Planetary and Space Science 47 (1999) 1475±1485
Meteoroid streams at Mars: possibilities and implications
Apostolos A. Christou*, Kevin Beurle
Astronomy Unit, School of Mathematical Sciences, Queen Mary and West®eld College, Mile End Road, London, E1 4NS, UK
Received 3 November 1998; received in revised form 25 May 1999; accepted 28 May 1999
Abstract
In order to assess the possibility of meteoroid streams detectable from the surface of Mars as meteor showers we have derived
minimum distances and associated velocities for a large sample of small body orbits relative to the orbits of Mars and the Earth.
The population ratio for objects approaching to within 0.2 AU of these two planets is found to be approximately 2:1. The
smaller relative velocities in the case of Mars appears to be the main impediment to the detection of meteors in the upper
atmosphere of that planet. We identify ®ve bodies, including the unusual object (5335) Damocles and periodic comet 1P/Halley,
with relative orbital parameters most suitable to produce prominent meteor showers. We identify speci®c epochs at which
showers related to these bodies are expected to occur. An overview of possible detection methods taking into account the unique
characteristics of the Martian environment is presented. We pay particular attention on the e€ects of such streams on the dust
rings believed to be present around Mars. # 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Meteoroid streams; Mars; Detection methods
1. Introduction
It is an established fact that comets, as they near the
perihelion of their orbits, generate clouds of particulates (Kirkwood, 1861) ranging in size from submicron
dust to metre-sized objects. These are ejected from the
cometary nucleus in a sublimating ice medium at typical speeds which are signi®cantly less than the comet's
orbital velocity (Whipple, 1950). As a result, these
meteoroids move in essentially the same orbit as the
parent comet, gradually spreading in longitude with
respect to the comet due to small di€erences in orbital
period.
If the orbit of a particular comet is physically close
to the orbit of the Earth Ð typically <0.2 AU Ð
then at certain times of the year the meteoroids can be
detected as they encounter Earth's atmosphere and
produce the luminous e€ect known as a meteor. It is
evident from this model that the study of meteors and
in particular the annual meteor showers (Jenniskens,
* Corresponding author.
E-mail address: [email protected] (A.A. Christou).
0032-0633/99/$20.00 # 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 2 - 0 6 3 3 ( 9 9 ) 0 0 0 6 4 - 1
1994) can provide valuable information on the nature
of comets.
If an association between a meteor shower and a
comet can be deduced due to orbital similarity then
the reduction of related meteor observation records
can yield insight into (a) the physical properties of the
ejected material (b) the nature of the ejection mechanism and (c) the behaviour of the parent comet over
hundreds or even thousands of years in the past.
One of the currently most active areas of study
focuses on comparing the past and present behaviour
of di€erent meteor showers as a means of inferring
actual variations in the intrinsic properties of the
parent body population. For example, the object
(3200) Phaethon associated with the December
Geminids, has been observed to be completely inert,
i.e. asteroidal, in stark contrast to the bulk of known
meteor shower producing bodies which exhibit the
typical characteristics of comets (Whipple, 1983;
Davies, 1985; Williams and Wu, 1993). Gustafson
(1989) traced the orbits of several Geminid meteoroids
back to Phaethon and concluded that this object is
probably an extinct cometary nucleus. Such insight on
the nature of Phaethon is only possible through the
1476
A.A. Christou, K. Beurle / Planetary and Space Science 47 (1999) 1475±1485
fortuitous orbital geometry of the Geminid stream
relative to the Earth. In this sense, our ability to derive
useful statistical conclusions is limited by the simple
fact that we can only sample meteoroid streams with
orbits which come suciently close to the orbit of the
Earth.
Recently, Adolfsson et al. (1996) (hereafter,
AGM96) showed that a fast-moving (r30 km/s)
meteoroid would produce a meteor of similar maximum luminosities in the atmospheres of Earth and
Mars. The height of maximum meteor intensity was
found to range between 90 and 50 km compared with
the 100 to 70 km height range found in the case of the
Earth in their work. Their study was initially motivated by the fact that at altitudes of 0120 km where
meteoroid ablation begins the atmospheric densities
for the two planets are comparable.
The undertaking of numerous unmanned missions
to Mars in the next 10 years (Mars Surveyor 98/01/03,
Mars Sample Return 2005, PLANET-B, Mars
Express, NETLANDER) creates an opportunity to
extend our knowledge of the meteoroid stream complex to a wider range of heliocentric distances than has
previously been possible. The detection of meteors in
the atmosphere of Mars by means of a surface camera
network or other means is, for these purposes, highly
desirable.
In this paper we present the results of a search for
individual comets or asteroids that are likely, in our
opinion, to generate observable meteor showers in the
atmosphere of Mars. We have computed, in a rigorous
way, the minimum orbit-to-orbit distances (MOODs
for short) between Mars and a suitable subset of the
known small body population. For objects which come
closer than a speci®c MOOD threshold we calculate
the relative velocity at the critical orbital longitude as
well as future epochs at which Mars is expected to
pass through that point in space.
