Earth and space sciences

For information on combustion, refer to Behind the science>Chemical sciences>Chemical reactions.

Bushfire behaviour

The intensity and rate of spread of a bushfire are dependent on three factors: the fuel available, the topography of the area, and the weather conditions. These three factors together are called the fire behaviour triangle. 

Figure 1: Fire behaviour triangle

Fuel availability 

Australia is a large country, and this means there are many different types of environments with very different types and amounts of combustion fuel. In general, the presence of large amounts of dry light fuel such as dried grasses, bark strips, and fallen leaves, allows oxygen to mix and cause fast combustion. Fire in bushland usually travels along the floor of the bush and up the bark ladder of the next tree. The overall fuel hazard is measured by considering the type of bark on a tree, the amount of fuel just above the ground (grasses, ferns, and shrubs), and the fine fuel on the ground surface. 

The exception to this is when the tops of the trees are very close together, then the fire can travel along the tree canopy. These fires are especially deadly on windy days when the wind and heat of the fire encourage thermal degradation and pyrolysis of the oils in many Australian trees. 
 

Topography 

A fire burns fast uphill because the flames can more easily reach the fuel in front of a fire. The radiant heat pre-heats the fuel, starting the degradation and pyrolysis process. 

Figure 2: Fire severity and intensity, by GRID-Arendal/Studio Atlantis. Source: https://www.grida.no/resources/15553

For every 10-degree slope, the speed the fire travels will double. For example, if the fire is travelling 10 km/hr on a flat surface, it will travel at 20 km/hr on a 10-degree slope. The fire will also increase its intensity as it travels faster. 

The opposite is also true for a downward slope. For every 10-degree slope downwards, the speed a fire travels will halve. 

Weather 

A humid day means that there is more moisture in the air. This slows the process of degradation and pyrolysis, therefore slowing the spread of the fire. 

In windy conditions, there is a greater supply of oxygen available for combustion. This means a fire can spread rapidly across a grassland because there is little to interrupt the flow and direction of the wind. The wind can also carry embers (burning bark, leaves, and seed pods) long distances. Sometimes, the wind high in the atmosphere (the outer edges of the troposphere, 1-10 km above the surface) can carry the embers in a different direction than that of the fire front. This means that houses away from the leading edge of the fire can still be in danger of being burnt. 

While many Australian trees can provide an explosive reaction in a bushfire, the wind speed is often greater in an open area where there is grassland and fewer trees. Flying embers light the fine fuel on the ground. These small fires then climb the ladder of materials up a tree truck to the canopy. Overlapping canopies have high surface areas and often strong winds, allowing the canopy fire to spread quickly to other trees. If the canopy is not overlapping, the embers will fall to the ground and spread to the next tree ladder. 

Extreme bushfires and fire-generated thunderstorms 

Australia has been experiencing more frequent and more severe bushfires in recent decades, including extremely dangerous cases where thunderstorms have been generated in the fire plumes. These fire-generated thunderstorms are sometimes referred to by scientists as pyrocumulonimbus, as a subtype of thunderstorm cloud, given that thunderstorm cloud types are classified as pyrocumulonimbus in meteorology. They were never documented and studied until relatively recently, with most of the known cases having occurred in recent decades. Fire-generated thunderstorms can cause severe hazards including extreme winds (e.g. strong enough to flip over a fire truck or rip roofs from buildings), lightning that can ignite new fires, hail, and tornadoes. The erratic and strong wind gusts that come from the thunderstorms can also influence the rate of spread of a fire, including based on changes in wind direction and speed as well as from transporting burning embers that land far ahead of the fire front and can ignite new fires (known as ‘spot fires’).  

Fire-generated thunderstorms form due to a combination of several factors. They require a sufficiently large and intense fire to release enormous amounts of heat causing air to rise rapidly in the smoke plume. As the plume rises, the atmospheric pressure reduces and causes the plume air to expand and cool. If it cools enough, the moisture in the plume air can condense and form clouds. The condensation process causes energy to be released that warms the cloud. If the surrounding air is relatively cold, the cloud is buoyant and can rise. If this process is strong enough a thunderstorm can be generated in the fire plume which can sometimes result in hazards such as lightning, hail, and tornadoes. Additionally, the evaporation of rain falling from the cloud can cool the air making it relatively heavy. This can lead to intense downbursts of strong and erratic winds that can create dangerous on-ground conditions. These strong and erratic winds can lead to more unpredictable fire behaviour that can make it very hard, if not impossible, for firefighters to control the fire, as well as being very dangerous for firefighter safety. 

