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Atmospheric Entry Heating of Micrometeorites at Earth and Mars: Implications for the Survival of Organics
Aaron P. Wilson1, Matthew J. Genge1, Agata M. Krzesińska2, Andrew G. Tomkins3
1. Department of Earth Science and EngineeringImperial College LondonExhibition RoadLondon SW7 2AZ, UK
2. Centre for Earth Evolution and DynamicsUniversity of OsloSem Sælands vei 2AOslo 0371, Norway
3. School of Earth, Atmosphere and Environment, Monash University, Melbourne, Victoria 3800, Australia
Corresponding Author: [email protected]
AbstractThe atmospheric entry heating of micrometeorites (MMs) can significantly alter their pre-
existing mineralogy, texture and organic material. The degree of heating depends
predominantly on the gravity and atmospheric density of the planet on which they fall. For
particles falling on Earth the alteration can be significant, leading to the destruction of much
of the pre-entry organics, however, the weaker gravity and thinner atmosphere of Mars
enhances the survival of MMs and increases the fraction of particles that preserve organic
material. This paper investigates the entry heating of MMs on the Earth and Mars in order to
examine the micrometeorite population on each planet and give insights into the survival of
extraterrestrial organic material. The results show that particles reaching the surface of Mars
experience a lower peak temperature compared to Earth and, therefore, experience less
evaporative mass loss. Of the particles which reach the surface, 68.2% remain unmelted on
Mars compared to only 22.8% on Earth. Due to evaporative mass loss, unmelted particles that
reach the surface of Earth are restricted to sizes <70 µm whereas particles >475 µm survive
unmelted on Mars. Approximately 10% of particles experience temperatures below ~800 K,
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i.e. the sublimation temperature of refractory organics found in MMs. On Earth this fraction
is significantly lower with less than 1% expected to remain below this temperature. Lower
peak temperatures coupled with the larger sizes of particles surviving without significant
heating on Mars suggests a much higher fraction of organic material surviving to the martian
surface.
Introduction Micrometeorites (MMs) and interplanetary dust particles (IDPs) are the dominant source of
extraterrestrial material falling on Earth with approximately 30 kt yr-1 thought to enter the
atmosphere (Love and Brownlee, 1993; Peucker-Ehrenbrink and Ravizza, 2000).
Micrometeorites are the fraction of particles <2 mm diameter which survive atmospheric
entry to be found on the Earth’s surface (Genge et al. 2008). With the total meteorite flux
estimated at ~50 t yr-1 (Zolensky et al. 2006a), the contribution from MMs and IDPs
dominates the mass accreted by the Earth. This is also thought to be the case for other
planetary bodies, including Mars. By accounting for the difference in heliocentric distance
and the smaller gravitational focusing effect of the planet, Flynn and McKay (1990)
calculated the ratio of the in-space MM flux on Mars to be ~0.17 of that at Earth, equivalent
to 5.1 kt yr-1, whilst estimates based on dynamical modelling of dust from different sources
suggests a ratio of 0.22 (e.g. Plane et al., 2018). The overall mass of material reaching each
planet’s surface, however, depends on the proportion of MMs that survive atmospheric entry.
The majority of micrometeorites are thought to be chondritic in composition and consist of
fragments of individual components of their parent bodies such as; CAIs, chondrules, fine-
matrix material and metal particles (Genge et al. 2008). Many MMs also contain
carbonaceous material, most of which is organic including aromatic and aliphatic compounds
(Engrand & Maurette, 1998; Clemett et al. 1993; Keller et al. 2004; Glavin et al. 2004;
Dobrica et al. 2011). It was suggested that prebiotic organic material was delivered to the
primitive Earth from an extraterrestrial source (Oró and Kamat, 1961; Anders, 1989; Chyba
and Sagan, 1992) and currently it is accepted that this delivery provided the essential material
needed for the development of life (Kebukawa et al. 2017)
Several different types of organic material have been reported in MMs. Clemett et al. (1993)
and Clemett et al. (1998), for example, found polycyclic aromatic hydrocarbons (PAHs) in
MMs and IDPs, Brinton et al. (1998) identified amino acids in some MMs and Matrajt et al.
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(2001; 2005) noted the presence of hydrocarbons associated with ketones similar to the
organic materials observed in carbonaceous chondrites. A small number of micrometeorites
are thought to have cometary origins and likely contain significantly higher contents of
organic matter (Noguchi et al. 2015). A small fraction of MMs have affinities with organic-
poor ordinary chondrites (Suavet et al. 2010, 2011a) and more rarely achondrites (Gounelle et
al. 2009; Taylor et al. 2007; Badjukov et al, 2010; Cordier et al. 2011).
Much of the inherent mineralogy and texture of micrometeorites is altered by heating
experienced during atmospheric entry (Genge et al. 2008) and thus affects the survival of
organic matter within these particles. Micrometeorites are classified depending on the degree
of thermal alteration they have experienced (Genge et al. 2008). They are divided into
melted, known as cosmic spherules (CSs), partially melted, known as scoriaceous MMs
(ScMMs) and unmelted particles. Cosmic spherules are further subdivided based on their
composition into S-type (silicate dominated), I-type (Fe dominated) and G-type (intermediate
between S-type and I-type) and unmelted particles are subdivided into fine-grained and
coarse-grained (Genge et al. 2008).
The degree of heating depends on a particle’s initial entry velocity, entry angle and size. In
general, an increased velocity, angle and initial particle size results in a greater degree of
heating and thus a greater degree of thermal alteration (Flynn, 1989; Love and Brownlee,
1991; Toppani et al. 2001; Genge, 2016). There are several factors that influence the entry
parameters. For instance, particles sourced from the main belt asteroids are brought sunward
by Poynting-Robertson (PR) light drag which slowly decreases the eccentricity and
inclination of particles orbits (Dohnayji, 1976) thereby acting to minimise entry velocities.
Hence, MMs derived from bodies from the asteroid belt are commonly found on Earth.
However, when released from their parent bodies cometary derived particles have larger
elliptical orbits similar to that of the parent body. Cometary particles might be expected to
have higher average entry velocities than that of asteroid derived particles. Dynamical models
of the orbital evolution of cometary dust, however, suggest that circularisation of orbits by
PR light drag and gravitational perturbations can decrease the geocentric velocity of cometary
dust, in particular that derived from Jupiter Family comets that have relatively low
inclinations (Liou and Zook, 1996; Nesvorný et al. 2010, 2011a,b). This, however, is contrary
to the much lower abundance of cometary particles (<2%) in collections of large MMs
(Dobrica et al., 2012; Noguchi et al., 2015).
