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Silica Aerogel: ISRU, Engineering, and Applications for Mars and Space Settlements



This research evaluates the methods, materials science properties, applications of the silica (SiO2) and aerogel life cycle on Earth in the early to mid stages of a Mars and space settlement. Silica Aerogels (SA) are highly effective insulation and thermal materials for rovers, EDL vehicles, heat shields, cryotanks, habitats, spacesuits, domes, radiators, electronics, and regolith/soil heating. SA is also an effective material to improve the performance of inflatables, pipes, heavy metal absorbents, sensors, nuclear waste containment, filtration, and bioengineering. Methods to tailor the physical, mechanical, chemical, hydrophobic properties and radiation refractive index of SA are reviewed. The addition of carbon fiber nanoparticles can increase the compressive modulus of SA by three fold and tensile stress by five fold. Nanofabrication and synthesis methods are referenced including the crosslinking with polymers, and deposition with sprayable coatings. Available data on hydrated silica bound minerals, maps, and native Martian minerals are referenced in candidate locations including Arcadia Planitia and Valles Marineris, paving a roadmap for industrial scale SiO2-based in-situ resource utilisation (ISRU) operations. Both the artificial and biological processes for the mining, extraction, separation, purification of silica and precursors from regolith are cited. The biogenic silica production on Earth with photosynthetic algae diatoms, radiolarians, sponges, and silicate solubilizing bacteria (SSB) are reviewed as novel organisms to biomine and produce SiO2. Microbial cell factories and algal photobioreactors also hold great potential to manufacture biomass, biodiesel, protein and bioplastics in situ. A 2-3 cm thick layer of silica aerogel could shield almost all of the high-cancerous-levels of UVA, UVB, and UVC radiation in the space environment without an external heat source. As a key motivator for the research study, SiO 2 aerogel sheets hold the potential to enable water ice mining and provide habitable environments for cyanobacteria, algae, and biomass cultivation while simultaneously transmitting visible light for photosynthesis, blocking ultraviolet radiation, raising temperatures, and reducing energy costs to induce solid state greenhouse effects. This research provides a literature review with over 130 references to design the silica ISRU and aerogel production process. This study investigates the past, present, and future engineering applications, challenges, and advancements of SiO2 aerogel to enable industrial scale production and habitable environments throughout the Solar system and beyond.
72nd International Astronautical Congress (IAC), Dubai, United Arab Emirates, 25-29 October 2021.
Copyright 2021 by Mars University. Published by the IAF, with permission and released to the IAF to publish in all forms.
Silica Aerogel: ISRU, Engineering, and Applications for Mars and Space Settlements
Kolemann Lutza, Paolo Pinob, Nadiir Bheekhun, PhDc
aCofounder, Mars University, Washington, DC, United States,
bPhD Researcher, Politecnico di Torino, Turin, Italy,
cFounder and Director, Aeronad, Port Louis, Mauritius,
This research evaluates the methods, materials science properties, applications of the silica (SiO2) and aerogel
life cycle on Earth in the early to mid stages of a Mars and space settlement. Silica Aerogels (SA) are highly
effective insulation and thermal materials for rovers, EDL vehicles, heat shields, cryotanks, habitats, spacesuits,
domes, radiators, electronics, and regolith/soil heating. SA is also an effective material to improve the performance
of inflatables, pipes, heavy metal absorbents, sensors, nuclear waste containment, filtration, and bioengineering.
Methods to tailor the physical, mechanical, chemical, hydrophobic properties and radiation refractive index of SA
are reviewed. The addition of carbon fiber nanoparticles can increase the compressive modulus of SA by three fold
and tensile stress by five fold. Nanofabrication and synthesis methods are referenced including the crosslinking with
polymers, and deposition with sprayable coatings.
Available data on hydrated silica bound minerals, maps, and native Martian minerals are referenced in candidate
locations including Arcadia Planitia and Valles Marineris, paving a roadmap for industrial scale SiO2-based in-situ
resource utilisation (ISRU) operations. Both the artificial and biological processes for the mining, extraction,
separation, purification of silica and precursors from regolith are cited. The biogenic silica production on Earth with
photosynthetic algae diatoms, radiolarians, sponges, and silicate solubilizing bacteria (SSB) are reviewed as novel
organisms to biomine and produce SiO2. Microbial cell factories and algal photobioreactors also hold great potential
to manufacture biomass, biodiesel, protein and bioplastics in situ.
A 2-3 cm thick layer of silica (SiO2) aerogel could shield almost all of the high-cancerous-levels of UVA, UVB,
and UVC radiation in the space environment without an external heat source. As a key motivator for the research
study, SiO2aerogel sheets hold the potential to enable water ice mining and provide habitable environments for
cyanobacteria, algae, and biomass cultivation while simultaneously transmitting visible light for photosynthesis,
blocking ultraviolet radiation, raising temperatures, and reducing energy costs to induce solid state greenhouse
effects. This research provides a literature review with over 130 references to design the silica ISRU and aerogel
production process. This study investigates the past, present, and future engineering applications, challenges, and
advancements of SiO2aerogel to enable industrial scale production and habitable environments throughout the Solar
system and beyond.
Keywords: Silica Aerogel, Greenhouse, Algae Mats, Space Settlement, In Situ Resource Utilisation
ASR = Alkali-Silica Reactivity
CRR = Carbothermal Reduction reactor
CVD = Chemical Vapor Deposition
HIRMS = High Intensity Rolling Magnetic separator
GCR = Galactic Cosmic Rays
ISRU = In Situ Resource Utilisation
IRMS = Induced Roller Magnetic Separation
PEG = Polyethylene Glycol
PI = Cross linked Polyimide
SA = Silica Aerogel
SSB = Silicate solubilizing Bacteria
TMOS = Tetramethyl Orthosilicate
IAC-21-A1.19 Page 1 of 18
1. Introduction
1.1. Challenges for Space Exploration and Settlement
Extreme thermal, radiative and atmospheric conditions pose challenges to the expansion of humans and life in
space, celestial bodies, and throughout the Universe as most biology as we know it has not evolved to survive far
from Earth. Aerogels and other insulative materials are highly effective at maintaining thermal operating
environments for biological and nonbiological systems and mitigating heat loss to keep on-board electronics at
approximately 20°C. With high cryogenic temperature gradients in space, Moon, Mars, and other celestial bodies,
the typical temperature range for space-qualified electronics components is -55°C to +125°C with a 15° C margin
on each side (-40° C to +110° C). As heaters are typically required to keep rovers and machine’s battery systems
from freezing, these external or self-heating components in energy storage devices add weight and constantly require
power for instruments and machinery. Lithium ion batteries and supercapacitors typically have the lowest operating
temperatures of -20° C to -40° C, limited by freezing points of electrolytes. These challenges suggest a need to
identify, implement, and advance alternative methods and insulative thermal materials to expand operating
1.2. Silica Aerogel Material Properties
Silica (SiO2) is chemically stable and refractory (resistant to
decomposition) at extreme temperatures and high thermal
gradients such as those on the Moon, Mars, and vacuum of
space. Silica aerogels (SA) are nanoporous networks of silicon
dioxide (SiO2). The physical properties of SA’s are highlighted
in detail in Table 1 from a 2008 study on Silica Aerogel;
Synthesis, Properties and Characterization [1]. Considering the
high density of silica around 2200 kg/m3or 0.003-0.35 g/cm3,
the aerogel’s peculiar porous structure forms one of the lightest
materials, around 50–200 kg/m3in mass with 5% solid
fraction. The SA pore size ranges from 5–100 nm and
constitute up to 99.9% of the total volume, which provides
aerogel with spectacular thermal insulative properties with an extremely low thermal conductivity from 0.03
W·m−1·K−1 [2] in atmospheric pressure to as low as 0.004 W·m−1·K−1 in a modest vacuum. SiO2aerogel has a high
R-values of R14 to R105 (US customary) or 3.0 to 22.2 (metric) for 3.5 in (89 mm) thickness, making it highly
resistant to the conductive flow of heat. Aerogels used in applications for buildings on Earth have densities closer to
150kg/m3[3] and require specific surface areas of up to 500 m2/g·m[4]. The poor mechanical properties of silica
aerogel result in a material that is typically extremely stiff and brittle [5,6], even though SiO2aerogel can sustain
loads up to 2000X its weight in applied force. When forces are not applied gently and uniformly, the aerogel tends to
fragment and pulverize. Moreover, the vibrations induced from a launch vehicle and entry descent and landing
(EDL) could impose fractures and cracks in aerogel if not monitored, which could result in system and mission
failure in extreme environments. Therefore, other materials, polymers such as plastic can be used to improve
performance with lower elastic young’s modulus (E) and rupture modulus (σ), or the measure of stress before it
yields in a bend test.
1.3 Aerogels, Insulation, and Coatings to Attenuate Radiation
On Earth, humans are exposed to 3 to 4 millisieverts (mSv) of ionising radiation a year, mostly from natural
sources like some kinds of rocks and the minimal galactic cosmic rays (GCR) through the atmosphere. Based
on measurements made by the Curiosity rover, the average natural radiation level on Mars for the surface of
Mars is approximately 230 mSv/year. [7] which is about 60-70X greater than the average on Earth. On the
Moon, the annual exposure caused by GCR on the lunar surface is roughly 380 mSv (solar minimum) and 110
mSv (solar maximum) [8]. Astronauts on the lunar surface in an uninsulated environment without radiation
protection would absorb 1,369 microsieverts per day, which is about 200 times the amount organisms receive
on Earth's surface.
