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Mon. Not. R. Astron. Soc. 000, 000--000 (0000) Printed 13 June 2001 (MN L A T E X style file v1.3)
NEO velocity distributions and consequences for the
Chicxulub impactor
S.V. Jeffers 1;2 , S.P. Manley 1 , M.E. Bailey 1 and D.J. Asher 1
1 Armagh Observatory, College Hill, Armagh, BT61 9DG
2 School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS
last update 2001 May 3
ABSTRACT
An ¨
Opik based geometric algorithm is used to compute impact probabilities and veloc­
ity distributions for various near­Earth object (NEO) populations. The resulting crater
size distributions for the Earth and Moon are calculated by combining these distri­
butions with assumed NEO size distributions and a selection of crater scaling laws.
This crater probability distribution indicates that the largest craters on both the Earth
and the Moon are dominated by comets. However, from a calculation of the fractional
probabilities of iridium deposition, and the velocity distributions at impact of each
NEO population, the only realistic possibilities for the Chicxulub impactor are a short­
period comet (possibly inactive) or a near­Earth asteroid. For these classes of object,
sufficiently large impacts have mean intervals of 100 Myr and 300 Myr respectively,
slightly favouring the cometary hypothesis.
Key words: minor planets, asteroids -- comets: general -- cratering -- Earth -- Moon.
1 INTRODUCTION
The Earth's geological record contains important details of
the planet's history, including evidence of major impact
events and abrupt transitions of life associated with mass
extinctions. The most famous example is the Cretaceous­
Tertiary (K/T) boundary, associated with both an excep­
tionally large crater, approximately 180 km in diameter, and
an anomalously high abundance of iridium and other plat­
inum group elements (Alvarez et al. 1980). Iridium is not
found to any significant degree elsewhere in the Earth's
crust, owing to its siderophile nature, but it is assumed to
be a constituent of undifferentiated bodies such as asteroids
and comets. The presence of a large crater together with an
anomalous Ir layer at the K/T boundary provides evidence
linking impacts with mass extinction. At the same time, the
detection of Ir provides evidence of an accretion event.
Not all mass­extinction boundaries show strong Ir sig­
natures, even when associated with large craters. In this
respect, the K/T boundary is exceptional, and the lack of
significant levels of Ir at other geological boundaries reopens
the question whether mass extinctions are mainly caused by
impacts or by purely terrestrial processes involving, for ex­
ample, vulcanism, climate and sea­level changes.
A further important issue for the impact hypothesis is
whether the largest craters are dominated by comets or as­
teroids. However, the efficiency of Ir deposition is a strong
function of the velocity and mass of the impactor, both gen­
erally higher for comets than asteroids.
The main purpose of this paper is to determine the im­
pact probabilities and velocity distributions for various NEO
populations. Resulting from this are detailed crater size dis­
tributions and calculations of the fractional probabilities of
Ir deposition for each population.
2 IMPACT PROBABILITIES AND VELOCITY
DISTRIBUTIONS
To create an accurate crater probability distribution for the
Earth and the Moon, separate impact probabilities and ve­
locity distributions were used for each near­Earth object
(NEO) population. It is important to include these parame­
ters for each contributory population as they determine the
resulting crater diameter and the frequency of impact.
2.1 Collision code
The ¨
Opik (1951, 1976) based geometric algorithm of Man­
ley, Migliorini & Bailey (1998) was used to calculate the
collisional probability per unit time between objects moving
in arbitrary elliptical orbits. The algorithm is an adaptation
of the Wetherill (1967) and Greenberg (Bottke & Green­
berg 1993) methods and avoids a number of approximations
made by ¨
Opik. Objects are assumed to be spherical and

2 S.V. Jeffers et al.
small compared to the size of their orbits. Also included is
gravitational focusing for encounters with large bodies such
as planets.