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The survival and preservation of pre-atmospheric mineralogy and texture is most likely in
particles with low entry angles. However, particles with extremely shallow entry angles
experience grazing incidences i.e., they encounter the atmosphere at a sufficiently low angle
that they escape back into space, having not experienced deceleration at lower altitudes to
below escape velocity. A proportion of grazing incidence particles can, however, be slowed
sufficiently that they may eventually be captured after re-entry but may experience several
passages (aeropasses) through the atmosphere (Love and Brownlee, 1991). The angle at
which this occurs varies depending on a planet’s gravity and a particle’s entry velocity and
initial size (Mcdonnell and Cook, 1997; Hunten, 1997). The discovery of rare MMs on Earth
with evidence for multiple heating events during atmospheric entry shows that at least some
of these particles are captured (Genge et al., 1996).
Atmospheric entry heating is expected to be less significant on Mars compared to Earth
owing to the lower possible entry velocities (~5 km s -1 compared with 11.2 km s-1) and the
lower atmospheric density. Thus, survival and preservation of micrometeorites and their
organic constituents should be enhanced on Mars. Here we use a numerical model to compare
the effect of atmospheric entry heating for micrometeorites falling on both the Earth and
Mars. The results extend those of Flynn (1996) who first modelled the atmospheric entry of
MMs on Mars and allow an estimation of the relative survival rate of particles. They also
give an insight into the likely size, mineralogy and textures of the surviving particles. In
particular, these results provide constraints on the abundance of micrometeorite types and
exogenic organic matter derived from MMs within martian soil.
Methods
Numerical Model
The atmospheric entry heating model used here is based on the original model developed by
Love and Brownlee (1991). The model simultaneously calculates a particle’s motion, mass
loss due to evaporation and heating rate at each instant during entry. The equations of motion
predict deceleration by considering the change in momentum, which is caused by collisions
with gas molecules within a cylinder of air with a diameter equal to that of the particle and
length derived from the velocity. Deceleration is thus a function of atmospheric density,
particle density, particle radius and velocity. Gas flow is considered to be within the free
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molecular flow regime which allows the direct collision of molecules with the surface and
prevents the formation of any shock structure (Love and Brownlee, 1991).
For particles falling on Earth, atmospheric density is calculated from the linear interpolation
of the 1976 U.S. standard atmosphere model. This requires the calculation of altitude by an
independent solution of equations of motion of the particle on orthogonal axes. For Mars,
atmospheric density is calculated using a scale height of 11.7 at altitudes <30 km and 7.9 at
altitudes >30 km (Seiff and Kirk, 1977).
The change in particle radius owing to evaporation influences their dynamic behaviour and is
therefore also included in the model. Evaporation rate is calculated using the Langmuir
formula with values of constants A=9.6, B=26,700 to evaluate vapour pressure and a mean
molecular mass of 45 similar to Love and Brownlee (1991).
The surface temperature of a particle is calculated from the heat flux caused by the energy
input from collisions with air molecules and the energy lost to evaporation and thermal
radiation. Some energy is also lost due to melting, but Love and Brownlee (1991) showed
this to be negligible, approximately 2 orders of magnitude less than the other mechanisms.
For the purpose of the modelling it is assumed that the particle is thermally homogenous.
This is due to the complexity of the thermal regime of a particle of varying size, shape and
composition and the difficulty in modelling this. However, in reality, thermal gradients can
be produced in micrometeorites owing to the endothermic decomposition reactions associated
with the breakdown of phyllosilicate materials and the increase in porosity due to
devolatilization of heated MMs allowing the formation of ‘cold spots’ inside the MMs
(Szydlik and Flynn, 1997; Flynn, 2001; Matrajt et al. 2006). The energy lost due to
evaporation can be calculated from the latent heat of vaporisation Lv (6.05 x 106 Jkg-1) and the
evaporation rate. The energy lost due to radiation was calculated with the assumption that the
particle radiates uniformly with black body properties from a spherical particle (i.e.,
emissivity = 1).
The expressions describing the dynamic and thermal behaviour of micrometeorites during
atmospheric entry were solved simultaneously by numerical integration using the Runge
Kutta 4th order method. In order to reduce the number of computationally expensive
simulations associated with rapid heating at extremely high velocities, a time step was chosen
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so that temperature changed by less than 10%. Typical timesteps varied from 0.2 to 3.125 x
10-4 seconds and most simulations were achieved in ~1000-6000 timesteps.
A full derivation of the equations used in this model can be found in Genge (2017a).
Particle Velocity, Size and Angle Distributions
In order to describe the initial population of interplanetary dust particles entering a planet’s
atmosphere, the pre-entry velocity, particle size and entry angle distributions are required. A
velocity distribution for Earth was developed by Southworth and Sekanina (1973) following a
radar study of meteors which were shown to approximately follow a power law above 13 km
s-1. This was further developed by Frandorf (1980) who produced a single power law which
considers all possible entry velocities (equation 1):
λ (v , v+dv )=1.791 ×105 v−5.394 dv 1.
Where v is velocity and λ is the fraction of particles within a specific velocity range (v, v _+
dv). This velocity distribution was also used by Love and Brownlee (1991) in their original
statistical analyses of MMs falling on Earth. Although more recent velocity distributions for
Mars and Earth have been developed from dynamical models (Liou and Zook, 1996;
Nesvorný et al. 2010, 2011a, b; Plane et al., 2018) an empirical approach is adopted in this
work. An evaluation of the flux and velocity distributions predicted by dynamical models and
their effects on the results of this study is given in the discussion.
The velocity distribution at Mars was calculated for this model applying the same methods as
Morgan et al. (1988) who extrapolated the distribution at Earth to the heliocentric distance of
Mercury. For our model, the velocity distribution at Earth was first corrected to an in-space
velocity distribution. This was achieved by removing the effect of gravitational focusing and
acceleration due to the Earth’s gravity. The resulting distribution was then transformed to the
heliocentric distance of Mars (1.53 AU). The gravitational focusing and Mars gravitational
acceleration were then used to produce a velocity distribution for particles entering the
martian atmosphere. The average entry velocity calculated from these velocity distributions
are for the Earth and Mars ~14.5 km s-1 and ~9.6 km s-1, respectively. The particles
considered covered all possible entry velocities which were separated into bins of varying
widths from 300 m s-1 at the lowest entry velocities and 7000 m s-1 at higher entry velocities.
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Particle radius distributions used in this study were obtained following method of Grun et al.
(1985). Using data from zodiacal light observations, in-situ spacecraft measurements and
oblique hypervelocity impact experiments and with considerations for the effects of mutual
collisions, the PR light drag effect, and radiation pressure ejection of meteoroids, Grun et al.
