 2003b]. Rotational
effects on the Earth, however, are more complex due to
factors such as the larger planetary radius, greater interactions of ejecta with impact-induced vapor, and the much
thicker atmosphere.
[4] The following contribution addresses the effect of the
Coriolis force on delivered distal ejecta on Mars. We first
review previous approaches to the problem. We then deCopyright 2004 by the American Geophysical Union.
0148-0227/04/2004JE002250$09.00
scribe a more complete approach including the Coriolis
terms in calculations of the flight paths of particles for
various ejecta velocities up to the Martian escape velocity.
Lastly, the Coriolis effect is incorporated into first-order
models of impact ejecta velocity/mass relationships.
2. Background
[5] One strategy of incorporating the effects of planetary
rotation on the ballistic flight of ejecta involves simply
shifting the ejecta arrival sites on the surface longitudinally
according to both the rotation rate of the body and the time
of flight of the particle. Such an approach, however, does
not fully account for the effects of the Coriolis force and
neglects the conservation of the angular momentum of the
ejecta.
[6] A rotating planet can be thought of in terms of two
different reference frames, a fixed inertial frame at the
center of the body and a moving reference frame near the
surface. Forces exerted on an object in !
flight can be
expressed in terms of a total effective force, F eff , measured
in the moving system:
!
!
F eff ¼ S þ m!
g 2m !
w !
vr :
ð1Þ
!
S represents the vector sum of external forces (e.g.,
impulse, electromagnetic force, and friction); m is the mass
of the particle in flight; !
g denotes the gravitational field
vector (allows for the consideration of the variation of
gravity with altitude as well as centrifugal force effects); !
w
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WROBEL AND SCHULTZ: DISTAL TEKTITE DEPOSITION ON MARS
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is the angular velocity of the planet; and !
vr is the vector
velocity of the particle (relative to the rotating axes)
[Marion and Thornton, 1995].
[7] The third term in equation (1) corresponds to the
Coriolis force. Outside of a rotating reference frame, the
Coriolis force is observed as an apparent deflection of
the motion of a particle that is in free flight within the
rotating coordinate system. The angular velocity vector, !
w,
is directed toward the north pole of a planet, producing a
vertical component, wz, that is directed upward in the
northern hemisphere and downward in the southern hemisphere. Consequently, a particle in flight experiences a
horizontal component of the Coriolis force with magnitude
2mwzvh, where vh is the horizontal velocity component of
the particle in the rotating frame. This force component
deflects the particle’s flight path to the right in the northern
hemisphere and to the left in the southern hemisphere
[Marion and Thornton, 1995].
[8] Inclusion of the Coriolis terms directly in a series of
detailed ballistic equations allows mapping the full consequences of planetary rotation on the trajectories and subsequent deposition of impact ejecta. The model here uses an
integrated approach by calculating the force as it is exerted
on an ejecta particle at every stage in its flight. Previous
studies demonstrated the Coriolis effect on ejecta deposition
with application to Mars [Lorenz, 2000; Wilbur and Schultz,
2002; Wrobel and Schultz, 2003b], Earth [Alvarez et
al., 1995; Kring and Durda, 2002; Wrobel and Schultz,
2003a], Mercury [Dobrovolskis, 1981], and Jupiter [Pankine
and Ingersoll, 1999]. The present study describes the complete rotational effects on distal ejecta deposition on Mars.
3. Approach: Model
[9] The velocity, !
v , of a particle launched from a site at
an angle q to the vertical and in a direction of azimuth f
(measured eastward with respect to local north) can be
resolved into eastward, northward, and vertical components:
vx ¼ j!
v j sin q sin f;
v y ¼ j!
v j sin q cos f;
ð2Þ
v z ¼ j!
v j cos q;
respectively, relative to a fixed frame of reference in the
rotating planetary body [Dobrovolskis, 1981].
[10] It is simpler to discuss the spherical ballistics of
particles in flight above a rotating planet in terms of an
inertial reference frame attached to the planet. Transforming
a particle velocity into this new frame of reference requires
the addition of the term
b ¼ j!
w jR cos y
ð3Þ
to the eastward component of the velocity, vx. This term, b,
defines the inertial speed of the surface at the point of
impact. R denotes the radius of the planetary body and y is
the latitude of the launch point of the particle, the location
of the source crater. The magnitude of the new speed of the
particle, jv!
o j, relative to the rotating system, is
ffi
! qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
vo ¼ !
v b sin q sin f:
v þ b2 þ 2!
