That glow you see at sunrise or sunset is caused by cosmic dust. For decades astronomers thought it came from asteroids but now they’re not so sure
John Leif Jørgensen didn’t set out to revolutionise the world’s understanding of space dust. In fact, the Danish astrophysicist wasn’t even looking for it. When, in 2011, he convinced his collaborators at Nasa to add a fourth camera to the Juno spacecraft, he hoped to use it to count asteroids that were too small to be detected by telescopes. But the particles that the cameras detected were far smaller: no more than 80 micrometres wide, not much bigger than the diameter of a human hair. Even stranger is the origin of the dust. In defiance of everything we thought we knew about space dust, these tiny particles seem to be flung from Mars.
We tend to think of paradigm shifts as big-picture transformations: from a religious worldview to a rationalist one; from an Earth-centred universe to a heliocentric one. But sometimes they start with something minute as dust. If Jørgensen and his team of researchers are correct, their findings may upend our understanding of the solar system. But first they will have to overcome forty years of contradictory research, and the scepticism of many experts, including one with a minor side gig as the guitarist for Queen.
Although you might not realise it, you may have seen the dust in question, or at least its effect: an eerie, conical glow emanating from the horizon just before sunrise or after sunset. The Persian poet and astronomer Omar Khayyam may have been referring to it when he wrote, in his 1132 Rubaiyat, of the “ false dawn”, but the phenomenon is more properly known as the zodiacal light. By the 1980s, scientists aided by satellites that could measure infrared emissions had figured out that the light came from particles that exist in bands circling the Sun. These particles, they thought, seemed to come from asteroids that shed dust as they travel into the inner solar system. “The satellite showed that there was structure to the zodiacal cloud,” says Stanley Dermott, one of the first scientists to develop the theory on the asteroidal origins of zodiacal dust bands, “And the modelling showed that they were linked to collisions between asteroids.”
More recent research has filled in other parts of the picture, including the contribution of comets, but consolidated around an explanation that these bodies were carrying the dust into the inner solar system. And that explanation, says Jørgensen, who is a professor at Denmark’s Technical University in Copenhagen, is precisely what his findings unexpectedly ran smack into. “It was totally serendipitous. We were looking for something else.”
In their hunt for smaller asteroids, Jørgensen and his team had programmed one of the four star tracking cameras they designed to photograph any celestial object that appeared in multiple images and track its velocity. It switched on when Juno was out around Mars, just before it swooped back towards Earth to gain the necessary boost in speed that would allow it to escape Earth’s gravity – a manoeuvre known as a ‘gravity assist.’ It then passed Mars again on its way to Jupiter. “We measured the area between Mars and Earth twice,” Jørgensen says. “Our measurements continue all the way to Jupiter. It’s the first time you have those.”
For the first three weeks while the spacecraft moved through the zone where they expected to see asteroids, there was nothing. And then, suddenly, there was a lot. In a statement from Nasa released following the March publication of the discovery in the Journal of Geophysical Research, Jørgensen described it looking like “someone shaking a dusty tablecloth outside the window.”
It took them a while to figure out what they were seeing; Jørgensen was even afraid for a short time that the streaks captured were evidence of a fuel leak. But once he and his team calculated the size and speed of the objects they realised something even more astonishing: they were minuscule pieces of their own spacecraft’s expansive solar panels, “liberated” by dust particles that had slammed into Juno at a velocity of 16,000 kilometres an hour. In both their speed and their size, the particles corresponded with the dust that makes up the zodiacal bands. Since the dust particles travel at speeds that makes them almost impossible to capture visually – they move at a velocity of 5-40 kilometres a second – they are measured by their impact; the fragments broken off move somewhat more slowly and are thus easier to image.
They didn’t find dust near Earth, and as Juno entered Jupiter,’s gravitational field the particles again disappeared. It was only while the craft was near Mars that it was prevalent. The dust’s nearly circular orbit pattern also fits neatly with Mars’. “At first we were sceptical about our own data,” says Jørgensen. “But no matter how we turned it around, we could see no other option than that the dust must come from the Mars system.”Most Popular
The finding is significant in part because it’s the first time that scientists have been able to measure the distribution of these dust particles in space. Until now, dedicated dust collectors, which are more limited in size than Juno’s vast solar panels and thus less sensitive to sparse collections of particles, have mostly counted the more abundant and much smaller interstellar dust particles, rather than the interplanetary particles Juno detected. But it may also change scientific consensus on the source of the zodiacal light. Jørgensen’s team developed a computer model to predict the light reflected with the dust cloud. In the words of Jack Connerney, a magnetometer researcher at Nasa’s Goddard Space Flight Centre, and Jørgensen’s deputy co-author, this model provides “a confirmation that we know exactly how these particles are orbiting in our solar system, and where they originate.”
