New Observations and Models of the Kinematics of the Zodiacal Dust Cloud
ABSTRACT We report on new observations of the motion of zodiacal dust using optical absorption line spectroscopy of zodiacal light. We have measured the change in the profile shape of the scattered solar Mg I 5184 line toward several lines of sight in the ecliptic plane as well as the ecliptic pole. The variation in line centroid and line width as a function of helio-ecliptic longitude show a clear prograde signature and suggest that significant fraction of the dust follows non-circular orbits that are not confined to the ecliptic plane. When combined with dynamical models, the data suggest that the zodiacal dust is largely cometary, rather than asteroidal, in origin.
- SourceAvailable from: John C Mather[show abstract] [hide abstract]
ABSTRACT: The orbital evolution of asteroidal, trans-Neptunian, and cometary dust particles under the gravitational influence of planets, the Poynting-Robertson drag, radiation pressure, and solar wind drag was integrated. Results of our runs were compared with the spacecraft observations of the number density of dust particles and with the WHAM observations of velocities of zodiacal particles. This comparison shows that the fraction of cometary dust particles of the overall dust population inside Saturn's orbit is significant and can be dominant. The probability of a collision of an asteroidal or cometary dust particle with the Earth during its dynamical lifetime is maximum at diameter about 100 micron.07/2006;
- [show abstract] [hide abstract]
ABSTRACT: Any theory of the origin of the particles that supply the zodiacal cloud must account for two key, well-established observations. These are: (1) the observed plane of symmetry of the cloud; and (2) the observed shape of the cloud, that is, the observed variation of the flux in a given waveband with ecliptic latitude for a given elongation angle. The dynamics of small asteroidal particles appears to account for the first observation (Dermott et al., Chaos, Resonance and Collective Phenomena in the Solar System, pp. 333–347. Kluwer, Dordrecht, 1992). However, asteroidal particles that spiral towards the Sun under the action of drag forces without significant disintegration due to dust-dust collisions do not, on their own, provide an explanation for the observed shape of the cloud, because asteroidal dust models provide insufficient flux at the Earth's ecliptic poles (Dermott et al., Chaos, Resonance and Collective Phenomena in the Solar System, pp. 333–347. Kluwer, Dordrecht, 1992). In an attempt to account for this major diserepancy, the dynamics of cometary particles is investigated. The orbital evolution of 9 μm diameter dust particles that originate from Comet Encke is described. The orbits of 5000 particles are integrated numerically in order to determine their spatial distribution in 1983. All planetary perturbations (except those due to Mercury and Pluto), radiation pressure, Poynting-Robertson drag, and solar wind drag are included in the calculation. The SIMUL code (Dermott et al., Comets to Cosmology, pp. 3–18. Springer, Berlin, 1988) is used to calculate the shapes of several model zodiacal clouds consisting of a range of combinations of cometary and asteroidal particles. By comparing the model results with IRAS observations in the 25 μm waveband, it is shown that the observed shape of the zodiacal cloud can be accounted for by a combination of about to asteroidal dust and about to cometary dust. This result is consistent with other work on the structure of the IRAS dust bands and the Earth's resonant ring (Dermott et al., Nature369, 719–723, 1994; Asteroids, Comets and Meteors, 1993, pp. 127–142. Kluwer, Dordrecht, 1994). It is also consistent with the conclusions of other workers that the cloud must be heterogeneous (Levasseur-Regourd et al., Icarus86, 264–272, 1990; Origin and Evolution of Interplanetary Dust, pp. 131–138. Kluwer, Japan, 1991).Planetary and Space Science 01/1995; · 2.11 Impact Factor
arXiv:astro-ph/0604229v1 11 Apr 2006
NEW OBSERVATIONS AND MODELS OF THE KINEMATICS OF THE ZODIACAL
G.J. Madsen1, R.J. Reynolds2, S.I. Ipatov3, A.S. Kutyrev4, J.C. Mather4, and S.H. Moseley4
1Anglo-Australian Observatory, P.O. Box 296, Epping, NSW 1710, Australia, Email: email@example.com
2Univ. of Wisconsin-Madison, Madison, WI, 53711, USA, Email: firstname.lastname@example.org
3Univ. of Maryland, College Park, MD 20742, USA, Email: email@example.com
4NASA - Goddard Space Flight Center, Greenbelt, MD, 20771, USA
We report on new observations of the motion of zodiacal
dust using optical absorption line spectroscopy of zodi-
acal light. We have measured the change in the profile
shape of the scattered solar Mg I λ5184 line toward sev-
eral lines of sight in the ecliptic plane as well as the eclip-
tic pole. The variation in line centroid and line width as a
functionof helio-eclipticlongitudeshow a clear prograde
signature and suggest that significant fraction of the dust
follows non-circular orbits that are not confined to the
ecliptic plane. When combined with dynamical models,
the datasuggestthatthezodiacaldustis largelycometary,
rather than asteroidal, in origin.
