Annual meteor showers at Venus and Mars: lessons from the Earth

Transcription

1 Mon. Not. R. Astron. Soc. 402, (2010) doi: /j x Annual meteor showers at Venus and Mars: lessons from the Earth A. A. Christou Armagh Observatory, College Hill, Armagh BT61 9DG Accepted 2009 November 23. Received 2009 November 19; in original form 2009 July 23 ABSTRACT We have generated a list of cometary bodies as well as known meteoroid streams that we consider to be prime candidates for producing significant meteor activity in the atmospheres of Venus and Mars. To compile this list, we defined a quantitative criterion based on catalogued properties of comets such as their dynamical class, orbital period and absolute brightness as well as the proximity of their orbits to the planetary orbits. This procedure improves over previous work that considered only this latter quantity as the sole criterion for meteor shower parentage at those planets. The list of Martian (Venusian) candidates contains six (eight) Halley-type comets, 11 (six) intermediate long-period comets, eight (nine) showers originating from known meteoroid streams of Encke or Jupiter-family type and one inert object in a comet-like orbit. Based on these findings, we conclude that (i) meteor shower activity at those planets would be variable on a seasonal scale, just as it is at the Earth, (ii) Venusian and/or Martian meteor showers from bright long-period comets, a population with no representatives in the Earth s vicinity, are a possibility, and (iii) numerous opportunities exist for sampling known Encke-type and Jupiter-family showers to probe their spatial structure far from the Earth s orbit. We calculate local observing circumstances of these showers to aid in their future observational confirmation and characterization. Key words: comets: general Earth meteors, meteoroids planets and satellites: individual: Mars planets and satellites: individual: Venus. 1 INTRODUCTION The annually recurring component of meteor activity in the Earth s atmosphere is composed of annual meteor showers and the sporadic background (see Campbell-Brown 2007 for a review). The former are highly directional, and can persist for periods of time ranging from hours to weeks, a typical duration being a few days. The latter persists throughout the year, the meteors emanating from broad regions of the celestial sphere principally near the ecliptic. Theoretical exploration of the physics of meteor ablation to date indicates strongly that these phenomena should occur with comparable intensity in the atmospheres of Mars and Venus (Apshtein, Pylyugin & Varmanian 1982; Adolfsson, Gustafson & Murray 1996; Christou & Beurle 1999; Christou 2004a; McAuliffe & Christou 2006b). However, very little information has been gleaned to date regarding meteor activity at those planets (Huestis & Slanger 1993; Selsis et al. 2005; Domokos et al. 2007). This is partly due to the fact that instrumentation specifically designed to detect meteors or their effects has yet to fly on Mars- or Venus-bound spacecraft. To facilitate such searches, predictions linking particular members of the comet population with putative Martian or Venusian meteor showers have been made. These mainly employ orbit-to-orbit prox- aac@arm.ac.uk imity criteria at various degrees of sophistication (Terentjeva 1993; Christou & Beurle 1999; Treiman & Treiman 2000; Larson 2001; Christou 2004a; Selsis, Brillet & Rappaport 2004), the most recent example being the work of Neslusan (2005). A byproduct of these works is that we now have a fairly strong grasp on the population characteristics of planet-approaching comets for Mars, Venus and the outer planets as compared to the Earth s (Ma et al 2002; Christou 2004b; Jenniskens 2006). However, even a cursory look at the Earth s strongest annual showers, and the small bodies that they are related to, shows that the minimum approach distance criterion cannot be the sole discriminator for meteor activity. While it works for, for example, the April Lyrids (C/1861 G1 Thatcher), August Perseids (109P/Swift Tuttle) and November Leonids (55P/Tempel Tuttle) in the sense that the minimum orbit-to-orbit distance (Christou & Beurle 1999) is <0.01 au for the parents of all these showers, it fails in several prominent cases such as the Geminids (3200 Phaethon), Taurids (2P/Encke), Ursids (8P/Tuttle), η Aquarids and Orionids (1P/Halley) where is typically of the order of 0.1 au or greater. In fact, our current understanding of the relation between comets, their debris and the path that the latter follows to the Earth s atmosphere show that both the physics and dynamics of streams and their parent bodies are key ingredients to the recipe for a strong meteor shower. This work takes on board the lessons learned from these Earthrelated studies and applies them to Venus and Mars. Our purpose C 2010 The Author. Journal compilation C 2010 RAS

2 2760 A. A. Christou is to identify that subset of the small body population that should yield reasonably high meteor activity in the atmospheres of these two planets (a quantification of high is given in the next section). 2 QUANTIFYING THE OBJECTIVE In this paper, we concentrate on annual showers, i.e. those that recur with the same intensity every Terrestrial ( d), Martian ( d) or Venusian ( d) year. These will be hereafter referred to as showers. A noteworthy consequence of this definition is that a Venusian shower can occur up to twice during a Terrestrial year. We do not contemplate meteor outbursts or the sporadic background; their modelling requires targeted large-scale numerical calculations (Christou, Vaubaillon & Withers 2007; Wiegert, Vaubaillon & Campbell-Brown 2009) and is outside the scope of this work. Further, we restrict our search to only those showers that achieve peak zenithal hourly rate (ZHR) equal to or greater than 10 at the Earth, roughly equal to the annually averaged level of sporadic activity. To do this, we need to take into account that intrinsic differences between the Terrestrial, Martian and Venusian atmospheres would yield a different visual meteor for a meteoroid of a given mass, density, composition and speed. Adolfsson et al. (1996) have shown that slow meteors at Mars would be fainter, although speedfor speed this loss in brightness is modest, mag (McAuliffe & Christou 2006a). We assume for simplicity that meteoroids entering the Martian atmosphere with a velocity higher than 15 km s 1 produce meteors as bright as those at the Earth while those slower than this value are not optically detectable. For Venus, we adopt the conservative assumption that meteors of any speed would be at least as bright as those at the Earth. In reality, the shorter scaleheight of the Venusian atmosphere at the theoretically predicted meteor ablation altitudes should translate into an intrinsically brighter meteor from a given meteoroid (Christou 2004a; McAuliffe & Christou 2006b). 