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The new launch angle, qo, and azimuth, fo, can be
calculated from
!
v
cos qo ¼ ! cos q;
vo
tan q
cos f;
cos fo ¼
tan qo
ð5Þ
respectively [Dobrovolskis, 1981].
[11] The flight of a particle in a rotating system can be
addressed in exactly the same manner as would be an object
in a ballistic path above a nonrotating sphere. Through a
series of complete ballistic equations, the specific angular
momentum of the particle, the distance traveled by that
particle, and the time of flight can be calculated. From this
information, the latitude and longitude of the final impact
can be found (incorporating rotation). The reader is directed
to the work of Dobrovolskis [1981] for a detailed description of the ballistic equations.
[12] Once terminal positions on the surface are known,
ejecta-scaling models from O’Keefe and Ahrens [1977] and
Housen et al. [1983] can be used to map the potential
thicknesses of deposits from individual craters as well as
cumulative deposits from a series of craters. For the moment,
the ultimate fate of the ejecta is ignored. The size of most
particles launched at velocities that are affected greatest by
the rotation of Mars (3 – 5 km/s) will be rather small, regardless of the size of the source crater, since the peak pressures
exerted on these particles will be large. This assumption is
consistent with particle size estimates for distal deposits from
major terrestrial craters (e.g., Chicxulub glasses and tektites
distributed across the Texas Gulf Coast). As a result of such
small particle sizes, along with the thin Martian atmosphere
(compared to Earth), the effects of phenomena such as
ablation, dynamic disruption, etc. are expected to be
minimal (100% (30%) survival rate for 500-mm-diameter
(1000-mm-diameter) particles) [Flynn and McKay, 1990;
Lorenz, 2000]. Thus only the total mass of ejecta projected
to arrive at a given area of the surface is examined.
[13] Gravity-scaling relations give approximations for
ejecta volume at different ranges from the crater center.
Mass balance between crater excavation and ejecta require
the following [Schultz et al., 1981]:
Vc ðve Þ
¼
VE
xe
RA
3
;
ð6Þ
where Vc(ve) is the total volume of ejecta with initial launch
velocity ve; VE denotes the total ejected volume or total
expected volume of ejecta (VE ffi 1/2VT [e.g., Stöffler et al.,
1975], where VT is the total displaced volume of the crater);
RA is the apparent radius of the crater, RA ffi RF/1.56
[Schultz, 1988; Melosh, 1989], where RF is the final radius;
and xe symbolizes the ejection position of each particle. The
value of VT is estimated from gravity-scaling relations using
the p2pv formulas for sand [Schultz and Gault, 1985].
[14] The ejection position of a particle, xe, measured
radially from the crater center, is obtained from assumptions
of gravity-controlled crater growth following Housen et al.
[1983]:
ð4Þ
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ve ffi 0:47
pffiffiffiffiffiffiffiffiffiffiffiffi xe 2:4
ðgRA Þ
;
RA
ð7Þ
WROBEL AND SCHULTZ: DISTAL TEKTITE DEPOSITION ON MARS
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Figure 1. Orthographic projections of Mars centered at (a) (0, 30E) and (b) the south pole (center
meridian through pole at 30E/150W). Black diamonds mark the terminal positions of ejecta launched
radially symmetric (ejection angles of 45) from the Lyot impact site (50N, 30E) at azimuth intervals of
15. Rotational effects are evident in the wrapping of ejecta arrival sites on the surface in the southern
hemisphere. Stars in Figure 1b highlight a few examples of terminal positions of ejecta with initial launch
velocities of 4 km/s.
where ve is ejection velocity and g is the surface gravity
(3.71 m/s2 on Mars). The resulting ratio xe/RF ranges from
1.0 (at the crater rim) to 0.0 (at the crater center). Specific
values of xe/RF will depend on ejecta launch velocities
(equation (7)).