The collision code is particularly suitable for determin­
ing impact probabilities for populations of bodies in which
the argument of perihelion ! and the longitude of the as­
cending
node\Omega take random, uniformly distributed values in
the range (0; 360 ffi ). The code explores all possible values of
! and
\Omega\Gamma taking as input the semi­major axis a, eccentricity
e and inclination i of the colliding objects.
Accounting for the Moon's orbit around the Earth intro­
duces a correction due to the gravitational field of the Earth
of approximately 1.4 km s \Gamma1 , and an additional velocity com­
ponent of approximately 1 km s \Gamma1 . This velocity component
is randomly distributed in the plane of the Earth's orbit,
ignoring the small inclination of the Moon's orbit to the
ecliptic.
As the Moon orbits the Earth at a distance of 0.0026 au,
it is necessary also to take account of the variation of the
Moon's heliocentric distance. This was modelled by increas­
ing the eccentricity of the target orbit (in this case the
Moon). This does not accurately take account of the true
geometry since the extrema in heliocentric distance only oc­
cur when the Moon has a particular configuration. However,
since the collision algorithm itself involves various approx­
imations, further minor improvements are not justified at
this time.
These factors lead to a small difference in the impact ve­
locity distributions between the Earth and the Moon beyond
that due to the difference in their respective gravitational at­
tractions. This is an important extension of the algorithm,
enabling it to be used also for impacts on satellites of other
bodies (e.g. the Martian, Jovian and Saturnian systems).
We have not implemented further possible refinements of
the model, for example considering the varying terrestrial
eccentricity over astronomical time­scales.
2.2 Velocity distributions on the Earth and the
Moon
The four component NEO populations considered are:
near­Earth asteroids (NEAs), short­period comets (SPCs),
Halley­type comets (HTCs) and long­period comets (LPCs).
In order to overcome the effects of observational bias
associated with NEAs (a few observed objects of low incli­
nation with extremely high collision probabilities at low ve­
locity), a synthetic distribution of orbital elements for 20000
asteroids was calculated. This used the probability distribu­
tion in a, e and i of Rabinowitz et al. (1994), and a Monte
Carlo (MC) numerical method.
LPCs, i.e. comets with periods P ? 200 yr, were mod­
elled using a MC simulation based on an assumed isotropic
parabolic source distribution. The same procedure was not
applied to the distributions of SPCs (with P!20 yr), and
HTCs (which we define as 20!P!200 yr; but see discus­
sion later), as they are not well modelled by an isotropic
source population. Instead, the populations of SPCs and
HTCs were derived from Shoemaker, Weissman & Shoe­
maker (1994) and Emel'yanenko & Bailey (1998).
The frequency distributions of impact velocities are
plotted for each component NEO population in Fig. 1, for
the Earth and Moon respectively. Each panel indicates the
total population of bodies estimated to exist (Pop); the
mean impact probability per object (MIP); the total impact
probability (TP) from the assumed population, i.e. Pop \Theta
MIP; and the mean impact velocity (MIV). The quantity
Pop depends on the assumed minimum size of object in the
population (see Section 3.2).
For NEAs, the mean impact probabilities with respect
to the Earth computed by Shoemaker, Wolfe & Shoemaker
(1990) and Steel (1998), 4.2\Theta10 \Gamma9 and 5.08\Theta10 \Gamma9 object \Gamma1
yr \Gamma1 respectively, are higher than our value (1.85\Theta10 \Gamma9 ).
This is because the previous calculations were for observed
asteroids, therefore biasing the results towards objects cur­
rently in orbits passing close to Earth.
The results for LPCs are in good agreement with
those of other authors such as Steel (1998) and Weissman
(1997), who respectively found values of 2.2\Theta10 \Gamma9 and 2.2--
2.5\Theta10 \Gamma9 per revolution. Results using observed LPCs are
slightly higher (3.5\Theta10 \Gamma9 ; Manley et al. 1998, Shoemaker
1984), because the observed LPC i­distribution is not quite
so isotropic, and there is a bias due to observed objects with
perihelion distances close to 1 au and a handful of objects
with inclinations close to zero or 180 ffi (Steel 1993).
