Mars down under

Last updated on September 16, 2023

Mars Down Under: Exploring the Phenomenon of Outback Dust Storms

Australia's vast and diverse landscapes are known for their captivating beauty and unique atmospheric phenomena. One such phenomenon is the occurrence of dust storms, which can transform the sky into a mesmerizing display of colors. Among these remarkable events, the "Mars Down Under" dust storm that occurred on September 23, 2009, stands out as an unforgettable spectacle that captured the attention of both locals and meteorologists alike.

The Mars Down Under dust storm was caused by fierce winds originating from inland New South Wales. These powerful gusts lifted massive amounts of dust from the ground, forming an enormous cloud that towered approximately 2 kilometers high. The cloud traveled all the way to Sydney, covering the city in a thick layer of dust. The visual impact was awe-inspiring, with the sky initially taking on an opaque blood-red hue before gradually fading to a dull gray as the sun ascended.

The red coloration of the sky during the Mars Down Under dust storm can be attributed to various factors. One significant contributor is Rayleigh scattering, a phenomenon responsible for the red hues observed during sunsets. During Rayleigh scattering, air molecules, fine dust particles, and other small particles in the atmosphere scatter blue light more strongly than red light. As a result, the unscattered light from the sun appears red, while the scattered light gives the sky its characteristic blue color. However, in the case of the Sydney dust storm, Rayleigh scattering alone cannot fully explain the vivid red sky due to the size and optical thickness of the dust cloud.

The primary cause of the striking red coloration during the Mars Down Under dust storm was selective absorption by the dust particles themselves. The cloud consisted of red-colored particles typical of the outback soil, which were larger than the wavelength of visible light. Consequently, these particles absorbed and selectively transmitted light, contributing to the overall red sky appearance. This selective absorption phenomenon played a crucial role in creating the remarkable visual spectacle witnessed by those in Sydney.

In addition to selective absorption, another factor contributing to the sky's coloration during the dust storm was the presence of smaller dust particles suspended in the air. As the event progressed, the cloud thinned, allowing glimpses of the sun with a bluish hue. These smaller micron-sized particles acted as Mie scatterers, causing minimal dependence on wavelength. Consequently, the sky appeared gray or diluted due to the scattering of light by these smaller particles. It is worth noting that when these particles are of similar size, they can occasionally produce the elusive "blue moon" or, in this case, a sun with a bluish tinge, as the scattered light becomes reddened.

The Mars Down Under dust storm serves as a fascinating example of how atmospheric optics can create breathtaking displays of color and light. The interplay between Rayleigh scattering, selective absorption, and Mie scattering all contributed to the unique visual experience witnessed by residents of Sydney on that memorable day. Understanding these phenomena not only deepens our appreciation for the beauty of nature but also provides valuable insights into the complex workings of our atmosphere.

The occurrence of dust storms in Australia's outback is not uncommon, as the arid and windswept regions create ideal conditions for their formation. These dust storms not only have a visual impact but can also affect air quality and pose health risks to individuals with respiratory conditions. Therefore, studying and monitoring these events is crucial for understanding their environmental and societal implications.

As technology advances, scientists and meteorologists have developed sophisticated instruments and models to predict and track dust storms more accurately. By analyzing data on wind patterns, soil conditions, and atmospheric dynamics, researchers can better anticipate the occurrence and behavior of these events. This knowledge enables authorities to issue timely warnings and take necessary precautions to minimize the potential impact on public health and infrastructure.

In conclusion, the Mars Down Under dust storm of 2009 stands as a testament to the captivating and dynamic nature of atmospheric optics. Through a combination of Rayleigh scattering, selective absorption, and Mie scattering, the sky over Sydney transformed into a mesmerizing display of colors, captivating onlookers and igniting curiosity among scientists. By delving deeper into these phenomena, we gain a greater understanding of the intricate workings of our atmosphere and the profound influence it has on our daily lives.

Outback Dust Storm imaged by Helen S of Sydney, Australia at 6:15 am on September 23, '09.

�Helen S, shown with permission.

Fierce winds from inland New South Wales lofted dust from the ground and carried it as an enormous cloud some 2 km high to Sydney (more images & reports). The sky colour was fading when this picture was taken but earlier it was an opaque blood red. As the sun climbed the colour slowly faded. By mid-day the atmosphere was still opaque but the sky was a dull gray.

How was the red colour produced? Red sky colours as at sunset are mostly produced by Rayleigh scattering. Air molecules, very fine dust and other particles all much smaller than the wavelength of light (lambda/15) scatter blue light more strongly than red and so the sun seen in the remaining unscattered light appears red. The same scattered light makes the sky blue. Rayleigh scattering is unlikely to have produced the Sydney sky because most of the the dust cloud particles were too large and also the cloud was so optically thick - notice how the background trees are partially obscured. Light is multiply scattered by thick clouds and any transmitted light is less dependent on the wavelength discrimination of a single scattering event.

More likely, part (not necessarily all) of the sky colouration came from the colour of the particles themselves. The cloud consisted of red coloured particles typical of outback soil and larger than the wavelength of light. The overall red sky colouration is then partly a consequence of simple selective absorption by the dust itself.

There is another effect. Clues to it are that the sky changed to gray later on and that other photos taken when the cloud was thin enough to reveal the sun show the latter with a bluish hue. This suggests that some of the dust in suspension (particularly towards the end of the event) was of smaller particle size. Micron sized particles act as Mie scatterers with little wavelength dependence - hence the gray or dilute sky colour. The same particles, if all of similar size, sometimes selectively produce the fabled rare 'blue moon' or in this case a bluish sun - the scattered light is reddened. There was a lot happening the morning of the 23rd!

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