3 COMETARY CLASSES AND METEOROID STREAMS Comets have been dynamically classified according to the value of their Tisserand constant with respect to Jupiter, T J, and their orbital period P (Levison 1996; Levison & Duncan 1997; Duncan, Levison & Dones 2004). The Tisserand constant of a comet with respect to a massive planet such as Jupiter is derived from the Jacobi integral of the particle s motion under the combined gravity of the planet and the Sun in the circular-restricted three-body problem. The circularity condition holds approximately true in the Sun Jupiterparticle system. In terms of the two-body heliocentric semimajor axis a, eccentricity e and inclination I, the Tisserand constant is expressed as T J = 1 α + 2 α ( 1 e 2) cos I, where α = a/a J is the ratio of the semimajor axis of the comet s orbit over that of Jupiter. Hereafter, we shall refer to T J as simply T. An alternative expression is T = 3 V 2,whereV is in units of the planetary orbital speed and represents the Keplerian energy of the comet with respect to the planet at encounter. In essence, T is a measure of the extent to which Jupiter s gravity changes the comet s orbit over time. It splits the cometary population into two broad classes, the so-called nearly isotropic comets (NICs; T<2) and the ecliptic comets (ECs; T>2). NICs are further divided into two subclasses according to their orbital period or, equivalently, P S Figure 1. Visualization of the meteoroid stream manifold as described in the text. A meteor shower results when the planet s trajectory intersects this structure. Its footprint on the planetary orbital plane, both pre- and post-perihelion, is highlighted by the red ellipses. The length of the planet s path through the larger ellipse (dashed segment) corresponds to the duration of the meteor shower. The section of the manifold indicated by the nearly parallel dashed segments lies below the planetary orbital plane. their semimajor axis: long-period comets (LPCs) with a>40 au (P >200 yr) and Halley-type comets (HTCs) with a<40 au. The subset of LPCs with orbital period less than 10 4 yr is sometimes referred to as the intermediate long period comets (ILPCs). On the opposite corner of the T-a domain, the EC class contains the Jupiter-family comets (JFCs;2<T <3) and the Encke-type comets (T >3, a<a J ). JFCs and HTCs are collectively called short-period comets. Jenniskens (2006) reviewed a large body of work on comet and meteor stream modelling that supports a relationship between the different dynamical classes of Earth-approaching comets and the ability to create strong showers. This is a reasonable thesis since the degree to which Jupiter s gravity perturbs these comets and their streams must have a bearing on whether a dense stream can form. In the application of this work to other planets, it is useful to think of a meteoroid stream as a volume of space enclosed by a two-manifold that bears a one-on-one and onto relationship with (i.e. is isomorphic to) a torus (Fig. 1). Although a manifold is a mathematical construct, we do not aim for mathematical rigour here, rather we offer this as a visualization aid. If, for example, we define a well-formed stream as one where the meteoroid orbits form a single dense bundle, then the stream manifold could be the surface containing a user-defined minimum fraction of these orbits. In its simplest manifestation, the long dimension of this manifold follows the orbit of the parent body while the short (or cross-sectional) dimension measures the intrinsic orbital scatter of particles within the stream. This scatter is created by the combined effect of the particle ejection conditions, orbital perturbations that occur on a time-scale faster than the dynamical evolution of the stream as a whole and particle scattering due to close encounters with the planets. 3.1 Nearly isotropic comets In the case of LPCs and ILPCs in particular, the orbital evolution of the parent due to planetary perturbations is negligible, so a stream manifold should form. However, due to the near-parabolic state of the parent, a significant fraction of particles ejected at perihelion are lost from the Solar system (and hence from the stream) altogether. The remaining particles are spread over a large volume of space due to their high values of a meaning that the stream is only sparsely populated. In other words, to create a dense stream the comet must generate particles at a sufficiently high rate and/or the orbital period must be relatively short. If a stream does form, it

3 Meteor showers at Venus and Mars 2761 Table 1. Earth-orbit-approaching comets used in Section 3 to quantify the strong (ZHR > 10) meteor shower parentage criteria used in this work. See text for details. Type Strong (ZHR 10) ILPC and HTC parent bodies 1P, 8P, 55P, 109P, C/1861 G1 Designations C/1739 K1, C/1846 J1, C/1852 K1, C/1853 G1, C/1854 L1, C/1854 R1, C/1871 V1, C/1874 G1, C/1894 G1, C/1907 G1, Weak (ZHR < 10) ILPC and HTC parent bodies C/1911 N1, C/1917 F1, C/1931 P1, C/1941 B1, C/1964 N1, C/1975 T2, C/1976 D1, C/1979 Y1, C/1983 H1, C/1987 B1, C/1991 L3, C/1995 O1, C/1999 A1 Earth-grazing JFCs 15P, 21P, 26P, 46P, 72P, 103P, P/2000 G1, 185P (P/2001 Q2), 209P (P/2004 CB) may have a typical width of a few hundredths of an au (cf. fig. 3 of Lyytinen & Jenniskens 2003). Another factor to consider is the number of revolutions that the comet has undergone in its present orbit since that is proportional to the amount of dust in the stream, assuming Whipple-type ejection. According to Jenniskens (2006), the supply rate can be quantified in terms of H 10, the magnitude of the comet at 1 au from the Earth and the Sun assuming that the