The velocity distributions for the SPCs and HTCs ap­
pear somewhat fragmented, a result of the small number of
objects used in each case to generate the distributions. The
NEA and LPC populations display a smoother velocity dis­
tribution because they originate from much larger synthetic
populations.
The mean impact probabilities for the Moon are gener­
ally lower than those on the Earth. This is particularly no­
ticeable for populations with low encounter velocities, e.g.
NEAs, as the stronger gravitational potential of the Earth
is then more significant. In the case of the Moon, 10% of
NEAs impact at less than 10 km s \Gamma1 , compared to none for
the Earth, the differences in the shapes of the two distribu­
tions being clear. The sharp peak at the lower end of the
frequency distribution of SPC (and to a lesser extent HTC)
impactors, demonstrates the existence of comets with rela­
tively low impact velocities. Whereas the HTC velocity dis­
tribution is comparable for the Earth and the Moon, the
difference between prograde and retrograde orbits leads to
a bimodality of the two curves.
An important feature is the generally higher impact ve­
locity associated with the majority of bodies in cometary
orbits compared to those of asteroids. This leads to a differ­
ent degree of gravitational focusing, and also influences the
final crater diameter through the velocity dependence of the
crater scaling laws (Section 3.1).
3 PREDICTED CRATER SIZE DISTRIBUTION
The crater size distributions, resulting from each NEO pop­
ulation impacting against the Earth and the Moon, were
obtained by combining the impact probabilities and veloc­
ity distributions with a range of crater scaling laws and the
size distribution of the relevant NEO population.
3.1 Crater diameter scaling
A review of the literature indicates a proliferation of dif­
ferent crater scaling laws. Table 1 lists eight of these laws

NEO velocity distributions and the Chicxulub impactor 3
Figure 1. Velocity distributions for NEAs, SPCs, HTCs and LPCs impacting the Earth. The normalized probabilities (per unit velocity
interval) have a velocity resolution of 0.1 km s \Gamma1 . Pop denotes the total number of bodies estimated to exist in the population (for SPCs
and HTCs the first number denotes active comets and the second the number of inactive bodies), MIP is the mean impact probability
per object per year (per revolution for LPCs), TP is the total impact probability (yr \Gamma1 ) and MIV is the mean impact velocity.
which have been selected from a list of 17 (Jeffers 2000), and
converted into a general form of equation (1).
log D t = k + kv log v i + kd log d i + k ae i
log ae i + k ae t
log ae t
+ kg log g t + k` log cos ` (1)
Here D t denotes the resulting transient or initial crater di­
ameter (m), v i the impactor velocity (m s \Gamma1 ) and d i the im­
pactor diameter (m). The constant input parameters for this
analysis are as follows; ae i (the impactor density) is 2000 kg
m \Gamma3 and 1000 kg m \Gamma3 (Harmon et al. 1999), for asteroids
and comets respectively; for the Earth and the Moon re­
spectively ae t (the target density) is 2600 kg m \Gamma3 and 2400 kg
m \Gamma3 and g t (the surface gravity) is 9.81 m s \Gamma2 and 1.62 m
s \Gamma2 . In the simulation, the evaluation of ` (the angle of im­
pact, measured from the vertical) is based on there being an
equal flux from equal angular areas for an isotropic source.
All units are SI and logarithms are to base 10. The corre­

4 S.V. Jeffers et al.
Table 1. Parameters for various crater scaling laws. The parameters are as presented in equation (1).
