High thermal inertia can also indicate the presence of exposed water ice. Theoretical models of water-equivalent hydrogen (WEH) from epithermal neutrons detected by the Mars Odyssey Neutron Spectrometer (MONS) instrument suggest that the regolith just below the surface on Alba's northern flank may contain 7.6% WEH by mass. This concentration could indicate water present as remnant ice or in hydrated minerals. Alba Mons is one of several areas on the planet that may contain thick deposits of near-surface ice preserved from an earlier epoch (1 to 10 million years ago), when Mars' axial tilt (obliquity) was higher and mountain glaciers existed at mid-latitudes and tropics. Water ice is unstable at these locations under present conditions and will tend to sublimate into the atmosphere. Theoretical calculations indicate that remnant ice can be preserved below depths of 1 m if it is blanketed by a high-albedo and low-thermal-inertia material, such as dust.