(1985) adapt the lunar microcrater flux to produce a mass distribution of interplanetary dust
particles for masses greater than ~10-14 g. The distribution gives the cumulative number of
particles above a given mass. In this study, the obtained masses were converted into radius
assuming a uniform density of 3 g cm-3 and the distribution was normalised to unity for the
particle size range considered. Particles were separated into size bins of varying widths from
5 µm at the lower sizes to 50 µm at the largest sizes considered. For the size distribution at
Mars little empirical data is available. It is likely that there may be a shift towards larger sizes
owing to the collisional evolution of the size population during inwards migration (Grun et al.
1985) Due to the lack of data, the distribution at Earth is also used for Mars. Particles with
initial radii from 10 µm to 500 µm are considered in this study incorporating >80% of the
total mass flux of meteoritic material in the 10-13 g to 106 g mass range incident on Earth
(Hughes 1978; Carrillo-Sanchez et al. 2016). The average initial radius of the particles
considered in this study was calculated to be ~34 µm.
The entry angle distribution can be computed assuming a random space distribution of
particles far from Earth. The distribution assumes no gravitational focusing by the planet.
When gravitational focusing is omitted, the angle distribution at Mars is equal to that of
Earth. The entry angles were binned with bin widths of 2.5° ranging from 5 to 90°. The
average entry angle is 45°.
The distributions above give a proportion of incoming particles within a certain entry
parameter range e.g. the probability of a particle entering with a velocity between V1 and V2.
The bin sizes used here gives those ranges. To calculate peak temperature distribution,
simulations were run using a representative value mid-way between the upper and lower bin
limit. Minimum bin sizes were chosen to reduce the number of numerically expensive
simulations required. In most cases, the peak temperature increase was <5% for each
increased entry parameter.
Grazing incidence particles
The maximum entry angle for a grazing incidence depends on a particle’s velocity and size
and an increase in either value causes an increase in angle (Mcdonnell and Cook, 1997;
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Hunten. 1997). Initial simulations were run to examine the maximum entry angle for a
grazing incidence at Earth and Mars. A minimum entry angle of 10° and 12.5° from the
horizontal was used for particles falling on Earth and Mars, respectively. Lower angle
particles were ignored due to the higher computational expense of their longer simulations.
The maximum effect on the proportion of unmelted particles introduced by this limit is 3%
for Earth and 5% for Mars.
Simulations
In total ~43000 simulations were run for particles with radii ranging from 10-500 µm and
entry angles ranging from 10-90° for both the entry conditions on the Earth and Mars.
Particles were assumed to be chondritic with a density of 3.0 g cm -3, corresponding to
ordinary chondrites. The entry velocities range from the escape velocity of Earth and Mars
(11.2 and 5 km s-1, respectively) to the solar system escape velocity at the planet’s distance
from the Sun (72 and 32.4 km s-1 respectively). A summary of the entry parameters
considered can be seen in table 1.
Results
Individual particle behaviour
The behaviour of individual particles falling on each planet at their respective average
velocities (14.5 km s-1 on Earth and 9.6 km s-1 on Mars) was modelled. The simulations were
run for particles entering the atmosphere at 45° and having an initial radius of 34 µm, (i.e. the
average size, as calculated using the method described in Grun et al. (1985)). The peak
temperature reached by a particle on Earth was 1728 K compared with 1342 K on Mars. Fig.
1. shows the temperature-time profile for these particles. Simulations show that the particle
falling on Earth lost >80% of its original mass whereas the particle falling on Mars lost <2%
of its original mass. These values correspond to a final radius of 19.5 µm for Earth and 33.8
µm for Mars. This greater mass loss and higher peak temperature seen on Earth can be
attributed to the higher entry velocity.
The influence of atmospheric density differences between Earth and Mars was investigated
through comparison of simulations conducted at the same entry parameters. Fig. 2. shows a
temperature-time profile for a 34 µm particle (the average size considered in this study)
entering at 45° with a velocity of 12 km s-1. The peak temperatures reached on Earth and
Mars are 1628 K and 1540 K, respectively. Additionally, the particle falling on Mars reaches
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peak temperature at a higher altitude than on Earth, at 94.6 km instead of 87.1 km. The mass
loss is also broadly similar at 42% on Earth and 20% on Mars. The maximum cooling rate for
the particle falling on Earth is higher than Mars at, 343.7 Ks -1 compared with 230.5 Ks-1,
which also leads to a shorter period spent above the solidus at 2.22 seconds compared to 2.4
seconds. The effect of the different atmospheric densities is, thus, significantly less than the
effect of entry velocity on heating behaviour.
Statistical analyses of the micrometeorite population
The fraction of the MM flux considered to have a grazing incidence encounter varies between
the Earth and Mars. On both planets, particles with entry angles < 5°, regardless of their size
and velocity, are seen to ‘skip’ and are lost to space. However, at lower velocities there is a
range of entry angles for which a particle shows/has a grazing incidence encounter but still is
captured by the planets gravity and eventually reaches the planet’s surface. As velocity is
increased, this range decreases and the maximum angle at which a particle will skip
increases. This is in agreement with the modelling work of Mcdonnell and Cook (1997). The
maximum angle for the Earth and Mars is 8.65° and 11.75°, respectively. Due to the higher
computational expense of the longer simulations associated with these low angle particles, all
particles with entry angles below 10° and 12.5° (the upper bin limit from which the cut off
angles fall) for the Earth and Mars respectively were excluded from this study.
A certain fraction of the particles falling on the Earth and Mars are completely vaporised
owing to the intense heating associated with the highest entry velocities. Approximately 3.2%
of particles falling on Earth and 0.9% of particles falling on Mars are predicted to completely
vaporise on entry.
To evaluate the peak temperatures reached by particles falling on each planet, the fraction
predicted to skip or vaporize are excluded. This gives an average peak temperature of 1571 K
for particles on Earth and 1226 K for particles on Mars. To give insight into preservation
potential of organic material, we investigated the fraction of particles which reach the
planet’s surface and remain below key temperatures including the sublimation temperatures
of organic compounds often found in MMs. The results are shown in table 2. Most
extraterrestrial organic materials are thought to sublime at temperatures around 473 K
however some, such as the polyaromatic hydrocarbon (PAH), coronene, have a boiling point
close to 800 K. In a pulse heating study undertaken by Matrajt et al. (2006) a small fraction
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(<0.1%) of the PAH coronene and the ketone 2-pentadecanone was seen to survive up to
1173 K (Matrajt et al. 2006). These temperatures have thus been chosen as upper limit
temperatures for the survival of organic materials.
