If the aim of travel is to view strange new sights and broaden the mind, NASA’s voyage to the asteroid belt to examine Ceres and Vesta has been well worth the trip. As Ceres’s dull, frozen surface came into view of the Dawn space probe’s cameras earlier this year, mysterious bright spots could be seen glinting from the surface. What might have caused them has kept the world guessing.
We might soon find out. On 3 June, the Dawn probe dropped its orbit to 4,400 kilometres above the dwarf planet’s surface for a closer look. The probe will map Ceres for 22 days and the higher resolution images it takes may well provide an explanation for the spots. The images will become available over the coming weeks.
But for now, we are in the dark. NASA has even created an online poll with six potential options where you can vote for what you think caused the spots.
To help make sense of NASA’s choices some information about Ceres might be useful.
Ceres is the largest object in the asteroid belt that lies between Mars and Jupiter, but it’s still less than 1,000 kilometres across – far smaller than our Moon. But like the Moon, it’s geologically cool – there’s little heat in its core to power volcanic eruptions or plate tectonics, for example. Its surface is -38 °C at warmest, and it’s likely that ice makes up a quarter to half the dwarf planet’s volume.
The dwarf planet’s surface is a dull, dark grey, covered in craters and a layer of dust of unknown thickness. The bright spots look brilliant white in many images, but are actually a much brighter shade of grey than the surrounding material.
Here are NASA’s online poll options explained:
The bright material could be a light-coloured rock that flowed across Ceres’ surface after volcanic eruptions. A difficulty here is that space weathering – the micrometeorites, cosmic rays and solar wind that pummel the dwarf planet – would slowly erode and dull its surface, so the volcano would have to have been active recently for the white patches still to be visible. This presents a challenge, as according to theory, Ceres should be geologically dead with no central hot core providing the energy for eruptions. But it would be exciting to find an active volcano on an asteroid!
This could also be called a cryo-volcano. Rather than an eruption of hot lava, water may explode from the surface – more like the Old Faithful geyser in Yellowstone National Park, Wyoming, rather than the Krakatoa volcano. If the spots were geysers they’d require less heat than a volcano to erupt. And the water might fall back as bright ice to cover the surrounding area.
We see this kind of icy volcano on the frozen moons of gas giants, including Saturn’s moon Enceladus. They’re powered by ‘tidal heating’ as the core of a moon is squeezed by the enormous gravity of the planet it orbits.
The challenge with Ceres is there is no nearby massive object to induce tidal heating, and the Sun’s heat (about 10% of what the Earth receives) isn’t strong enough to cause pockets of ice to erupt.
On Earth, rocky outcrops can be strikingly lighter in appearance than surrounding material, especially rock with high silicate content (which isn’t unusual in asteroids). Over time, space weathering dulls the rock on the dwarf planet. This means the spots would have needed to be formed recently to stand out as much as they do. But, as discussed in the volcano option, there’s not much heat left in Ceres to power geological activity, so recent upthrusts are unlikely.
As much as half of Ceres’ volume might be ice so this might seem the obvious answer, but any of that ice existing at the surface isn’t a given. While the Sun’s energy striking Ceres shouldn’t be strong enough to power any geysers, it would be strong enough to make ice at the surface unstable and prone to a process called sublimation, in which ice is converted directly into a vapour that can escape into space.
This means that if the spots are ice, they must have only recently been uncovered. One possible candidate is a meteorite impact – the craters on Ceres are a testament to a violent past – but it’s probable that the grey rock and dust thrown up in the impact would rain back down and cover the area again.
There may be eruptions of salt water from subsurface ice in Ceres, as there are in Australia’s great salt lakes. The water then escapes into space, leaving the salt behind as a curiously reflective coating on the surface until it is covered with dust. But, as with the geyser explanation, the difficulty is what drives the eruption of water to the surface in the first place.
The favourite online option, with 40% of the vote ( the next most popular ice is at 30%) this catchall answer is likely the correct one. We explore these alien worlds in order to uncover surprises such these bright spots, which might turn out to have a surprising explanation.
For what it’s worth, my bet is some combination: a meteorite strike either shook covering material off the salty ice or heated it so that salty water rose to the surface as a geyser. The water escaped into space and now, only the salt remains. Unfortunately, the NASA poll doesn’t allow multiple answers!
Chances are we won’t have long to wait for the answer. Over the next month Dawn, at its new low-altitude mapping orbit, will take photos with three times the resolution than before and – we hope – uncover the dwarf planet’s spotty secrets.
Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne. He was Lead Scientist of The Royal Institution of Australia from 2017 to 2021. Twitter | @astroduff
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