The motion of interplanetary dust particles contains im-
portant information about their origin, distribution, and
orbital evolution. At optical wavelengths, dust with radii
≈ 10-100µm that lie within ≈ 3 AU of the Sun scatters
the incident solar radiation to produce zodiacal light, and
the relative motion of the dust modifies the location and
shape of solar spectral lines [1-3]. The fraction of zodi-
acal dust with a cometary or asteroidal origin is not well
constrained at present [4-5], and the kinematics of these
two components may shift the velocity and widths of the
spectral features in unique ways. However, the low sur-
face brightness of zodiacal light has, until recently, lim-
ited the observability of this effect, requiring a combina-
tion of high sensitivity and high spectral resolution. Pre-
vious work has shown the dust to be on prograde orbits,
but different groups reported uncertain and contradictory
results regarding the details of the orbital properties of
the dust [6-7]. Here, we report on new measurements
of scattered solar Mg I λ5184 absorption line in zodiacal
light with the Wisconsin H-AlphaMapper(WHAM), and
compare the observations with predictions from dynami-
cal models of the zodiacal dust cloud.
WHAM consists of a 15cm, dual-etalon Fabry-Perot
spectrograph coupled to a 0.6m siderostat, and produces
an average spectrum over a 1◦circular field of view with
a 12 km/s resolution within a 200 km/s spectral window.
It is located at the Kitt Peak National Observatory in Ari-
zona, and is entirely remotely operated. It is specifically
designed to study extremely faint, diffuse optical light at
high spectral resolution . We have recorded spectra
centeredontheMg Iline at 5183.6˚ Atoward49directions
along the ecliptic equator, with two directions at high
ecliptic latitude on the nights of 2002 November 4 and
5 . The unprecedentedcapabilities of WHAM allowed
sion lines that probably affected the results of previous
investigations [6-7]. The terrestrial lines are stronger on
the red side of the Mg I line. If they are not accounted
for, they shift the line centroid to negative velocities and
produce asymmetries in the line profile shape.
A sample spectrum demonstrating the high resolution
and sensitivity of the observations is shown in Figure 1.
This spectrum was taken toward the north ecliptic pole,
with the brightness of the line given in units of milli-
Rayleighs per km/s. The shaded solid line is a spec-
trum taken at twilight, where the light is scattered off the
Earth’s atmosphere which is at rest relative to the zodia-
cal cloud. The two weak absorption lines in the twilight
spectrum near +50 km/s are other solar lines of Fe I and
Cr I. The line centroid, width, and area of each spectrum
were measured using a least-squares fitting technique,
with the twilight spectrum used as a template. Figure 2
shows the change in velocity centroid with solar elonga-
tion (helio-ecliptic longitude, ǫ) for directions along the
ecliptic equator. The data show a clear prograde kine-
matic signature. We see no evidence for a net radial out-
flow (at ǫ=180◦) or an east/west asymmetry as reported
by previous authors [6-7].