The fraction of particles reaching the surface of each planet which are unmelted, partially
melted or melted can be estimated by assuming a solidus temperature of 1414 K and a
liquidus temperature of 1700 K (Genge. 2017a). Simulation results for particles with a radius
of >50 µm falling on Earth indicate that ~80.6% of them are melted, 18.8% partially melted
and 0.6% unmelted. For particles with radius of 25-50 µm ~49% are melted, 46% partially
melted and 5% unmelted. These simulated values are similar to those of Antarctic
micrometeorite collections (Maurette et al. 1991; Taylor et al. 2000; Genge et al. 1997a). For
particles >50 µm falling on Mars, 20.0% are melted, 40% partially melted and 40% unmelted
and for particles 25-50 µm radius ~10.0% are melted, 27.0% are partially melted and 63%
unmelted (Fig. 3). Therefore, of the total MM flux reaching the ground, the fraction of
unmelted particles on Mars is much greater than that of Earth. Approximately 68.2% of
particles are predicted to be unmelted on Mars compared to only 22.8% for Earth. The
fraction thought to be entirely melted is much greater for Earth with 29.3% of particles
predicted to completely melt compared with 9.8% for Mars.
The size range of particles reaching the surface on Mars is much greater than for
micrometeorites on Earth. The largest surviving particle on Earth has a final radius of
~265 µm compared with ~475 µm on Mars. This doesn’t consider particles with initial radii
>475 µm, which are beyond the maximum considered, and those particles which have
grazing incidences. The modelled average final radii for particles falling on the Earth and
Mars are 19.8 µm and 28.3 µm, respectively. Fig. 3. shows the fraction of each size range
that will be melted, partially melted or unmelted. The initial peak in Fig. 3. seen in melted
and partially melted particles <10 µm is a result of the limit in particle size considered in this
study. All particles with radii <10 µm were generated through the mass loss of larger
particles, which requires higher peak temperatures. For both Earth and Mars, graphs follow a
similar pattern, an increase in the fraction of melted particles is observed as particle size
increases. However, the partially melted particles are seen to increase in relative abundance
with decreasing size on Earth, whilst on Mars the opposite is observed. The likely cause of
this effect is the greater mass loss experienced by melted particles on Earth since they
experience higher peak temperatures at equal sizes. This effect is most pronounced at large
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sizes. Fig. 4. shows the fraction of particles in each size range along with the proportion of
unmelted, partially melted and entirely melted particles.
Toppani and Libourel (2003) suggest a 50% melt fraction as a cut off for spherule formation.
This allows the prediction of the abundance of cosmic spherules, scoriaceous and unmelted
particles reaching the surface of each planet. Fig. 5. shows the relative abundance of each
type in each size range. For particles falling on Earth in the 50-100 µm radius size range,
approximately 89% are predicted to be cosmic spherules. This is significantly higher than that
which was predicted by Genge (2017a) (60%). This is likely a result of the higher velocities
considered in this study. In the equivalent size range on Mars, only ~26% are predicted to be
cosmic spherules.
When looking at the whole MM population the most striking feature of these data are the
high abundance of scoriaceous and unmelted particles predicted for Earth, 36% and 22.8%
respectively (Fig. 5 and 6). These fractions fall within the range of some of the MM
collections found on Earth, however, there is considerable variation in the abundance of
unmelted and scoriaceous particles between MM collections. This variation can be attributed
mainly to varying preservation conditions and terrestrial residence times, and the size range
of the collection. For example, the fraction of unmelted and scoriaceous particles in the Cap
Prud’homme collection are 50.4% and 13.6% respectively for a suite of 550 MMs dominated
by particles in the 50 – 100 µm size range (Genge et al. 2018). This is very different from the
predicted fraction of 89% cosmic spherules in this size range which is likely due to the
preservation mechanism in this area favouring unmelted and scoriaceous particles. In the
CONCORDIA collection 34% are unmelted and 22% are scoriaceous for a suite of 1019
particles ranging in size from 13 - 300µm. (Dobrica et al. 2010). However, cosmic spherules
comprise >96% of the Larkman Nunatak moraine collection which consisted of 634 MMs
ranging in size from 60-450µm (Genge et al. 2018) and the Transantarctic Mountain
collection, which consisted of ~3500 MMs with sizes ranging from 100-1600 µm (Suavet et
al. 2009). In earlier studies of deep-sea collections, 100% of the particles found were cosmic
spherules (Parashar et al. 2010; Prasad et al. 2013) However a more recent study found
unmelted and scoriaceous particles in a deep sea collection at a concentration of 14% and
27% respectively with a total of 474 MM and sizes ranging from 70 – 732 µm (Prasad et al.
2018). The variation in MM type fractions between collections make it difficult to compare to
the model results.
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Mass Loss and Accretion Rate
Evaporative mass loss during heating primarily leads to a decrease in particle size. As
modelled in this study, the average mass lost by particles falling on Earth is 43% and only
16% for particles falling on Mars. It is also worth noting that ~60% of particles falling on
Mars lose <1% of their original mass whereas only ~8.8% of particles on Earth are in this
category.
The total mass of material reaching the surface of the Earth and Mars is calculated to be
~16,040 t and 4,390 t respectively. These values are obtained after subtracting from the total
mass flux the fraction of particles predicted to skip and vaporise and including the average
mass lost on entry. Assuming a uniform distribution over the planet’s surface, that is
equivalent to 0.0314 g m-2 yr-1 and 0.0302 g m-2 yr-1 for the Earth and Mars, respectively. This
shows that accretion to both planets differs by <4%. The enhanced survival of
micrometeorites on Mars is thus balanced by the effects of its smaller gravitational cross-
section.
Molten time and cooling rate
The period for which particles will remain above the melting temperature (the solidus) can
affect the thermal decomposition of MM components owing to non-equilibrium behaviour.
The time spent molten for particles falling on Mars is higher than on Earth. The average
molten time is 2.6 s for particles on Mars and 1.9 s on Earth. Fig. 7. shows the time spent
molten by the particles for both the Earth and Mars, not including those which remain below
the melt temperature.
As well as the longer average molten time, the cooling rate for particles on Mars is slower
than on Earth, 202 K s-1 compared with 447 K s-1, respectively. This results in particles
remaining at high temperatures for longer periods during atmospheric entry on Mars than on
Earth. This enables a closer approach to equilibrium and enhanced thermal alteration of
micrometeorite components on Mars.
DiscussionThe simulation results indicate a much higher survival rate of particles falling on Mars
compared to Earth. Overall, the lower flux at Mars that results from its smaller cross-section
and gravity, is largely compensated by the higher survival rate. This leads to an almost equal
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mass accreted at the surface of the Earth and Mars, only 4% more reaching the surface of
Earth. This is significantly larger than the in-space micrometeorite flux ratio suggested by
Flynn and McKay (1990) of 0.17 highlighting the more significant mass loss for particles
falling on Earth.