comet brightens as the inverse fourth power of its heliocentric distance. In this sense, the fact that the only LPC meteor shower with ZHR > 10 known at the Earth is the April Lyrids (LYR), originating from a relatively bright comet (C/1861 G1 Thatcher; H 10 =+5.5) of 400 yr period, a modest value among LPCs, is not surprising. Weak showers (ZHR < 10) are associated with LPCs of longer period than Thatcher s comet such as C/1983 H1 IRAS Araki Alcock (P 1000 yr) and C/1911 NI Kiess (P 2000 yr). These comets, along with other LPCs that so far have not been associated with detectable annual activity, have H 10 > + 7 and/or >0.012 au (cf. fig. 6.2 of Jenniskens 2006). As will be discussed later in the paper, the Earth s orbital position in the Solar system is not ideal for constraining, through meteor observations, the balance between meteoroid source and sink for bright comets of long period. For HTCs, planetary perturbations induce significant apsidal and nodal precession on the comet s orbit as well as oscillations in either a, e or I arising from proximity to mean motion resonances. Differential perturbations on the initial orbital elements of ejected particles tend to distort the stream manifold, principally its short dimension, but do not destroy it; thus a stream can form. This expanding of the cross-sectional area of the stream renders the intersection condition easier to satisfy for a larger. Inthatcasea large, recently active, cometary nucleus is required to establish high dust density within the stream. Examples of such streams are (a) that of comet 1P/Halley, generating two distinct showers of ZHR > 10 at the Earth, the η Aquariids in May and the Orionids in October. The orbit of the comet is at present 0.06 and 0.15 au, respectively, from that of the Earth at those locations and (b) that of comet 8P/Tuttle, associated with the December Ursids; the orbit of this comet is, at present, 0.1 au from the Earth s orbit. Hence, several key characteristics of ILPCs and HPCs namely the orbital period P, the orbit-to-orbit approach distance and the absolute magnitude H 10 taken together may be employed to define domains where Mars- and Venus-approaching objects genetically related to strong shower at those planets should exist. To determine if this statement can be translated into a quantitative test, we searched the Catalogue of Cometary Orbits 2008 (Marsden & Williams 2008) for Earth-orbit-approaching ILPCs and HTCs using a two-phase procedure. In the first phase, we pre-screened candidate comets with T < 2 using the 3D orbit visualization tool at In the second phase, we calculated using the algorithm of Christou & Beurle (1999) and rejected those comets for which >0.1 au. Five of these objects orbits had indeterminate periods; their orbital eccentricities were calculated under the assumption that P = 10 4 yr. This left us with a sample of 23 ILPCs and HTCs. Where orbits were given for multiple perihelion passages of the same comet, the orbit corresponding to the most recent such passage was used. Absolute magnitudes for comets with a most recent perihelion passage before 1974 were taken from Vsekhsvyatskii (1963, 1964, 1967) and Vsekhsvyatskii & Il ichishina (1974). For other cases, the absolute magnitude was calculated from observations given in IAU circulars through the formula m = H log r + 5logδ, where m is the comet s apparent magnitude while r and δ are the heliocentric and geocentric distances, respectively. We then checked that none of these 23 objects are associated with strong annual shower activity as reported in appendix 7 of Jenniskens (2006). We refer to this as the weak population which we compare to the following six cases making up our strong population: C/1861 G1 (Thatcher; LYR), 109P/Swift Tuttle (PER), 55P/Tempel Tuttle (LEO), 1P/Halley (EAQ), 8P/Tuttle (URS) and 1P/Halley (ORI) (Table 1). Plots of those 29 objects periods versus their absolute magnitudes (Fig. 2) and values (Fig. 3), respectively, shows a good separation between the strong and weak populations. Moreover, within the uncertainties imposed by small number statistics, the offsets between the two populations are in the directions one would expect. Brighter LPCs with shorter periods are more likely to produce strong showers. Similarly, longer period LPC streams have relatively small cross-sectional areas, being fairly impervious to planetary perturbations; thus, needs to be smaller for a strong annual shower to exist. This can be represented quantitatively by fitting slopes to the strong population: this yields d log P/d H and d log P/dlog = 0.35 in the least-squares sense. The respective y-intercept constants are B (log H 10 = 0) = log P 0 = 3.06 and D (log = 0) = log P 0 = Our criteria for strength can then be defined through appropriate values of these latter constants to demarcate regions of H 10 P space that contain as many members of the strong population as possible and a userdefined contribution from weak population members. Indeed, the domain described by (B <3.8, D<1.75) (Domain I) contains the entire strong population but only one member of the weak population. The slightly larger domain (B <4.4, D<2.5) (Domain II) contains an equal number (six) of strong and weak cases. Thus, planet-approaching comets within Domain I will constitute the strongest candidates for shower parentage; they will be referred to hereafter as Class A candidates. In a similar vein, Class B