We try to estimate the likelihood of these objects
being meteoroid stream parents by carrying out the
same procedure for the Earth where a number of
known meteoroid stream parent bodies exists.
Subsequently, we re®ne the likelihood of Martian
meteoroid streams and their parent bodies by examining the latter's orbital characteristics.
Finally, we discuss the e€ects of meteoroid stream
¯ux on the Martian environment and how the various
detection methods can be used to glean information
about the planet and its moons.
2. Search methodology
2.1. Relation to previous work
Searches for potential meteoroid stream parents as-
sociated with planets other than the Earth have been
carried out in the past. In unpublished work referred
to in AGM96, Fox showed that 18 of the cometary
orbits listed in Belyaev et al. (1986) come within 0.1
AU of the orbit of Mars as opposed to only eight for
the case of the Earth. More recently, Steel (1998) derived the impact velocity distribution for a sample of
over 600 known asteroids and 100 ®ctitious parabolic
comets. Beech (1998) conducted a search for bodies as
possible sources of ®reballs in the dense atmosphere of
Venus by comparing the nodal distances of their orbits
with the semimajor axis of that planet. In this work
we have preferred a more rigorous approach than that
employed by Beech for two main reasons:
. The non-negligible inclination of the Venusian orbit
(038) or, in our case, the signi®cant eccentricity of
the Martian orbit (0.093) may a€ect the results for
the closest approachers.
. The fact that some asteroid/comet orbits have small
inclinations means that the minimum orbit-to-orbit
distance will not always be attained at the nodal
crossing point. This is important when one tries to
determine the actual epochs at which the planet
passes through a postulated meteoroid stream.
In order to avoid such problems we have considered
the cartesian representation of a Keplerian orbit in
space.
2.2. Minimum orbit-to-orbit distance
The following have been taken, for the most part,
from Murray and Dermott (1999). Consider an orbit
with known semimajor axis a, eccentricity e, inclination I, argument of pericentre o and longitude of
ascending node O. Using the rotation matrices
0
cos o
P1 …o† ˆ @ sin o
0
ÿsin o
cos o
0
1
0
0A
1
1
1 0
0
P2 …I † ˆ @ 0 cos I ÿsin I A
0 sin I cos I
…1†
0
0
cos O
P3 …O† ˆ @ sin O
0
ÿsin O
cos O
0
1
0
0A
1
…2†
…3†
we can move from the orbit-speci®c reference frame in
which the position of the body is given by the true
anomaly f to an orbit-independent frame where the
position of an object is given by the cartesian co-ordinates
A.A. Christou, K. Beurle / Planetary and Space Science 47 (1999) 1475±1485
0
1
0
1
X
r cos f
r ˆ @ Y A ˆ P3 P2 P1 @ r sin f A
Z
0
…4†
where
rˆ
quantities.
L1 …I, o, O† ˆ cos O cos o ÿ sin O sin o
cos I
a…1 ÿ e2 †
:
1 ‡ e cos f
…5†
For the purposes of this investigation we shall regard
the above co-ordinates as functions of the parameters
a, e, I, o, O and the free variable f, that is
X ˆ X…a, e, I, o, O;f †
…6†
Y ˆ Y…a, e, I, o, O;f †
…7†
Z ˆ Z…a, e, I, o, O;f †:
…8†
1477
…11†
L2 …I, o, O† ˆ ÿcos O sin o ÿ sin O cos o
cos I
…12†
M1 …I, o, O† ˆ sin O cos o ‡ cos O sin o
cos I
…13†
M2 …I, o, O† ˆ ÿsin O sin o ‡ cos O cos o
The distance Dij between two orbits i and j is then
given by
D2ij … fi , fj †
2
2
ˆ …Xj ÿ Xi † ‡ …Yj ÿ Yi † ‡ …Zj ÿ Zi †
2
…9†
where for each comparison the parameters are ®xed
and the local/global minima are obtained along with
the associated values of fi and fj.
2.3. Relative velocity at the critical epoch
Apart from the minimum distance criterion, the
other quantity which governs the generation of a
meteor from a meteoroid is the speed at atmospheric
entry. For fast meteoroids, this is essentially the magnitude of the relative velocity Vrel between the planet
and the parent at the critical epoch. We make a distinction between this de®nition of speed which is given
by the vector di€erence of the velocities of the two
Keplerian orbits at the point of closest approach and
the actual speed V1 of the meteoroid in that orbit at
the top of the Martian atmosphere. This may be
expressed as
q
…10†
V1 ˆ V 2rel ‡ V 2esc
where Vesc is the escape velocity at a height of 120 km
above the surfaces of Mars or the Earth; the respective
values for the two planets used here are 4.95 and
11.1 km/s. Flynn and McKay (1989) found that slowmoving meteoroids smaller than 1 mm will not be
heated to ablation temperatures Ð a necessary condition for the luminous e€ect Ð due to their large surface-area-to-mass ratio. This upper limit is shifted to
0.04 mm for fast meteoroids (AGM96).