Two main factors contribute to the enhanced survival of particles on Mars. The most
influencing are the lower possible entry velocities. On Earth, the largest particle that
remained below the melting temperature had an initial radius of 65 µm at the lowest possible
entry velocity (11.2 km/s) and entry angle considered in this study (11.25°). For Mars, ~10%
of particles in the largest size range considered in this study survived without reaching the
melting temperature (Fig. 3) but all of these unmelted particles had entry velocities below the
minimum possible entry velocity on Earth. The velocity distribution calculated for Mars
suggests that 65% of particles have entry velocities lower than the minimum possible on
Earth and 85% have velocities lower than the average on Earth.
The second factor is the atmospheric density profile of the two planets. The individual
simulations show the peak temperature of two identical particles (entry velocity, angle and
size) is higher for the particle falling on Earth (Fig. 2). However, this is not caused by a
greater atmospheric density. Although the atmospheric density at the surface of Mars is
~0.6% that of Earth, the atmospheric densities encountered at high altitude in the atmosphere
are almost identical due to the weaker gravity of Mars (Fig. 8). The average altitudes of peak
heating for Earth and Mars were 90 and 98 km respectively where the martian atmospheric
density is marginally greater than that of Earth. Above these altitudes the martian
atmospheric density is significantly greater than that of Earth’s. The lower peak temperature
seen for the particle on Mars is caused by more gradual heating and deceleration. By the time
the particle on Mars reaches the point of maximum heating it has experienced greater
deceleration than on Earth owing to the thicker atmosphere it encounters at higher altitudes.
Particles on Mars, therefore reach their peak temperature earlier and at greater altitudes
compared to particles on Earth. This thicker atmosphere also produces slightly longer heating
pulse durations and slower cooling rates on Mars.
The accuracy of the scale height model for Mars must be evaluated when considering the
veracity of differences in micrometeorite heating. The scale height model for Earth provides
an approximation to measured values highlighting the simplification potentially arising from
the use of a scale height model for Mars (Fig. 8). A study by Forget et al. (2009) attempted to
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determine the CO2 density profile of Mars using data from the SPICAM instrument on board
Mars Express. As CO2 is the predominant gas in the martian atmosphere (~95.3%) this
provides a good estimate of the overall atmospheric density. Although a full profile is not
available, these data does show that atmospheric density varies depending on latitude and
with seasonal variations caused by fluctuating temperatures. Atmospheric density was seen to
vary by a factor of 3 below 70 km and even more at higher altitudes. These variations are
significant enough that the CO2 partial pressure is in a similar range and sometimes higher
than that seen on Earth at equivalent heights.
It is thus expected that the peak temperature achieved by a particle entering the martian
atmosphere varies with time of year and entry latitude and may occasionally reach
temperatures greater than equivalent particles on Earth. Until more accurate density profiles
for Mars are determined, however, an average scale height model is the most accurate way of
determining the average atmospheric density.
Relative abundance of micrometeorite types
The most abundant MMs reaching the surface of the Earth and Mars are in the 10-15 µm
radius range, approximately 36.8% and 34.4% of particles respectively (Fig. 4 and 6).
However, for Mars, 84% of the particles in this size range are unmelted compared with 45%
on Earth (Fig. 5). Considering the whole MM population, a much larger fraction of particles
on Mars are expected to be unmelted; 68.2% compared to the Earth, 22.8%. The fraction of
cosmic spherules is, therefore, much higher on Earth compared to Mars at 41.2% and 14.8%,
respectively. The unmelted fraction on Mars is here predicted to be slightly lower than that of
Flynn (1996) who estimated 82% of particles in the mass range considered in this study
would remain unmelted. This is a result of the lower solidus temperature chosen in this study
of 1414 K compared to 1600 K used in Flynn (1996), however, the fraction of particles
remaining below 1600 K in this study is approximately 82%.
The textures of cosmic spherules are dominated by quench textures associated with peak
temperature and cooling rate (Genge et al. 2008). The nature of the precursor is also thought
to influence the textures (Van Ginneken et al. 2017). Porphyritic spherules form by partial
melting (>50 vol%) of coarse-grained components such as chondrules, whilst barred olivine
textures are predominantly derived by melting of fine-grained, carbonaceous chondrite-
related particles (Van Ginneken et al. 2017). Cryptocrystalline cosmic spherules, dominated
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by sub-micron olivine dendrites within a glassy mesostasis, and V-type spherules composed
mainly of glass form by heating to supra-liquidus temperatures (Genge et al. 2008).
Porphyritic and barred cosmic spherules may, therefore, be significantly more abundant on
Mars than on Earth owing to the lower peak temperatures experienced by most of the melted
particles on each planet. Additionally, in light of our simulations, the size range of spherules
is likely to be different on Mars. While large particles are removed by evaporation on Earth, a
significant proportion survives atmospheric entry on Mars, even at diameters of 1 mm.
Slower cooling rates on Mars are also likely to influence the textures of spherules, because
cooling rate primarily controls the morphology of quench-formed olivine crystals
(Donaldson, 1976). Dendritic crystal morphologies are common within cryptocrystalline and
barred olivine spherules on Earth. At the lower cooling rates on Mars formation of
cryptocrystalline and barred olivine spherules would probably require higher peak
temperatures to form, since these induce supercooling through destruction of crystal nuclei
(Donaldson, 1976).
The textures and mineralogy of I-type cosmic spherules on Earth are predominately formed
as a result of oxidation during entry with most I-types being composed of a combination of
wüstite and magnetite (e.g. Genge et al. 2017). As the martian atmosphere is composed
predominately of CO2 the atmospheric oxidation processes, if any, will be less significant
than for those particles falling on Earth. This most likely causes enhanced formation of I-type
cosmic spherules that contain significant residual, unoxidised metal on Mars. The effects of
atmospheric oxidation, however, were not included in the current model.
Effect of entry velocity and dust flux model
An empirical entry velocity and flux model is adopted in this paper based on Earth meteoric
observations and spacecraft measurements. This model corresponds to that used in Flynn
(1996) and Flynn and McKay (1990) allowing direct comparison to these studies. More
recently studies, however, have examined the dynamical orbital evolution of interplanetary
dust derived from particular sources and reveal that cometary particles can evolve to low
eccentricity orbits similar to asteroidal dust (Liou and Zook, 1996; Nesvorný et al. 2010,
2011a,b). Based on assumed dust production rates these dynamical models predict that
interplanetary dust in the inner solar system is dominated by particles derived from Jupiter
Family Comets (50-70% of dust) with only a minor proportion (<20%) of asteroidal particles
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(Nesvorný et al. 2010). These models can successful reproduce the infrared spectral signature
of the zodiacal cloud determined by IRAS (Nesvorný et al. 2010) and produce cosmic
spherule fluxes at the Earth’s surface compatible with data from the South Pole Water Well
micrometeorite collection (Carrillo-Sanchez et al., 2016).
Some significant evidence exists, however, that brings into question whether low velocity
cometary dust from Jupiter-Family comets dominate dust accreted by the terrestrial planets.