Figure 3 shows the change in the full-width at half-
Figure 1. Spectrum of the zodiacal light taken toward
the north ecliptic pole, centered on the Mg I λ5184 ab-
sorption line. The solid line shows the Mg I line from
the twilight sky, and represents an unperturbed reference
spectrum. Note the flat-bottomed profile and enhanced
line width relative to the twilight spectrum. This suggests
that a significant population of the zodiacal dust particles
are on non-circular orbits.
maximum of the lines as a function of elongation for di-
rections along the ecliptic equator. The solid horizontal
line near 58 km/s is the width of the unperturbedtwilight
spectrum. The Figure shows that along the ecliptic plane,
the line is broadened by 10-20 km/s relative to the so-
lar line, with no significant trends with elongation. The
large intrinsic width of the solar line makes these mea-
surements difficult to quantify accurately and contributes
to the large uncertaintycompared to the velocity centroid
If all the dust particles were on pure circular orbits cen-
tered on the Sun, the zodiacal and twilight line profiles
would be nearly identical toward the anti-solar direction.
The broadening of the zodiacal profiles can only be ex-
plained by particles having radial components to their or-
bital motion. The data imply that a significant fraction of
the dust follows elliptical orbits.
Furthermore, the substantial broadening of the profile to-
ward the ecliptic pole (Figure 1), which is even greater
than that toward the anti-solar direction (see ), implies
a populationof particlesthat have significantcomponents
of their orbital velocities projected perpendicular to the
ecliptic plane. For an orbital speed of 30 km/s near 1
AU, and neglecting the radial motion associated with ec-
centricity, the approximately±15–20km/s broadeningat
the base of the profile suggests a distribution of inclina-
tions extending up to ∼30◦-40◦with respect to the eclip-
tic plane .
Figure 2. Velocity centroid of the Mg I absorption line as
a function of helio-ecliptic longitude. The observational
data are shown as solid circles, with model predictions
are shown as lines. The data indicate the dust follows
prograde orbits, with no significant net radial outflow or
east/west asymmetry. The observations are better fit by
models in which the dust follows elliptical or cometary-
like orbits (see text).
3.COMPARISON TO MODELS
The relationship between the observational data and the
kinematic properties of the zodiacal dust cloud is a clas-
sical inversion problem. The shape of the observed line
profiles is determined by the population of dust particles
of varying size, radial distance, scattering function, and
relative motionalong the line of sight. To infer the orbital
properties of the particles that comprise the dust cloud,
the data need to be compared to a range of predictive dy-
namical models. There has been considerable work done
in the field of modeling the zodiacal dust cloud that take
into account a wide range of observationalcharacteristics
[e.g., 10-12]. However, few models explicitly consider
the dynamics of the cloud that can be directly compared
to our data. Below we discuss a few models from the
literature, including some new models, that estimate the
change in both the velocity centroid and line width with
Some of the models that compare favorably to the data
are shown in Figures 2 and 3. The solid black line is a fit
to a model from . This model describes particles on
prograde, elliptical orbits with eccentricities uniformly
distributed between 0 and 1, with randomly distributed
perihelions. Their model did not include the influence of
radiation pressure, and the particles were confined to the
ecliptic plane. The model fits the centroid data well (Fig-
ure 2), but strongly overestimates the width of the lines
(Figure 3). The inclusion of radiation pressure and/or in-
clined orbits could provide a better match to the observa-
Figure 3. Full-width at half-maximum of the Mg I ab-
sorptionlineas afunctionofhelio-eclipticlongitude. The
observations are shown as solid circles with the model
data shownas variouslines. Thesolid horizontalline rep-
resents the width of the absorption line taken at twilight.
The motion of the zodiacal dust broadens the line by ≈
20 km/s relative to the twilight spectrum, with no sig-
nificant changes in line width with elongation. With the
exception of the model of , the models are generally
well-matched to the observations, within the uncertainty.
The colored lines in Figure 2 and 3 are models from [15-
16], which trace the motion of different populations of
dust particles subject to gravity, radiation pressure, and
drag forces. The individual dashed and dotted lines rep-
resent particles with asteroidal and various cometary tra-
force of β = 0.002. Some models considered particles
with different β-values, and some with trans-Neptunian
orbits, but those models are generally poorly matched to
the data and are omitted for clarity. In Figure 2, we see
that a good match is provided by the cometary particles
on inclined, eccentric orbits compared to the asteroidal
particles. In Figure 3, all of the models fall within the
large scatter in the data, do not allow us to discriminate
between the different models. Future observations of in-
trinsically narrower lines, such as Fe I, will aid in using
line widths to assess the asteroidal and/or cometary-type
orbits of the zodiacal dust.