In what follows we present expressions from which
the components of the relative velocity vector can be
derived. Following Roy (1988) we de®ne the auxiliary
cos I
…14†
N1 …I, o† ˆ sin o sin I
…15†
N2 …I, o† ˆ cos o sin I
…16†
with the help of which the velocity vector in the orbitindependent frame may be expressed as
0 1
2 0
1
L2
X_
B _ C na 4 @
b M2 A cos E ÿ a
V ˆ @YA ˆ
r
_
N2
Z
1
3
L1
@ M1 A sin E 5
N1
0
…17†
where b denotes thep
semi-minor
axis of the Keplerian

ellipse equal to a …1 ÿ e2 † and E is the eccentric
anomaly given by
1 ÿ r=a
r sin f
‡i
, i2 ˆ ÿ1:
…18†
E ˆ arg
e
b
In this work we make use of bold notation to denote
vectors whose moduli are simply given by the Roman
type symbol. The relative velocity vector Vrel of body i
with respect to body j will be equal to ViÿVj.
2.4. Sample selection
In order to measure, in a statistically meaningful
way, the likelihood of existence of meteoroid streams
at Mars compared to that for the Earth we have chosen to apply the algorithms described in the previous
section to a suitable sample of known asteroids and
comets. In the case of comets, we have simply collected
the orbits of 134 numbered short-period comets listed
1478
A.A. Christou, K. Beurle / Planetary and Space Science 47 (1999) 1475±1485
Table 1
Adopted ecliptic J2000 orbital elements for the Earth and Mars generated by the HORIZONS ephemeris system for the epoch
JD2451000.5
Planet a (AU)
e
I (8)
o (8)
O (8)
t (JD)
Earth 0.999352 0.017354 0.000420 93.2437 8.9702 2,451,181.311
Mars 1.523594 0.093384 1.849930 286.4286 49.5622 2,4508,20.898
in JPL's DASTCOM database (Chamberlin et al.,
1998).
On the other hand, the large number of known
asteroids dictated the use of a ®ltering procedure.
Firstly, we have rejected unnumbered asteroids in
order to ensure the reliability of the orbits present in
the sample. In this way, we avoid large errors in cases
where the calculated minimum distance Dmin is very
small (010ÿ2 AU). Furthermore, we require that the
perihelia distances of the selected asteroid orbits are
within 0.2 AU of the aphelion distance of Mars. This
ensures that our sample encompasses all the orbits
which pass suciently close to Earth or Mars for a related meteoroid stream to be detected. Note that all
orbits with perihelia inside that of the Earth have corresponding aphelion distances greater than 1 AU. In
this way we have selected 1371 out of a total of 9246
numbered asteroids listed in Bowell's `astorb.dat' ®le
available on the web at ftp://ftp.lowell.edu/pub/elgb/
astorb.html as of 30 September 1998. The elements for
Earth and Mars were computed with JPL's
HORIZONS ephemeris generation tool (Chamberlin et
al., 1998) and are shown in Table 1.
3. Expected meteor brightness
Before we go any further it would be instructive to
comment on how bright a meteor of a given velocity
and mass one would expect to see at Mars.
In their analysis AGM96 reached the conclusion
that fast (r30 km/s) meteors penetrate to lower altitudes at Mars than they do at Earth making their
detection from the surface more favourable in the case
of that planet. A subtle feature in their investigation
was that in comparing the magnitudes of meteors at
the two planets they adjusted the atmospheric impact
speed at Mars for the expected fall-o€ of Keplerian
speed with heliocentric distance and the weaker gravitational acceleration at that planet with respect to the
Earth. In this way they were able to address the issue
of the relative overall brightness of meteors from the
surfaces of the two planets.
Since here we are interested in individual cases
whose velocities/speeds are known we would like to
have estimates for meteor brightnesses at the same
speed as the Earth and also look at the intrinsic as
well as the apparent brightness. This is important
when one considers meteor searches from vantage
points other than the surface of the planet (see Section
5).
For a meteor of a given speed and mass the di€erence in magnitude between the two atmospheres was
given in AGM96 as
dM ˆ ÿ2:5 log‰…HE =HM †…hE =hM †2 Š
…19†
where H and h denote the atmospheric scale height
and altitude of maximum luminosity for Mars and the
Fig. 2. Same as Fig. 1 but for cometary orbits. The high speed outlier present in the plot for Mars is comet 1P/Halley.