Solid silica is transparent to visible radiation but opaque to UV wavelengths shorter than 200-400 nm, and to
infrared wavelengths longer than 2 µm. Thus, aerogels hold great potential to block ionising UV-B, which damages
DNA from sunlight wavelengths less than 320 nm and UV-C Radiation [9]. In a 1996 study, silica aerogel
Cherenkov nuclear radiators samples were irradiated up to 9.8 Mrad of equivalent dose. Deteriorations in
IAC-21-D3-1.8 Page 2 of 18
transparency and changes of refractive index were observed to be less than 1.3% and 0.001 at 90% confidence level,
respectively. Thus, Silica aerogels can be used in high-radiation environments, such as B-factories, nuclear and
heavy-ion experiments, space-station and satellite experiments, without any fear of radiation damage (Sahu, 1996).
However, aerogel alone is generally not suitable to attenuate GCR’s, protons, and gamma rays that bombard the
Martian surface with intensities around 156.4 mSv/year (at solar maximum) to 273.8 mSv/year (at solar minimum).
Aerogels could isolate the biological effects of GCR's and attenuate 1.0 - 5.0 MeV in alpha particles, but will require
additional resources, nanomaterials, and insulation to shield cosmic rays, solar wind, and small impactors.
NASA has investigated GCR active and passive shielding materials including: (i) Carbon nano-materials with
adsorbed H • (ii) Metal hydrides: LiH, MgH2, LiBH4, NaBH4, BeH2, TiH2and ZrH2, (iii) Pd (and alloys) with
absorbed H, (iv) Hydrocarbons (polyethylene or (CH2)n ) with boron, and (v) Quasi-crystals (TiZrNi) [10].
Multilayer cosmic-ray shielding with ultralight multifunctional materials, including metallic microlattice,
aerographite, and aerogels containing or filled with hydrogen. Aerographite is the lightest, and all of them are
capable of holding a high volume of hydrogen, up to 99% of total weight. Aerographite is produced via single-step
chemical vapor deposition (CVD) and synthesized by growing porous carbon nanotubes around a sacrificial
template made from mixing and heating Zn and polyvinyl butyral powders at 900 °C [11]. Ten layers of 10-cm thick
aerographite, results in a 1.0-m-thick protective layer around the entire habitat and would weigh only 113 kg.
Whereas heavy materials such as lead or depleted uranium absorb all alpha, beta, gamma radiation, materials with
low atomic numbers (low-Z) and hydrogen rich materials are ideal materials for radiation shielding because they do
not easily break down to form secondary radiation sources. Hydrogen filled GCR multilayer insulation composed of
microlattice, aerographite, and aerogel could reduce the dose equivalency of GCR to small values, 5–10 cSv /yr
(50-100 mSv /year) on the surface of Mars.[12] Multilayer Cosmic-Ray Shielding design holds the potential to
yield a 56-78% reduction in average radiation on Martian surface. Although a GCR dose of 50 mSv/year is
bearable, it is still significantly greater than the 3 to 4 millisieverts (mSv) of radiation a year on Earth’s surface.
Therefore, improvements in radiation shielding material science, coatings, and nanoparticles may further reduce the
dose equivalent radiation.
1.4 Improving Aerogel Mechanical Properties.
A 2018 study from Ma et al. used the finite volume method (FVM) to develop and analyse the stress strain curve
of the SiO2aerogel and simulate compression behavior, elastic modulus and yield stress.[13] Scanning electron
microscopy (SEM) and Brunauer Emmett Teller (BET) analysis were used to derive the geometrical properties of
silica aerogel such as pore size, ligament diameter, and particle size. The overall elastic modulus and Poisson’s ratio
are obtained under an overall linear compression strain of 5 × 10−3. The compressive experiment shows that the
elastic modulus E0is 0.042 GPa and the compressive yield stress σ0is 0.023 GPa. SiO2aerogel studied has a tensile
strain (deformation or elongation of a solid body) of 5%, 35% and 65% for the linear elastic, plastic yielding and
densification, respectively.
The mechanical properties of aerogels may be described by two parameters, Young’s modulus (E) and Poisson’s
ratio (v), which is highlighted in Equation 1. The absolute value of the ratio between the longitudinal strain and
transverse strain is called Poisson's ratio, which is expressed as follows:
Eq (1)
The compression or Bulk modulus (k), determined from Eq X, measures the stiffness of the material or the ability of
the material to withstand changes in length when subjected to compressive loads, where (P) pressure and (V) is
initial volume of the substance.
IAC-21-D3-1.8 Page 3 of 18
k = -V Eq (2)
The mechanical properties (elastic modulus E and yield stress σ) have a power–law dependence on the relative
density ρ [14,15] where m and n both represent constant as highlighted below:
E x ρ mEq (3)
σ x ρnEq (4)
Studies detailing the scaling of mechanical properties with the density of aerogels were put forth by Fricke and
Pekala et al.[16] Young’s modulus (G) was found to scale with bulk density (Fb) in a power law relationship, G
Fba, where the exponent, a value varying two and four [17]. Gelation, aging, and shrinkage all play roles in defining
the final mechanical properties of aerogels.
The R-value, Lankford coefficient, or Plastic Strain Ratio is the resistance of a material to thinning or thickening
when put in tension or compression. The Plastic Strain Ratio can be determined using Equation 5 below where
in-plane plastic strain (deformation) in numerator and plastic strain through the thickness in denominator.
Eq (5)
Copolymerization or cogellation of silanes with an organic polymer can improve mechanical aerogel
reinforcement. Polydimethylsiloxane (PDMS) or aeromosils were synthesized with TEOS to demonstrate a 4-fold
increase in the compressive strength over TEOS-based aerogels and recovered their original shape from a state of
30% compressive strain [18]. Moreover, when the relative density of silica aerogel is moderate and fixed around
.08-.12, the pore size and ligament diameter to acquire
larger elastic modulus and yield strength to optimise the
mechanical behavior of SiO2aerogels. When increasing
the relative density to greater than 0.16, the mechanical
behavior of SA becomes much greater. The tensile
properties of porous aerogels are largely dependent on the
particular orientations among ligaments and the length
scale of ligaments. Liu et al. [19] found that the neck
radius and the strength and stiffness of the particle chains
are inversely related and decrease with greater ligament
lengths. Moreover, a 2013 study from South Korea
mentions the negative pressure rupturing method proposed
by (Kieffer and Angell 1988) can improve the porous
structure and improve stability when density of SiO2
aerogel is above 0.8g/cm3.
Most aerogels experience compressive and bending stresses and small strains less than 25%, which suggests the
flexibility of aerogel becomes critical in such applications. A 2011 ACS study highlights leading methods to
improve the Mechanical Properties of Aerogels for Aerospace Applications[20]. As the use of organo-silanes hold
great promise to improve mechanical SA properties, aerogels can be strengthened by reacting the hydroxyl groups
on the silica gel surface with organic moieties carrying isocyanate groups (-NCO), followed by supercritical fluid
extraction after exchanging CO2solvent. [21] With a coating of polymer on the silica aerogel backbone, the resultant
aerogels demonstrate large increases in mechanical strength compared to unreinforced silica aerogels when
evaluated by three point bending tests. For example, the isocyanate-reinforced aerogels supported stresses up to 800
kPa in a three-point bending test, compared to 20 kPa for a native aerogel of the same density of 280 mg/cm3[20].
18-25 repeat units of polymer produced the largest enhancement in mechanical properties at lower densities than
IAC-21-D3-1.8 Page 4 of 18
previously reported.[22] The thermal conductivity of optimized aerogels varied from 19 to 36 mW/(m K) as
characterized using laser flash method.
It should be noted that the ambient humidity present during testing can also alter the mechanical properties of
silica aerogels, especially for hydrophilic aerogels. [20] Ambient humidity has less of an effect on mechanical
properties of hydrophobic aerogels. Miner et al.[23] noted a 60% increase in Young’s modulus, from 0.5 to 0.8 MPa,
and a 10% increase in mass of hydrophilic silica aerogels due to water vapor uptake. As insulations may degrade
due to moisture absorption or condensation when they are exposed to humidity, a 2004 study from UVA evaluated
the effects of ambient humidity on the mechanical properties on hygroscopic SA. Hygroscopic aerogels were tested
in a controlled humidity chamber to study the effects of adsorption of H2O in gel on Young's modulus and
non-recoverable strain. Results indicate that at 70% relativity humidity, the samples failed and several absorption
lines indicative of hydrogen bonding between water and silica were seen to increase with increasing humidity.
A 2013 study reviews the literature on several methods to improve SiO2 aerogel synthesis and mechanical
reinforcing strategies [24]. Reinforcing of silica aerogels with fiber blankets inside sol before gelation is simpler and
more effective when compared to the reinforcement of aerogels with separate and non-woven fibers. Reinforcing
through a fiber blanket can be achieved by immersing the fiber blanket inside of the sol before gelation, facilitating
large scale production [24]. Meador et al. [25] studied the use of the carbon nanofibers as reinforcement agents for
polymer cross-linked aerogels. They examined the effect of incorporation of 5 wt.% carbon nanofibers in
di-isocyanate cross-linked silica aerogels. in their established model, it is possible to obtain a three-fold increase in
compressive modulus without any increase in density of the monoliths by increasing the fiber concentration up to
5%. And also, a five-fold increase of tensile stress at break was obtained by incorporation of 5% fiber, at lowest
concentration values of total silane and cross-linker agents [25].
