Evidence for Water Ice Near Mercury’s North Pole from MESSENGER Neutron Spectrometer Measurements

Abstract

Measurements by the Neutron Spectrometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft show decreases in the flux of epithermal and fast neutrons from Mercury's north polar region that are consistent with the presence of water ice in permanently shadowed regions. The neutron data indicate that Mercury's radar-bright polar deposits contain, on average, a hydrogen-rich layer more than tens of centimeters thick beneath a surficial layer 10 to 30 cm thick that is less rich in hydrogen. Combined neutron and radar data are best matched if the buried layer consists of nearly pure water ice. The upper layer contains less than 25 weight % water-equivalent hydrogen. The total mass of water at Mercury's poles is inferred to be 2 × 10(16) to 10(18) grams and is consistent with delivery by comets or volatile-rich asteroids.

How thick are Mercury’s polar water ice deposits?

“…The deficit in neutron flux observed over the poles is consistent with a localised hydrogen-rich layer extending down for tens of centimetres beneath a 10-20 cm thick layer that is less rich in hydrogen (Lawrence et al 2013). These observations suggest that nearly pure water ice, and not an alternative volatile such as sulphur (Sprague et al 1995), is responsible for the radar features.…”

Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy

“…MCNPX is widely used for planetary nuclear spectroscopy (e.g. 35,[37][38][39][40]. The collision of highenergy GCR ions with nuclei in the regolith makes secondary particles via spallation and evaporation.…”

The Main Belt Comets and ice in the Solar System

“…Beyond Earth, it has long been recognised in the outer planets and comets, and is now also observed throughout the terrestrial planet region. Evidence for water is found with now almost monotonous regularity on Mars, but more surprisingly, ice has also been identified in permanently shadowed craters of Mercury (Lawrence et al 2013) and the Moon (Colaprete et al 2010). While these deposits could plausibly have been delivered by comet impacts in the geologically recent past, evidence for ice in smaller bodies is more difficult to attribute to an exogenous source.…”