Admittedly, some big questions remain, and as Jørgensen and his co-authors note, chief among them is how, exactly, the dust got there. Although Mars is known to be a dusty place, and dust storms are not uncommon, for that dust to leave the planet altogether it’d have to escape Mars’ gravity, and scientists haven’t identified the precise mechanism by which that happens. It’s possible, the authors conclude, that the dust is emanating from two of Mars’ moons, which have weaker gravity.
Many space dust experts – and yes, there are lots of them – remain unconvinced by Jørgensen’s dusty hypothesis. Among them is Brian May, who first started working on the subject in the 1970s, but whose career as an astrophysicist was interrupted by a detour as Queen’s guitarist. May returned to astrophysics in 2007, and focused on the velocity of dust particles in the Zodiac cloud for his PhD. But, as he told the Danish science website Videnskab in a brief statement, “These data from Juno do not seriously challenge the hypothesis that the detailed structure of the zodiacal light seen from Earth is due to collisions between members of certain asteroid families in the Asteroid Belt.”
Others are less dismissive, but still see weaknesses in the work. Dermott faults the research for failing to account for the form the dust takes. “We know that there’s a discrete structure in the zodiacal cloud, and any model has to explain why the source accounts for that structure. I didn’t see that.” And Petr Pokorny, a Nasa researcher, is appreciative of the data, calling it “an important reservoir that will serve the community for years to come,” but is unpersuaded by Jørgensen’s interpretation. He notes that Juno’s flat, one-directional solar panels were effectively “blind” to the impact of dust particles that came from directions perpendicular or behind the solar panels, which means it could not capture the full complexity of the dust environment, and he faults the authors for failing to compare their model to existing ones. “Without any formal model I can only guess now, but I cannot see a way how dust generated at Mars or in its vicinity could explain various dust related phenomena,” he wrote in response to emailed questions. “I also do not know about a mechanism that could produce the amount of dust needed to sustain the current shape of the zodiacal dust cloud or at least its small portion. We have rovers on Mars, satellites orbiting Mars and they did not detect any dust-worthy activity.”
Although he remains sceptical, Pokorny recognises that, if upheld, Jørgensen’s theory would transform the field. “I am all for new revolutionary ideas, because that’s what propels our field and humankind forward, but each finding needs to be very well reasoned and scrutinised by the community,” he writes. “ I couldn’t be more thrilled if Dr. Jørgensen’s findings are correct. That would represent a big paradigm shift in our field. But currently, I personally don’t see a way to fit Martian dust into the story, at least not at the moment.”Most Popular
For his part, Jørgensen suggests that some of the discrepancies can be explained by the innovative source of the data, and he accepts that resistance to a new theory is natural when it contradicts an entrenched understanding. “For the last 40 years, there has been consensus that the dust is coming from the asteroid belt. But one must keep in mind that the idea stems from observations made with telescopes from Earth or measurements from satellites in orbit around the Earth. We are the first to have actually been out to test whether the dust is now also out at the asteroid belt. We’re stepping on a lot of toes.”
Yet even his critics see value in the implications of the findings for how spacecraft are outfitted. In order to lessen their weight, Nasa has equipped some – such as Lucy, which is scheduled to launch this fall – with the thinnest possible solar panels and other devices. But given the size and velocity of the particles that Juno detected, that may be the wrong strategy. “What they need, Jørgensen says, “is a bulletproof vest.”
In that sense, says Dermott, the findings are “absolutely a paradigm shift.” Pokorny agrees. “I couldn’t agree more with Jørgensen’s conclusions. Everyone wants to build the spacecraft as light as possible to save money for instruments, so assessing the right amount of shielding and damage mitigation scenarios is a part of every space mission. Future missions with bigger and better solar arrays should definitely take into consideration that there are hyper-velocity dust particle impacts in our solar system and they are quite frequent and difficult to avoid,” he says.
Jørgensen believes that using the star tracker cameras to assess where dust is and is not may even one day open new possibilities for exploring exoplanets for inhabitable conditions, since scientists have until recently been hindered in their search for life-supporting gases by their belief that the extent of dust clouds would make that detection impossible. Currently, he and his team are busy collecting new data coming in from star tracker cameras on a UAE craft that reached Mars in February, and are awaiting possible corroboration from data collected on an earlier Nasa mission to Jupiter. But in the meantime, he welcomes the criticism his potential paradigm shift has awakened. “It has to be as challenging,” he says of the pushback. “Otherwise, it’s no fun.”