As the aging process can also be used to enhance mechanical SA properties, ways to expedite the aging process
include heat treatment in water, [26] and soaking in alcohol [27,28] with and without additional TEOS. These
processes increased the elastic modulus of the final aerogel products by roughly a factor of 2. Additionally, low
dielectric Polyimide Aerogels can be used as substrates for lightweight patch antennas for the transmission of
electrons with minimal electrical power loss.The polyimide aerogel antennas made from DMBZ and BPDA cross
linked with TAB exhibited broader bandwidth, higher gain, and lower mass than the antennas made using
commercial substrates, which is very encouraging for aerospace applications [28]. Moreover, silica aerogel is a
promising candidate for replacing the conventional micron-sized silica to improve the mechanical properties of
epoxy-based nanocomposites. Mechanical tests showed improvements in flexural modulus and strength by ~ 80%
and ~ 40%, respectively, as compared with those of pure epoxy. Based on this study, water-glass based silica aerogel
hold great promise as a low-cost filler in polymer composite. [29]
1.5 Silica Aerogel Precursors and Synthesis
The manufacturing process to form silica aerogels comprises
two steps: the formation of a wet gel by sol–gel chemistry, and the
drying of the wet gel, when the liquid within the gel is removed
leaving only the linked silica network. The significant change from
the liquid to the solid stage is termed the sol–gel transition. Acids or
base catalysts are used to modifysol-gel reaction time at room
temperatures [30,31].The amount and type of the used catalysts
play key roles in the microstructural, physical and optical properties
of the final aerogel product. Three main routes are commonly used
for drying: (1) freeze-drying, in which the solvent inside of pores
needs to cross the liquid–solid then the solid–gas equilibrium curve;
(2) evaporation, which implies the crossing of the liquid–gas equilibrium curve of the solvent; (3) supercritical fluids
drying (SFD).
The most common of the silicon alkoxides are the tetramethyl orthosilicate (TMOS, Si(OCH3)4) and
tetraethylorthosilicate (TEOS, Si(OCH2CH3)4)[32], with a common chemical formula of Si(OR)4. The most
common technique used for producing silica gels today involves the reaction of a silicon alkoxide with water,
usually in the presence of basic, acidic, and/or fluoride-containing catalyst. The silicon alkoxide reacts with water to
form silanols, which then condense to form the silica network. The key to TMOS was the use of methanol as the
solvent, which was then replaced with liquid CO2 as the solvent, which alleviated the material property challenges
from the supercritical drying of methanol.[34] By heating the system to 32 C, liquid CO2can be maintained at
supercritical conditions with lower pressure. The lower temperature in the drying step and nonflammability of CO2
make the process safer and less expensive.
IAC-21-D3-1.8 Page 5 of 18
Figure X. Silica Aerogel Synthesis with TEOS [ 35].
Traditional production steps and equipment to manufacture silica aerogels include: Reactors for mixing of
water/alcohol and alkoxides (fluidized bed/ mechanical stirring), moulds for gelling and ageing, Methanol pools for
washing and ion exchange, Supercritical drying reactor, Atmospheric CO2liquefiers and storage tanks, Methanol
recycler. SA can also be obtained starting from waterglass, a substance that can be extracted from martian minerals
with the use of ionic liquids. In this case, SA can be formed with the following reactions: Na2SiO3+ H2O + 2HCl →
Si(OH)4+ 2NaCl.
1.6 Established SA Applications for Space Systems, Exploration, and Settlement
Aerogels are widely employed on Earth and find numerous applications in space as well. As previously
mentioned, they are extremely successful as thermal insulators. SA is used in pipeline insulation (especially in the
petrochemical and oil and gas industries) [36], cryo-insulation, buildings and constructions, windows and glasses,
shipping containers and refrigeration. In construction applications, it has been shown that addition of granular
aerogels to plaster and mortars leads to consistent reduction of thermal conductivity [37] [38]. Ibrahim, et al. tested
the hygrothermal performance of walls with patented aerogel-based insulating rendering assessing the assembly
water content, drying rate, mold growth, condensation risk, ASHRAE-160 moisture criterion, and heat losses.
Results show that interior thermal insulation systems can cause several moisture problems: inability to dry out over
the years, condensation risk, etc. Adding the aerogel-based rendering on the exterior surface of the un-insulated or
the internally insulated walls removes or significantly reduces the moisture risks and heat losses [39]. On Earth, SA’s
are also used for cryogenic storage and fuel cell catalysis. In 2007, NASA developed aerogel based insulation for
ambient pressure environments for liquid hydrogen (LH2) storage.Long-duration tests (up to 10 hours) showed that
the potentially harmful nitrogen mass taken up inside the hydrogen storage tankis reduced by a factor of 3X for the
aerogel insulated case compared to the un-insulated case [40]. In space, aerogels have been used onboard Martian
rovers for thermal control and for cosmic dust collection.The aerogel structure can also be used for hypervelocity
particles to gradually slow particles and capture them intact for further analysis in situ or upon EDL on Earth.
Silica aerogels can be applied to collect aerosol particles [41], to protect space mirrors, and to design tank
baffles [42, 43]. Aerogels have also been used for maintaining the Seismic
Experience for Interior Structure (SEIS) instrument on the NASA InSight mission
to Mars. [44] Aerogels have been utilised on most Mars rovers including the
Pathfinder mission, where a stable 21° C temperature was maintained in a -67° C
environment. Other applications include light and efficient re-entry vehicles, heat
shields, insulation of cryotanks, habitats and crew rovers, air revitalization and
EVA spacesuits. However, current aerogel composites flake apart under rigorous
cyclic loading-unloading tests and lose insulation quality over time.[45,46] Robust
aerogel composites may also be used for baseline insulation materials of inflatable
decelerators for entry, descent, and landing (EDL) vehicles [47]. ISRU production
of SiO2and aerogel manufacturing facilities on Moon and Mars holds the potential
to reduce launch costs, and risk of aerogel and glass fracture from launch and EDL
landings. Integrating aerogel insulation and coatings into domes and greenhouses
could also retain thermal heat and costs. As highlighted in the 929 pg aerogel
handbook published in 2011, aerogels have also proven effective and are continuously being developed to support
containment of nuclear waste, CO2trapping, water repellent coatings, chemical sensors, heterogeneous catalysts,
metal casting molds, acoustic transducers, energy storage devices, thermites, pharmaceutical drug carriers, non
flammable cryogenic insulator, and confinement media to study the interactions in superfluids. [48] Aerogels are
also effective materials to improve the performance of cables, dopants, pipes, plumbing, catalysts and filters,
IAC-21-D3-1.8 Page 6 of 18
adsorption, filtration, air revilitazation, rust and corrosion prevention, absorbents of heavy metal & contaminants in
water, and bioengineering (antibacterial activity, wound healing, angiogenesis).
Cryogel is an insulation material used at temperatures as low as -200°C. Cryogel insulation blankets are made
with a patented nanotechnology form of silica aerogel insulation with a non-woven, glass-fibre batting to strengthen
the material. [49]. Developed and manufactured by Aspen Aerogels, Cryogel Z is a flexible aerogel blanket
laminated to a vapor retarder with zero water vapor permeance. As highlighted in the figure on the right, cryogel Z
has a 5-10 mm thickness with a .16g / cubic centimeter, and 116 m2roll size. With excellent acoustic and thermal
protection, the aerogel blanket can be used for cryogenic pipelines,vessels and equipment, gas liquefaction &
re-gasification facilities, which provide 50% less heat gain and boil off.