Scaling Law Parameters
No Source k k v k d k ae i
k ae t
k g k `
1 Melosh 1989 \Gamma0.091 0.440 0.790 0.330 \Gamma0.330 0.0 0.0
2 Moore et al. 1980 \Gamma1.446 0.555 0.833 0.278 0.0 0.0 0.0
3 Schmidt & Housen 1988 0.047 0.430 0.780 0.330 \Gamma0.330 \Gamma0.220 0.0
4 Grieve & Shoemaker 1994 0.064 0.440 0.780 1=3 \Gamma1=3 \Gamma0.220 0.0
5 Shoemaker & Wolfe 1982 \Gamma1.733 0.588 0.882 0.588 \Gamma0.294 \Gamma1=6 0.0
6 Wetherill 1989 0.169 0.433 0.781 0.336 \Gamma0.336 \Gamma0.216 0.0
7 Zahnle et al. 1999 0.066 0.440 0.780 0.333 \Gamma0.333 \Gamma0.220 0.44
8 Dence et al. (D ! 2400m) 1977 \Gamma2.398 2=3 1.0 1=3 0.0 \Gamma0.188 0.0
Dence et al. (D ? 2400m) \Gamma1.694 0.588 0.882 0.294 0.0 \Gamma0.188 0.0
sponding constants, k, kv , kd , k ae i
, k ae t
, kg , k` are tabulated
in Table 1.
3.2 NEO size distribution: asteroids and comets
The NEA size distribution adopted in this analysis is that of
Rabinowitz et al. (1994) and Moore, Boyce & Hahn (1980)
for impactors ?10 m and !10 m in diameter respectively.
The size distribution of Rabinowitz et al. was chosen as
it also includes an orbital element probability distribution
from which it was possible to simulate the Monte Carlo dis­
tribution of NEAs used in the previous section. The size
distribution of Moore et al. was chosen because it is valid
for small sizes that are generally not included in other laws
primarily because an extrapolation may be misleading. In
agreement with Moore et al. (1980) the fireball data of Ce­
plecha (1988) would suggest that the power­law index of the
cumulative size distribution decreases for asteroids with di­
ameters !5 m. The size distributions of Rabinowitz et al.
(1994) and Moore et al. (1980) are tabulated below. The
uncertainty that applies to this size distribution is believed
to be less than a factor of 2 for 1 km asteroids.
N(? d) / d \Gammaff
8
? !
? :
ff = 2:34 0:001 ! d ! 0:010 km
ff = 3:5 0:010 ! d ! 0:070 km
ff = 2:0 0:07 ! d ! 3:5 km
ff = 5:4 3:5 km ! d
(2)
In the case of the Earth, NEAs down to d= 70 m are consid­
ered, a total of 2.75\Theta10 5 objects in the above distribution.
This is approximately the minimum size for an asteroid ca­
pable of reaching the Earth's surface. In practice, iron as­
teroids smaller than ¸70m may occasionally penetrate the
Earth's atmosphere and make a crater, but they are much
smaller in number than the more friable chondrites. NEAs
down to d = 1 m are included in the case of the Moon, hence
the larger total population `Pop' in Fig. 1.
For comets the size distribution of Donnison (1986,
1987), based on an analysis of cometary magnitudes, is fol­
lowed. It is assumed that a single power law can be used
to describe cometary observations. The results of Donnison
(1986, 1987) comprise two parts, one power law for LPCs
and another for SPCs, indicated below in equation (3). The
error that applies to these values is \Sigma 0.15 for LPCs and
\Sigma 0.42 for SPCs, indicating that they could also be consis­
tent both with each other and the value ff=2 (cf. asteroid
law in equation 2).
N(? d) / d \Gammaff
ae
ff = 2:13 LPC
ff = 2:07 SPC;HTC (3)
The flux of LPCs within 1 au has been calibrated by Man­
ley (2001), where following the work of Kres'ak (1978), an
orbital distribution for LPCs on Earth approaching orbits
was derived. A list was determined of 15 LPCs that had an
absolute visual magnitude H0 ! 7 (' 4 km) and were on an
Earth crossing orbit with impact parameter to the Earth less
than 0.2 au, over a period of 300 years. It is assumed that
such objects would not have been missed even using naked
eye observational techniques, and the derived LPC orbital
distribution then gave the total flux of Earth crossing LPCs
brighter than H0 = 7.