A.A. Christou, K. Beurle / Planetary and Space Science 47 (1999) 1475±1485
1479
Fig. 1. Two-dimensional density histogram of numbers of asteroidal orbits passing within 0.2 AU of Mars (a) and Earth (b). Gravitational acceleration at a height of 120 km above the respective surfaces of the two planets has been incorporated into the speed calculation. The approach
distances and speeds of the individual orbits have been binned with a resolution of 0.01 AU and 4 km/s, respectively. The brightness scale translates in counts as follows: dark grey: 1, light grey: 2, white: r3. Maximum count for an individual resolution element is 14. A 30 km/s cut-o€
point in the case of Mars is evident in this and the following ®gure.
Earth, respectively. If we adopt those authors' estimate
of the scale height ratio (20.8) and set hE/hM=1 we
®nd dM0 +0.25m. Therefore, the intrinsic brightness
of Martian meteors should be only marginally lower
than that for the Earth. In order to derive an estimate
of the relative apparent magnitudes from the surface
for the speeds we consider in this investigation we
compare their Fig. 2 with their Eq. (25) for v1,E equal
to 72 and 30 km/s. This gives hE/hM 2 1.2 and if we
assume a maximum luminosity altitude of 0100 km
for the Earth (Hughes, 1978) we ®nd hM 2 80 km. A
straightforward substitution in Eq. (19) gives
dM=ÿ0.25m as viewed from the surface of the planet.
A simple expression for the apparent magnitude as
function of mass and velocity may be derived by
adapting Eq. (19) in Hughes (1978) according to our
magnitude di€erence estimate to give
M ˆ ‡24:8m ÿ 2:5 log…tmV 31 =L†
…20†
where t is the luminous eciency, m is the mass and L
is the fraction of the trail over which the observer
measures the magnitude. For V1=25 km/s,
t=2 10ÿ3 cm/s, m = 1 g and L = 7.5 km which is
relevant to this work, Eq. (20) gives M 2ÿ1.7m.
4. Results
4.1. Population comparisons
We have proceeded to measure the current MOODs
between the orbits of those small bodies and the orbit
of Mars/Earth using the method described in Section
2. This revealed a total of 297 asteroids and 51 comets
which can approach Mars to within 0.2 AU compared
to 156 and 24, respectively, for the case of the Earth.
An important point to be made here is that in some
cases a particular object is able to produce two meteor
showers as its orbit crosses the orbit of the planet near
both its ascending and descending node. We have treated such opportunities as separate events, since we are
interested in the frequency of shower occurrence at
Mars.
In order to gain insight into the di€erences between
the respective populations of potential meteoroid
stream parents we have computed the atmospheric
impact speeds of these objects at the point of closest
approach using Eq. (10). Subsequently, we gather our
results in distance/speed bins which we then display in
a two-dimensional density histogram format for the
asteroidal (Fig. 1) and cometary (Fig. 2) components
separately. One striking di€erence between these two
plots is immediately obvious. Although the spread in
speeds for asteroidal orbits is similar for the two planets, this is not observed to be the case for cometary
orbits, with a distinct de®cit of high speed Mars
approachers. The theoretical relative speed upper limit
for Mars, achieved for a comet in a retrograde parabolic orbit with a pericentre at the orbital distance of
the planet, is 58 km/s. Only one object, 1P/Halley, is
found to approach this with the remainder of our
asteroidal and cometary orbit sample staying under
1480
A.A. Christou, K. Beurle / Planetary and Space Science 47 (1999) 1475±1485
Table 2
Comparison of the number of asteroids for a given atmospheric
impact speed V1 (km/s) and minimum orbit-to-orbit distance Dmin
(AU) for Mars and the Earth
V1 < 15 15 < V1 < 25 25 < V1 < 35 V1 > 35
Dmin < 0.05
54 (11)a
0.05 < Dmin < 0.1 43 (11)a
0.1 < Dmin < 0.15 46 (9)a
0.15 < Dmin < 0.2 81 (11)a
a
30
10
14
15
(35)a
(20)a
(14)a
(15)a
1
1
1
0
(7)a
(5)a
(8)a
(7)a
0
0
0
0
(2)a
(1)a
(0)a
(0)a
30 km/s. We note that the respective upper limit for
the Earth is 72 km/s and this is approached for the
most prominent annual showers such as the Perseids
and the Leonids.
In Tables 2 and 3 we show the actual relative statistics of Mars and Earth approachers for cometary
and asteroid orbits, respectively. In combination with
Figs. 1 and 2, the following two features are apparent:
1. There are approximately ®ve times more asteroids
and comets encountering Mars at low velocities
(<15 km/s) than the Earth. This is partly due to
the mean Keplerian speed drop-o€ with heliocentric
distance but also due to the weaker gravitational
acceleration for meteors hitting the atmosphere of
Mars (4.95 km/s for a parabolic meteoroid compared with Earth's 11.1 km/s).
