The solved mystery of Phobos, the Martian moon

The mission Mars Express The ESA has revealed how Mars’ largest moon, Phobos, interacts with the solar wind of charged particles thrown by the Sun.

When performing a series of real and ‘fake’ overflights, This orbital mission has detected an elusive process that has only been seen in Fobos once before.

The solar wind leaves our star, filling the Solar System with energetic particles. Earth’s Moon reflects these particles continuously, and the same ‘backscatter’ is expected on Mars’ moon Phobos, given the similarities between the two (both are rocky, lack atmosphere and magnetic field, and orbit terrestrial planets in the Inner Solar System). However, ESA’s Mars Express has only seen this backscatter once (in 2008), despite having come close to Phobos many times.

Researchers now report the second successful detection of solar wind particles reflected off Phobos, detected during a moon flyby in January 2016.

“The relationship of Phobos to the solar wind has long been a enigma“, says in a statement Yoshifumi Futaana from the Swedish Institute of Space Physics (IRF), and lead author of the new article on the 2016 flyby. “We know that Phobos must be interacting with these particles, but we are not seeing them, why? Why is Phobos behaving like such a different way to the Moon when the two seem to be quite similar?

“For the first time in eight years of flybys, we are excited to see again signs of these reflected particles in the largest moon on Mars. “

However, since this backscatter is so intermittent and is rarely seen on Phobos, scientists wondered if the phenomenon could have been caused by Mars Express reflecting off particles from the solar wind. During the 2008 flyby, the spacecraft shifted its solar array and moved to point its instruments at Phobos, a maneuver that may have affected the behavior of the surrounding particles.

“The same criticism held for the 2016 flyby: how do we know that this detection is actually a reflection of Phobos, and not the Mars Express itself? “Yoshifumi adds.

To explore this possibility, the researchers conducted three unprecedented special operations, called ‘sham’ flybys, with the spacecraft in 2017. Using exactly the same sequence of operations, control maneuvers, and settings for solar panels, Mars Express flew in a patch of space filled with solar wind energy but without the presence of Phobos, essentially conducting a flyby, just without its target.

“In essence, we were completing a kind of laboratory experiment on Mars,” says co-author Mats Holmström, also from IRF and principal investigator for the Mars Express ASPERA-3 instrument, which observed the reflected particles. “The ‘fake’ flybys they allowed us to explore how Mars Express influences the solar wind in a more controlled environment, so we can look for signs that the spacecraft itself is the cause of the reflection of the particles. “

The ‘bogus’ flybys revealed no signs that Mars Express produced or will scatter incoming particles, suggesting that Phobos did reflect the particles detected in space during the 2008 and 2016 flybys.

Despite this, backscattered particles have only been detected in two of more than a dozen Phobos flybys, and even then, the signals are sporadic and intermittent. This is completely different from what we see on the Moon, another body that lacks both an atmosphere and a magnetic field, so it would be expected to behave in a similar way. Why this difference?

Yoshifumi and his colleagues consider a number of possibilities, from processes that perhaps take place in spatial scales or different storms than those captured by Mars Express, up to possible magnetism in Phobos, differences in the composition of the surface of Phobos and the Moon, and more.

“In general, intermittent particles are likely to be reflected off the surface of Phobos, but we cannot rule out another mysterious origin“Yoshifumi adds.” However, the ‘fake’ flybys helped us understand the situation significantly better, explicitly showing that Mars Express was not the source.

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“To find out more, we need more Phobos flybys on Mars Express in various configurations. Even if no reflected particles are seen during those flybys, even lack of signal it will provide valuable statistics. “

The behavior of the solar wind on Phobos and on the Moon implies that the surfaces Each of them have evolved differently, raising intriguing questions about how the Mars system differs from ours.

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