Figure 3: Fire-generated thunderstorm formation and associated hazards. Source: When bushfires make their own weather - Social Media Blog - Bureau of Meteorology

Examples of fire-generated thunderstorms in Australia include the disastrous Canberra fires in January 2003 with several hundred homes destroyed and several lives lost, with the fire-generated thunderstorm also causing a tornado as well as black hail (due to the ash in the thunderstorm as the hail formed). The fires near Melbourne on 7 February 2009 known as Black Saturday killed 173 people and had several fire-generated thunderstorms, including generating lightning that ignited new fires far ahead of the fire front (about 100 km away). The fires in the 2019/2020 summer known as the Black Summer had a large number of fire-generated thunderstorms, producing almost as many as had been observed in the previous 35 years combined. Examples such as these have raised the question about if climate change is influencing the occurrence of extreme fires including fire-generated thunderstorms. 

Several factors contribute to the occurrence of extreme bushfires, some of which are being exacerbated by human-caused climate change. For example, observations show a clear trend towards more dangerous weather conditions for bushfires in recent decades as compared to the past. Figure 4 shows this trend using the Forest Fire Danger Index (FFDI) which is calculated based on temperature, humidity, wind, and rain. This trend towards more dangerous weather conditions for bushfires is due to increasing greenhouse gas concentrations in the atmosphere primarily due to burning fossil fuels, resulting in increased temperatures in combination with drier air (i.e. reduced relative humidity) and more variable rainfall. 

These trends observed over recent decades are very likely to increase in the future. Climate modelling for the coming decades this century shows more dangerous weather conditions for bushfires throughout Australia due to increasing greenhouse gas emissions. In addition to weather conditions such as those represented by the Forest Fire Danger Index (FFDI), there are several other factors that contribute to the occurrence of extreme bushfires including those with thunderstorms generated in their fire plumes. These factors include having suitable fuel conditions, such as dense vegetation available to burn, as well as an ignition source such as lightning or human influences, noting considerable uncertainties around how factors such as these potentially could change in the future. 

Figure 4: More dangerous weather conditions for bushfires in recent decades as compared to the past. Red regions are where there has been an increase in the number of days with dangerous weather conditions for bushfires. This is based on data from July 1950 to June 2020, using the Forest Fire Danger Index (FFDI). The FFDI is calculated based on temperature, humidity, wind and rain. The values in this figure show how many more days are in the top 10% of dangerous days for the second half of this study period as compared to the first half.

Source: Dowdy A.J, Ye H, Pepler A, Thatcher M, Osbrough S.L, Evans J.P, Di Virgilio G and McCarthy N. 2019. Future changes in extreme weather and pyroconvection risk factors for Australian wildfires. Scientific Reports, 9. https://www.nature.com/articles/s41598-019-46362-x

Cultural burning 

Cultural burning is different from prescribed/hazard reduction/planned/controlled burns. The latter usually involves setting fire along a line at the edge of a grassland or bushland area. The purpose of this hot fire is to reduce the amount of fuel that can burn in the area, and it usually burns all the small or loose fuel or removes pest plants in the area. This impact is temporary as the plants in the area rapidly regrow. When an area is burnt often, it can select plants with rapid growth cycles, which has the potential to rapidly increase the fuel load for future fires (Zylstra et. al. 2022). 

A cultural burn is part of the cultural practices of First Nations Peoples. It is important to notice the plurality of this. There are over 700 different cultural groups across Australia, all of whom use fire in different ways and for different purposes.  

Cultural burning can be used to:

• create areas for good hunting.
• clean up campsites.
• control pests, plants and animals.
• provide grassy pathways and walking tracks.
• encourage the growth of particular plants.

First Nations Australians consider fire a gift to Country and (in general) use small spot fires in a mosaic pattern that allows the animals and insects to move out of the way into the canopy. These fires are usually cooler so that it does not damage the soil or larger trees. 

Local knowledge of when and how to burn is sometimes protected knowledge within a community. It is important not to make assumptions, and instead to respectfully work with your local First Nations People. 

References 

Deadly Science, <https://deadlyscience.org.au>.

Dowdy A.J, Ye H, Pepler A, Thatcher M, Osbrough S.L, Evans J.P, Di Virgilio G and McCarthy N. 2019. Future changes in extreme weather and pyroconvection risk factors for Australian wildfires. Scientific Reports, 9. <https://www.nature.com/articles/s41598-019-46362-x>.