2. At the other end of the speed spectrum, the situation is reversed. We see in Tables 2 and 3 that a
total of 12 comets and 30 asteroids enter Earth's atmosphere with V1 > 25 km/s compared with only
three comets and three asteroids, respectively for
Mars.
Thus there appears to be a de®cit of intermediate
period (Halley-type) Mars-approaching comets. The
reason for this state of a€airs is not clear to us. It
would be hard to justify it in terms of observational
bias since any comet that can approach Mars should
be detectable from the Earth.
4.2. Criteria for parent body candidates
We now proceed to identify individual bodies which,
Table 3
Same as Table 2 but for cometary orbits
V1 < 15 15 < V1 < 25 25 < V1 < 35 V1 > 35
a
(2)a
(2)a
(1)a
(1)a
3
1
1
6
(4)a
(1)a
(0)a
(1)a
1
0
0
1
(0)a
(2)a
(0)a
(3)a
0
1
0
0
Values for the case of the Earth are given in parentheses.
Object
VX
VY
VZ
V1 Dmin (Mars) Dmin (Earth)
(2102) Tantalus ÿ14.3 ÿ9.7 ÿ20.6 27.3 0.060
(5335) Damocles
7.5
2.3 ÿ28.3 29.8 0.049
(5660) 1974 MA ÿ20.0 10.3 11.7 25.8 0.024
1P/Halley
17.2 ÿ50.6
4.5 53.9 0.067
13P/Olbers
2.4 ÿ19.0 18.8 27.3 0.021
0.045
±
0.158
0.066
±
a
Values for the case of the Earth are given in parentheses.
Dmin < 0.05
14
0.05 < Dmin < 0.1 12
0.1 < Dmin < 0.15 4
0.15 < Dmin < 0.2 7
Table 4
Closest approach data for the most favourable meteoroid stream
candidates for Marsa
(2)a
(2)a
(0)a
(3)a
The cartesian components of Vrel, denoted as (VX, VY, VZ), are
given in km/s and are referred to the J2000 ecliptic frame. The column labelled V1 gives the actual impact speed on the Martian atmosphere. Closest approach distances are given in AU. Respective
closest approach distances with the Earth are provided for comparison. A dash should be interpreted as that object coming no closer
than 0.2 AU to the planet. In the case of comet 1P/Halley and the
Earth, the closest approach distance corresponding to the more productive Z Aquarid shower is given.
at least on dynamical grounds, should produce prominent meteor showers at Mars. Our experience with the
Earth dictates that these will be in orbits with small
closest approach distance with Mars and high relative
speed. However, as we saw in the previous section,
there are virtually no objects which approach the orbit
of Mars with a relative speed greater than 30 km/s. As
a compromise between our results and the high speed
requirement we have chosen a relative velocity
threshold of 25 km/s and a close approach distance
upper limit of 0.1 AU. As pointed out in Section 3,
this speed threshold should be sucient to produce
bright, easily observable meteors.
4.3. Candidate identi®cation and description
By using these distance/speed criteria we have identi®ed ®ve objects which, assuming the existence of
ejected particulates or collisional debris along their
orbits, we expect to be the parent bodies of prominent,
bright meteor showers at Mars. These are listed in
Table 4 along with their relative velocity components
at Mars and MOODs for Mars and the Earth for comTable 5
Future meteor shower detection/observation opportunities from the
surface of Mars for the next six years (Earth dates are given)a
Object
1999
(2102) Tantalus
(5335) Damocles
(5660) 1974 MA
1P/Halley
13P/Olbers
19/8
a
1/10
2000
11/9
28/3
5/6
2001
6/7
18/8
2002
30/7
13/2
23/4
2003
24/5
6/7
2004
16.6
1/1
10/3
The orbits of the ®ve candidate parent bodies have Dmin < 0.1
AU and V1 > 25 km/s ensuring a strong luminous e€ect during atmospheric ¯ight of the postulated meteoroids.
A.A. Christou, K. Beurle / Planetary and Space Science 47 (1999) 1475±1485
parison purposes. In Table 5 we present their epochs
of occurrence in the next six years for the convenience
of Mars mission planners and scienti®c investigators.
We also note that the respective lists for the Earth,
which are not shown here, contains all the known prominent meteor shower parents such as comets 55P/
Tempel±Tuttle, 109P/Swift±Tuttle and minor planet
(3200) Phaethon. In what follows we discuss these
objects in some detail.
Minor planet (2102) Tantalus is a Q-type, 3.5 kmsized object spectrally similar to H6 chondrites (Hicks
et al., 1998; Lupishko and Di Martino, 1998; Binzel et
al., 1996). The prominent Q-types (1862) Apollo and
(1566) Icarus have been associated, albeit weakly, with
minor meteoroid streams by Olsson-Steel (1987). The
absence of a stream association with Tantalus in
Olsson-Steel's work was given to be due to lack of observations during the critical epoch. Furthermore, a
strong association between several Prairie network ®reballs and Icarus was shown in Drummond (1982). We
thus conclude that further observations from Mars
and the Earth at the proper epochs can be used to
determine the state of this object and near-Earth asteroids in general as meteoroid stream parents.