4 2762 A. A. Christou Period (yr) H_10 ZHR > 10 ZHR < 10 JFCs B=3.8 B=4.4 Figure 2. Absolute magnitude H 10 as a function of orbital period (yr) for Earth-approaching comets showing the boundaries between the different domains described in the text. Domain I, projected on (H 10, P ) space, lies to the left of the grey dotted line. The area between the grey and black dotted lines indicates Domain II projected on the same space. A sample of nine JFCs (Table 1) with <0.1 au has been superimposed on the plot to emphasize their non-compliance with the trends evident for ILPCs and HTCs and discussed in Section 3.1. Period (yr) Delta (AU) ZHR > 10 ZHR < 10 JFCs D=1.75 D= Figure 3. Minimum orbit-to-orbit approach distance (au) as a function of orbital period (yr) for Earth-approaching comets. See caption of Fig. 2 for details. candidates are defined as those residing within Domain II but outside Domain I, thus having a chance of being good candidates. Finally, we define as Class C candidates those comets that satisfy one of the parametric constraints of Domain II (i.e. either B<4.4 or D<2.5) but not both at the same time. The chance of those objects corresponding to strong showers is worse than but they should nevertheless be considered as plausible candidates to allow for our coarse statistics. All remaining comets were excluded from further consideration. 3.2 Ecliptic comets For ECs with T<3 (i.e. JFCs), the rapid and stochastic nature of the parent s orbital evolution is such that a stream manifold does not exist in general. Indeed, there are no strong (ZHR > 10) annual showers associated with such a comet in the Earth s meteor year (see also caption for Fig. 2), although for completeness we note that strong JFC-related outbursts do occur. Encke-type comets are related to some of the strongest daytime or nighttime meteor showers of the year. Their aphelia are decoupled from Jupiter, meaning that they do not suffer from the disruptive effects of close approaches with that planet. Their orbit undergo fast, albeit deterministic, evolution, an example of which is the Kozai cycle, where the eccentricity and inclination exhibit anticorrelated oscillations that follow the evolution of the argument of pericentre ω while a remains constant (Kozai 1962; Hamid & Youssef 1963). The evolution of such an orbit means that the stream manifold can intersect the Earth s orbit more than twice and up to eight times (Babadzhanov & Obrubov 1992). The dynamical process that decouples these orbits from Jupiter ineffectreducinga is thought to be slow (Levison et al. 2006), consistent with the many of the identified parents of Encke-type meteor showers being inert (Jenniskens 2007; Babadzhanov, Williams & Kokhirova 2008) and thus difficult to associate genetically with streams unless a shower is directly observed. In addition, cometary nucleus fragmentation is now thought to be an important process in the formation of Encke-type meteoroid streams (Jenniskens & Lyytinen 2005; Jenniskens 2008). This results in either impulsive events of dust injection in the stream able to sustain a strong shower for centuries, or the gradual feeding of meteoroids into the stream from the fragments themselves through additional fragmentation or ice-sublimation-driven ejection. As fragmentation does not occur uniformly over a precession cycle of the angular elements (ω or ), only certain parts of the stream manifold, corresponding to certain values of these angles, are populated by dust dense enough to produce meteor showers at the Earth. Consequently, one cannot use orbit-to-orbit proximity or an object s brightness to determine whether an Encke-type stream dense enough to produce a strong shower exists. Here, we used the orbits of known streams as measured through their showers at the Earth as the principal discriminators. The showers that are intense enough to concern us here can be split into three groups according to their common orbital evolution and progenitor heritage. These are: (i) the Phaethon Geminid complex shower group, containing the Geminid (GEM) and daytime Sextantid (DSX) meteor showers (ii) the Encke group, containing the Northern and Southern Taurids (NTA and STA), daytime β Taurids (BTA) and daytime ζ Perseids (ZPE) and (iii) the Machholtz complex shower group, containing the Quadrantids (QUA), Southern δ Aquarids (SDA) and daytime Arietids (ARI). Although the Quadrantids are in a Jupiter-family orbit, they are thought to be related to the progenitor of the HTC 96P/Machholtz and the Marsen/Kracht Sunskirter groups (Sekanina & Chodas 2005). 4 SEARCH AND RESULTS Hence, we proceeded as follows: we (i) do not consider JFCs as strong candidates for Martian/Venusian meteor shower parentage apart from a limited number of cases reported in Section 5.4 that we regard as exceptional, (ii) use the orbits of substreams corresponding to strong Encke-type showers in order to determine whether these also manifest themselves at Mars and Venus and (iii) use the dynamical (orbital period P, orbit-to-orbit distance ) and physical characteristics (absolute cometary magnitude H 10 ) of known HTCs and ILPCs with periods less than 2500 yr to establish their association with Martian and Venusian showers at present through the classification scheme introduced in the previous section.

5 Meteor showers at Venus and Mars 2763 Table 2. Mars ILPC and HTC meteor shower candidate parent bodies. See Section 3.1 for classification scheme. Reference orbits were taken from Marsden & Williams (2008) with the exception of C/1854 L1 Klinkerfues (Vaubaillon & Jenniskens 2007) and 5335 Damocles (Giorgini et al. 1996). Additional results for 1P/Halley are taken from Christou et al. (2008). An asterisk ( ) denotes an exceptional case discussed separately in Section 5.4. a H 10 b Period Assigned v c L S d Dates e RA f Dec. f SZA g LST h Name ( 10 3 au) (yr) class (km s 1 ) ( ) (2010) (2011) ( ) ( ) ( ) ( ) 1P A : : :17 13P A : P A : P C : P/2006 HR B : C/1769 P C : C/1854 L A : C/1932 G B : C/1940 O C : C/1942 X C : C/1952 H C : C/1974 O C : C/1979 Y B : C/1984 U C : C/1998 U C : C/2007 D C : C/2007 H C : B : : a Minimum orbit-to-orbit distance as defined in Christou & Beurle (1999). b Magnitude at a distance of 1 au from the Earth and from the Sun during the most recent apparition assuming a fourth-power brightness dependence on heliocentric distance. c Meteor speed at a height of 120 km from the planetary surface. d Solar longitude as defined by the geoscience community (Clancy et al. 2000). It is related to the astronomical solar longitude λ through the relationship L S λ = at 12:00 UTC on 2000 January 1. e Epochs of peak activity (DDMM:HH) calculated for 2010 and f Right ascension and declination of the theoretical shower radiant in the IAU_MARS reference frame as defined in Davies et al. (1995). The reference plane in this frame is the J2000 Martian equator; the reference direction (x-axis) is that of the ascending node of the Martian equator on Earth s equator. g Solar zenith angle; the angle between the planetocentic directions of the Sun and the shower radiant at the epoch of maximum activity. SZA > 90 implies a nighttime radiant. h