Research on micrometeorites, published in the journal Icarus, sheds light on the composition and origin of cosmic dust.

When Icarus flew too close to the Sun, his wings melted and he fell to his death. This ancient Greek myth was a cautionary tale warning against the dangers of arrogance but today finds relevance as it foretells the fate of comets. New research shows that approximately 3-9% periodic comets will form liquid water as they pass close to the Sun during the end stages of their lifetimes. The research is published in the journal Icarus. The research team included Dr Matthew Genge from Imperial College London and colleagues at the University of Pisa, and was led by Dr Martin Suttle from the Natural History Museum.

This surprising discovery goes against our previously established expectations of comets and could help answer a long standing question in meteoritics and planetary science: How much cosmic dust found at the Earth’s surface comes from asteroids and how much from comets?

Asteroids and comets are both solid celestial bodies, formed during the early solar system 4.5 billion years ago. Asteroids tend to be rockier and reside exclusively in the Asteroid Belt (between Mars and Jupiter). In contrast, comets come from the outer solar system and contain lots of water-ice and other volatiles. Both bodies send material to Earth, in the form of centimetre-sized (and bigger) fragments as well as a huge quantity of cosmic dust (.40,000 tonnes per year). Once caught by our planet’s gravity and if they survive passage through our atmosphere this material falls to the surface. They are termed meteorites or in the case of cosmic dust: micrometeorites – being smaller than a grain of sand at 25–400 μm, no more than twice the width of a human hair.

Cometary dust may be more altered than previously expected, and this alteration would be a recent feature, not formed during the early solar system. Dr Martin Suttle and Dr Matthew Genge

How much cosmic dust found at the Earth’s surface comes from asteroids and how much from comets remains an open question. In general, astronomers, dynamical modellers and some scientists studying cosmic dust feel that comets are the dominant source of extraterrestrial dust. While others argue that asteroids play the biggest role. So how can we answer this question? How can we tell an asteroid from a comet under the microscope?

Mineralogy of comets

Paper author Dr Martin Suttle, who works as a researcher at the Natural History Museum having completed his PhD at Imperial, says: “The presence of liquid water is significant because it drives a range of chemical reactions that generate new minerals, alters the primordial composition, and cements the comet together making it stronger.”

The researchers suggest that some cometary dust grains are altered due to presence of water before release from their parent body, and therefore appear asteroid-like, which confuses assessments of relative contributions from different sources. Liquid water will rapidly alter the composition and mineralogy of a comet. This process is termed aqueous alteration and is largely associated only with asteroids.

Suttle and Genge say, “Cometary dust may therefore be more altered than previously expected, and this alteration would be a recent feature, not formed during the early solar system.”

Previously, scientists thought the clay-bearing micrometeorites were from asteroids, but we now understand that some of the micrometeorites with clay mineralogy are from comets.

Comets and the Icarus Effect

This discovery is important to the planetary scientists who attempt to resolve the relative contributions of cosmic dust from different sources and may help resolve the outstanding question of whether asteroid dust or cometary dust is the biggest contributor.  By exploring the conditions in the surface layers of comets as their orbit comes close enough to the sun, the ‘perihelion’ point in its orbit, the research found it can get hot enough and sustain high enough pressures to form liquid water.

“We show that both the temperature and pressure conditions necessary for the generation of liquid water are possible for bodies that pass inside the orbit of Mars,” says Suttle. They estimate that liquid water can form in 3-9% of the current comet population but this process will affect many more comets in the future. This is because new comets are continually brought into the inner solar system by gravitational interactions with the gas giants (Jupiter and Saturn).

Warming up

This research paves the way for solar heating to be considered more seriously as a geological process among the planetary science community. To further explore the activity of comets requires a combination of new research including by space missions to comets paired with telescope and laboratory research.

The ideal comet candidate that is expected to heavily affected by solar radiant heating, is a body named 2P/Encke. This comet orbits very close to the sun and has had its north pole point continuously at the Sun for the last 300 years. It is also the source of the Taurids meteor shower and therefore sends abundant cosmic dust to Earth. The Icarus study is also relevant to the upcoming Japanese space mission (JAXA’s DESTINY+) which will visit Phaethon – the sun-baked parent body responsible for the Geminids meteor shower.

Observing Meteors

Dr Martin Suttle researches the dust under a scanning electron microscope, and also watches meteors from a camera he installed on his parents’ roof. The camera is part of the UK Fireball Network (UKFN), the UK branch of the Global Fireball Observatory (GFO) in collaboration with the Desert Fireball Network (DFN) in Australia, and multiple institutions around the UK.
Sarah McMullan, a PhD student at Imperial College London, coordinates the network together with Dr Luke Daly from the University of Glasgow.

McMullan says, “The aim is to capture fireballs over the UK and calculate their physical properties, so we are able to calculate their orbit and see if or where there is a meteorite on the ground. We currently have six cameras installed covering the majority of England, Wales and Southern Scotland, with plans to install a further 5 in the next few years to provide full coverage of the skies over the UK”.

There are four other meteor networks in the UK (both amateur and academically funded) plus countless individual citizen scientists imaging the night sky.  

Citizen Science

Together with UK planetary scientists and National museums, the networks and individuals have formed the UK Fireball Alliance (UKFAll), a collaborative data-sharing initiative. “A meteorite hasn't been recovered in the UK for nearly 30 years, but together we plan to change that,” says McMullan. Anyone can get involved: Follow the UK Fireball Network on Twitter or register for updates. McMullan says, “You may just be contacted to participate in citizen science, when we have a rock on the ground and need a recovery team.”

McMullan’s research is on numerically modelling the atmospheric entry of meteoroids 1-100 m in size. The impact frequency of objects in this size range is poorly understood as they are too small to be easily observed astronomically but enter the atmosphere relatively infrequently.

Objects >10 m diameter have the potential to cause damage on the ground, so understanding their impact frequency is important.

“The more cameras we have capturing and recording fireball properties, the more data we have available to improve our knowledge of these objects,” says McMullan. 

With thanks to Martin Suttle and Sarah McMullan for their contributions.