[15] Combining equations (6) and (7) thus gives
Vc ðve Þ
¼
VE
1:25
ve
R0:625
:
pffiffiffi
A
0:47 g
ð8Þ
Assuming ejection velocities ranging from 0.25 km/s to
5.02 km/s (escape velocity) in increments of 0.01 km/s,
deposit thickness (volume/area) values were obtained for
ejecta arrival sites using equation (8) by averaging the
volume of ejecta emplaced at a site over a 1 1
spherical polygon centered on the location of deposition.
4. Results
[16] The Coriolis effect was applied to several different
cases. First, the landing positions of ejecta launched with
45 ejection angles from different locations on Mars were
mapped in order to assess the overall effect on distal ejecta
deposition. Second, the consequences of planetary rotation
on particles in flight with much lower (oblique impacts) and
higher (plume-driven ejecta) ejection angles were evaluated.
Third, the potential rotational effects on ejecta distributions
from a selected crater (Lyot) were considered.
4.1. Effects of Latitude
[17] The Coriolis force is strongest at the poles of a
rotating body and zero in magnitude at the equator [Marion
and Thornton, 1995]. As a result, ejecta launched northward, in the northern hemisphere, are deflected eastward
from the path of a great circle. Ejecta launched eastward are
more gently diverted southward. Particles launched westward experience the opposite deflection. In the southern
hemisphere, all deflections mirror those from the northern
hemisphere.
[18] The Coriolis force thus redirects (from the path of a
great circle) trajectories of ejecta particles. The symbols in
Figures 1 – 3 display final arrival locations of ejecta. The
actual flight paths of these particles are not indicated, just
the location of origination, the source crater, and the final
arrival sites. A direct consequence of the Coriolis deflection
can be observed in these figures: terminal positions of
distal ejecta originating from the same location, but from
different directions, can ultimately overlap on the surface
[Dobrovolskis, 1981].
[19] Figures 1a and 1b are projections of Mars showing
arrival sites of ejecta from the large Hesperian-aged Martian
crater Lyot (220 km in diameter at 50N, 30E). All ejecta
arrival sites mapped in these figures (likewise for Figures 2
and 3) are for particles that were launched with increasing
velocities (0.01 km/s increments) at ejection angles of 45
in azimuthal intervals of 15. Only a fraction of all possible
landing sites for ejected particles is mapped in these figures.
All other potential deposition locations can be interpolated
between these mapped sites.
[20] The Coriolis force does not substantially alter ejecta
arrivals in the northern hemisphere near Lyot (Figure 1a).
Hence ejecta-thickness decay relations for proximal deposits
should not be significantly affected (for gravity-controlled
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Figure 2. Orthographic projections of Mars centered at (a) (0, 82W) and (b) the north pole (center
meridian through pole at 82W/98E). Black diamonds indicate the landing positions of ejecta launched
radially symmetric (ejection angles of 45) from the Lowell crater (53S, 82W) at azimuth intervals of
15. Rotational effects are manifest in the wrapping of the ejecta landing sites in the northern hemisphere.
Stars in Figure 2b denote examples of arrival sites of ejecta with initial launch velocities of 4 km/s.
excavation). The trajectories of arrival sites (Figure 1b) of
more distal (high-velocity) ejecta, however, wrap in a
counter-clockwise direction around the south pole due to
the cumulative effects of the Coriolis force acting throughout
the long ballistic flight time.
[21] For comparison, Figures 2a and 2b reveal the effects
of planetary rotation on ejecta from the 190-km-diameter
crater Lowell, a high-latitude impact located in the southern
hemisphere of Mars (53S, 82W). Just as for Lyot, the
ejecta arrival sites wrap around the pole opposite from
impact, in this case clockwise around the north pole.
[22] The Bakhuysen crater (180 km in diameter), a lowlatitude southern hemisphere crater (23S, 16E), is shown
in Figure 3, accompanied by mapped ejecta arrival sites.
The ejecta deposition locations again wrap in the hemisphere opposite from impact (clockwise in the northern
hemisphere). This wrapping now occurs, however, as a
band just on the opposite side of the equator (rather than
around the pole) from the source crater.