Lindenmayer, D., & Bowd, E. (2022). Cultural burning, cultural misappropriation, over‐simplification of land management complexity, and ecological illiteracy. Ecological Management & Restoration, 23(3).  

Zylstra, P. J., Bradshaw, S. D., & Lindenmayer, D. B. (2022). Self-thinning forest understoreys reduce wildfire risk, even in a warming climate. Environmental Research Letters, 17(4), 044022. 

The Earth comprises a series of interdependent systems that constantly interact. Although different systems may be examined at different times, the core concept relates to the interaction between these systems, and the influence of human activity on key processes, cycles, and relationships.

The rock cycle 

Essentially the rock cycle reinforces the concept that matter is neither created nor destroyed but is transformed into different configurations.

If any rock materials are weathered and eroded they will form sediment, which can be cemented and compacted to become a (clastic) sedimentary rock. Other sedimentary rocks form either by precipitation from a supersaturated solution (chemical), or due to the accumulation of plant or animal matter (biogenic). 

If any rock materials are subjected to sufficient heat, the composite minerals melt, and magma (hot molten rock) forms. When this magma cools it forms igneous rocks. If magma cools slowly below the surface there is time for larger crystals to form and the rocks are called plutonic rocks (e.g. granite). Magma may erupt at the surface to violently eject block and ash material (volcanic sediments) or it may erupt to produce lava flows that cool quickly to form fine-grained volcanic rocks (e.g. basalt). 

Metamorphic rocks have been changed over time by extreme pressure and heat (over 150°C). These rocks can be formed from other rocks by the high pressures deep under the Earth's surface and from the extreme heat caused by magma. Uplift and erosion help bring metamorphic rock to the Earth's surface. 

Examples of metamorphic rocks include anthracite (from coal), quartzite (from sandstone), marble (from limestone), and slate (from shale or mudstone). 

All rocks on Earth can be broadly categorised into three groups; sedimentary, igneous, and metamorphic. These groups of rocks have different characteristics that relate to how they are formed. 

Minerals are the pure substances, or chemicals, that rocks are made from. Most rocks contain a mixture of several minerals, as individual crystals and grains. All rocks on Earth are made from a limited number of different minerals. Minerals can be identified using several observable diagnostic properties. Diagnostic properties are used to distinguish between and identify different minerals. 

Soils 

Soil is the area of the earth where plants can grow. It is a mixture of organic matter, sand, minerals, and clay. Regolith (from rhegos—soil, lithos—rock) is all matter that covers the unfragmented rock on the surface of objects in the solar system. It forms as a result of natural weathering and erosion. 

Lunar regolith 

The fine dust or grains that make up the lunar regolith were formed from the regular meteorite impacts. Scientists estimate that the regolith is 4-15 m deep. It does not contain organic matter. The fine lunar grains are abrasive due to the jagged edges. The lack of water and minimal atmosphere (containing helium, argon, neon, ammonia, methane, and carbon dioxide) causes the lack of weathering that could smooth surfaces. The lunar dust sticks to the astronauts' suits, gets inside the lunar module and irritates the astronauts’ eyes and lungs. 

The limited atmosphere means that meteorites of all sizes hit the regolith. The mineral fragments are often held together by glass-like silicates (60-70%) and the concentrations of the elements will vary depending on the location of sampling. 

Martian regolith 

The regolith on Mars is finer than that found on Earth’s moon. This is most likely due to the once-flowing water that covered its surface. The dust can remain suspended in the low-density atmosphere, contributing to the red colour often seen in photographs. It is thought that water and carbon dioxide ice are frozen within the regolith. 

Martian soil contains high concentrations of toxic perchlorate (containing perchlorate ion, $\require{mhchem}\ce{ClO4-}$) compounds. A perchlorate level of 0.5 g/L similar to that on the surface of Mars) was found to accumulate in the leaves and reduce the amount of chlorophyll in plants. Perchlorates at this level are toxic to humans. While some bacteria (Dehalococcides mccartyi) can break down perchlorates, UV light can break down the products into other toxic compounds. The indoor growth of plants may be required.  

The carbon cycle 

Carbon is the fourth most common element in the universe. Most of the carbon on Earth is stored in the rocks, soil, and fossil fuels of the geosphere, while the rest cycles through the hydrosphere (ocean), and biosphere (living animals and plants). The carbon cycle can be described as both a slow cycle and a fast cycle. 