Most fine-grained micrometeorites >50 um in size are dominated by phyllosilicates and are
thus derived from chondritic parent bodies that experienced significant aqueous alteration
similar to CI1 and CM2 chondrites, which are to be derived from C-type asteroids (Genge et
al., 2008; Taylor et al., 2012; Genge et al., 2017). In contrast phyllosilicate was not detected
by either the Rosetta or Stardust missions in JFCs 67P/Churyumov–Gerasimenko (Davidsson
et al., 2016) and Wild-2 (Zolensky et al., 2006b) respectively, consistent with the expectation
that these objects, ultimately derived from the Kuiper Belt, have not experienced post-
accretion temperatures sufficient to melt water ice (e.g. Davidsson e al., 2016). The suggested
detection of the phyllosilicate nontronite in trace amounts in JFC Temple 1 by the Deep
Impact mission (Lisse et al., 2006), furthermore, is not consistent with a parent body
extensively altered to phyllosilicate. Although there are mineralogical, chemical and oxygen
isotope similarities between fine-grained MMs and Stardust samples (Dobrica et al., 2010),
these properties are also similar to carbonaceous chondrite meteorites. Furthermore, particles
interpreted as cometary in origin are present amongst large MMs but are in low abundances
(<2%; Dobrica et al., 2011; Noguchi et al., 2015). Thus, primitive asteroids, rather than JFCs,
are the most likely parent bodies of the majority of large MMs on Earth contrary to the
predictions from dynamical modelling of large influxes of low velocity JFC dust.
Although it is difficult to reconcile the prediction of dynamical models with the observations
of large MMs, the consequences for delivery of organic molecules is relatively minor. The
total relative mass of MMs accreted by Mars predicted by Flynn and McKay (1990) and used
in this study is 0.17x that of the Earth, whilst the relative mass predicted from dynamical
models is 0.22x (derived from data in Plane et al., 2018). If JFC dust does dominate the flux,
then it must consist of similar materials to large MMs and must have experienced significant
aqueous alteration to many C-type asteroids. A similar abundance and inventory of organic
materials would thus be likely.
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A more significant difference is in the distribution of entry velocities shown in Fig. 9.
Dynamical models predict a larger proportion of higher entry velocity dust entering the
atmosphere of Earth and Mars than the empirical observational data from Southworth and
Sekanina (1973) for Earth and its extrapolation to Mars. High velocity dust represents ~30%
more of the flux to Mars predicted from dynamical rather than the empirical model and
would result in a lower survival of organic matter to the surface than predicted here. The
difference is, however, likely to be <30%, since the higher total flux predicted by the
dynamical models partially compensates for the increased heating at higher velocity.
Furthermore, if JFC dust has lower density than used here, less atmospheric heating would
occur.
Implications
Micrometeorite survival and accumulation on the martian surface
As discussed above, almost the same mass of MMs per unit area are accreted in time on Mars
and Earth. However, generally much lower soil production rates on Mars (1 m Ga -1) (Flynn
and McKay, 1990), are likely to result in significantly higher concentrations of MMs on the
martian surface over time. Large nickel content in martian soil was observed by Mars
Exploration Rovers (Yen et al. 2006) and is attributed to meteoritic material. This material
comprises approximately 1-3% of the martian soil. Additionally, martian basaltic breccias,
NWA7034, NWA 7533 and NWA 7475 contain highly siderophile elements in significantly
elevated amounts, which is attributed to admixture of 5% of carbonaceous chondrite material
to the martian regolith (Humayun et al. 2013; Wittmann et al. 2015).
Micrometeorite collections on Earth tend to be from areas with a minimal influx of terrestrial
debris such as the Transantarctic Mountains (TAM) and Antarctic ice sheets, the Greenland
ice sheet and deep-sea sediments (Robin et al. 1990; Taylor et al. 2000; Rochette et al. 2008;
Prasad et al. 2013). These terrestrial environments are all surrounded by large bodies of water
meaning dust must be transported large distances via surface winds. For Antarctica, this is
limited further due to dust transportation in the region being dominated by midlatitude
circumpolar winds which move westerly around the continent but rarely cross over. Models
show only a small fraction of dust transported via this system reaches the continent (Neff and
Bertler, 2015). On Mars, the areas in which high concentrations of MMs are likely to be
found will be different. Significant fluvial activity is broadly accepted to have terminated on
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Mars around 3 billion years ago (Carr, 2012) and in general negligible geological activity has
affected the martian surface over the past 2 million years (Neukum et al. 2004). Current
active surface processes on Mars are dominated by aeolian transport within an extremely arid
environment (Greeley et al., 1992). The heavily cratered surface of Mars and the large
ancient fluvial channels may provide areas in which aeolian processes are negligible,
allowing the accumulation of high concentrations of MMs. Micrometeorite placers found in
the Atacama Desert (Hutzler et al. 2016) may provide a good terrestrial analogue for the type
of environment where MMs may be expected to concentrate on Mars.
A further possible location for the natural concentration of MMs are the polar ice caps. On
Earth, placer deposits of MMs such as those produced from the melting of the Greenland ice
sheet can concentrate 100-1000 times more MMs than that of deep-sea deposits (an area of
little terrestrial sedimentation input but no means of concentrating the MMs) (Maurette et al.
1984). Yada et al. (2004) documented MM concentrations in Antarctic ice with a ratio of
MMs to glacial sand of 1:17. Significant concentrations of MMs have also been found in
moraines in the Antarctica, that formed by a combination of ice-sublimation from below and
aeolian transport and concentration in placers (Genge et al., 2018; Harvey and Maurette,
1991). On Mars, the semi-permanent water ice caps seen at the poles are thought to be
approximately 3 km thick (Carr, 2012). There is evidence for liquid water in the form of
subglacial lakes being present under the polar ice caps on Mars (Orosei et al. 2018),
indicating that on Mars, analogue processes may operate to those in Antarctic environments.
It is possible that, in periods of a warmer climate some of martian ice sublimates or melts,
producing placer like deposits of MMs similar to those found on Earth. As a consequence,
MMs could be concentrated in these subglacial lakes on Mars.
A crucial factor influencing the concentration of MMs on the surface of Mars is, however,
their weathering rate. In Antarctica, weathering of silicate and metallic phases occurs owing
to transient exposure to water with significantly more alteration occurring in coastal areas at
higher latitudes (Van Ginneken et al., 2017). Residence times of MMs in the Transantarctic
mountain range, however, can exceed 1 Myr (Rochette et al. 2008; Folco et al. 2009; Suavet
et al. 2011b; Folco et al. 2011; Genge et al. 2018) indicating survival over geologically
significant timescales under arid conditions. Considering the hyper-arid nature of the martian
surface in the most recent epochs (Carr et al. 2012) the preservation of MMs within martian
sediments is likely to significantly enhance their abundance. However, the preservation of
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MM material, specifically organics, in such environments on Mars is perhaps further
complicated by the highly oxidative nature of the martian soil, but this is beyond the scope of
this paper.