1.6 Silica Aerogel Mats for Algae Biomass Production, Water Mining, and Greenhouses
Aerogels can also provide thermal insulation on the surface of Mars and
planetary bodies to maintain thermal conditions to heat soils, to melt water ice,
and to sustain habitable induced greenhouses, which presents a key motivator
for this SA literature review. In 2019, Wordsworth et al. [50] conducted an
experiment that demonstrated a 2–3 cm-thick layer of silica aerogel will
simultaneously transmit sufficient visible light for photosynthesis, block
hazardous ultraviolet radiation and raise temperatures underneath it
permanently to above the melting point of water, without the need for any
internal heat source in Mars environment conditions. Moreover, less smoke and
light could hold the potential to increase temperature into hundreds. Although,
heat was lost in experimental setup via sidewall and base thermal losses and
convection. Also measured transmission of aerogel particles and tiles in
ultraviolet and found strong attenuation of UV-A and UV-B and near total
attenuation of most hazardous UVC radiation. However, galactic cosmic rays
(GCR) and higher energetic radiation particles would still penetrate through the aerogel down to around 1-meter
depth. Moreover, a 2019 study demonstrated that a low scattering non-evacuated transparent aerogel could heat the
environment below to intermediate temperatures(120–220 °C) and induce the greenhouse effect. The solar receiver
aerogel has a scattering center diameter (which dictates optical transparency of aerogel) around 6 nm, much smaller
than previously reported values of around 20 mm aerogel. Without the need for costly optical and mechanical
components, the thicker aerogel layer reduces heat loss without incurring a significant optical loss,enabling a
pathway to promote solar thermal energy utilisation. [51]
2. Silica minerals on Mars and processing techniques
2.1. Native Silica (SiO2) on Mars
The separation of metals from SiO2is a common process throughout the Solar System. The relatively rapid
separation of silicate and iron-rich inner planets and meteorites implies both components were once both fluid
(Stevenson 1990). Mars has a central core made up of metallic iron and nickel surrounded by a less dense, silicate
mantle and crust. Silicon comprises around 23% on average of the Martian Crust [53], which is considered to be the
second most common element on Mars, after oxygen. Pathfinder’s soil samples were normalized to 44% wt of Silica
for the purpose of comparison in chemical composition of Martian soil and rock analysis [53]. While silica can
comprise up 90% of the composition of some of the rocks, the
majority of silica is currently mixed with other elements and
metal oxides. Although, there may have been geological
processes that have concentrated silica into more easily usable
The figure to the right highlights NASA JPL map of
Mid-latitude Martian silicon concentrations, which was observed
from the gamma ray spectrometer onboard the 2001 Mars
Odyssey orbiter. The region with the highest silicon is in the high
latitudes north of Tharsis (centered near 45 degrees latitude, -120
degrees longitude) and northwest of Valles Marineris [54]. In Valles Marineris, there is evidence for a volcanic field
on the floor of the deepest trough of Coprates Chasma. Spectral data reveals an opaline-silica-rich unit associated
with at least one of the 130 individual structures resembling dark-colored volcanic rock scoria and tuff or volcanic
ash cones that are associated with units that are interpreted as lava flows.[55] Hydrated Fe sulfates, including H3
IAC-21-D3-1.8 Page 7 of 18
O-bearing jarosite, and are found in finely stratified deposits exposed on the floor of and on the plains surrounding
the Valles Marineris. A 2008 study on the opaline silica in young deposits on Mars, mentions the silica signatures
are associated with stratigraphically lower Fe sulfates, consistent with the mineral sequence expected from
evaporation of fluids produced by acidic dissolution of basalt [56]. In 1997, Mars pathfinder rover observed much
higher silicon content in some of the nine rocks at Ares Vallis, Chryse Planitia compared to martian meteorites. If
the high silicon-based andesites (extrusive volcanic rock) are representative of the highlands, they suggest that
ancient crust on Mars is similar in composition to continental crust on Earth. This similarity would be difficult to
reconcile with the very different geologic histories of the two planets. Alternatively, the rocks could represent a
minor fraction of high-silicon rocks on a predominantly basaltic plain. [53] NASA's Curiosity rover has found much
higher concentrations of silica at some sites it has investigated in the past seven months than anywhere else it has
visited since landing on Mars 40 months ago. Multispectral data from MER Spirit rover at Gusev Crater (14.5°S
175.4°E)\, a 166km wide crater, near northern most party of Elysium Planitia found silica-rich deposits and hydrated
minerals are bound with H2O or OH with 1009 nm pancam wavelength. Observations of the brightest exposure of
soil by the rovers Alpha Particle X-Ray Spectrometer (APXS) instrument show that its composition is 90.1 wt.%
SiO2 (98 wt.% SiO2 when corrected for dust contamination). Spectra also reveals a suite of sodium silicate minerals
(magadiite (NaSi7O13(OH)34(H2O)), sodium metasilicate pentahydrate (Na2SiO35H2O), and sodium metasilicate
nonahydrate (Na2SiO39H2O)) as geologically reasonable silica-rich material components
As SpaceX reaffirmed prioritization of Arcadia Planitia in 2019, a 2015 study from UArizona Researchers
analysed the Shallow Subsurface Water Ice and minerals at two Exploration Zones (EZ) in the northern
mid-latitudes of Mars in the vicinity of Arcadia and Amazonis Planitia. The uppermost surface at both of these two
locations is rich in iron and silicon, 14 and 18-20 wt. % respectively, which are also of interest for ISRU.
Researchers proposed a landing site centered at 192.1°E, 39.0°N near Erebus Montes and confirmed the detection of
metal silicon at SRO1-1 and SROI-2 [52,57].
2.2.1 Hydrated Silica
There may be substantial deposits of silica gel and hydrated silica (SiO2• xH2O) within aqueously altered
mineral suites and opaline mineralization by-products in hydrothermal metamorphic and weathering locations in the
Martian regolith [58]. Biogenic silica (bSi), also referred to as opal, biogenic opal, or amorphous opaline silica,
forms one of the most widespread biogenic minerals. bSi is hydrated silica (SiO2·nH2O), and is essential to many
plants and animals. Aqueous free silica is a product of basalt weathering, when the interaction of water with mafic
(i.e., Mg- and Fe-rich, silica-poor) rock rapidly dissolves olivine, pyroxene, and glass. Hydrated silica crystallinity is
correlated with the geochemistry of associated minerals. Highly crystalline hydrated silica is found with
Fe/Mg-phyllosilicates, moderately crystalline hydrated silica is associated with Al-phyllosilicates, and poorly
crystalline phases are associated with sulfates. [59]In a 2008 study on the detection of silica-rich deposits on Mars,
opaline or biogenic silica deposits (as much as 91 weight percent SiO2) have been found in association with volcanic
materials by the Mars Rover Spirit [60]. The deposits are present both as light-toned soils and as bedrock and likely
formed under hydrothermal conditions, which are strong indicators of a former aqueous environment.
Using near-infrared spectral data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM)
aboard the Mars Reconnaissance Orbiter (MRO), B.L. Ehlmann et al. found spatially widespread and
mineralogically diverse minerals west of Isidis basin with large exposures of both mafic minerals and iron
magnesium phyllosilicates in stratigraphic context. [61] Observed minerals with greater concentrations of silica can
be categorised into smectite clays, phyllosilicates, felsic minerals, and mafic minerals. Mafic minerals are silicate or
igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic
minerals include Olivine (Mg,Fe)2SiO4), Pyroxene (XY(Si,Al)2O6), Augite ((Ca,Na)(Mg,Fe,Al)(Si,Al)2O6),
Pigeonite ((Ca,Mg,Fe)(Mg,Fe)Si2O6). Phyllosilicates, or parallel sheets of silicate tetrahedra include: Kaolinite
(Al2Si2O5(OH)4}, Montmorillonite ((Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O), micas such paragonite
NaAl2[(OH)2|AlSi3O10], margarite (CaAl2(Al2Si2)O10(OH)2), and Serpentine ((Mg,Fe)3Si2O5(OH)4). Felsic minerals
are usually light in color rich in Quartz (SiO2), Feldspar (KAlSi3O8– NaAlSi3O8– CaAl2Si2O8), and Maskelynite.
The presence of phyllosilicates on Mars has been previously suggested on the basis of in situ elemental analyses
by the Viking Landers 4.An unambiguous detection of water-bearing phyllosilicates has been reported over large
areas [62]. The detection of phyllosilicates in small areas of Arabia Terra and northern Terra Meridiani suggests that
the alteration processes could have been intense over this entire region. [63] The identification of hydrated silicate is
first based on the detection of the 1.9-µm absorption band, calculated using spectral channels at 1.93 µm for the
band centre and at 1.86 and 2.14 µm for the continuum.A major outcome of the present work is that phyllosilicates
are detected in only a very restricted number of areas, commonly in association with two types of terrains: dark
deposits and eroded outcrops. phyllosilicates, or sheet silicates, are an important group of minerals that includes the
IAC-21-D3-1.8 Page 8 of 18
micas, chlorite, serpentine, talc, and the clay minerals.Most phyllosilicates contain hydroxyl ion, OH-, with the OH
located at the center of the 6 membered rings, as shown here. Thus, the group becomes Si2O5(OH)-3.