The slow carbon cycle can take hundreds of thousands of years and describes the movement of carbon between the geosphere, hydrosphere, and hydrosphere. 50 million years ago, the uplift of the Himalayan mountains exposed a fresh source of carbon in the rocks. Water in the atmosphere can combine with carbon dioxide forming weak carbonic acid. The acid chemically weathers the rock, releasing ions such as calcium, magnesium, and sodium. The calcium ions form calcium carbonate that sinks to the bottom of the ocean. This, together with the shells of dead ocean organisms and corals are cemented together into sedimentary rocks such as limestone. Organic carbon can become part of this cycle through dead matter being trapped in layers of mud and forming fossil fuels. Volcanoes can return carbon to the atmosphere through gas vents and eruptions. Many components of the slow carbon cycle are called carbon sinks because they can store carbon for a long time. 

The fast carbon cycle describes the movement of carbon describes how the carbon cycle moves within the time of a single human lifespan. Carbon is one of the main components of the complex organic molecules found in the biosphere. When these complex molecules are broken down into simpler molecules, energy is usually released. This makes the complex carbon molecules a good way to store energy.  

This process of using carbon dioxide and water to produce carbon-based sugar (glucose) occurs through photosynthesis in plants and phytoplankton (microscopic organisms found in the ocean). The carbon (and energy) is transferred and transformed when animals eat the plants or phytoplankton and use the chemical reaction of aerobic respiration to break down the carbon-based sugar to produce carbon dioxide and water. If plants are allowed to grow, they can become a carbon sink and store carbon for longer than our lifetime. 

Combustion of fossil fuels causes the fast release of carbon which is usually part of the slow carbon cycle. This has had an impact on atmospheric carbon dioxide levels. 

Ice cores 

Glaciers are slow-moving masses or rivers of ice. They are formed by layers of snow being buried by more snow each year. Over time, each layer with unique compositions and textures becomes compacted. The layers of ice form a record of the climate conditions including snowfall, temperature, atmospheric gases, and volcanic activity.

Scientists use hollow rotating drills to remove a cylinder of ice vertically from a glacier to examine the ‘time capsule’ of information on global changes.

Other analyses include:

• the number of layers as an indication of age.
• solid and dissolved impurities in ice: environmental changes, measure dust, salts, and pollutants (humans and volcanoes).
• air bubbles: Measure $\require{mhchem}\ce{C14}$ in fossil methane and carbon monoxide (caused by burning fossil fuels).
• proportion of hydrogen and oxygen isotopes in melted and refrozen water is different from that of continuously frozen water. Measuring this is an indication and record of changing temperatures over time.
• analyse beryllium-10, chlorine-36, and carbon-14 as indications of solar radiation over time.

Global cycles of carbon dioxide and temperature 

Global variations in temperature in the past 800,000 years have largely been a result of periodic changes in the orbit of the Earth around the Sun. These changes fall into three types of orbital cycles (Precession, Obliquity, and Eccentricity) called the Milankovitch cycles. These cycles change the Earth’s axis of rotation, the tilt of the Earth’s axis, and the shape of the Earth’s orbit respectively. All three impact the average temperature of the Earth, resulting in ice ages that trap much of the water on the Earth’s surface as ice.

In past ice ages, the water level dropped by up to 120 metres from current levels. Exposing more land allowed plant life to expand and take up more carbon dioxide. The colder water also absorbed higher levels of carbon dioxide. This is the reason why graphs show a decreased level of carbon dioxide when global temperatures decrease.

The increase in glacial ice formed a positive feedback system where the white ice reflects a greater amount of sunlight into the atmosphere, allowing the Earth’s temperature to decrease further. The glacial ice also grounds surrounding rocks, releasing nutrients into the water and supporting food webs.

In the past, the average temperature of the Earth increased as its orbit changed once again.

Current global warming is different from this slow process. This time it is being driven by the increase in carbon dioxide as a result of industrial combustion and is occurring at a faster rate (50-100 years instead of 1000s of years).