[23] An impact directly at a pole of Mars will exhibit
similar ejecta distribution patterns as the high-latitude
craters with ejecta arrival sites wrapping around the pole
in the opposite hemisphere. Impacts very near the equator
produce somewhat different results. There, ejecta will be
deposited symmetrically with respect to the equator
[Dobrovolskis, 1981]. Those particles launched toward
the northern (southern) hemisphere will wrap clockwise
(counter-clockwise) and form a band of arrival sites
similar to that formed from Bakhuysen, except smaller
and just north (south) of the equator.
[24] An intuitive (but incomplete) explanation of these
results can be made. Particles launched at 45 from a
Figure 3. Orthographic projection of Mars centered at (0,
16E). The black diamonds indicate the terminal positions
of ejecta launched radially symmetric (ejection angles of
45) from the Bakhuysen crater (23S, 16E) at azimuth
intervals of 15. Rotational effects are observable in the
equatorial wrapping of ejecta arrival sites on the surface in
the northern hemisphere. Stars indicate examples of landing
sites of ejecta that were initially launched at 4 km/s.
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WROBEL AND SCHULTZ: DISTAL TEKTITE DEPOSITION ON MARS
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Figure 4. Orthographic projections of Mars centered at (a) (0, 36W) and (b) (0, 144E). Terminal
positions of ejecta launched 325 east of north from the Hale impact site (36S, 36W) are represented by
black diamonds. Ejection angles corresponding to each trajectory of ejecta landing sites are indicated.
A solid black line depicts a great circle through Hale in the direction of 325 east of north. As ejection
angles increase, rotational effects become more significant.
spherical body with increasing velocities achieve greater
distances to apoapsis without significantly increasing the
ballistic range on the surface (from launch to redeposition).
Consequently, particles with high ejection velocities remain
in flight for progressively longer times while the planet
rotates below. Along with this rotational effect, the calculations here also incorporate angular momentum terms and
the decrease in gravitational forces with distance from the
surface.
[25] As a result of the vastly different travel times among
the highest-velocity ejecta, the intersections of particle
trajectories with the surface (the ejecta arrival sites) eventually spread out in bands that center around the latitude
opposite from the source crater. Such particular ejecta
deposition patterns thus actually constrain the location of
the source crater.
[26] To summarize, the effects of the rotation of Mars on
ejecta emplacement from symmetrical excavation are latitudinally dependent. The landing sites of distal ejecta wrap
in the hemisphere opposite from the source crater: poleward
for high-latitude craters (45 latitude and above) and equatorially for low-latitude craters (below 45 latitude). Consequently, large impacts (craters >100 km) at high latitudes
could contribute a significant amount of distal ejecta (ve 3 km/s) to near-polar regions.
4.2. Effects of Ejection Angle
[27] Analytical approximations for crater excavation
(equations (6)– (8)) provide a useful, first-order assessment
of the delivery of distal ejecta across Mars. Computational
models would be required to completely address melt
fractions and additional complexities created at early times
in the cratering process. Nevertheless, the possible effects of
such complexities can be generally assessed by examining
two distinct ejecta components: low-angle, downrange trajectories (during oblique impacts) [see Schultz and Gault,
1990; Schultz, 1996] and high-angle trajectories (vaporentrained particles) [see Jones and Kodis, 1982; Schultz,
1996]. The effects of the Coriolis force on these various
ejecta components are radically different.
[28] Figures 4a and 4b illustrate arrival sites for ejecta
launched over a range of different angles from the crater
Hale (136-km-diameter crater at 36S, 36W). All ejecta
were launched downrange 325 east of north, for this
appears to be the most probable direction of the path of
the projectile that impacted the surface to form Hale
[Schultz and Mustard, 2004].
[29] The Coriolis force does not dramatically alter the
arrival positions for ejecta with low ejection velocities
(Figure 4a) or high-velocity ejecta with low ejection angles
(e.g., 5 in Figure 4b) (trajectories of terminal ejecta
positions are not significantly different from the path of a
great circle). The minimal Coriolis deflection occurring
under this latter set of conditions reflects the fact that ejecta
launched at low angles require higher velocities to reach a
particular location on the surface, thereby minimizing the
time these particles are in flight above the rotating planet.