Local solar time defined as the angle between the projections of the radiant vector and the position of the Sun at the given epoch on the Martian equator. It is positive on the left half of the planet ( evening ) and negative on the right half ( morning ) as seen from a hypothetical observer looking down on the planet from the north pole of the reference frame with the Sun towards the top. This definition is independent of the planet s sense of rotation. For near-circular planetary orbits the morning and evening hemispheres as defined here coincide with the leading and trailing hemispheres of the planet in its motion around the Sun. Table 3. Mars JFC and Encke-type meteor shower candidates. MG refers to the Marsden group of sunskirting comets. See Table 2 for details. v L S Dates RA Dec. SZA LST Name ( 10 3 au) (km s 1 ) ( ) (2010) (2011) ( ) ( ) ( ) ( ) Ref NTA : Porubcan & Kornos (2002) 2004 TG : Giorgini et al. (1996) STA : Porubcan & Kornos (2002) ZPE : Sekanina (1976) BTA : Sekanina (1976) ARI : Campbell-Brown (2004) MG : Sekanina & Chodas (2005) GEM : Betlem (2001) GEM : :00 Ryabova (2007) : Giorgini et al. (1996) DSX : Galligan & Baggaley (2002) 2005 UD : Giorgini et al. (1996) SDA : Betlem (2001) The results for ILPCs and HTCs for Mars and Venus are given in Tables 2 and 4 whereas those for known EC-type streams in Tables 3 and 5, respectively. The process of sifting through planetapproaching cometary candidates was essentially identical to the one used to identify Earth-approaching comets in the previous section. For each candidate classified as A, B or C, we have determined the value of, the impact velocity v in the planetary atmosphere and the date of closest approach of the planet to the cometary or

6 2764 A. A. Christou Table 4. Venus ILPC and HTC meteor shower candidate parent bodies. Reference orbits were taken from Marsden & Williams (2008). See Table 2 for details. H 10 Period Assigned v λ a Dates RA b Dec. b SZA LST Name ( 10 3 au) (yr) class km s 1 ( ) (2010) (2011) ( ) ( ) ( ) ( ) 1P A : :06, 2010: : : : :16, 1810: :09 12P A :23, 2211: : P A :22, 2011: : P B :23, 0310: :15, 2712: P A :14, 2511: : P/2005 T C :07, 2410: : C/1857 O B :19, 2410: : C/1858 L B :04, 2709: :14, 2112: C/1881 K C : :03, 0411: C/1888 D C : :14, 2010: C/1917 F A :01, 0112: : C/1937 D C :08, 0811: : C/1939 B C :21, 1312: : C/1964 L C :09, 1109: :19, 0412: a Astronomical solar longitude defined as 180 ϖ M, the heliocentric longitude of perihelion, longitude of ascending node and mean anomaly of Venus, respectively. b Right ascension and declination of the theoretical shower radiant in the IAU_VENUS reference frame. See Table 2 for details. stream orbit for 2010 and Dates of subsequent approaches can be calculated by adding appropriate multiples of the planetary orbital period (see Section 2) to these dates. In addition, we provide the local observational circumstances, namely the right ascension (RA) and declination (Dec.) of the theoretical shower radiant with respect to a local frame referenced on the planetary equator as well as its solar zenith angle (SZA) and local solar time (LST). The latter is measured with respect to the direction of the Sun and the direction of motion of the planet in its orbit (see also footnote in Table 2). We expect that this type of information will be crucial for any statistical exercise attempting to link observational evidence for meteor activity to our theoretical predictions. In the case of known Encke-type streams, we have opted for orbits determined from optical observations in the case of nighttime showers, and radar-determined orbits for daytime showers. We have also considered (i) asteroidal or cometary objects for which a case for genetic linkage with these showers has been made in recent literature and (ii) a few cases where a stream s structure at either Mars or Venus has been explicitly addressed [e.g. GEMs, κ Cygnids (KCG)]. 5 DISCUSSION OF INDIVIDUAL CASES 5.1 Periodic comets (i) 1P/Halley (Mars+Venus). This is the archetype for the Halley-type group of comets. It is a Great Comet according to D. K. Yeomans ( Great Comets in History, and produces two nighttime meteor streams at the Earth, the northerly Orionids in October and the southerly η Aquarids in May. Also mentioned as a potential Martian/Venusian shower parent by Christou & Beurle (1999), Treiman & Treiman (2000), Christou (2004a), Selsis et al. (2004), Neslusan (2005), Jenniskens (2006), Domokos et al. (2007). The Orionid branch, of relevance here, lasts for about a week and peaks at ZHR 23 around October 21. The encounter conditions of a numerical model of the 1P/Halley stream at Mars and Venus were investigated by Christou, Vaubaillon & Withers (2008). At Mars, fast (54 km s 1 ) Halleyid meteors would impact on the morning hemisphere of Mars, slightly south of the equator. The shower should last a minimum of 8 of solar longitude at Mars and 12 at Venus. At Venus, the Halley shower will consist of fast meteors (80 km s 1 ) that would also impact primarily on the morning side. (ii) 12P/Pons Brooks, 122P/de Vico and 27P/Crommelin (Venus) are thought to be responsible for forming a cluster of meteor showers within a short interval of solar longitude (Christou 2004a,b; Jenniskens 2006). Speeds are moderate for 27P (27 km s 1 )butfast for the other two (50+ km s 1 ). These three comets were also mentioned in Beech (1998). In addition, 27P and 122P ware mentioned in Selsis et al. (2004) while 27P alone is mentioned in Neslusan (2005). (iii) 13P/Olbers (Mars) is a bright HTC (P = 70 yr, H 10 = +5.5). Previously identified as a potential Martian meteor shower parent (Christou & Beurle 1999; Treiman & Treiman 2000; Christou 2004a; Selsis et al. 2004; Neslusan 2005; Jenniskens 2006). Its meteors would be moderately fast (27 km s 1 ), impacting the southern hemisphere of Mars well into the nightside. (iv) 35P/Herschel Rigollet (Venus) is a moderately bright HTC in an orbit relatively far from that of Venus. Mentioned by Beech (1998), Christou (2004b) and Jenniskens (2006). (v) 161P/Hartley IRAS (Mars) is a moderately bright HTC crossing the Martian orbital plane outside the orbit of Mars. 161P meteoroids hit at a fast 44 km s 1 primarily on the Southern hemisphere, morning side. The orbital characteristics of this candidate parent comet are reminiscent of those of 8P/Tuttle, the parent of the December Ursids at the Earth. On the other hand, this object s orbit approaches that of Jupiter; it may have arrived recently at its present location. In any case, we expect that proximity to first-order mean motion resonances with Jupiter will have acted to spread the stream sufficiently to allow Mars orbit encounters. Previously mentioned in Neslusan (2005). (vi) 177P/Barnard (Mars) is a faint HTC. 177P meteoroids hit Mars primarily on the Southern hemisphere, morning side at a moderate 22 km s 1. (vii) P/2005 T4 SWAN (Venus) is a faint, retrograde HTC discovered in SOHO SWAN images (Matson et al. 2005). Its meteors