Nitrogen cycle 

Nitrogen is an essential element for all living organisms, including amino acids, proteins, and nucleic acids (DNA and RNA). However, atmospheric nitrogen ($\require{mhchem}\ce{N2}$) is not usable by most organisms. Certain bacteria (such as Rhizobium in root nodules of legumes or nitrogen-fixing bacteria in the soil) convert atmospheric nitrogen ($\ce{N2}$) into forms that plants can use including ammonia ($\ce{NH3}$) or nitrates ($\ce{NO3-}$). The plants then absorb the nitrogen and incorporate it into proteins and other molecules. Herbivores and carnivores consume the plants or other animals, obtaining the nitrogen they need. Decomposers break down dead plants and animals, releasing nitrogen back into the soil in forms that can be used by plants. Some bacteria convert nitrates($\ce{NO2-}$) back into nitrogen gas ($\ce{N2}$), returning it to the atmosphere. 

Phosphorus cycle 

Phosphorus is vital for energy transfer (as part of ATP) and is a key component of DNA, RNA, and cell membranes. Phosphorus is released from rocks through weathering, primarily in the form of phosphate ions ($\ce{PO4^{3-}}$). Plants absorb phosphate from the soil and incorporate it into organic molecules such as ATP, DNA, and RNA. When animals consume the plants (or other animals), they obtain phosphorus and use it for growth and energy storage. Decomposers break down dead organisms, releasing phosphorus back into the soil or water. Over time, phosphorus can be deposited in sediments, and in some cases, it can become part of new rocks. 

The Earth is part of a much larger celestial system that can affect our everyday life.

Gravitational force

Gravitational force is linked to mass. Celestial objects such as stars, planets, and moons have different masses. The space between the celestial masses can flex and ripple. The greater the mass of a star or planet, the more space curves around it. This curving of space generates a gravitational force. Therefore, the larger the mass, the greater the gravitational force that is generated. This leads to the following learning ideas:

• Gravitational force is the attraction between two masses.
• As the distance between objects increases, the gravitational force decreases.
• Weight = the size of the gravitational force acting on an object.
• The size of the gravitational force is not affected by the surroundings (water, air etc.).
• The orbit of a planet around a star is affected by the combined gravitational forces of all surrounding celestial objects.

Moon and tides

The Moon is the closest celestial object to Earth. This exerts a gravitational force on Earth's ocean water that is closest to the Moon, causing it to bulge towards the Moon. The water levels also bulge on the opposite side (furthest away from the Moon) due to inertia. This creates two bulges on the opposite sides of the Earth that align with the Moon.

As the Earth rotates, it moves through each bulge, generating a high tide when in the middle of the bulge, and a low tide when away from the bulge.

The gravitational force of the Sun can also affect the size and position of the tidal bulges.

Microgravity

The space station is not in outer space. It is only 400km from Earth’s surface. At this distance, Earth exerts a strong pull and an astronaut weighs 89% as much as their weight on Earth’s surface.

The reason astronauts experience microgravity in the space station is because the space station is in continual freefall towards Earth as it travels at a tangential velocity of 27,600 km/h. At this speed and distance from Earth, its path adopts an orbit around the Earth. This is a concept best left to explore in Year 11 physics.

Dark matter

It was a surprise to astronomers to discover that the motion of our own galaxy could not be accounted for by all the detectable matter it contained. These include stars and regions of dust and gas. There appeared to be something ‘invisible’ exerting a gravitational force and affecting the rate at which different regions appeared to be orbiting the centre of the galaxy.

Amazingly this missing mass appears to account for around 96% of the mass of our galaxy. Theoretical ideas about ‘dark matter’ and ‘dark energy’ have been proposed to account for this, but the nature of these materials is not yet clear. The 4% that is most noticeable refers to all the stars in the galaxy.

The history of the universe

Approximately 13.7 billion years ago, the universe underwent cosmic inflation and expanded faster than the speed of light for $10^{-32}$ of a second. At the time, the universe was a mix of light and sub-atomic particles and was extremely hot (10 billion degrees Celsius). Over the next 3 minutes, atomic nuclei of hydrogen, helium, and traces of lithium formed when the protons and neutrons collided. The electrons remained free as the universe was still too hot to form atoms.

Over the next 380,000 years, the universe cooled enough that electrons were captured by the atomic nuclei releasing energy in the form of light. The glow from these first atoms is detectable today as cosmic background radiation.

As sections of the universe cooled (200 million years after the cosmic inflation), denser clouds of gas were formed. As the mass grew, it exerted a gravitational force on the surrounding matter. As the centre of the clouds became more compact, heat was generated. Eventually nuclear fusion began to occur and the first stars were formed.

Further information will be provided in the future.