The glow could tell us what there is on the surface, says S.Ananthanarayanan.

The first satellites of planets in the solar system that were seen were the four moons of Jupiter, which Galileo spotted through his telescope in 1610. And the smallest of the four, slightly smaller than the earth’s moon, is Europa.

Europa displays a surface which is the smoothest of the known solid objects in the Solar System. The smoothness of the surface suggests that below the surface, there is an ocean of liquid water, kept liquid by internal heat and tidal flexing. This, and the high oxygen content of the thin atmosphere makes Europa a strong candidate as a host of extra-terrestrial life.

The surface of Europa, however, is battered by intense radiation, which was thought to prevent the survival of any life-form till deep below the surface. While there are studies to say that life could exist at a depth of just a centimetre, there is now a paper in the journal, Nature Astronomy, that puts a different slant on the onslaught of radiation. Murthy S. Gudipati, Bryana L. Henderson and Fred B. Bateman, from NASA’s Jet Propulsion Laboratory, Pasadena and the National Institute of Standards and Technology, Gaithersburg, Maryland, write that the radiation would affect the salt and ice on the surface to give off a greenish glow, which could help sensitive analysis of the composition of the surface. Bombardment of pieces of material with radiation, for analysis, is standard laboratory technique. It would appear that we could carry out these studies of the material on Europa without the need to go there and fetch samples.

The reason for the radiation that strikes Europa is the strong magnetic field of the parent planet, Jupiter. Jupiter’s magnetic field is fourteen times stronger than the earth’s and the planet has a rotation period of nine hours. The strong and rapidly changing magnetic field reacts with the solar wind - charged particles, mainly electrons, protons and alpha particles, that stream forth from the sun. These, along with the rich outpouring of charged particles from Io, Jupiter’s volcanic moon, create a rotating ring of charged particles. The influence of the radiation, which is called the magnetosphere of Jupiter, extends seven million kilometres towards the sun and another seven million kilometres, almost to the orbit of Saturn, in the opposite direction. This is the largest magnetosphere in the Solar System and its existence was first inferred from strong radio signals that come from Jupiter, as a rotating radio beacon

The result is that the moons of Jupiter (there are 79 in all) are blasted with radiation. In fact, even spacecraft that were sent near Jupiter and to fly past Europa needed special shielding and could remain in the radiation only for short periods.

The paper in Nature Astronomy observes that the radiation that strikes the surface of Europa, which is essentially made of silicate rock and covered by a crust of water-ice, could create energy-rich chemicals at the surface. These, if transported down to the ocean below the surface, could promote chemical reactions, which, in turn, could support life processes. In this context, the paper says, it is important to understand the chemical constituents of Europa’s surface, particularly, sodium and magnesium, and chloride, carbonate and sulphate ions, which would help assess the salinity of the water ocean and the nature of the interaction between the ocean and the surface.

The major moons of Jupiter, including Europa, have been extensively studied with the help of a number of conventional tools. These include telescopic observation from the earth and orbiting stations like the Hubble Space Telescope and observations by the Pioneer and Voyager fly-by spacecraft, the Galileo orbiter (to Jupiter) that mapped near infrared emission from Europa over 8 years, and New Horizons, which flew past on its way to Pluto. And there are more missions planned, including Europa-Clipper, to orbit Europa. However, “many salts of interest are relatively featureless in the near infrared spectral region that is typically used for compositional analysis,” the paper says. The near IR spectra seen can thus be corelated to many combinations of chemical components. Even observations of the visible and UV spectra, using the Keck Observatory and the Hubble Space Telescope have not given significant results. “For definitive salt identification, complementary methods are needed,” the paper says.

Laboratory studies conducted by the group shows that the radiation induced ice-glow can provide this complementary information. The laboratory studies used a piece of equipment built at the Jet Propulsion Laboratory, called the Ice Chamber for Europa High-Energy Electron and Radiation-Environment Testing (ICE-HEART). Ice cores held in an aluminium tube were cooled down to the temperature of Europa’s surface. They were then bombarded with controlled electron beams from the Medical and Industrial Radiation facility at Gaithersburg, in Maryland, USA. Electron beams of different energies were used with a large number of ice-core compositions. Any emission from the ice-cores was recorded with all other lighting switched off, to simulate the night-time conditions of the water-ice on Europa.

The trials, building on earlier studies, showed that electron bombardment of water-ice and salts produced a glow with spectral characteristics that changed according the salt component. The radiation that bathes the surface of Europa would then produce similar emission. Imaging the glow, from short range, as from the proposed Europa-Clipper orbiter, could then provide information about the distribution of salinity, patches with fresh water ice, and other characteristics.

The Europa Instrument Set aboard the proposed orbiter includes a Wide-Angle Camera and a Narrow Angle Camera, for selective, high resolution imaging. Night-time imaging of the radiation-induced glow could then produce a map of the chemical composition of the Europa surface. The night-time data could be combined with the daytime observations for better identification of the chemical composition and geological features, now of Europa and later of the Io, Ganymede and Callisto, the other moons of Jupiter, the paper says.

Heading

It is not reflected light
It is not unusual to analyse reflected light, for instance the light that comes to us from the planets, to identify the elements in the planet’s atmosphere, through which the light has passed. This is even done with starlight which passes through the atmosphere of a heavenly body that is just about to block the star from view.
In the case of Europa, the glow that we see is not reflected light. What shines on Europa is not light, but high energy radiation of charged particles – electrons, protons and alpha particles. These cause the elements on the surface of Europa to emit light, which could help us understand what these elements are. As for the atmosphere, there is none to speak of on Europa. And not a bad thing, as the glow at the surface can be viewed with less scattering.

------------------------------------------------------------------------------------------ Do respond to : response@simplescience.in -------------------------------------------