A lower limit of 0.4 km on the diameter was imposed on
the cometary populations (cf. Fern'andez et al. 1999). This
figure is in agreement with the observations of Lowry et
al. (1999) for the smallest possible comet nuclei, based on
observed lower limits of nuclear sizes for active comets. The
numbers at d = 1 km estimated by Shoemaker et al. (1994)
and Emel'yanenko & Bailey (1998) are extrapolated to this
assumed lower limit to give the overall population sizes. An
upper limit of 1000 km is enforced as this is the order of the
size of the largest objects in the Edgeworth­Kuiper Belt.
3.3 Frequency distribution of crater diameters
Using appropriate input parameters for any given crater di­
ameter scaling law, it was a simple Monte Carlo process to
generate predicted crater size distributions from any given
source population. The probability distributions for the indi­
vidual NEO populations were then summed, weighting each
population by the appropriate normalized impact probabil­
ity. This results in a total crater size distribution for each
crater diameter scaling law, for both the Earth and Moon.
In order to represent the frequency of asteroid impacts
of all sizes, the asteroid size distribution is divided into sepa­
rate diameter regimes: 1--10m (5.47\Theta10 10 asteroids), 10--70m
(2.5\Theta10 8 asteroids) and ?70m (2.75\Theta10 5 asteroids), the first
two ranges applying to the results for the Moon only. It can
be seen that the predicted distributions of crater diameters
for the Earth and the Moon, Fig. 2 parts (a) and (c) respec­

NEO velocity distributions and the Chicxulub impactor 5
Figure 2. Log--log cumulative plot of relative frequency versus crater diameter, for various crater scaling laws. (a) Earth. (b) Earth
excluding inactive comets. (c) Moon. (d) Moon excluding inactive comets. The crater scaling laws are in the following order, at relative
probability 10 \Gamma3 : Grieve & Shoemaker (1994), Wetherill (1989), Zahnle et al. (1999), Shoemaker & Wolfe (1982), Melosh (1989), Schmidt
& Housen (1988), Moore et al. (1980) and Dence et al. (1977).
tively, retain strong elements of the size distributions of the
underlying populations that generated them. The different
velocity distributions generally serve merely to spread the
distribution and dilute the dependence on impactor size.
The dependence on the size distribution is illustrated
by the presence of a knee in the distribution. This occurs
at probabilities of about 10 \Gamma3 to 10 \Gamma4 on the Earth and at
about 10 \Gamma8 on the Moon, where the number of asteroids with
sizes ?3.5 km begins to drop off rapidly. The precipitous
drop is halted when the curve catches up with the probabil­
ity distribution for comets. Here, there is a clear indication
of the effects of different populations of contributing bodies,
i.e. comets dominate the production rate for large craters on
both the Earth and the Moon.
The crater size distribution for the Earth starts with
larger craters, as a result of the decision to remove any ob­
ject with size below 70 metres. On the Moon, as the atmo­
sphere does not affect the incoming population, there is an
abundance of small craters. This is particularly important
for the calibration of these Monte Carlo results, as statis­
tics for small craters will be far superior to those of the
larger craters. In the size range where there are craters on
the Earth, the weaker gravitational focusing of the Moon
leads to differences in the cratering distributions between
the Earth and the Moon. Overall, at a given diameter the
cratering rate per unit area is typically ¸30% less on the
Moon.
Excluding the contribution of `dark' inactive comets

6 S.V. Jeffers et al.
proposed by Shoemaker et al. (1994), Emel'yanenko & Bai­
ley (1998), and Bailey & Emel'yanenko (1998), Figs 2, parts
(a) and (c) now have the appearance of parts (b) and (d),
for the Earth and Moon respectively. The resulting effect is
that the comets do not begin to make an appearance until
lower probabilities are reached.
