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A Ph.D. student from Lancaster University has used historical data from the Cassini spacecraft to calculate the optical depth of Saturn's rings. To avoid doubt, "optical depth" is a measure of the transparency of an object, which reveals how far light can pass through it without being absorbed or scattered. The study was conducted in collaboration with the Swedish Institute of Space Physics and is an exciting use of historical data to make discoveries about our celestial neighbors.
George Xystouris, the Ph.D. student in question, analyzed historical data from the Langmuir Probe onboard Cassini. The probe measured cold plasma- consisting of low-energy ions and electrons in Saturn's magnetosphere. They focused on the solar eclipses of Cassini, which are periods when it (the spacecraft) was in the shadow of Saturn or the main rings. The Langmuir Probe recorded dramatic changes in the data during each eclipse.
NASA/JPL-Caltech
Cassini's mission began in 2004 and ultimately ended in 2017 when it plunged into Saturn's atmosphere after gathering considerable amounts of data about the planet, moons, and rings.
“As the probe is metallic, whenever it is sunlit, the sunlight can give enough energy to the probe to release electrons. This is the photoelectric effect, and the electrons that are released are so-called ‘photoelectrons. They can create problems, though, as they have the same properties as the electrons in the cold plasma around Saturn, and there is not an easy way to separate the two," explained Xystouris.
“Focusing on the data variations, we realized that they were connected with how much sunlight each ring would allow to pass. Eventually, using the properties of the material that the Langmuir Probe was made of and how bright the Sun was in Saturn’s neighborhood, we managed to calculate the change in the photoelectrons number for each ring, and calculate Saturn’s ring optical depth," he added.
“This was a novel and exciting result! We used an instrument that is mainly used for plasma measurements to measure a planetary feature, which is a unique use of the Langmuir Probe, and our results agreed with studies that used high-resolution imagers to measure the transparency of the rings.”
Saturn's main rings extend up to 86,992 miles (140,000 km) from the planet but have a thickness of only 0.62 miles (1 km). Starting in 2025, these rings will be difficult to view from Earth because they will be tilted edge-on to Earth. However, during the next phase of Saturn's 29-year orbit, they will tilt back towards Earth and become more visible and brighter until 2032.
"It is always good to see a postgraduate student involved in using space probe instrumentation in an unusual and inventive way. Innovation of this kind is just what is needed in astronomical research – and an approach which many former students who are in a variety of careers are applying to help address the world's problems," added Professor Mike Edmunds, the President of the Royal Astronomical Society.
You can read the study in the journal Monthly Notices of the Royal Astronomical Society.
Study abstract:
A Langmuir Probe (LP) measures currents from incident charged particles as a function of the applied bias voltage. While onboard a spacecraft the particles are either originate from the surrounding plasma or emitted (e.g. through photoemission) from the spacecraft itself. The obtained current–voltage curve reflects the properties of the plasma in which the probe is immersed, but also any photoemission due to illumination of the probe surface: As photoemission releases photoelectrons into space surrounding the probe, these can be recollected and measured as an additional plasma population. This complicates the estimation of the properties of the ambient plasma around the spacecraft. The photoemission current is sensitive to the extreme ultraviolet (UV) part of the spectrum, and it varies with the illumination from the Sun and the properties of the LP surface material, and any variation in the photoelectron irradiance can be measured as a change in the current-voltage curve. Cassini was eclipsed multiple times by Saturn and the main rings over its 14-year mission. During each eclipse, the LP recorded dramatic changes in the current–voltage curve, which were especially variable when Cassini was in shadow behind the main rings. We interpret these variations as the effect of spatial variations in the optical depth of the rings and hence use the observations to estimate the optical depth of Saturn’s main rings. Our estimates are comparable with UV optical depth measurements from Cassini’s remote sensing instruments.