7 Meteor showers at Venus and Mars 2765 Table 5. Venus JFC and Encke-type candidate meteor showers. See Table 2 for details. v λ Dates RA Dec. SZA LST Name ( 10 3 au) (km s 1 ) ( ) (2010) (2011) ( ) ( ) ( ) ( ) Reference NTA :11, 1611: : Porubcan & Kornos (2002) 2004TG :08, 1311: : Giorgini et al. (1996) STA :18, 1111: : Porubcan & Kornos (2002) BTA : :14, 0111: Sekanina (1976) ZPE : :10, 1610: Sekanina (1976) ARI : :02, 2210: Campbell-Brown (2004) MG : :22, 2510: Sekanina & Chodas (2005) CAP :19, 2509: :05, 1812: Jenniskens (2006) 169P :22, 3009: :08, 2412: Marsden & Williams (2008) GEM :05, 0112: : Betlem (2001) GEM : :20, 1507: :06 Ryabova (2007) 0212: : :23, 0212: : Giorgini et al. (1996) DSX :10, 0410: :20, 2712: Galligan & Baggaley (2002) 2005UD :11, 0810: :21, 3112: Giorgini et al. (1996) KCG :20, 1709: :06, 1012: Porubcan & Gavajdová (1994) KCG : :12, 2904: :21, Jenniskens & Vaubaillon (2008) 1709: : : : ED :01, 2009: :11, 1412: Giorgini et al. (1996) enter the Venusian atmosphere at a fast 80 km s 1 on the morning side of the planet. 5.2 Single apparition comets (i) C/1769 P1 Messier (Mars) is one of the great comets of the 18th century, observed over a period of 4 months. Its best-fitting orbit, by F. Bessel, is one of long period (P 2100 yr). Meteors from this comet would impact primarily on the North hemisphere of Mars near the midnight direction at a fast 41 km s 1. (ii) C/1857 O1 Peters and C/1937 D1 Wilk (Venus) are relatively faint comets straddling the boundary between ILPC and HTC status. C/Wilk has been mentioned in Beech (1998) as a potential parent of a Venusian meteor shower; both comets are mentioned in this respect in the work of Neslusan (2005). (iii) C/1858 L1 Donati (Venus) is regarded as a great comet and is the first comet to be recorded photographically. It reached a magnitude of 1 in 1858 July at a geocentric distance of 0.5 au, exhibiting a tail 60 long (see Kronk 2003 and references therein). The relatively long observational data arc ( 270 d) allowed the fitting of an elliptical orbit with a period of 2000 yr. The encounter distance of its orbit to that of Venus ( au) is the closest any Great Comet s orbit currently approaches a terrestrial planet s. A strong morning meteor shower at Venus, primarily on the Northern hemisphere, is a good possibility unless the comet is a new arrival to the inner Solar system. (iv) C/1881 K1 Tebbutt (Venus) is another Great Comet with a period similar to that of C/1858 L1, albeit slightly fainter, that approaches the orbit of Venus to within 0.01 au. As for C/Donati, the time-span of observations was sufficient to determine an elliptical orbit. It was also the subject of the first photographic recording of a comet s spectrum. Its radiant declination of 73 at Venus is essentially circumpolar. (v) C/1888 D1 Sawerthal (Venus) is intrinsically slightly fainter again than C/Tebbutt. It was observed to split into at least two fragments during its 19th century apparition. Its meteors will be similar to those of C/Tebbutt in that they impact Venus at high southern latitude. (vi) C/1917 F1 Mellish (Venus) is a HTC with a period of 145 yr. At the Earth, this comet is associated with the weak (ZHR 2) December Monocerotid shower (Ohtsuka 1989; Lindblad & Olsson- Steel 1990). Fox & Williams (1985) have shown that the orbit of the Mellish stream is stable over a period of >2000 yr; a very slow precession of the node causes the nodal distance to linger near 1 au for several millenia. Its distance to Venus orbit is <0.01 au compared to 0.06 au at the Earth implying that the resulting shower at Venus will be more intense. A rough estimate of the level of Venusian activity can be made if we assume that the spatial density of meteoroids giving rise to visible meteors decreases with distance from the orbit of the comet as 10 B /( X), where a typical value for the slope B at the Earth is 0.2 (Jenniskens 1994) and X= au is the distance travelled by the Earth along its orbit in 1 d. This yields an enhancement factor of 4 for the Mellish meteor shower at Venus and a ZHR of 10. Given the intrinsic ability of the Venusian atmosphere to produce brighter meteors for a given meteoroid than the terrestrial one, this figure should be regarded as a lower limit. Thus, the annual meteor shower at Venus could be reminiscent of the annual Lyrids or Leonids at the Earth. The stream encounter characteristics at Venus (as at the Earth) are very similar to those of the GEMs (discussed in Section 5.3), so precise optical observations would be required to distinguish between members of

8 2766 A. A. Christou these two showers. Previously mentioned in the works of Beech (1998), Neslusan (2005) and Jenniskens (2006). (vii) C/1932 G1 Houghton Ensor and C/1940 O1 Whipple Paraskevopoulos (Mars) are ILPCs with orbital periods similar to that of the brighter April Lyrid parent C/1861 G1 Thatcher. Their orbital approach distances to Venus are, however, an order of magnitude greater than Thatcher s comet at the Earth. C/Houghton Ensor meteors impact primarily the South hemisphere of Venus on the morning side while those of C/Whipple Paraskevopoulos are Northern circumpolar. Both comets were also identified as potential Venusian shower candidate parents in Neslusan (2005). Houghton Ensor is also mentioned in Treiman & Treiman (2000). (viii) C/1939 B1 Kozik Peltier (Venus) is a faint comet (H ) in an orbit that approaches the orbit of Venus to au. Its orbital period, physical parameters and Venus orbit encounter distance make it similar to Earth-approaching ILPC C/1911 N1 (Kiess). (ix) C/1942 X1 Whipple Fedtke