New parent bodies
Unmelted micrometeorites provide samples of parent bodies with relatively minor changes to
mineralogy and texture occurring during atmospheric entry heating. Most unmelted MMs on
Earth are broadly chondritic in composition and show affinities to CI, CM and CR chondritic
matrices (Kurat et al. 1994; Genge et al., 1997; Genge et al. 2008; Rudraswami et al. 2015;
Folco and Cordier, 2015). However, it is likely that, due to the much higher fraction of
particles on Mars predicted to be unmelted, samples of parent bodies not represented in any
abundance on Earth may be preserved on Mars.
In micrometeorite collections, there appears to be a correlation between MM size and
potential source. The fraction of particles with affinities to ordinary chondrites becomes
larger as size increases (Suavet et al. 2010; 2011a; Prasad et al. 2013; Cordier and Folco,
2014). This has been attributed to the more friable nature of carbonaceous chondrites (Flynn
et al. 2009), which results in an increase in their abundance at smaller size ranges. On Earth,
this causes a preferential survival of carbonaceous chondrite-derived MMs due to the higher
peak temperatures seen for larger particles. Thus, it seems reasonable to assume that, due to
the increased survival rate of larger particles on Mars, more particles with affinities to
ordinary chondrites should be present on the surface, likely increasing the number of parent
bodies represented in the MM population. In future sample return missions, MMs collected
from the martian soil could hold a wealth of pristine samples of asteroidal and cometary
parent bodies not yet sampled on Earth.
Survival of extraterrestrial organic material
Polycyclic aromatic hydrocarbons (PAHs), amino acids and a carbonyl group (C=O) mainly
associated with a ketone have all been found in MMs (Clemett et al. 1998; Glavin and Bada
2001; Flynn et al. 2003; Matrajt et al. 2005). Most of these organic materials sublime at
temperatures around 473 K although some refractory varieties can survive up to 823 K
(Matrajt et al. 2006). Matrajt et al. (2006) also showed that a small fraction (<0.1%) of the
PAH coronene and ketone 2-pentadecanone could survive up to 1173 K.
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Result of this study show that no particles remain below 473 K on either planet, thus most
organic material perhaps sublimes during atmospheric entry heating. However, ~10.4% of
particles falling on Mars remain below 823 K (compared to 0% on Earth) and 47.6% below
1173 K (compared to only 3.5% on Earth). This enables preservation of refractory organic
matter on Mars. Peak temperatures less than 473 K are possible on Earth and Mars for
particles on grazing incidence trajectories which are not subsequently lost to space. These
comprise a maximum of 1.9% of the terrestrial flux and 3.2% of the martian flux. The
presence of volatile organic compounds, such as amino acids documented within some MMs
found on Earth, therefore, indicates they underwent grazing incidence encounters during
atmospheric entry. Given the lower peak temperatures experienced by MMs in the martian
atmosphere it is certain that the survival of organic material is higher on Mars than on Earth.
In addition to the higher fraction of particles remaining below the pyrolysis/sublimation
temperatures of organic materials on Mars there is also a higher survival rate of large
particles. Less than 7% of particles reaching the surface of Earth are predicted to be >50 µm
radius compared with approximately 12% for Mars. In large MMs a significant temperature
gradient can form inside the particle during atmospheric entry (Flynn, 2001; Genge, 2006;
Matrajt et al. 2006) owing to the presence of material such as organics and phyllosilicates
which undergo an energy absorbing phase transition. Given that large thermal gradients
preserve primary textures within the cores of particles (Genge, 2006), this is likely to assist
the survival of indigenous organic matter.
Cometary derived particles are rare in micrometeorite collections. Only a small fraction (<
2% of the CONCORDIA collection (Dobrica et al. 2010)), known as ultracarbonaceous
Antarctic micrometeorites (UCAMMs), are thought to be cometary derived based on their
high quantities of carbonaceous material which can be upwards of 85 wt % (Sandford et al.,
2006; Dobrica et al. 2010; Duprat et al. 2010; Dartois et al. 2017). However, a greater
fraction of interplanetary dust particles (IDPs), the flux of extraterrestrial material which
remains suspended in the Earth’s stratosphere, are thought to be derived from comets
(Brownlee et al. 1993; Bradley et al. 1996; Aleon et al. 2009). These particles exhibit high
porosity, low density and high abundance of carbonaceous material that are expected for
cometary materials (Zolensky et al. 2006b). A small proportion of MMs (~1%) have
mineralogical similarities to anhydrous IDPs and are likewise probably cometary in origin
(Noguchi et al., 2015).
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Although most anhydrous IDPs are < 50 µm in diameter (Brownlee, 1985) they are still
subject to heating upon entry. For example, a 25 µm diameter particle with a density of 3 g
cm-3 entering at 12.5 kms and 45° entry angle reaches peak temperatures of ~1400 K. It is
likely that dense cometary particles larger than this experience peak temperatures great
enough to cause significant mass loss and, in most cases, complete vaporization. Comet-
derived particles are likely to have much lower densities of ~1 g cm-3 (Joswiak et al. 2005).
For a 100 µm particle entering at 12.5 km s-1 at an angle of 45°, this lower density causes a
difference in peak temperature of >200 K. This lower density may be what allows small
cometary derived particles on Earth to survive without significant melting, however, for
larger particles lower density will not cause a significant enough difference in peak
temperature to affect the survival of organic material. Although cometary particles are
preserved on Earth in the form of IDPs, the smaller sizes of these particles make it difficult
for any significant thermal gradient to form which is the main mechanism by which organics
are preserved in extraterrestrial particles (Matrajt et al. 2006; Genge, 2006) and so cometary
derived organic material is likely in low abundance on Earth.
Treiman and Treiman (2000) analysed the orbits of known comets to identify those which
had Mars-crossing orbits and could potentially provide cometary dust to Mars. They found 50
such comets and calculated the entry velocities of dust derived from these. The average
velocity of long period comets was 38.4 km s-1 (Treiman and Treiman 2000). Therefore, it
can be assumed that the majority of these particles would be vaporised on entry. The average
Mars-centric velocity of short period comets is 12.1 km s-1 and ranges from 5–25 km s-1. The
encounter velocity of cometary particles with planets, however, is decreased by their orbital
evolution after release from cometary nuclei with the circularisation of orbits occurring by
PR light drag and gravitational perturbations (Liou and Zook, 1996; Nesvorný et al. 2010,
2011a,b). Dynamical models suggest that the entry velocities of cometary particles, in
particular those from Jupiter-Family and Kuiper Belt object, can be as low as those of
particles derived from asteroids, albeit with a large proportion of higher velocity particles
(e.g. Plane et al., 2018). If comet-derived particles do have similar velocity populations to
asteroidal particles, then their lower densities would suggest enhanced survival at any
particular size. Furthermore, dynamical modelling studies suggest that 50-70% of
interplanetary dust in the inner solar system is derived from Jupiter-Family comets. With
entry velocities of cometary material possible at 5 km s-1 and with the lower temperatures
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achieved by lower density particles, it is reasonable to assume that more cometary particles
should be preserved on Mars than Earth. These large cometary derived particles reaching the
surface of Mars could provide the best environments for the survival of large quantities of
organic material.
