We present maps of Ganymede's surface composition with almost complete longitude coverage, acquired using high spatial resolution near-infrared (0.95 to 1.65
) observations from the ground-based VLT/SPHERE instrument.
Observed reflectance spectra were modelled using a Markov Chain Monte Carlo
method to estimate abundances and associated uncertainties of water ices,
acids, salts and a spectrally-flat darkening agent. Results confirm Ganymede's
surface is dominated by water ice in young bright terrain (impact craters,
sulci), and low-albedo spectrally-flat material in older dark terrain (e.g.
Galileo Regio). Ice grain size has strong latitudinal and longitudinal
gradients, with larger grains at the equator and on the trailing hemisphere.
These trends are consistent with the effects of the latitudinal thermal
gradient and global variations in radiation driven sputtering. Sulphuric acid
has a low abundance and appears potentially spatially correlated with plasma
bombardment, where Ganymede's poles are exposed to the external jovian magnetic
field. Best-estimate abundances suggest a mixture of salts could be present,
although their low abundances, spectral degeneracies and associated
uncertainties mean individual salt species cannot be detected with confidence.
If present, sodium magnesium sulphate and magnesium chlorate appear tentatively
correlated with exogenic plasma bombardment, while magnesium chloride and
sulphate appear tentatively correlated with younger terrain, implying a
possible endogenic origin. MCMC modelling was also performed on Galileo/NIMS
data, showing comparable distributions. The high spatial resolution of SPHERE
allows the precise mapping of small scale (<150km) surface features, which
could be used along with higher spectral resolution observations to jointly
confirm the presence and distribution of potential species.