Asteroid (5335) Damocles was discovered in 1991 by
R. H. McNaught and colleagues at which time it was
given the provisional designation 1991 DA. Its calculated orbit was typical of that of an intermediateperiod comet (e.g. 1P/Halley) taking the object from
the aphelion distance of Mars out to the orbit of
Uranus. However, further observations revealed no
cometary activity. Based on a combination of the
above facts and subsequent numerical work, Steel et
al. (1991) and Asher et al. (1994) concluded with high
probability that Damocles is, in fact, an inactive or
extinct cometary nucleus. If that is the case, a detection or otherwise of a meteoroid stream associated
with this body is of major scienti®c importance.
Results from observations at the critical epoch can be
used to infer the true nature of Damocles and relate it
to objects with similar characteristics such as (944)
Hidalgo, the Centaur/Trojan groups or even (3200)
Phaethon, the parent of the Geminids. The calculated
MOOD of Damocles with Mars is 0.05 AU compared
with 0.6 AU for the Earth, meaning that Mars is the
only planet of the two where meteoroids ejected from
Damocles will be detectable.
Comet 1P/Halley is a known meteoroid stream
parent body. Furthermore, it is unique in being the
only comet responsible for two distinct prominent
meteor showers during the year, the Orionids in
October, visible from the northern hemisphere, and the
Z Aquarids in May, mainly a southern-hemisphere
shower (Hajduk and Buhagiar, 1982; Hughes, 1987).
Its calculated MOODs to Earth are 0.15 AU for the
October shower and 0.07 AU for the May event with
1481
a large relative speed of 066 km/s owing to the
comet's retrograde orbit. These MOODs are well represented by the observed peak rates of 25 and 37, respectively, (Jenniskens, 1994). The above means that
the calculated MOOD value to Mars of 0.07 AU combined with a relative speed of 54 km/s should yield a
respectable meteor shower in the atmosphere of Mars.
Thus, the predictable behaviour of comet Halley as a
meteor shower parent render the existence of a related
shower in the Martian atmosphere highly likely.
We note here that a lowering of the peak ¯ux with
respect to the Earth is expected since the orbit of Mars
is further away from the comet's perihelion than the
Earth is. However, this argument could be inverted in
the sense that observation of the actual ¯ux at Mars
could provide constraints on the range of orbital longitudes at which meteoroid ejection is taking place.
Comet 13P/Olbers has an orbit which is in some
ways similar to that of 1P/Halley (a 018 AU,
e 0 0.93) apart from its inclination (0458) which
implies a lower encounter velocity with Mars (Table 4)
and its perihelion distance of 01.18 AU meaning that
Mars is the only planet of the two where meteors associated with that comet will be observable. Its last
apparition was in 1956 and the next one will take
place in 2024.
(5660) 1974 MA is a minor planet in a comet-like
orbit (e 0 0.76, i0 388) which approaches the orbit of
Mars approximately six times closer than that of the
Earth according to Table 4. Mars is, therefore, a far
more suitable place than the Earth to detect an associated stream and indeed provide insight on the nature
of the object in a manner similar to the Phaethon±
Geminids association.
5. Detection methods and implications
Concerning the issue of actual observation of such
phenomena, it would seem prudent to employ, if possible, methods which do not necessitate the manufacture of expensive, customized hardware. Ideally, the
deployment of one or, preferably, several wide-angle
surface cameras observing the Martian sky at night as
proposed in Morgan et al. (1995) and AGM96 would
be more than sucient to characterize any existing
annual meteor showers and provide counts as well as
orbital information for the individual meteors. The
most important impediment here is the high data rates
that are required to transmit the generated image data
volume to the Earth. The development of suitable data
compression techniques such as Hough transforms
may alleviate this problem in the near future (Trayner
et al., 1996).
Data sent back from the Mars Path®nder have
suggested that the opacity of the Martian atmosphere
MS98 Orbiter? MS01 Orbiter? Mars Express?
Mars Express/Beagle 2? NETLANDER? Mars
Surveyor craft?
PLANET-B? MS98 Lander? Mars Express?
PLANET-B? Beagle 2? NETLANDER?
PLANET-B
Small detector area (100 cm2)
Requires at least two stations. Limited
direction/velocity information.
Requires active sounding apparatus.
Applicable only to the largest (>0.1 m)
meteoroids.
For camera searches: two-dimensional CCD
detector array and multi-second exposure
capability necessary.
Mars Surveyor Landers? Beagle 2?
Orbital search with camera or radar
Radar/LIDAR sounding
ELF/VLF wave emission
Ionization trail radio re¯ection
Density enhancement in Phobos/Deimos
ejecta
Large detection area (the Martian
atmosphere). Accurate directional
information.