Tevzadze (Mars) is a bright ILPC approaching the orbit of Venus at au. Its meteors at Mars will be slow, straddling the lower cut-off imposed in this paper for detectable Martian meteors. They impact slightly South of the equator, primarily at nighttime. Previously mentioned in Treiman & Treiman (2000). (x) C/1952 H1 Mrkos and C/1974 O1 Cesko (Mars) are similar to C/Houghton Ensor and C/Whipple Paraskevopoulos but with slightly longer orbital periods and slightly smaller at the orbit of Mars. Meteors from C/Mrkos will impact the Northern hemisphere at daytime while C/Cesko meteors impact on the evening side of the planet. C/Cesko should also produce very fast meteors at Mars (62kms 1 ). Previously mentioned by Treiman & Treiman (2000) and Neslusan (2005). (xi) C/1964 L1 Tomita Gerber Honda (Venus) is an interesting case due to its extremely small value of, comparable to the size of Venus itself. The planet s atmosphere may thus be sampling the core of a long-period-type stream. The meteors will be fast (80 km s 1 ) and impact the morning side of the planet. (xii) C/1979 Y1 Bradfield (Mars) is a comet of 300 yr period. This would be a morning, Southern hemisphere shower. (xiii) C/1984 U2 Shoemaker (Mars) is a faint ILPC on a relatively short (270 yr) orbit. Its value of at Mars is small, 10 3 au. Comparing it with C/1911 N1 Kiess, the parent of the weak September Aurigids (AUR) shower, we find that, on one hand, its period is shorter but the comet itself is fainter. Its meteors will be relatively slow, impacting the Northern hemisphere of the planet in deep night. (xiv) C/1998 U5 LINEAR (Mars) is a brighter (H 10 =+8), longer period (P 1000 yr) version of C/1984 U2. It is similar to C/Kiess in brightness and orbital proximity to the planet but with half the orbital period. It should produce a moderately strong shower slightly below or at a ZHR of 10, the fast (53 km s 1 ) meteors impacting primarily on the morning side of the Southern hemisphere of Mars. Previously mentioned in Treiman & Treiman (2000), Larson (2001), Selsis et al. (2004) and Domokos et al. (2007). (xv) C/2007 D2 Spacewatch (Mars) is a faint ILPC with a 400 yr period in a retrograde orbit. Its orbital plane is essentially the ecliptic, meaning that it can have close approaches with Jupiter and the other giant planets both post- and pre-perihelion. Consequently, it may not have existed in its current orbit for more than a few revolutions. The effect of these close approaches may be mitigated on account of its retrograde motion. On Mars, it should produce fast meteors (53 km s 1 ) on the morning side of the planet. (xvi) C/2007 H2 Skiff (Mars) is the faintest Mars-orbitapproaching comet in our list and the one with the smallest value of. Its orbital period is similar to that of C/1861 G1 Thatcher. Its current orbit is very close to that of Saturn. Its meteors at Mars would impact the daytime hemisphere of the planet south of the equator. 5.3 Dense Encke-type streams with known orbits (i) The STA and NTA are the nighttime, pre-perihelion manifestations of the well-researched Taurid meteoroid stream related to comet 2P/Encke (Porubcan, Cornos & Williams 2006). They are two of the longest lasting nighttime showers at the Earth extending from October to December [full width at half-maximum (FWHM) 22 d or 0.4 au], being typically rich in slow, bright meteors and fireballs. Their activity maximum is reached during the first week of November with a combined ZHR of 10. Their characteristics at Mars and Venus would be similar with radiants slightly north of the nightside equator and impact speeds of 27 (Mars) and 32 (Venus) km s 1, respectively. Also mentioned in Treiman & Treiman (2000) and Domokos et al. (2007). (ii) The ZPE and BTA are the daytime, post-perihelion manifestations of the Taurid stream, the former being the summertime counterpart of the STA and the latter of the NTA [see e.g. Porubcan et al. (2007) for a review of past work]. The two showers peak on June 9 and 28at ZHR 20 and 10, respectively. Their FWHM is similar to their nighttime counterparts, 18 d or 0.3 au along the Earth s orbit. They should both be active as daytime showers at Mars and Venus. The speeds of the two showers at the Earth are somewhat different, with BTA slightly faster than ZPE. This feature is reproduced at the other two planets (ZPE-Mars: 23 km s 1 ;ZPE- Venus: 30kms 1 ; BTA-Mars: 27 km s 1 ; BTA-Venus: 35kms 1 ). The ZPE are also mentioned in Treiman & Treiman (2000). (iii) The GEM is a strong (ZHR 100) nighttime shower in December with FWHM 1 d. It shares an orbit with minor planet 3200 Phaethon, a fact that elevated the latter to the status of an extinct/dormant comet or a main belt comet (Williams & Wu 1993; Licandro et al. 2007). The stream, or rather the orbit of the part of the stream that encounters the Earth, is not close to Mars; however, recent work by Ryabova (2007) indicates that the GEM should also encounter that planet as well as Venus where has a smaller value. By measuring figs 5 and 6 of that paper, we find that the peak of the shower for the particle masses modelled by Ryabova should occur somewhere in the interval L S = at Mars and λ = for Venus. Note that the former range lies 5 in solar longitude away from the nominal peak of the stream calculated using the orbit from Betlem (2001). This is an equatorial nighttime shower at all three planets. The speed varies from 28 km s 1 at Mars to 42 km s 1 at Venus. (iv) The DSXs are active at the end of September/beginning of October at the Earth. They are regarded as the daytime branch of the well-known GEM shower, their peak rate being 1/3 that of the GEM (Ohtsuka et al. 1997). Recently, associations were reported with near-earth asteroids 2005 UD (Ohtsuka et al. 2005) and 1999 YC (Ohtsuka et al. 2008), now thought to be fragments of the dormant/extinct comet 3200 Phaethon (Jewitt & Hsieh 2006; Ohtsuka et al. 2006; Kasuga & Jewitt 2008). At Venus and Mars, the Sextantids would also be a daytime shower. The small distance to Venus orbit implies a similar level of activity at that planet. The case of Mars is different in that, given the larger approach distance, the shower may be less active or (our favoured hypothesis given Ryabova s results for the GEM) Mars may be sampling a different part of the stream.

