Cassini caught in Hyperion's electron beam
16 October 2014Static electricity is known to play an important role on the airless, dusty Moon, but evidence of surface charging on other objects in the Solar System has been elusive. However, a new analysis of data from the international Cassini mission has revealed that the orbiter was briefly bathed in a beam of electrons coming from the electrostatically charged surface of Saturn's moon, Hyperion.
Hyperion is an irregular outer moon of Saturn, with a mean radius of 133 km. It has a low mean density, suggesting that it may consist primarily of water ice, with an unusually porous interior, resulting in a bizarre, sponge-like appearance.
Hyperion tumbles chaotically as it moves around Saturn at a distance of 1.48 million km – four times further than our Moon is from Earth. The distant, elliptical orbit often causes it to cross from the planet's magnetosphere - the invisible bubble generated by Saturn's internal magnetic dynamo – and enter the solar wind. This is partly due to its changing orbital position and partly due to expansion and contraction of the planet's magnetosphere.
As a result, Hyperion is sometimes exposed to plasma (charged particles – electrons and ions) trapped in the magnetosphere and sometimes it is bathed in the solar wind of particles ejected from the Sun. The surface of the airless satellite is also bombarded by solar ultraviolet (UV) light. This exposure to the hostile space environment causes some unusual effects.
Spacecraft observations and theoretical studies have previously shown that incoming solar radiation and charged particles can generate a static electrical charge on Earth's Moon. Now a team led by Tom Nordheim, a PhD student at Mullard Space Science Laboratory (MSSL), University College London, has shown that similar effects can occur on other small, airless objects, such as Hyperion. Their results are published in Geophysical Research Letters.
During a close encounter with Hyperion on 26 September 2005, unexpected measurements from several instruments on board the Cassini spacecraft indicated that something strange was taking place in the particle – plasma environment.
The new analysis of the data shows that Cassini was magnetically connected to the surface of Hyperion for a brief period, which enabled it to be bathed in a beam of electrons coming from the moon's surface.
"We know that objects in space, including Earth's Moon, may become electrostatically charged by exposure to solar ultraviolet light and incoming charged particles. This is comparable to what happens when you rub your hair against a balloon, or when a shirt or blouse rubs against a sweater," said Nordheim.
The Cassini data show that a similar process can take place on Hyperion. Due to its interaction with solar UV light and charged particles from Saturn's magnetosphere, the moon's surface may acquire a net electric charge. This is precisely what was found by Cassini's instruments.
Approximately 6 minutes before its closest approach to Hyperion, the Electron Spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) instrument, detected a sharp increase in the number of negatively charged particles. Cassini's magnetometer showed that these electrons were coming in a beam along the magnetic field lines, from the direction of Hyperion.
At the same time, the Radio and Plasma Wave instrument detected intense plasma wave fluctuations caused by the electron beam and the Magnetospheric Imaging Instrument observed the absorption of other particles by Hyperion.
Analysis of the CAPS-ELS data indicates that it remotely detected a strongly negative surface potential (-200 volts) on Hyperion, consistent with the predicted electrostatic charge in regions near the moon's terminator – the day-night boundary.
"The large difference in potential between the surface and the spacecraft resulted in a flow of electrons being accelerated from Hyperion toward Cassini," said Tom Nordheim. "It was rather like Cassini receiving a 200 volt electric shock from Hyperion, even though they were over 2000 km apart at the time."
"The alignment between the two was just right for us to be able to detect this fairly rare event. If Cassini had been in a different location during the flyby, the electron beam would not have been detected."
The first confirmed detection of surface charging on an object in the outer Solar System has wide-ranging implications. This fundamental process is predicted to occur on many different bodies, including asteroids, moons and the surface of comets.
Scientists have previously suggested that surface features observed on the asteroid Eros and several Saturnian moons are due to the motion of charged dust across their surfaces. On small objects with low gravity, dust grains might even be able to overcome the force of gravity and escape into space.
In terms of human exploration of planetary objects without atmospheres, such as the Moon, strong electric charging effects may also prove to be a hazard to astronauts, who might be subjected to strong electrostatic discharges.
"Surface charging as a fundamental phenomenon affecting planetary objects is currently not well understood and while it has been observed on Earth's Moon, the Saturn system presents us with an opportunity to study this effect in an environment where many parameters are completely different," said Geraint Jones of MSSL, who co-supervised Tom Nordheim's research.
"Our observations show that this is also an important effect at outer planet moons and that we need to take this into account when studying how these moons interact with their environment."
"After 10 years in orbit around Saturn, Cassini continues to demonstrate its importance in probing the physics of the highly complex, interconnected system made up of the giant ringed planet, its moons and their immediate space environment," said Nicolas Altobelli, ESA's Cassini-Huygens Project Scientist.
"We see once again that the knowledge gained by this remarkable explorer can be applied to other places in the Solar System and beyond."
Detection of a strongly negative surface potential at Saturn's moon Hyperion, by T. Nordheim et al., is published in Geophysical Research Letters, 2014; DOI: 10.1002/2014GL061127
The Cassini–Huygens mission is a cooperative project of NASA, ESA and the Italian Space Agency (ASI). Launched in 1997, Cassini arrived in the Saturn system in 2004 and is studying the ringed planet and its moons. The Huygens probe was released from the main spacecraft and, in 2005, parachuted through the atmosphere to the surface of Saturn's largest moon, Titan.
NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C.