3.4 Fractional probabilities of iridium deposition
The question of a cometary versus asteroidal nature for the
impactor that produced the famous Chicxulub structure has
often been debated. The geological evidence is not yet con­
clusive (Shukolyukov & Lugmair 1998; cf. Kyte 1998), and
the physical nature and composition of cometary nuclei is
another significant unknown. The calculations in this paper
nevertheless impose constraints on the nature of the im­
pactor. Fig. 2 shows it to be in the comet dominated size
range, and therefore --- considering the crater's size alone ---
it was probably generated by a comet. However, the asso­
ciated Ir signature provides an additional piece of evidence,
namely that the object should deposit sufficient mass to
match the observed iridium.
In fact, provided the body is in the multi­kilometre
size range, the fraction of the impactor's mass deposited
is a sharply decreasing function of the impactor velocity.
From Vickery & Melosh (1990), for silicate impactors with
V ¸ 20 km s \Gamma1 in the size range of relevance for the K/T Ir
layer, the impactor deposits ¸80% of its original mass. How­
ever, impactors with V ¸ 25 km s \Gamma1 deposit only 15--20%,
i.e. the fraction of the impactor's mass that is retained by
the Earth drops rapidly as the impact velocity rises through
the range 20 km s \Gamma1 to 25 km s \Gamma1 , and above 25 km s \Gamma1 essen­
tially none of the impactor mass is retained. This means that
the initial mass required to explain any given Ir anomaly in­
creases rapidly with increasing impact velocity above 20 km
s \Gamma1 .
Thus, we assume that impactors arriving at the Earth's
surface with speeds greater than 25 km s \Gamma1 will not cre­
ate a significant Ir anomaly, and that impactors arriving
at 25 km s \Gamma1 need to have a diameter 1.6 times greater
than those at 20 km s \Gamma1 . Assuming that an asteroid that
could generate the K/T Ir layer has a minimum diameter
d ' 6:6 km (Alvarez et al. 1980), one with an impact ve­
locity V ' 25 km s \Gamma1 would have d ' 10:5 km (and hence,
other things being equal, would make a much larger crater).
Similarly, the minimum size of a cometary impactor would
be 8.3 km for V ! 20 km s \Gamma1 , owing to the lower density of
cometary material (cf. Section 3.1). Indeed, one could argue
for an even greater increase in the diameter of the cometary
nucleus necessary to yield a given amount of Ir, owing to
comets containing a smaller proportion of chondritic ma­
terial (Sharpton & Mar'in 1997). However, this refinement
is omitted in Table 2 since the proportion in the various
populations of bodies is rather uncertain. We also note that
active comets may be relatively richer in ices, at the expense
of chondritic material, compared to extinct comets and as­
teroids.
The numbers of potential impactors at these various
sizes can be evaluated from the size distributions of each
population (Section 3.2). The fraction of asteroids and
comets with an impactor velocity (i) Ÿ 20 km s \Gamma1 and (ii)
20 ! V Ÿ 25 km s \Gamma1 can be found from the calculations of
Section 2.2, as can the impact probabilities. Thus a frac­
tional probability for the relevant impactor size and veloc­
ity ranges was calculated, equal to the inverse of the impact
interval. These probabilities for an impactor to leave suffi­
cient mass to account for the observed Ir layer at the K/T
boundary are given in Table 2.
The calculated results confirm that the population of
NEAs has a substantial probability of generating the irid­
ium anomaly as a result of their low velocities, as was found
by Vickery & Melosh (1990). However, Table 2 also shows
that this group is more than matched by inactive SPCs,
with a mean interval to generate an equivalent Ir signature
of approximately 100 Myr. The HTCs and LPCs have es­
sentially zero probability of depositing sufficient mass to
generate the K/T anomaly, and the velocity distributions
therefore strongly argue against the K/T impactor coming
directly from either of these populations.
4 DISCUSSION
Was the Chicxulub event caused by a low­velocity comet or a
rare, large asteroid? On the one hand, comets are the main
source of the largest terrestrial and lunar impact craters,
as they have generally larger sizes and higher impact ve­
locities than NEAs. However, their large impact velocities
( ? ¸ 25 km s \Gamma1 ) make them poor candidates for depositing
significant amounts of Ir. Therefore, whilst the lack of Ir (or
other siderophile elements) at geological boundaries does not
rule out impacts as the cause of extinctions, its presence at
the K/T boundary would seem to strengthen the asteroidal
hypothesis.
