Hydration on the lunar surface was first
reported in 2009 by three spacecraft and manifested as a
strong absorption at 3 microns. The hydration at 3 microns is
produced by hydroxyl (OH) attached to metal cations and/or
molecular water (H2O). Measurements of the 3 micron band were
revolutionary but the returned spacecraft data have
limitations in wavelength coverage, spatial resolution, global
coverage, and lunar time of day. To bridge the gap, we use two
Earth based observatories to characterize the true nature of
the 3 micron band and to unambiguously detect molecular water.
Using the SpeX infrared cross-dispersed spectrograph at the
NASA InfraRed Telescope Facility (IRTF) at Maunakea
Observatory we address diurnal variability of the 3 micron
band and with the NASA/DLR Stratospheric Observatory For
Infrared Astronomy (SOFIA) we observe the Moon at 6 microns
where a purely H2O feature is exhibited.
Observations with the IRTF reveal total water (OH + H2O)
abundances ranging from 0 to ~500 ppm H2O. From this new data
set with improved thermal removal, we find diurnal variations
of the 3 micron band along with variations with latitude and
composition. We observe a decrease in abundance with
increasing lunar local time, an asymmetric trend about the
equator that favors the South, and higher abundances in
highland regions. Data from SOFIA of the Clavius crater and
surrounding region reveal abundances of ~100 to 412 ppm H2O
from the 6 micron emission band that we attribute to molecular
water on the Moon. All spectra acquired at the Clavius region
exhibit a 6 micron emission band. We are unaware or any other
lunar material that may exhibit an isolated 6 micron band.
This is the first direct, unambiguous detection of H2O on the
Moon outside the permanent shadows at the lunar poles.
About the Speaker:
Dr. Casey Honniball is a lunatic (lunar
scientist) research fellow in the NASA Postdoctoral Program at
NASA's Goddard Space Flight Center. Her current research
focuses on characterizing water on the Moon and testing field
portable instruments for use by astronauts during an
extravehicular activity on planetary surfaces. She earned her
Ph.D. in Earth and Planetary Science from the University of
Hawaii in 2019. Recently Dr. Honniball and her team announced
the direct detection of water molecules on a sunlit portion of
the Moon using the air-borne telescope SOFIA.