Potential use as diagnostic of the physical
properties of Phobos/Deimos regolith and
the dynamical evolution of dust.
Sensitive to faint (>+7m) meteors.
Straightforward implementation.
Provides orbital information
Omni-directional. Potential use for magnetic
®eld mapping.
Does not require a landing station. Broad
search area.
Surface camera network
Restrictions
Advantages
Detection method
Table 6
List of meteor/meteoroid detection methods applicable at Mars
Requires high data rate transmission
capability.
A.A. Christou, K. Beurle / Planetary and Space Science 47 (1999) 1475±1485
Implementation platform
1482
may increase during the night due to the formation of
high altitude water clouds in the early hours of the
morning (Smith et al., 1997). However, optical surveys
from the surface may be supplemented by other
methods which make use of instrumentation that is
currently operating or is due to come on-line in the
next few years. A synergy among these will eventually
provide the necessary justi®cation (or otherwise) for
the deployment of dedicated instrumentation in later
missions.
These methods are summarised in Table 6 along
with their perceived advantages and disadvantages. As
a high speed meteoroid passes through the atmosphere,
it leaves behind an ionization trail which re¯ects radio
waves (McKinley, 1949; Hawkins, 1963). This property
of the trail can either be exploited by a radio transmitter/receiver system for simple meteor counting or a
radar experiment to glean size/range information. In
addition, the presence of a planetary magnetic ®eld
such as that of the Earth means that electromagnetic
energy is emitted as the ®eld lines are distorted by the
presence of ionized material and then allowed to relax
to their original con®gurations (Bronshten, 1983). The
frequency of the emitted radiation tends to depend on
the nature and speed of the meteoroid but is typically
in the 1 to 10 kHz range (Keay, 1985; Watanabe et al.,
1988). This radiation can be detected, for example, by
means of a riometer system (Morgan et al., 1995;
Detrick et al., 1997) and can be used to investigate the
Martian crustal magnetic ®eld anomalies discovered
recently by Mars Global Surveyor (Acuna et al.,
1998).
The study of meteor trains in the Martian atmosphere can also yield useful data on the upper layer of
the atmosphere itself, as demonstrated for the case of
the Earth (Whipple, 1938). It is conceivable that an
optical search for meteors can be conducted using a
wide-®eld camera aboard an orbiting spacecraft passing above the nightside of the planet. This was ®rst
proposed by Khotinok (1985) as a method of deriving
the ¯ux of meteorite-sized bodies at the heliocentric
distance of Mars. The typical altitudes of meteors in
the atmosphere of Mars are estimated by AGM96 to
be 080 km. Thus, meteors observed from orbit by
nadir-pointed cameras will appear 02.2 and 3 magnitudes fainter from 300 and 400 km altitudes, respectively, than they would from the surface. The sensitivity
of modern CCD detectors in combination with the
expected dark background should ensure that satisfactory results will still be obtained. The main issue here
is the requirement for a two-dimensional detector
array Ð unlike the linear CCD detector which is
employed in the camera aboard Mars Global Surveyor
Ð with multi-second exposure capability.
The authors are aware of two instruments with speci®cations which may satisfy the requirements for an
A.A. Christou, K. Beurle / Planetary and Space Science 47 (1999) 1475±1485
orbital meteor search. One is the wide angle mode of
the MARCI camera on board the Mars Climate
Orbiter which was successfully launched in December
1998 and is expected to reach the planet in September
1999. The centremost portion of the CCD detector in
this instrument is partitioned into 20-pixel-high sections which are overlaid by di€erent colour ®lters. A
colour image is constructed by allowing the scene to
sweep through the detector, coinciding sequentially
with the di€erent ®ltered sections (Malin et al., 1999).
The data that accumulates into the remainder of the
1024X1024 CCD array will be normally discarded.
The other possibility lies with the THEMIS visible/IR
imaging device slated to ¯y aboard the 2001 Mars
Surveyor orbiter (Christensen et al., 1999).
In light of the above, we would like to encourage
the instrument investigators to consider seriously, the
feasibility of observations of the nightside of the planet
at or near the epochs listed in Table 5. As a ®nal note,
this observational mode has also been proposed as a
means of detecting electrostatic discharges thought to
occur during dust storms (Melnik and Parrot, 1998).
One possibility that has so far been neglected in the
literature is the use of a dust detector in orbit around
Mars such as the one aboard the recently launched
Nozomi (PLANET-B) spacecraft. It is known that a
meteoroid stream will lose its dust-sized (R10ÿ9 g)
particles very quickly due to the action of radiation
pressure (Dohnanyi, 1978). One is then left with a
reduced ¯ux (R1 particle/km2) of mm-sized particles
(Babadzhanov et al., 1991) which is unlikely to register
in the 0100 cm2 of detector surface in the hours or
days during which passage through the stream takes
place. We caution here that this ¯ux estimate is based
on visual observations of bright meteors and does not
necessarily extend to particle sizes less than 0.1 mm for
which corresponding meteors would be faint (>+9m).
