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The Hindu
The Hindu
Unnati Ashar

In zodiacal dust mystery, PRL Ahmedabad study points to a familiar source

Say you send a spacecraft into the outer reaches of the Solar System on a scientific mission. But on its way, somewhere around Mars, fast-moving dust particles hit its body, including the solar panels, chipping off little pieces. Fortunately the spacecraft is robust and it continues its journey towards Jupiter relatively unfazed. But the solar panels have suffered important damage.

This is the fate that befell Juno, a spacecraft that NASA launched in 2011 to study the gas-giant Jupiter and its moons. Juno entered into a polar orbit around the planet on July 5, 2016. But before it did, according to data reported in a 2021 paper by a group of researchers from Denmark and the U.S., dust particles struck the solar panels attached to the Juno spacecraft.

A pocket of dust

A scientist at the Physical Research Laboratory, Ahmedabad, decided to make the best of Juno’s situation.

In a paper published in the Monthly Notices of the Royal Astronomical Society, Jayesh P. Pabari used the data in the 2021 paper to calculate the number of dust particles Juno might have encountered between 1 and 5 AU.

‘AU’ stands for ‘astronomical unit’, which is the distance between the earth and the Sun. Mars is at a distance of 1.52 AU and Jupiter at 5.2 AU from the Sun.

The 2021 paper reported a peak in the number of dust particles impacting Juno at 1.5 AU. Dr. Pabari used this data to calculate the flux of dust between 1 and 5 AU. (The flux is the number of dust particles flowing through a given area per second.) He found the flux at 1.5 AU to be 10-times higher than at other distances.

Scientists have known that this dust is the source of zodiacal light. Zodiacal light is sunlight scattered by interplanetary dust. From the earth, it is visible as a faint, diffuse glow on completely dark nights. Zodiacal light is present across the entire path of the ecliptic, which is the path along which the Sun moves in the sky over the course of a year.

Where could this dust be coming from? This is an open question in astronomy. In his paper, Dr. Pabari compared the flux of dust in the vicinity of Mars, and the number of particles escaping the two moons of Mars and concluded that these moons could be the dust source. He also found no other phenomenon in the neighbourhood of this area that could release as much dust.

Gods of dread and panic

Mars’s two moons are called Deimos and Phobos. Mars in Greek mythology is the god of war and the planet’s moons are named for his twin sons, the gods of dread and panic, respectively. The American astronomer Asaph Hall discovered both of them in 1877.

Phobos is the bigger of Mars’s two moons. It is drifting closer to Mars at a rate of six feet per century. Eventually, astronomers expect it will either crash into the planet or break up into a ring around it.

The most prominent feature on Phobos is a 10-km-wide crater named in honour of Hall’s wife Angeline Stickney. In 1877, Hall had almost given up studying Mars’s surroundings when Stickney – a mathematician – encouraged him to continue. He did so and spotted Phobos the next day and Deimos six days later. Stickney crater is in fact half as wide as the entire moon.

On its day-side, the temperature on Phobos is around -4 degrees C, while just a few kilometres away on the night side, the temperature often drops to an even-lower -112 degrees C. This large temperature difference (around 108 degrees C) arises because the surface of Phobos is covered with fine dust that lacks the ability to hold heat. Phobos also has no atmosphere that can trap heat.

Deimos is different: astronomers believe its actual surface is buried under almost 100 metres of dust.

Dusty welcome

In his study, Dr. Pabari incorporated the shapes of the two Martian moons along with the gravitational effects of Mars, incoming and outgoing dust particles, the effect of spacecraft ejecta on the velocities of dust particles, and other parameters in his models of dust. Based on their output, he estimated the net rate of mass influx at Deimos and Phobos.

This in turn he combined with observational data and finally found a mechanism that could explain how Deimos and Phobos could be contributing to the zodiacal dust.

Micrometeorites are very small dust particles. They weigh no more than one-ten-thousandth of a gram. But they can move really fast, and when they do they can pack a punch. Dr. Pabari found that such micrometeorites fly into Mars’s moons just as they do into the earth. In the latter case, they burn up and disintegrate in the atmosphere. But Deimos and Phobos don’t have atmospheres, which means most micrometeorites slam into their surfaces and kick up small clouds of dust.

These dust particles can easily escape Phobos and Deimos because of the moons’ low gravity. (The more gravity a planetary body has, the more spherical its shape. Deimos and Phobos are not at all spherical.) In this way, Phobos has lost more dust.

The smaller of these dust particles escape into space whereas Mars’s gravity pulls in the larger ones. The latter collect in the form of a dust ring around Mars. Over time, they drift closer towards or away from the planet but stay in orbit.

According to Dr. Pabari’s work, a future mission to Phobos and Deimos could confirm his study’s findings – especially whether they are really losing more mass than they are gaining and whether his calculations are correct. We can only hope the mission spacecraft’s solar panels will be strong enough to withstand the moons’ dusty welcome.

Unnati Ashar is a freelance journalist.

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