9 Meteor showers at Venus and Mars 2767 (v) The daytime ARI is the strongest daytime shower of the year, peaking at ZHR 60 at the beginning of June with a FWHM of 9 d (Campbell-Brown 2004). It is thought to originate from the Machholtz complex of comets (named after their largest member, 96P/Machholtz) likely the product of progressive fragmentation of a large parent body currently taking place along a Kozai cycle of dynamical evolution. The ARI in particular are related to the Marsden group of sunskirting comets (Sekanina & Chodas 2005). Martian ARI would appear at L S = 180, Venusian ARI at λ = 71 although the critical longitude corresponding to the Marsden Sunskirters is somewhat offset, at 75. The speed of the meteors brackets the shower s at the Earth, being 32 (Mars) and 46 km s 1 (Venus). The parent body orbit used in our calculations is the average of C/1999 N5 and C/1999 J6 in tables 6 and 8 of Sekanina & Chodas (2005). (vi) The SDA is a summer nighttime stream peaking at a modest ZHR in the last few days of July, occasionally producing rates as high as on certain years (Johahnnik et al. 2008). As mentioned earlier, it is related to the Machholtz complex of comets. The stream s prominence in radar surveys, rivalling that of the GEMs, points towards a relatively large population of smaller meteoroids in the stream (Brown et al. 2008), resulting in a stealth component to this shower. At Mars, it will also be a nighttime stream, probably manifesting itself at the Terrestrial level of activity, given its relatively large width (FWHM 6 d or 0.1 au). Also mentioned in Treiman & Treiman (2000). 5.4 Other exceptional cases (i) The KCG is a potential highlight of the Venusian meteor year. This relatively weak shower at the Earth (ZHR 2) attracted attention recently due to an outburst in 2007 containing many bright fireballs (Jenniskens et al., CBET 1055), prompting a search for the parent body. This resulted in the identification of NEA 2008 ED69 (Jenniskens & Vaubaillon, CBET 1453). Modelling of the stream led Jenniskens & Vaubaillon (2008) to conclude that...the κ Cygnids will be an important shower at Venus.. We find that the present orbit of 2008 ED69 approaches that of Venus at a very small distance ( au). Even if that were not the case in the past, analysis of fig. 4 in that paper shows that the cores of all the model streams of meteoroids ejected from 4000 BC to 1000 AD neatly intersect the Venusian orbit. Thus, the case for annual activity at Venus with a duration of >3 in orbital longitude appears strong. It is also noteworthy that, with a declination of +80, the Venusian KCG is a Northern circumpolar shower. This unusual feature could be used to aid observations. (ii) P/2006 HR30 Siding Spring (Mars) is a large, weakly active, cometary nucleus in a Halley-type orbit (Hicks & Bauer 2007; Demeo & Binzel 2008). Its orbit is relatively stable with no approaches closer than 1 au to Jupiter for the past 400 years or 20 revolutions of the comet (Giorgini et al. 1996) and probably longer. Thus, a strong shower related to this comet should be dependent on its (unknown) history of past activity. (iii) C/1854 L1 Klinkerfues (Mars) is a relatively bright comet with a poorly determined orbit. It is associated with a weak September shower at the Earth, the ɛ Eridanids. The nominal orbit solution for this comet is parabolic, yet Vaubaillon & Jenniskens (2007), in an attempt to link Klinkerfues with an outburst of the shower observed in 1981, derived an elliptical HTC-type orbit assuming that comet C/962 B1 was a previous apparition of the same object. This latter orbit was used here to calculate the Mars-encounter circumstances listed in Table 2 and was found to approach the orbit of Mars to au, 10 times closer than the Earth value. Hence, the observational confirmation of this stream at Mars would be an excellent test of the Vaubaillon & Jenniskens hypothesis. (iv) 5335 Damocles (Mars) is a large (H +11) asteroid in a Halley-type orbit, the archetype of the Damocloid class of suspected inactive comets (Asher et al. 1994; Jewitt 2005). Its orbit, which has been stable for the past 10 4 years, has an aphelion at Uranus orbit and is near several high-order mean motion resonances with Jupiter (Asher et al. 1994). Therefore, it is reasonable to expect a population of Mars-orbit-crossing meteoroids originating from Damocles if this suspected cometary nucleus has been active in its present orbit. Previously mentioned by Christou & Beurle (1999) and Jenniskens (2006). (v) The JFC 169P/NEAT (Venus) was initially discovered as minor planet 2002 EX12 by Ticha et al. (MPEC 2002-F30). An association between this object and the α Carpicornids (CAP), a weak (ZHR 2) shower rich in bright meteors, was proposed by Wiegert & Brown (2004) and Jenniskens (2006) EX12 was subsequently found to be weakly active by Warner & Fitzsimmons (2005). Its low aphelion distance (4.6 au) translates into a sequence of shallow encounters with Jupiter (>0.9 au) for the past 400 yr (Giorgini et al. 1996) and possibly longer. The comet s orbit is currently twice as close to Venus as it is to the Earth s while the speed of the meteors is similar in both cases ( 20 km s 1 ). 6 SPECIFIC FEATURES OF THE MARTIAN AND VENUSIAN METEOR YEAR 6.1 Meteor activity seasons At the Earth, strong meteor showers are not distributed evenly throughout the year. The Quadrantids in early January are followed by a 100-day lull in activity until the April Lyrids (cf. fig. 2 of Brown et al. 2008). In the top panel of Fig. 4, we have simulated this feature of the distribution using a toy model where Encke-type Solar Longitude (deg) Figure 4. Distribution of known and predicted cometary streams as a function of solar longitude at the Earth (upper panel), Mars (middle panel) and Venus (bottom panel). Vertical red line segments of length 3, 2 and 1 correspond to showers related to specific ILPCs and HTCs classified as A, B and C in Tables 2 and 4 in this paper. Vertical blue line segments of length 4 correspond to known streams at the Earth as listed in Tables 3 and 5.