Satellite imagery of Mars surface indicates primarily mafic rock with limited patches of light-colored rock
containing feldspar and quartz. Geochemical data and images of 22 specimens analysed by the Curiosity rover
suggest the rocks belong to two distinct geochemical types: alkaline compositions containing up to 67 wt% SiO2and
14 wt% total alkalis (Na2O + K2O) with fine-grained to porphyritic textures on the one hand, and coarser-grained
textures consistent with quartz diorite and granodiorite. Silica-rich magmatic rocks may constitute a significant
fraction of ancient Martian crust and may be analogous to the earliest continental crust on Earth. [64]
In addition to warming, fracturing, and materials brought to the surface, the meteor impactors also imply a
certain amount of glassification. These newly formed glasses, along with the likely presence of greater hydrogen
ions in the subsurface water, are likely to produce varying quantities of silica gel. The amount of gel produced would
be dependent on the amount of silica, the amount and degree of alkalinity of the subsurface water, and the
temperature of the materials under-going the Alkali-Silica Reaction(ASR).. ASR is a simple acid-base reaction
between calcium hydroxide or Ca(OH)2, and silicic acid (H4SiO4or Si(OH)4). If the bi-products of ASR are
widespread on Mars, ASR may have been significant in the weathering and erosion process. ASR reaction will tend
to take place in similar conditions as life; warm, wet conditions. This similarity may make ASR Gels and products a
good place to find evidence of life. [58]
High spatial and spectral resolution reflectance data acquired by the Mars Reconnaissance Orbiter Compact
Reconnaissance Imaging Spectrometer Mars (CRISM) instrument reveal the presence of H2O- and SiOH-bearing
phases on the Martian surface. The spectra are most consistent with opaline silica and glass altered to various
degrees. Acid-sulfate steam condensates produced by fumaroles have the capacity to leach metal cations from
basaltic rocks, leaving behind a residue of opaline silica. [65] Knowing the types of opal is important, as different
types may indicate different aqueous environments on Mars. Vivian Z. Sun and her colleagues distinguished
between opal-A and more crystalline-hydrated silica on Mars by comparing CRISM spectral data over opal silica
deposits with laboratory measurements. They found that opal-A is mostly associated with bedrock, whereas more
crystalline-hydrated silica is mostly associated with aeolian sediment. Opal-A occurrences on Mars are commonly
associated with bedrock exposures, whereas more crystalline hydrated silica (opal-CT and quartz/chalcedony) is
primarily observed in unconsolidated sediments.[66]
2.3 ISRU: Mining, Extracting, and Processing Silica and Precursors from Regolith
On Earth, silica is generally not pure when mined and tends to be associated with iron hydroxide and
oxy-hydroxide impurities which lowers its industrial value and requires purification before use. With high
concentrations of silicon dioxide in the Martian regolith, SiO2could be extracted, processed, and stored locally
nearby the settlement and production facilities. As silicon in minerals is typically highly concentrated, it still needs
to undergo a separation and costly and energy-intensive purification process before industrial use. Considering SiO2
is bound to several other mineral constituents and rocks on the surface of Mars, most mineral extraction and
processing is likely to amass feedstocks of SiO2mixed with minerals, unpurified, and purified silica. If silicate
minerals are cleaned without impurities, the cost of mineral extraction drops considerably. Traditionally, silicon rock
is melted in a furnace at 4,000° F and the silicon purified even further, similar to the steel production process, which
is why this molten rock is often referred to as metallurgical silicon. Silicon dioxide can also be melted and
chemically transformed to manufacture glass. Before glass production, SiO2can be heated in the presence ofH2in a
cyclonic mixer and passed through a magnetic separator to remove any remaining iron oxide. Silica production for
glass has an embodied energy of 6-15 MJ/kg. Dust collectors and atmospheric treatment systems will also be
advantageous in production areas.
2.3.1 SiO2Separation and Purification.
NASA Marshall Space Flight Center (MSFC) investigated using sodium silicate as a potential binder for martian
regolith grains for additive construction. Metal silicates and sodium can be extracted from sodium silicate Na2SiO3,
which has been observed in Martian regolith, with ionic liquids (ILs), which are organic salts that are liquid at or
near room temperature. [68] For physical separation 5 kg raw sand samples were dried in the sun to remove
moisture. Separation of heavy, medium and light minerals was carried out depending on specific gravity or density
of the minerals using laboratory shaking tables. After the gravity separation and drying, the light fractions of Padma
River sand was run into the high intensity roll magnetic separator to separate the magnetic and nonmagnetic
fractions. The samples were separated at 60 rpm (magnetic fraction) and 140 rpm (paramagnetic fraction) speed by
High Intensity Rolling Magnetic separator (HIRMS). The nonmagnetic part was separated again by Induced Roller
Magnetic Separation (IRMS) for high precision. The remaining light fraction was separated into ferromagnetic,
IAC-21-D3-1.8 Page 9 of 18
paramagnetic and nonmagnetic fractions with an Induced Roll Magnetic Separator (IRMS) run at 0.3 A (2000
Gauss) and 3.0 A (20 000 Gauss).This separation was done to remove feldspar and other aluminosilicates mineral
form silica sand. As silica sand is non-conductor it is easily separated by this process, electrostatic plate separator
(ESPS) separates the fine conductors from coarse nonconductor rich streams. The non conductive portion obtained
from the ESPS is mostly the silica. The fractions separated by the Induced Roll Magnetic Separator (IRMS) were
then processed by using an electrostatic plate separator (ESPS) operating at 25 kV and a feed rate of 20 rpm to
separate the conductive and nonconductive minerals. The nonconductive sand particles contain high amounts of
silica. It is observed that the silicon oxide (SiO2) content is significantly degraded in the magnetic fraction and
upgraded in the nonmagnetic fraction [69]. This separation was done to remove iron bearing magnetic minerals from
silica sand.
Following the separation of SiO2, silicon purification methods include the smelting and secondary refining of
metallurgical-grade silicon and acid leaching treatment. Other methods to refine silicon include the solvent refining,
vacuum treatment, plasma refining, and electron beam treatment. Developed for a variety of materials, chemical and
metallurgical processes, a fluidized bed reactor (FBR) is currently used for silane pyrolysis to heat Si particles.
FBR’s provide lower energy consumption at around 600-650° C at lower costs and high efficiencies for uniform
particle mixing, temperature gradients, and continuous operations. [70]
SiO2can also be purified via organic or polymer-based reduction as highlighted from the following four steps:
(1), placing a certain amount of silicon dioxide treated with organic matter or superpolymer and sulfur in a quartz
tube, feeding chlorine gas carried by nitrogen for 2 to 4 hours under 600 to 1100° C, absorbing tail gas with lime
water, and cooling to 100 to 200° C; (2), pouring the cooled silicon dioxide to 5-10% hydrochloric acid, stirring
fully, washing the mixture with deionized water until the mixture is neutral, and baking to obtain purified silicon
dioxide. The method provided by the invention is simple in process and mild in condition and is applicable to
industrialized production; silicon dioxide purified through the method can meet actual requirements on production
of special quartz glass products for semiconductors and the photovoltaic industry. [71] The purified silicon can later
be used as a precursor for silanol (SiH4O) and aerogel synthesis.
2.3.2 Carbothermal Reduction Reactors
In order to separate SiO2into constituent silicon, multipurpose carbothermal reduction reactors (CRR) can also
be used to reduce metal oxides (primarily SiO2) with carbon as the reducing agent at temperatures of several
hundred degrees Celsius. In 2009, NASA developed a carbothermal reduction process for usage on the Moon, where
methane is used as a source of carbon, and concentrated solar energy or a laser beam is used as a heat source [72].
This process also works for silicon. As highlighted in figure, a carbothermal reactor can be used to convert regolith
through which methane flows continuously over the molten regolith
zone. The extraction of gaseous oxygen from the oxides by reduction
using hydrogen (Taylor and Carrier, 1992), molten salt electrolysis
(Tripuraneni-Kilby et al., 2006) and carbothermal processing of
regolith (ORBITEC, 2006) are currently being pursued by NASA
(Sanders, Larson and Sacksteder, 2007). Carbothermic Reduction of
Silica in an Arc Furnace has been implemented at large scales by Dow
Corning, US-based conglomerate that manufactures hyperpure
polycrystalline silicon to produce materials. Improvements in purity
levels and large scale metallurgical reactions have been obtained in
continued developments by Elkem-Exxon, Solarex, and Elkem [73]. In
2020, researchers from South Korea published a study on the ultrafast carbothermal reduction of silica to silicon
using a CO2laser beam. Carbothermal reduction took place within a few seconds of the laser beam illuminating the
silica/carbon mixture Laser beam supplied heat energy to the mixture of silica and carbon black at an intensity above
~4.29 × 106W/m2. The intensity of the laser beam and N2gas flow during the process were critical to obtaining
silicon [74]. By utilising local abundant CO2, CRR’s provide a low-mass, low power method with several
advantages over existing systems to yield high purity silicon for SiO2aerogel synthesis.
2.3.3 Microwave Regolith Processing
Alternatively, microwave processing is also a prime candidate for regolith processing of rocks, metals, silica
sand, and ceramics [75,76,77,78,79,80,81,82]. Susceptor-assisted microwave heating [83] utilises a susceptor to
absorb electromagnetic energy and convert it to passively heat and activate silicon dopants [84]. The
susceptor-assisted microwave approach yields the advantage of becoming more independent of the regolith
composition and universally applicable. Variations in iron oxide content influences processing time or processing
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temperature. However, there are typically greater energy requirements and thermal losses from susceptor-assisted
heating than regular microwave heating. A microwave kiln was utilised in combination with a commercially
available 2.5 GHz microwave oven with a maximum operating power of 1 kW. Microwaves hit the susceptor
material which then starts radiating infrared heat. Since microwave processing of material occurs at high
temperatures (> 1100 °C), high-temperature materials like carbon or silicon carbide [85,86] can be used as
crucibles, both of which are also excellent susceptor materials for heating. [87]
2.4 Industrial Silica Life Cycle and Applications
Silica has many uses and is extensively utilised in the steel industry. Silica is the starting material for the
production of ceramics and silicate glasses and is often used with molds and cores to make metal castings. Silica is
also used for refractory bricks and ramming masses used in steel plants, foundries, and cement plants. Silica is also
a common building material used with concrete, grout, and plaster. Abrasive blasting or sandblasting uses
compressed air or water to direct a high velocity stream of an abrasive material to clean an object or surface, remove
burrs, apply a texture or prepare a surface for painting.