Moreover, Kyte (1998) has presented evidence for a fos­
sil meteorite in deep sea sediments associated with the K/T
boundary layer, arguing for an asteroidal origin for the im­
pactor. However, the internal structure and composition of
a comet (or comet fragment) capable of producing the K/T
event is not known, and the dynamical process of transport
from the main belt to the NEA region would favour smaller
bodies, more boulder­like than large asteroids. A 6.6 km as­
teroid fragment would be unlikely to gain the necessary ve­
locity from a collision to be ejected directly into a resonance
from the dynamically stable main belt, and would have to be
transported by long­term secular processes (e.g. Menichella,
Paolicchi & Farinella 1996; Zappal`a et al. 1998; Farinella,
Vokrouhlick'y & Hartmann 1998).
The difficulty of dynamically transporting a sufficiently
large asteroid from the main belt on to an Earth­crossing
orbit suggests that a comet, possibly assisted by outgassing
( ¨
Opik 1963; Harris & Bailey 1998; Asher, Bailey & Steel
2000) should be considered as an alternative candidate for
the K/T impactor. A cometary hypothesis would be con­
sistent with the arguments of Zahnle & Grinspoon (1990),
and by the geological evidence for prolonged environmental
change on either side of the K/T boundary (e.g. Bailey et
al. 1994).
Such a near­Earth object might have been an active
Jupiter­family comet, but more probably would have been
an inert or devolatilized object, perhaps originating from
a JFC orbit (cf. 2P/Encke) or from the HTC population
(cf. 96P/Machholz 1). The latter object has P ¸ 5.25 yr, and
hence a SPC classification according to the scheme of this

NEO velocity distributions and the Chicxulub impactor 7
Table 2. Impact probabilities per year for objects in different orbits with large enough diameters to produce the observed Ir at the K/T
boundary. For SPCs and HTCs, values are given both for all objects, and for active comets. Note that bodies arriving with high impact
velocities (? 20km s \Gamma1 ) must be much more massive in order to deposit enough Ir to explain that observed. Such bodies, whether comets
or asteroids, are very rare, i.e. the mean impact interval between them is exceptionally long. This indicates a low­velocity impactor (i.e.
V ! ¸ 20km s \Gamma1 ).
Velocity: V Ÿ 20 20 ! V Ÿ 25 Total (V Ÿ 25)
Object Min Fraction Impact Min Fraction Impact Fraction Impact
d(km) Prob yr \Gamma1 Interval d (km) Prob yr \Gamma1 Interval Prob yr \Gamma1 Interval
NEA 6.6 3.290\Theta10 \Gamma9 304\Theta10 6 10.5 1.508\Theta10 \Gamma10 6.63\Theta10 9 3.441\Theta10 \Gamma9 290\Theta10 6
SPC(all) 8.3 1.050\Theta10 \Gamma8 95.3\Theta10 6 13.2 6.694\Theta10 \Gamma10 1.49\Theta10 9 1.117\Theta10 \Gamma8 89.6\Theta10 6
SPC(act) 8.3 5.095\Theta10 \Gamma10 1.96\Theta10 9 13.2 3.236\Theta10 \Gamma11 30.9\Theta10 9 5.419\Theta10 \Gamma10 1.85\Theta10 9
HTC(all) 8.3 3.425\Theta10 \Gamma11 29.2\Theta10 9 13.2 1.723\Theta10 \Gamma11 58.0\Theta10 9 5.148\Theta10 \Gamma11 19.4\Theta10 9
HTC(act) 8.3 2.277\Theta10 \Gamma12 439\Theta10 9 13.2 1.149\Theta10 \Gamma12 871\Theta10 9 3.426\Theta10 \Gamma12 292\Theta10 9
LPC 8.3 9.994\Theta10 \Gamma12 100\Theta10 9 13.2 4.385\Theta10 \Gamma12 228\Theta10 9 1.438\Theta10 \Gamma11 69.5\Theta10 9
paper, but dynamical characteristics closer to the Halley
family (cf. classifications involving the Tisserand parameter:
Carusi et al. 1987, Levison & Duncan 1997), indicating an
origin in the Oort cloud.