There have been occasions in which dust detectors
aboard interplanetary probes have registered temporally enhanced ¯uxes of micron-sized dust particles
(Alexander et al., 1971; Ho€mann et al., 1976). In this
paper we concentrate our attention on the indirect
e€ects of a stream on the dust environment at Mars.
Namely, the possibility of dust rings around Mars
may still enable the detection of such streams using
suitable detectors in orbit. These rings were ®rst proposed by Soter (1971) as a product of micrometeroid
sputtering of the surfaces of Phobos and Deimos. The
weak gravity ®eld of the Martian moons implies that a
large fraction of the impact ejecta will spread along
their orbits and create a kind of dust ring or torus,
similar to the ones believed to be associated with the
Saturnian moons Mimas and Enceladus (Hamilton,
1994; Showalter and Cuzzi, 1993). Although the unambiguous detection of such ring material is still pending
Ð this is in fact the primary purpose of the Nozomi
1483
Table 7
List of observation/detection opportunities related to very-closeapproaches (Dmin < 0.01 AU) between Mars and the orbits of a
given object (Earth dates)a
Object
1863
2101
2135
2201
3040
4503
4503
5641
2004
14/7
25/10
28/1
7/8
2005
26/7
22/3
16/9
23/4
15/12
Object
5645
6130
6178
6322
8014
9162
9P/
114P/
2004
25/9
24/12
21/10
11/3
2005
8/1
27/6
7/3
26/2
a
At those instances the dust environment around Mars may be
disturbed due to enhanced meteoroid ¯ux on the surfaces of the
Martian moons. Additionally, the occasional physical proximity of
the parent to the planet may result in meteor outbursts (Jenniskens,
1995; Jenniskens et al., 1997). All such opportunities during the
nominal lifetime of the Nozomi (PLANET-B) probe, expected to
reach Mars in December 2003, are shown.
dust detector (Ishimoto et al., 1997) Ð extensive studies of its nature and dynamics have already been carried out (Krivov, 1994; Ishimoto and Mukai, 1994;
Hamilton, 1996; Krivov and Hamilton, 1997). In particular, the number density of particles in the ring has
been shown to depend sensitively on the assumed
micrometeroid ejecta yield at the surfaces of the
moons (Krivov, 1994; Ishimoto, 1996). This quantity is
currently poorly constrained.
In view of the above, it is possible that periods of
greatly enhanced meteoroid ¯ux, such as times of passage of Mars through a meteoroid stream, may have
dramatic consequences for the spatial and temporal
structure of the ring/torus. For example, for the
above-mentioned upper ¯ux limit which can be
attained during a Leonid-type meteor storm we expect
0400 meteoroids r1 mm in size to impact the surface
of Phobos every hour at velocities ranging from 5 to
55 km/s depending on the particular stream being
encountered, delivering 0104 times the number of
same-sized sporadic background particles received by
the satellite over the same period of time. These estimates assume that (a) Phobos presents a 20 24 km
¯at elliptical target to the stream with a surface area
of p 10 12 0400 km2 and (b) the ¯ux of mm-sized
meteoroids in the sporadic background at the heliocentric distance of Mars can be extrapolated from the
value of 10ÿ11 particles mÿ2 secÿ1 (2p str)ÿ1 reported
in Table 3 of McDonnell (1978). These are only
intended to serve as order-of-magnitude estimates.
The Mars Dust Counter aboard Nozomi (Tsuruda
et al., 1996) is well-suited for the study of such
phenomena. In contrast to previous experiments on
Mars-bounded missions (Alexander et al., 1965;
Nazarova and Rybakov, 1976), it is intended to stay in
1484
A.A. Christou, K. Beurle / Planetary and Space Science 47 (1999) 1475±1485
the vicinity of the planet for at least one Martian year
(1.88 Earth yr). It should be able to correlate likely
periods of enhanced interplanetary (hyperbolic)
meteoroid ¯ux (see Table 7) with any subsequent disturbances in the circum-Martian dust environment.
These should manifest themselves as sudden enhancements in dust density in the vicinity of the satellites
which gradually spread along the circumference of
their orbits.
Our hope is that the inevitable detection of meteoroid streams by direct or indirect means in the vicinity
of Mars will encourage the research community to
regard meteor science on other planets as a valuable
source of information in the interdisciplinary study of
the solar system.
Acknowledgements
The authors wish to thank Iwan Williams, Carl
Murray, Bo Gustafson and Lars Adolfsson for their
constructive and enlightening remarks concerning the
physics of meteoroid streams and the Martian environment; David Hughes and Peter Jenniskens whose valuable suggestions greatly improved the paper. This
work was supported by PPARC grant GR/L39094.
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