2.5 Biological Silica Life Cycle on Earth
The biological silica life cycle on Earth provides a novel analog biosphere to emulate on Mars and other
planetary bodies. As the primary silica reservoir is from silicate rocks in Earth’s crust, silicic acid (H4O4Si) is
delivered to the ocean through six pathways as illustrated in the diagram, which all derive from the weathering of
the Earth's crust.[88] Marine biological production of
biogenic opaline silica primarily comes from diatoms [89],
unicellular photosynthesising algae found in almost every
aquatic environment including fresh and marine waters,
soils, in fact almost anywhere moist. The major sink of the
terrestrial silica cycle is exported to the ocean by rivers.
Silica that is stored in plant matter or dissolved can be
exported to the ocean by rivers. The rate of this transport is
approximately 6 Tmol Si yr−1.[90] Moreover, plants
assimilate Si as soluble monosilicic acid resulting in
strengthening of the cell wall through various mechanisms
[91;92]. Higher accumulation of silicon improves plant
resistance to diseases, insect attack, and adverse climatic
conditions in various plant species like rice, oat, barley, wheat, cucumber, and sugarcane [93,94,95,96], which is
favorable for crops grown in silica rich basalts around 40-50% wt SiO2on the Moon and up to 60% wt on Martian
2.5 Bacteria and Algae to Biomine and Purify Silica and Silicon
On Earth, a variety of organisms utilise silica as a building material, including hexactinellid sponges,
radiolarians, and diatom phytoplankton [97,98]. Diatoms have a silica rich, hard cell wall (frustule), composed
almost purely of silica, made from silicic acid, coated with a layer of organic substance known as pectin. As diatoms
account for around 40% of all marine carbon sequestration, diatoms store more CO2from the atmosphere than all the
world’s rain forests put together and produce 20% of all oxygen in the atmosphere. Biogenic silica production in the
photic sunlight zone is estimated to be 240 ± 40 Tmol Si year −1.[88] Dissolution in the surface removes roughly 135
Tmol Si year−1, while the remaining Si is exported to the deep ocean within sinking particles.[90] Diatoms are
primarily between 20-200 microns in diameter, and occasionally 2 millimeters in length. The diatomic near pure
silica cell wall may discourage ingestion by grazing organisms, provide necessary support for the large vacuole,
buoyancy for cells to access nutrient and light enriched surfaces [99] facilitate light harvesting, increase nutrient
uptake, and protect the cell against UV radiation. A 2019 study on silicified cell walls in diatoms found that a 6X
increase in silica content leads to a 4X decrease in copepod grazing suggesting thickening of silica walls is an
effective defence strategy against being consumed by copepods. Hence, silica deposition in diatoms decreases with
increasing growth rates, suggesting a possible cost of defence. [100] The silicon accumulation gene enables
organisms to absorb and store silicon and is found in many algae species including cyanobacteria. As diatoms
uptake and sense silicic acid from seawater via silicon transporter (SIT) proteins, a 2016 study evaluated the SIT
gene family to identify potential genetic adaptations that enable diatoms to thrive in the modern ocean [101.102]
Diatoms are known to have high potential for bionanotech applications such as such as gel filtration (purification of
proteins), biosensors, immunoisolation bioencapsulation, microfabrication (fibriles, tubules, nano drug delivery,
IAC-21-D3-1.8 Page 11 of 18
lithographic masks [103]. Biomolecules such as proteins, enzymes or antibodies can be encapsulated within the
silica matrix to form hybrid biosensors and bioreactors. Diatom frustules can be utilised as 3D hierarchically
structured materials for photonic devices or microfluidics. [104]Diatoms such as Phaeodactylum tricornutum
species are also ideal candidates for microbial cell factors and biomanufacturing bulk commodity products (biomass,
biodiesel, protein and bioplastics) and specialty chemicals (eicosapentaenoic acid, docasahexaenoic acid,
fucoxanthin and recombinant proteins, e.g., recombinant antibodies) while enabling carbon sequestration [105,106].
Some suggest fertilising oceans with iron could promote diatom production or blooms that extract CO2out of the
air. In 2004, Smetacek et al. dissolved seven metric tons of iron sulfate in acidic seawater and spewed the solution
into the ship's propeller wash, which is the equivalent of adding 0.01 gram of iron per square meter. Haetoceros
atlanticus, Corethron pennatum, Thalassiothrix antarcticus and nine other species of diatoms grew in abundance
down to depth of 100 metres and carbon fell 34 times as fast as natural rates for nearly two weeks. The
geoengineering approach holds the potential to sequester one billion tons of CO2per year to the ocean floor for a
few centuries on Earth. As dissolved iron oxide would be more readily bioavailable in water ice melted aquatic
environments on Mars, a similar particle dispersion approach to distribute other minerals could aid the growth of
diatoms, algae, and aquatic organisms [107].
Radiolarians are also silica-secreting, single-celled organisms that dwell in open-ocean with a diameter of
0.1–0.2 mm, orders of magnitude larger than diatoms, and a skeleton composed of silica. On Earth, they occur
throughout the water column from near the surface to great depths. Some surface-dwelling radiolarians also have
algal symbionts. Silica-forming sponges also contain a silica glass skeleton such as the marine sponge, hexactinellid
sponge Euplectella sp, that filters bacteria and plankton from the surrounding water to provide important habitats for
marine life. The Venus' flower basket siliceous sponge utilises silicatein to extract silicic acid from surrounding
seawater, which is then converted into complex 3D silica structures at ambient temperatures underwater [108].
While sponges catalyze silica using a specific enzyme known as silicatein, diatoms do not use silicateins but rather
small specialised peptides called silaffins which attach long chain polyamines (LCPAs) to lysine groups, outlined in
a study on the role of proteins in biosilicification [109].The intracellular diatom silicification process occurs under
physiological conditions at temperatures between 0 and 37°C, neutral pH, and ambient pressure, and biosilicification
is around 106times faster than the corresponding abiotic process [110], something human engineering capabilities
are unable to replicate without the use of high-temperature. Thus, diatom algae could hold the potential to become a
leading biomanufacturing method to extract high quality silica on planetary bodies. Further research will
demonstrate the efficacy and adaptability of organisms to bioleach, to process, and to manufacture silica and silicon
in-situ on Mars and beyond Earth.
Silicate solubilizing bacteria (SSB) are in soil, water, aquatic sediments and in silicate minerals on Earth and
increase the bioavailability of silicates, P and K in the soil while protecting plants against pathogenic fungi.
Numerous bacterial strains of genus Bacillus, Pseudomonas, Proteus, Rhizobia, Burkholderia, and Enterobacter
release silicone (R2SiO)xfrom silicates and promote plant growth [111,112,113,114,115,116]. As organic acid
production (gluconic, succinic, fumaric, tartaric, and maleic acid) is the most common mechanism for P and Si
solubilization, the role of acidic phosphatase during Silicon solubilization has been firstly reported in a 2020 study
on Silicon-Solubilization for Characterization of Bacteria and Mitigation of Biotic Stress[117]. Dominance of
Pseudomonas and Bacillus spp. for the function of Si solubilization was observed during diversity analysis of Si
solubilizers isolated from different rhizospheres. Functional diversity studies show genetic relatedness of Bacillus
and Pseudomonas sp, a gram negative bacteria similar to extremophile cyanobacteria, a perchlorate reducer and is
also capable of nitrogen fixation (e.g. strain A1501). [117] Feldspar, NBRISN13 plant bacteria, and other
unexplored methods also have combinatorial effect of on the immune response through (i) increased Silicon uptake,
(ii) reduced disease severity, (iii) modulation of cell wall degrading and antioxidative enzyme activities, and (iv)
induced defense responsive gene expression. Moreover, 16S rRNA gene sequencing demonstrated that the Silicate
solubilizing bacteria (SSB) UPMSSB7 Enterobacter sp showed the highest solubilization of insoluble silicate,
phosphate (P) and potassium (K) at 5 and 10 days and inhibition of root diseases. [118] SSB suppressed disease and
may also be used to produce silicon-solubilizing microbe based biofertilizers and nanocoatings to dissolve SiO2 in
aqueous environments. Bioreducing bacteria, shewanella strains are efficient at bioleaching silica sand and can
reduce Fe(III) from silica sand. Shewanella strains (S. putrefaciens CIP8040, S. putrefaciens CN32, S. oneidensis
MR-1, S. algae BrY and S. loihica) were found to remove up to 17.6% of the iron bearing impurities (~117mg of
bioreducible Fe2O3 per 100g of silica sand) after 15 days, and Shewanella algae BrY was the most efficient [119].
Growing and resting cells of Rhodococcus erythropolis strains PD1, R1, and FMF, and R. qingshengii
heterotrophically removed of sulfur and bioleached iron and removed most silica impurities from coal. Results of
XRF X-ray fluorescence (XRF) indicate growing cells of strain PD1 bioleached 46% of the iron and 14% of the
silicate after 7 days of incubation.[120]
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Algae photobioreactors hold great potential to dissolve, bioleach, and extract silicon from SiO2. A 2008 study
investigated the effect of silicates with chalcopyrite (CuFeS2) and a complex multi-metal sulfide ore, on heap
bioleaching in column bioreactors. Microbial inhibition and liquid flow from acid consumption, release of trace
elements, and increasing the viscosity of the leach solution resulted in a negative impact of silicate mineral
dissolution on heap bioleaching. Silicate minerals are present in association with metal sulfides in ores and the SiO2
dissolution occurs when the sulfide minerals are bioleached in heaps for metal recovery. Algae photobioreactors
provides an affordable, low power and mass method to catalyze the decomposition of ore (without grinding) such as
newly mined/run-off-the-mine (ROM) materials (intermediate grade oxides and secondary sulfides). Both the
artificial and biological method become avenues to in situ manufacture, process, and utilise abundant silicon on
Moon and Mars, as a stepping stone to apply to other celestial bodies.