As ejection angles increase, the Coriolis term becomes more
significant (Figure 4b).
[30] The initial stages of a hypervelocity impact may
generate significant vapor, whether due to high impact
velocity or to volatile components in the substrate [e.g.,
Schultz, 1996; Stewart et al., 2000]. While a forward model
is required to incorporate complete details, the effects of
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Figure 5. Orthographic projection of Mars centered at
(45N, 15E). Landing sites for ejecta launched radially
symmetric at 80 (from the horizontal) from the Lyot impact
site (50N, 30E) are shown (black diamonds). Effects of
rotation on high-angle ejecta trajectories are evident in the
sharp wrapping of terminal positions just east of the crater.
Stars denote a few examples of arrival sites of ejecta with
initial launch velocities of 4 km/s.
vapor expansion on early-time ejecta can be revealed clearly
by simply assessing the effects of high ejection angles, as
observed in experiments [Schultz, 1996] and theory [Jones
and Kodis, 1982]. Arrival sites of ejecta launched at an
angle of 80 from the Lyot crater are illustrated in Figure 5.
Deposition is severely limited east of the crater but greatly
enhanced west of the crater (as well as immediately north,
south, and east) as a direct expression of the longer
planetary rotation time beneath particles whose flight trajectories obtain greater distances to apoapsis. An application
of the O’Keefe and Ahrens [1977] model provides estimates
of up to meters of vapor-entrained particles being deposited
west of the Lyot crater.
4.3. Ejecta Deposit Thicknesses
[31] The amount of distal ejecta that might be concentrated
near the pole opposite from the crater Lyot could be substantial. In the following estimates, the incremental volume of
deposits as a function of ejection velocity is estimated on the
basis of gravity-controlled flow (equations (6) –(8)) modified
by the Coriolis effect (equations (1) – (5)). Figure 6 is a
map of distal ejecta thicknesses from Lyot based on the
arrival sites mapped in Figure 1 and the mass balance
equation (equation (6)). Due to the wrapping of the ejecta
arrival sites, particular locations on the surface receive
ejecta in stages. The first stage, representing direct delivery, is followed by a second deposit that is composed of
high-velocity components that travel large distances to
apoapsis. This results in a band of enhanced deposition
(5 to 75S, 75 to 150E) from 30 cm to 5 m thick
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(no bulking or ablation losses assumed), as revealed in
Figure 6.
[32] The study of Lorenz [2000] addressed the likelihood
of the delivery of microtektites to polar locations on Mars
from the crater Lyot and estimated that the south pole could
receive millimeter-thick deposits. These approximations,
however, appear to be significantly underestimated, perhaps
due to an incomplete account of rotational effects in the
model. Differences in the thickness equations used by
Lorenz (corresponding to equations (6) and (7) here) also
could contribute to volume estimates up to 10 times smaller
than those presented here. Such inconsistencies, however,
cannot solely account for the three-orders-of-magnitude
difference in the estimated deposit thicknesses for the south
polar region, i.e., millimeters (Lorenz) vs. meters (this
study). Regardless, the study by Lorenz does conclude that
distal ejecta originating from Lyot could be preserved in
layered terrains near the south pole. This conclusion is
reinforced by the results of our study.
[33] Another significant result illustrated in Figure 6 is the
absence of ejecta deposits projected for the area that is
antipodal to the Lyot crater. In a nonrotational model, all
directions of launch supply ejecta to the position antipodal
to Lyot, 50S, 150W. Consequently, a multitude of depositional events occurs over time exactly 180 from the
source crater. This results in an enhanced accumulation of
deposits at the antipode, as is predicted in standard radial
decay models (Figure 7) [Moore et al., 1974].
[34] Incorporating rotational effects, however, reduces the
antipodal concentrations since the trajectories of ejecta in
flight within a rotating system do not follow the path of a
great circle. For some ejection azimuths, the antipode is
never reached since the particle velocities exceed the escape
velocity of Mars before reaching this distance. Thus antipodal deposits are considerably thinner than predicted in
nonrotational models. Specifically, the gravity-scaled excavation model of Housen et al. [1983] would predict an
accumulation of 40 cm antipodal to Lyot for a nonrotating
body (depicted by the dashed line in Figure 7). Incorporating rotational effects into this model results in predictions of
only 1 cm of antipodal ejecta deposition.