Favouring an Oort cloud connection is the argument
that geological disturbances arise periodically at approxi­
mately 30 Myr intervals following passages of the solar sys­
tem through the Galactic plane (e.g. Smoluchowski, Bah­
call & Matthews 1986). To be detectable, enhancements in
the cometary collision rate with the Earth would need to be
strong compared to the background level of asteroid impacts
from the main belt. However, if a ¸30 Myr cycle of mass
extinctions is accepted, then further investigations into the
evolution of Oort cloud comets captured into SPC orbits
and their wider geological effects would be merited.
Fluctuations in the NEO population are also expected
to occur stochastically, for example following the close pas­
sage of a star through the Oort cloud (Hills 1981) or an
exceptional collision in the main asteroid belt (Kortenkamp
& Dermott 1998, Zappal`a et al. 1998). Each of these sce­
narios would lead to a substantial short­term (10 5 --10 6 yr)
increase in the comet or asteroid population in near­Earth
space, accompanied by a significantly increased abundance
of interplanetary dust which would be accreted by the Earth
over a similar time­scale. Such a picture is consistent with
3 He data from interplanetary dust associated with the late
Eocene (Farley 1995, Farley et al. 1998), but not with the
K/T boundary (Mukhopadhyay, Farley & Montanari 2001).
The latter have argued for a one­off asteroid impact or a lone
comet and against there being a significant enhancement of
the comet/asteroid population in near­Earth space at this
time.
Finally, we note that various alternative sources of K/T
iridium, not directly associated with the Chicxulub im­
pactor, have also been proposed. Clube & Napier (1984)
have pointed out that a large disintegrating comet may sub­
stantially enhance the stratospheric dust load through in­
tense bombardment by `Tunguska­sized' sub­kilometre me­
teoroids (cf. Napier 2001). A similarly enhanced rate of dust
accretion at the K/T boundary with a time­scale of the or­
der of 10 5 yr, associated with the evolution of a giant comet,
has been independently proposed by Zahnle & Grinspoon
(1990). Scenarios in which the K/T iridium is not directly
associated with the Chicxulub impactor would require the
latter to have a moderate to high impact velocity, strength­
ening the argument for a cometary provenance.
A quite different hypothesis for the K/T iridium ex­
cess has been proposed by Yabushita & Allen (1997),
namely that the Earth encountered a dense molecular cloud
¸65 Myr ago. However, no isotopic anomalies attributable
to interstellar material have yet been detected at the K/T
boundary.
5 CONCLUSIONS
The principal conclusions of this paper are:
(i) the presence of a strong iridium signature, if associated
with the Chicxulub crater, suggests that the K/T projectile
had an impact velocity V ! ¸ 20 km s \Gamma1 (Vickery & Melosh
1990), and therefore cannot have been a Halley­type or long­
period comet;
(ii) the presently observed number of comets and aster­
oids on Earth­crossing orbits, together with the crater's
large size, suggest that the projectile was most probably
a devolatilized or inert short­period comet (P ! 20 yr); and
(iii) such a body could have evolved from the Jupiter­
family, Halley­type or long­period populations, leaving
open the question of its ultimate source, whether in the
Edgeworth­Kuiper belt or the Oort cloud.
ACKNOWLEDGMENTS
We thank Alan Fitzsimmons (Queen's University Belfast)
for helpful discussions during the course of this work, and
Duncan Steel (University of Salford) and Bill Napier for sug­
gestions to improve and clarify the paper. This research was
supported by the Irish Department of Education, the NI
Department of Culture, Arts and Leisure, and the Particle
Physics and Astronomy Research Council.
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