3. Alternatives and future perspectives
3.1 Methods to Improve Performance of SiO2Aerogels
Silica aerogel demands improved mechanical and structural properties for many applications. Alternative
nanomaterials can aid or replace silica-based aerogel. Polyethylene glycol (PEG), or H−(O−CH2−CH2)n−OH is
found to be very effective additive, mixed during stirring process before aging, for the improvement of the
mechanical properties of aerogel, which broke down at 21,924.6 N/m2with elongation of 0.8 cm. [121]. Notably, a
three-fold increase in compressive modulus with 5% carbon nanofiber was observed for aerogels with a silica
backbone of di-isocyanate cross-linked silica aerogels. A 5X increase of tensile stress at break is predicted by
including 5% fiber when total silane and di-isocyanate concentration are low without any change in the density or
porosity of the aerogels. Perhaps the great effect of including carbon fiber in the aerogels may be an improvement in
the strength of the initial hydrogels before cross-linking. [122] The high surface density of silanol groups provides
for easy synthesis of silica aerogel, which is particularly important in the preparation of hydrophobic hydrogels [33].
Hydrogels are crosslinked hydrophilic polymers that do not dissolve in H2O and can swell in water and hold a large
amount of water. They can be formed with the following reactions: Na2SiO3+ H2O + 2HCl → Si(OH)4+ 2NaCl.
A separate 2021 study found optimal parameters of the carbon fiber-silica aerogel composite for silica aerogel
reinforced with 10 vol.% of carbon fibers. [123] Thin film aerogels can also be reinforced with electrospun flexible
nanofibers to bridge cracks and hold structures together before the aging and drying process [124]. A variety of
nanomaterials and particles may also be fused into aerogels to help impede and potentially harvest incoming
radiation. The radiation, refraction index (bending of a high energetic particle) and ability to block ionising particles
could be improved by filling and compacting greater quantities of hydrogen atoms inside a porous aerogel network.
An aerogel skeleton or 3D printed lattice with geometric pores could be designed to leverage the spin and collisions
of hydrogen atoms. Mulltiscale channeled macro and mesopore networks inside aerogels could be reservoirs of
nanoparticles and hydrogen-rich materials to potentially improve the radiation shielding properties. Moreover,
hydrogen-rich water vapor could potentially be adsorbed from beneath SiO2 aerogel to form compact
hydrogen-dense structured layers to reflect radiation GCR’s. Moreover, smart aerogels have recently been
developed. Synthesis of shape memory aerogels by means of a shape memory polymer as reinforcement agent is a
promising area of investigation. For example, a polyurethane block copolymer as a shape memory polymer can be
used as a reinforcing agent for the silica aerogel network [125]. As this polymeric system is able to change its shape
in response to external stimuli, the aerogels can be stored in deformed state aboard a spacecraft. [126]
3.2 Cross-linked polyimide (PI) Aerogels
Aerogels can be cross linked with plastic-like polymers to form covalent bonds and to join polymer chains
together and reinforce the desired material and structural properties. All polyimide (PI) nano-aerogels (polymer with
repeating amide bonds) are as strong or stronger than polymer reinforced silica aerogels at the same density. By
varying the structure of diamine (PPDA, ODA,BAX) and dianhydrides (BTDA and HFDA), the material properties
such as flexibility, thermal oxidative stability, mechanical properties, and thermal conductivity) can be tailored to the
desired application.[124] Moreover, cross linked polyimide hydrogels are 500 times stronger than silica aerogel and
more flexible. NASA Glenn Research Center synthesized polyimide aerogel by cross-linking through an aromatic
triamine or polyhedral oligomeric silsesquioxane. Formulations made using 4,4′-oxydianiline or
2,2’dimethylbenzidine can be fabricated into continuous thin films using a roll to roll casting process. The 2012
study entitled Tailoring Properties of Cross-Linked Polyimide Aerogels for Better Moisture Resistance, Flexibility,
and Strength, found that replacing ODA with 50 mol % of DMBZ maintains the flexibility of thin films, while the
moisture resistance of the aerogels is greatly improved [18]. The films are flexible enough to be rolled or folded
back on themselves and recover completely without cracking or flaking, and have tensile strengths of 4-9 MPa. As
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highlighted in a 2010 patent entitled Highly porous and mechanically strong ceramic oxide aerogels, polymer
reinforcement doubles the density and results in two orders of magnitude increase in strength, and reduces surface
area by 30-50% [127].Another area of interest is to study the effectiveness of cage-shaped silanes available in the
form of polyhedral oligomeric silsesquioxane (POSS), which cross-linking during the synthesis of flexible and
foldable polyimide aerogels.[128] Polymer-reinforced silica aerogel can be streamlined to manufacture flexible thin
sheets to be wrapped around pipes, tanks or other assemblages needing insulation, or used as flexible insulation for
space suits or inflatable structures [20]. Cross-linked polyimide aerogels are a viable approach to higher temperature
resistant, flexible insulation for inflatable decelerators.
3.3 Sprayable Aerogel Nanocoatings
Sprayable aerogel nanoparticles can also be thermally sprayed to improve the deposition, placement, time
duration, and installation process. The insulation structure includes aerogel agglomerates formed by combining
ceramic particles with aerogel particles. The method for forming an insulation structure includes spray-drying and
post-drying a mixture of ceramic particles, aerogel particles, water, and a binder. [129] Researchers from China
developed a novel Frozen Spray-Coating method to prepare a graphene aerogel (GA) sponge mat with enhanced
mechanical, and electrochemical properties. GA mat has prominent potential in the fields of pollution absorption,
supercapacitors, battery electrodes, and electromagnetic shielding. [130] Danny Ou, et al elaborate in a 2013 study
on a sprayable aerogel insulation with silica aerogel beads yields a great mechanical integrity and provides lower
thermal conductivity than incumbent polyurethane spray-on foam insulation, at similar or lower areal densities, to
prevent insulation cracking and debonding in an effort to eliminate the generation of inflight debris. Silica aerogel
beads with a packing density of 0.03 to 0.05 g/cm3were added in a mixture with binders or foams cto form complex
shapes, or sprayed onto panels. The aerogel compositions can withstand repeated cycles of high enthalpy shear
flows of 20 to 100 Pa at temperatures tested up to 370 °C without losing mechanical integrity. The aerogel bead
bindersprayed panel, with a thermal conductivity of 20 to 25 mW/mK, outperformed the commercial foam by 30 to
40 percent in the 10 to 100 °C temperature range. Compression modulus for the aerogel bead/foam composite was
60 percent higher than the one from the foam without aerogel dopant. The sprayable insulation can be utilised in
various thermal management systems that require low mass and volume, such as cryogenic storage tanks, pipelines,
space platforms, and launch vehicles. [131]
3.4 Future Research to Advance Silica Life Cycle, Aerogels, Coatings, Nanoparticles
A 3-cm thick layer aerogels sheet, or cyanobacteria algae roof, holds the potential to transmit sufficient visible
light for photosynthesis, block hazardous ultraviolet radiation and induce greenhouse effect to create habitable
aqueous environments on the surface of Mars and planetary bodies without the need for any internal heat source.
Future research with Mars University will focus on the design, engineering, and experiments of SiO2shields for
algae biomass production, water ice mining, and greenhouses. Research may prioritize the deposition of
heat-responsive microcapsules loaded with polymerized organosiloxane as a novel technique to anchor SiO2aerogel
sheets to regolith and other methods to reduce losses from convection and sidewall and base conduction. Novel
methods and experiments in analog environments will guide the development of sprayable SA nanocoating, aerogel
material testing in Mars Environment chamber, detailed modeling of microfluidics, and microbial interactions in
aqueous habitable environment underneath SiO2aerogel. Moreover, carbon nanotubes are probably the next most
efficient GCR shielding element after hydrogen. Nano-carbons storage of large amounts of hydrogen seems
well-documented at 6% wt and claims of up to and exceeding 20% have been published [132]. Further research may
focus on novel methods to synthesize, etch, and increase the radiation refractive ability with hydrogen in SiO2
aerogels. Additionally, diatoms evolving in marine environments at greater depths and freezing waters provide
analog organisms and environments to observe effects of limited or no sunlight on diatom growth, which could
unlock clues to the biological mechanisms to mimic or adapt photosynthetic algae to the 44% solar irradiance on
Mars surface. In addition to evaluating diatom microbial cell factories and algal photobioreactors, research may
emphasize the efficacy, genes, bioengineering pathways, effects of ⅜ Gs, and modeling of diatom algae species as
Mars microbial candidates.
