In a few million years, Mars’s moon Phobos will be shredded into pieces that will settle into a flat ring like Saturn’s. But bits of Mars’s two moons may already be circling the Red Planet, some of it in the form of nascent rings.
Astronomers have long thought it was possible for Mars to be encircled by rings made of bits of rock kicked up from its moons Phobos and Deimos, but no one had ever observed them. This may be because the rings lie in planes not easily viewed from Earth or space telescopes – or perhaps they aren’t there at all.
After it arrived at Mars in 2013, the Mars Atmosphere and Volatile Evolution (MAVEN) satellite spotted a cloud of high-altitude dust surrounding the planet. The MAVEN team could not determine how big the dust particles were or their source, but they were spread out uniformly rather than being concentrated into rings. That suggested they were coming from interplanetary space.
A fresh analysis of the MAVEN data from suggests Mars is also surrounded by dust in the form of proto-rings, and some of it is coming from its moons.
Jayesh Pabari of the Physical Research Laboratory in Ahmedabad, India, and his colleagues have compared the MAVEN dust measurements with models based on existing assumptions about how many meteoroids hit Mars and its moons. They argue that smaller particles thrown up by the meteoroid strikes are typically swept away by the solar wind, while Mars’s gravity collects the larger particles into proto-rings located along each moon’s orbit.
Dust from these rings might be able to get to Mars’s upper atmosphere. Pabari’s team found that while the most of the dust cloud is interplanetary, about 0.6 per cent of it could be coming from Phobos and Deimos.
“The bigger ring particles can reach Mars over a period of time, in addition to the interplanetary dust particles,” Pabari writes.
The dust shedding due to meteoroid impacts would continue even as Mars’s gravity pulls Phobos (pictured above) inward during the next 20 to 70 million years and starts to break it apart. That means there will be less of Phobos left when it finally collapses. “This essentially can reduce the [final] ring dust density,” Pabari writes.
The MAVEN team are not convinced that any proto-rings exist. MAVEN sidled up to Phobos in 2016, and spacecraft managers didn’t see any increase in dust along its orbit, says principal investigator Bruce Jakosky at the University of Colorado Boulder.
As for the dust cloud around Mars, it’s hard to be sure of its origin since MAVEN was not designed to look for dust, and no dust-collecting probe has visited the planet. A Japanese probe called Nozomi was packing a dust-measuring instrument, but it experienced electrical failures and did not reach Mars orbit as planned in 2003. Pabari is proposing a future dust investigation mission called the Mars Orbit Dust Experiment (MODEX).
“To really say anything definitive about the dust, you really need to have a dedicated dust detector,” says Laila Anderssen, also at the University of Colorado Boulder. She has been studying the dust using MAVEN’s electrical sensing instruments, and is still examining the data.
“We still haven’t seen a good indication that there is significant material in the vicinity of the moons. So I think it’s a long shot,” Anderssen says, “but one should never say never.”
Journal reference: Icarus, DOI: 10.1016/j.icarus.2017.01.023