5. Implications and Conclusions
[35] By simple definition, the Coriolis force is primarily
just the apparent deflection of the path of a particle in flight
above a rotating body. Although adjusting the landing
positions of ejecta longitudinally by the amount wT, where
T is the time of flight of the particle, may appear sufficient
for incorporating this force, such a method will completely
account for the Coriolis effects only if the calculated
momentum of the ejecta also includes the eastward boost
imparted on them due to the planetary rotation. A particle in
flight will experience a pseudoforce, such as the Coriolis
force, at different magnitudes, from varying directions,
throughout its trajectory. Therefore an accurate depiction
of the effects of the Coriolis force requires an integrated
approach.
[36] Due to the effects of planetary rotation, the deposition of distal ejecta is concentrated at specific locations in
the hemisphere opposite from the source crater: around the
pole for high-latitude craters and in an equatorial band for
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WROBEL AND SCHULTZ: DISTAL TEKTITE DEPOSITION ON MARS
Figure 6. Map of deposit thicknesses for ejecta from the Lyot crater based on both the terminal
positions of ejecta, as mapped in Figure 1, and the mass balance equation (equation (6)). Terminal
positions used were limited to those corresponding to particles with ejection velocities >2 km/s. This was
done to observe the rotational effects on the deposition of distal ejecta and to assure that this image would
not be saturated by kilometer-thick proximal deposits. Purple indicates deposits 500 cm thick or greater,
and black denotes a complete lack of deposits (0 cm). The antipode of Lyot is located at 50S, 150W.
Figure 7. This graph compares the Housen et al. [1983] gravity-scaled excavation model applied to a
nonrotational body to the rotational model used in this study. The dashed line (nonrotational model)
depicts a general decay of deposit thickness with increasing range from the source crater (Lyot)
accompanied by an antipodal buildup of deposits. The green and red lines (rotational model) delineate
predicted ejecta thicknesses for given ranges from Lyot for two particular azimuth directions (red, 225
east of north; green, 135 east of north), illustrating contrasting effects of the Coriolis force for different
ejection directions. A symmetric distribution of ejecta does not occur in models incorporating planetary
rotation due to Coriolis deflections of ejecta flight paths. Rotational models, also, do not predict an
enhanced accumulation of deposits antipodal to Lyot.
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WROBEL AND SCHULTZ: DISTAL TEKTITE DEPOSITION ON MARS
low-latitude craters. In general, most distal deposits should
consist of primarily high-velocity (but late-arriving) ejecta
that should be rich in melt and/or glass, including tektites
and microtektites. For example, the Australasian tektite
strewn field on Earth is believed to have a source in
southeast Asia [e.g., Glass and Pizzuto, 1994], implying
the delivery of tektites to locations 1000 km to 5000 km
from the crater. If it is assumed that similar ejection
velocities reflect similar peak pressures, then tektites and
impact melts should comprise most of the ejecta launched
over the velocity range considered for Mars in the present
study (ve 3 km/s). Thus high-latitude craters the size of
Lyot (220 km in diameter and larger) could supply a
considerable amount (on the scale of meters) of melt and
glass products to the opposite pole, providing an alternative
hypothesis for the origin of the dark circumpolar materials
(as opposed to unweathered volcanics, pyroclastics, exposed bedrock, etc.) [Wrobel and Schultz, 2003b; Schultz
and Mustard, 2004]. More ancient polar deposits should
have accumulated increased contributions due to the higher
flux rate in the past. Exhumation and lag deposits from such
accumulations may have further contributed to the present
circumpolar erg fields. The dry, cold conditions of the
current polar regions would ensure the survival of such
deposits over hundreds of millions, if not billions, of years.
[ 37 ] Acknowledgments. The authors would like to thank
A. Dobrovolskis and an anonymous reviewer for their constructive comments. This research was supported by NASA grant NAG5-11538 and a
NASA Rhode Island Space Grant Scholarship.
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P. H. Schultz and K. E. Wrobel, Department of Geological Sciences,
Brown University, Box 1846, Providence, RI 02912, USA. (kelly_
[email protected])
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