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The article presents the synthesis of silica aerogel from a much cheaper precursor of water glass that was reinforced with short pitch carbon fiber by way of ambient pressure drying. Before being added to the silica gel, the carbon fibers were surface modified to increase adhesion at the interfacial border. We were able to obtain stable structures of the composite with the amount of fibers above 10% by volume. The presence of fibers in the silica matrix resulted in lower synthesis time of the composite, improved adhesion of fibers to the aerogel nanostructure, and increased mechanical and structural parameters. An additional effect of the presence of fibers in excess of 10% by volume was a new function of the nanocomposite—the ability to conduct electric current. The most optimal parameters of the composite, however, were obtained for silica aerogel reinforced with 10 vol.% of carbon fibers. This material indicated relatively low density and good physical parameters. The paper also analyzes the results on the synthesis of fiber-reinforced silica aerogels that have appeared in recent years and compares these to the results gained in presented work.
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We report the extraction of silicon via a carbothermal reduction process using a CO2 laser beam as a heat source. The surface of a mixture of silica and carbon black powder became brown after laser beam irradiation for a few tens of seconds, and clear peaks of crystalline silicon were observed by Raman shift measurements, confirming the successful carbothermal reduction of silica. The influence of process parameters, including the laser beam intensity, radiation time, nitrogen gas flow in a reaction chamber, and the molar ratios of silica/carbon black of the mixture, on the carbothermal reduction process is explained in detail.
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Carbon dioxide (CO2) is a major greenhouse gas responsible for climate change. Diatoms, a natural sink of atmospheric CO2, can be cultivated industrially in autotrophic and mixotrophic modes for the purpose of CO2 sequestration. In addition, the metabolic diversity exhibited by this group of photosynthetic organisms provides avenues to redirect the captured carbon into products of value. These include lipids, omega-3 fatty acids, pigments, antioxidants, exopolysaccharides, sulphated polysaccharides, and other valuable metabolites that can be produced in environmentally sustainable bio-manufacturing processes. To realize the potential of diatoms, expansion of our knowledge of carbon supply, CO2 uptake and fixation by these organisms, in conjunction with ways to enhance metabolic routing of the fixed carbon to products of value is required. In this review, current knowledge is explored, with an evaluation of the potential of diatoms for carbon capture and bio-based manufacturing.
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Silicon (Si), the second most abundant element on earth, remains unavailable for plants' uptake due to its poor solubility. Microbial interventions to convert it in soluble forms are well documented. However, studies on discrimination of Si and P solubilizing microbes due to common estimation method and sharing of solubilization mechanism are still obscure. A defined differential media, i.e. silicon-solubilizing media (NBRISSM) is developed to screen Si solubilizers. NBRISN13 (Bacillus amyloliquefaciens), a Si solubilizer, exhibiting antagonistic property against Rhizoctonia solani, was further validated for disease resistance. The key finding of the work is that NBRISSM is a novel differential media for screening Si solubilizers, distinct from P solubilizers. Dominance of Pseudomonas and Bacillus spp. for the function of Si solubilization was observed during diversity analysis of Si solubilizers isolated from different rhizospheres. Sphingobacterium sp., a different strain has been identified for silicon solubilization other than Pseudomonas and Bacillus sp. Role of acidic phosphatase during Si solubilization has been firstly reported in our study in addition to other pH dependent phenomenon. Study also showed the combinatorial effect of feldspar and NBRISN13 on elicited immune response through (i) increased Si uptake, (ii) reduced disease severity, (iii) modulation of cell wall degrading and antioxidative enzyme activities, and (iv) induced defense responsive gene expression.
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The low temperatures1,2 and high ultraviolet radiation levels³ at the surface of Mars today currently preclude the survival of life anywhere except perhaps in limited subsurface niches⁴. Several ideas for making the Martian surface more habitable have been put forward5–8, but they all involve massive environmental modification that will be well beyond human capability for the foreseeable future⁹. Here, we present a new approach to this problem. We show that widespread regions of the surface of Mars could be made habitable to photosynthetic life in the future via a solid-state analogue to Earth’s atmospheric greenhouse effect. Specifically, we demonstrate via experiments and modelling that under Martian environmental conditions, a 2–3 cm-thick layer of silica aerogel will simultaneously transmit sufficient visible light for photosynthesis, block hazardous ultraviolet radiation and raise temperatures underneath it permanently to above the melting point of water, without the need for any internal heat source. Placing silica aerogel shields over sufficiently ice-rich regions of the Martian surface could therefore allow photosynthetic life to survive there with minimal subsequent intervention. This regional approach to making Mars habitable is much more achievable than global atmospheric modification. In addition, it can be developed systematically, starting from minimal resources, and can be further tested in extreme environments on Earth today.
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Due to the unique properties such as 3-dimensional nanoporous structure and high surface area, silica aerogel is a promising candidate for replacing the conventional micron-sized silica to improve the mechanical properties of epoxy-based nanocomposites. In the present study, the water-glass based silica aerogel was first synthesized by the low-cost sodium silicate and cheap ambient pressure drying method and then used as the filler in the epoxy system. Finally, rheological and mechanical properties of the silica aerogel-epoxy nanocomposite were investigated. The introduction of silica aerogel powders impacted the rheological properties of epoxy dispersion and improved the mechanical performance of the corresponding nanocomposite. The dispersion microstructure has been characterized by its rheological properties and has been used to determine the critical silica aerogel weight fraction of the network formation (φ*) for the silica aerogel-epoxy dispersion. At the critical filler concentrations (φ*), the overall mobility of the polymer chains is restricted in both dispersion and solid nanocomposites. Therefore, the network structure and the interface surrounding nanoparticles increases and this results in an improvement of the mechanical properties. Mechanical tests showed improvements in flexural modulus and strength by ~ 80% and ~ 40% respectively as compared with those of pure epoxy. Based on this study, water-glass based silica aerogel hold great promise as a low-cost filler in polymer composite.
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Diatoms contribute nearly half of the marine primary production. These microalgae differ from other phytoplankton groups in having a silicified cell wall, which is the strongest known biological material relative to its density. While it has been suggested that a siliceous wall may have evolved as a mechanical protection against grazing, empirical evidence of its defensive role is limited. Here, we experimentally demonstrate that grazing by adult copepods and nauplii on diatoms is approximately inversely proportional to their silica content, both within and among diatom species. While a sixfold increase in silica content leads to a fourfold decrease in copepod grazing, silicification provides no protection against protozoan grazers that directly engulf their prey. We also found that the wall provides limited protection to cells ingested by copepods, since less than 1% of consumed cells were alive in the faecal pellets. Moreover, silica deposition in diatoms decreases with increasing growth rates, suggesting a possible cost of defence. Overall, our results demonstrate that thickening of silica walls is an effective defence strategy against copepods. This suggests that the plasticity of silicification in diatoms may have evolved as a response to copepod grazing pressure, whose specialized tools to break silicified walls have coevolved with diatoms.
Many applications of silica aerogel require desired mechanical and structural properties. Glycerol and polyethylene glycol were used for the synthesis of silica aerogel to improve mechanical and structural properties. Methyltrimethoxysilane was used as a precursor in various concentrations to explore the effects on the properties of silica aerogel. Polyethylene glycol-based aerogel exhibited high tensile stress-withstanding capacity, at the same time high surface hydrophobicity with contact angle 148°. The swelling and de-swelling behavior of aerogel were also analyzed. This lightweight material has low bulk density, which is found to be affected by the presence of additives. The density of aerogel varied from 0.07 to 0.09 g/cm3 with increasing amount of additive. Polyethylene glycol is found to be very effective additive especially for the improvement of the mechanical properties of aerogel, as it has exhibited the breakdown at 21,924.6 N/m2 with elongation of 0.8 cm. The aerogel samples were analyzed for their surface hydrophobicity and thermogravimetry analysis. The elemental analysis of aerogel confirmed the purity of aerogel while thermogravimetric analysis and differential thermogravimetric analysis revealed its high thermal stability up to 519 °C. Graphical abstract
A switch from a petroleum-based to a biobased economy requires the capacity to produce both high-value low-volume and low-value high-volume products. Recent evidence supports the development of microalgae-based microbial cell factories with the objective of establishing environmentally sustainable manufacturing solutions. Diatoms display rich diversity and potential in this regard. We focus on Phaeodactylum tricornutum, a pennate diatom that is commonly found in marine ecosystems, and discuss recent trends in developing the diatom chassis for the production of a suite of natural and genetically engineered products. Both upstream and downstream developments are reviewed for the commercial development of P. tricornutum as a cell factory for a spectrum of marketable products.
Heat at intermediate temperatures (120-220 °C) is in significant demand in both industrial and domestic sectors for applications such as water and space heating, steam generation, sterilization, and other industrial processes. Harnessing heat from solar energy at these temperatures, however, requires costly optical and mechanical components to concentrate the dilute solar flux and suppress heat losses. Thus, achieving high temperatures under unconcentrated sunlight remains a technological challenge as well as an opportunity for utilizing solar thermal energy. In this work, we demonstrate a solar receiver capable of reaching over 265 °C under ambient conditions without optical concentration. The high temperatures are achieved by leveraging an artificial greenhouse effect within an optimized monolithic silica aerogel to reduce heat losses while maintaining high solar transparency. This study demonstrates a viable path to promote cost-effective solar thermal energy at intermediate temperatures.