One potential issue with the survival of organic matter is the longer heating pulses of
particles falling on Mars. Kinetic effects have been observed in pulse heating studies of layer-
lattice silicates (Sandford and Bradley, 1989) implies that short heating pulses are too short to
enable the complete sublimation/pyrolysis of organic material allowing some to survive.
However, this was proven not to be the case in a pulse heating study on coronene (a
refractory PAH) by Matrajt, et al. (2005). They showed that the coronene completely
disappears at temperatures >650°C regardless of heating duration. Instead, survival of organic
material was attributed to the porosity and devolatilization through endothermic
decomposition reactions of phyllosilicates during entry heating providing localised ‘cool’
areas inside MMs, an effect known as “the heatshield effect” (Matrajt et al. 2006; Genge,
2006). Although the heating pulse duration does not directly relate to the sublimation/
pyrolysis of the organics themselves, it is likely that longer heating pulses eventually lead to
complete devolatilization and migration of vesicles to the surface of the particle thus
removing these localised ‘cool’ areas. This concept is supported by results of pulse heating
experiments on MM analogues (Toppani et al. 2001) which show that longer heating pulses
result in an increase in vesiculation and more significant partial melting.
It is difficult to determine to what extent these longer heating pulses affect the survival of
organic matter in martian MMs. The degree of devolatilization and vesicle migration will
vary depending on entry parameters, pre-existing mineralogy and particle spin rates (Genge,
2017b). However, it is still worth noting that increased heating duration could result in a
decrease in organic material survival.
ConclusionThe simulation of micrometeorites falling on the Earth and Mars allows for a direct
comparison between the two MM populations and an estimate of the abundance of cosmic
spherules, scoriaceous and unmelted MMs on each planet. Results clearly show a much
higher abundance of unmelted particles on Mars and a larger maximum size. This, coupled
with the lack of any extensive resurfacing processes on Mars, leads to the assumption that the
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martian regolith should be highly enriched in meteoritic material compared to soil on Earth,
likely including a wealth of new parent body samples. The higher abundance of particles
remaining below the maximum hydrocarbon sublimation temperature on Mars also suggests
that there should be a high abundance of extraterrestrial organic material on the martian
surface. However, the oxidation of MMs under the high CO2 concentrations seen on Mars,
along with the slightly acidic martian soil, could result in the destruction of much of the
organic material in MMs. If organic material does survive for long periods on the surface,
areas which act to concentrate micrometeorites could provide ideal locations for future
sample return missions such as Mars 2020 to search for evidence of past or current life.
Acknowledgements
This study was funded on UKSA/STFC grant ST/M003167/1. This paper was written by the
lead author however credit for much of the model must first be given to Stanley Love and
Don Brownlee following their pioneering work and then Matt Genge for his major
contribution in writing the model used in this study. All authors contributed to review and
discussion of content. George Flynn, Luigi Folco and Don Brownlee are all acknowledged
for their insightful comments on this manuscript.
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Figures
Table 1. Summarising the entry parameters considered.
Range Bin Sizes Average
Velocity – Earth (km s-1) 11.2 – 72 0.5 – 7 14.5
Velocity – Mars (km s-1) 5 – 32.4 0.5 – 2 9.6
Size (µm) 10 – 500 2.5 – 50 34
Angle – Earth (°) 10 – 90 2.5 45
Angle – Mars (°) 12.5 – 90 2.5 45
Table 2. Fraction of micrometeorites reaching key temperature on the Earth and Mars.
Temperature (K)
Reference Earth (%) Mars (%)
Average Organic Sublimation
473 (Matrajt et al. 2006) 0 0
Coronene (PAH) 800 (Matrajt et al. 2006) 0 10.4
Max organic found 1173 (Matrajt et al. 2006) 3.5 47.6
Chondritic solidus 1414 (Genge et al. 2017) 22.7 68.2
Average Pyroxene 1650 (Hall, 2007) 61.8 86.4
chondritic liquidus 1700 (Genge et al. 2017) 70.6 90.1
Nickel 1728 N/A 75.8 91.8
Iron 1811 N/A 88.3 96.1
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Olivine (Fa50:Fo50) 1832 (Leclerc and Benoist, 1993)
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Fig. 1. Temperature vs time profiles of a micrometeorite with a radius of 34 µm falling on the Earth (solid line) and Mars (dashed line) with an entry angle of 45° and entry velocities equal to the average on each planet (14.5 and 9.6 km s-1 respectively).
Fig. 2. Temperature vs time profiles of a micrometeorite with a radius of 34µm falling on the Earth (solid line) and Mars (dashed line) with an entry angle of 45° and an entry velocity of 12 km s-1.
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Fig. 3. Relative size distributions of unmelted (black), partially melted (grey) and entirely melted (white) particles falling on the Earth (top) and Mars (bottom).
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Fig. 4. The fraction of the total micrometeorite flux reaching the surface of the Earth (top) and Mars (bottom) which are predicted to be unmelted (black), partially melted (grey) and entirely melted (white).
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Fig. 5. The abundance of unmelted (black), scoriaceous (grey) and cosmic spherules (white) on the Earth (top) and Mars (bottom).
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Fig. 6. The fraction of the total micrometeorite flux reaching the surface of the Earth (top) and Mars (bottom) which are predicted to be unmelted (black), scoriacious (grey) and cosmic spherules (white).
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Fig. 7. The time spent molten for particles on the Earth (Black) and Mars (White).
Fig. 8. The measured atmospheric density profile for Earth (big dash) and the atmospheric density profiles of Mars (solid line) as calculated using a scale height of 7.9 above 30 km. This also shows the atmospheric density profile for Earth (dashed line) as calculated using a scale height of 8.
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Fig. 9. Comparison of the entry velocity and flux model for Earth (grey) and Mars (black) derived from the empirical data of Southworth and Sekanina (1973) (solid) with those derived from dynamical models derived from Plane et al. (2018) (dashed). The dynamical models suggest that high velocity dust (>10 km s-1) comprises ~30% more of the population than the model used in the current work.
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