One of the aspects of our study of the universe that fascinates me is the hunt for dark matter. That elusive material that doesn’t interact with much makes it difficult but not impossible to detect. Gravitational lenses are one such phenomena that point to its existence indeed it allows us to estimate how much there is in galaxy clusters. A paper now suggests that observations of Jupiter by Cassini in 2000 suggest we may be able to detect it using planets too.
Dark matter is as its name suggests, mysterious and elusive. It is believed to account for about 27% of the universe’s mass and energy. However unlike ordinary matter – of the like that makes up you and me; the stars and planets, dark matter doesn’t emit, absorb or reflect light making it invisible and difficult to detect. Its very existence is only inferred from the effect its gravity has on visible matter and the large scale structure of the universe.
The foundations for an interesting twist in the search for dark matter were laid in 1997 with the launch of the Cassini spacecraft from Cape Canaveral in the US. A seven year journey began that would take the probe from Earth to Saturn utilising gravitational slingshots from Venus, Earth and Jupiter. On board was a plethora of instruments to record data from radio waves through to extreme ultraviolet. En-route to Saturn, Cassini would be used to observe the planets using multiple wavelengths.
Of particular interest to the mission was using the Visual and Infrared Mapping Spectrometer (VIMS) to measure levels of hydrogen ions known as trihydrogen cations. They are a common ion found across the universe and are produced when molecular hydrogen interacts with cosmic rays, extreme ultraviolet radiation, planetary lightning, or electrons accelerated in planetary magnetic fields.
The team explore how dark matter can also produce trihydrogen cation in the atmosphere of planets. Any dark matter that is captured by planetary atmospheres – in particular the ionosphere – and is consequently annihilated, can produce detectable ionising radiation.
Using data from Cassini VISM system, the team have searched for dark matter ionisation in the ionosphere of Jupiter. Due to Jupiter’s relatively cool core, it was identified as the most efficient dark matter captor in the Solar System allowing dark matter particles to be retained. The challenge was to identify the signals over the background ‘noise’ from other radiation so the team had to use data from 3 hours either side of Jovian midnight. Choosing this time meant solar extreme ultraviolet irradiation was at a minimum. They also focussed on lower latitudes, keeping away from the high magnetic fields around the polar regions.
The detection of dark matter ionisation in the Jovian atmosphere reveals a whole new method for understanding this strange and mysterious cousin of normal matter. It is not just planets in our Solar System though, exoplanets are a new possible source especially those based in dark matter rich regions of the Galaxy.