4.SUMMARY AND FUTURE WORK
Observations of scattered solar absorption lines in the zo-
diacal light are a powerful technique for exploring the
kinematics of the zodiacal dust cloud. Our data are fit
well by models that contain particles on elliptical orbits
that are inclined to the ecliptic plane. This suggests that
most of the dust in the zodiacal cloud has a cometary ori-
servations is highly model dependent, and we emphasize
the importance of exploring a range of predictive mod-
els in assessing our conclusions. Higher signal-to-noise
observations covering a larger fraction of the ecliptic sky
that include other, more intrinsically narrow, absorption
lines will provide a more complete picture of the kine-
matics of the zodiacal dust cloud. New dynamical mod-
els that investigate a wider range of dust properties and
orbital parameters can provide strong, quantifiable con-
straints on the nature of the zodiacal dust cloud when
compared to these new observations.
This research has been generously supported by the Na-
tional Science Foundation through grant AST 02-04973
to the University of Wisconsin, and an MPS Distin-
01416 to G.J.M.
1. Gr¨ un E. et al. Dust in Interplanetary Space and in
the Local Galactic Environment, in Astrophysics of Dust,
ASP Conf. Series, 309, 245-264, 2004.
2. Clarke D. et al. On the line profiles in the spectra of the
zodiacal light, Astronomy & Astrophys., 308, 273-278,
3. James, J. F. Theoretical Fraunhofer line profiles in the
4. Dermott S.F. et al. Sources of Interplanetary Dust,
in Physics, Chemistry, and Dynamics of Interplanetary
Dust, ASP Conf. Series 104, 143-153, 1996.
5. Liou, J.-C., Dermott, S.F., Xu Y.L. The contribution of
cometary dust to the zodiacal cloud. Planet. Space Sci.
43, 717, 1995.
6. Fried J.W. Doppler shifts in the zodiacal light spec-
trum, Astronomy & Astrophys., 68, 259-264, 1978.
7. East I. R. & Reay N. K. The motion of interplane-
tary dust particles. I - Radial velocity measurements on
Fraunhofer line profiles in the Zodiacal Light spectrum,
Astronomy & Astrophys., 139, 512-516, 1984.
8. Reynolds R.J. et al. The Wisconsin H-alpha Mapper
(WHAM): A brief review of performance characteristics
and early scientific results, Pub. Astr. Soc. Of Australia,
15, 14-18, 1998.
9. Reynolds R. J., Madsen G. J. & Moseley, S. H. New
Measurements of the Motionof the Zodiacal Dust, Astro-
phys. J., 612, 1206-1213,2004.
10. Kelsall, T. et al. The COBE Diffuse Infrared Back-
groundExperimentSearch forthe Cosmic InfraredBack-
ground. II. Model of the Interplanetary Dust Cloud, As-
trophys. J., 508, 44-73, 1998.
cloud, in The Extragalactic Infrared Background and its
Cosmological Implications, IAU Colloq. 204, eds. M.
Harwit & M.G. Hauser, 17-34, 2001.
12. Dikarev, V., et al. Upgrade of Meteoroid Model
to Predict Fluxes on Spacecraft in the Solar System and
Near Earth, in Dust in Planetary Systems, in press, 2006.
13. HirschiD.C. &BeardD.B. Dopplershifts inzodiacal
light, Planet. Space Sci., 35, 1021-1027,1987.
14. Rodriguez G. L. & Magro C. S. The Doppler shift
from Zodiacal Light, Astronomy & Astrophys., 64, 161,
15. Ipatov S. I. et al. Dynamical zodiacal cloud models
constrained by high resolution spectroscopy of the zodi-
acal light, 36th LPSC, (#1266), 2005
16. Ipatov, S.I. & Mather, J.C. Migration of Dust Parti-
cles to the Terrestrial Planets, in Dust in Planetary Sys-
tems, in press, 2006.