Sky & Telescope - February 2021 - 11

FROM THE DPS
Observations
of Clavius
Crater show
water trapped
in the lunar
regolith.

THE MOON

Water on the Moon Not
Just in Polar Craters

N ASA / DA NIEL RU T TER

PERMANENTLY DARK CRATER
FLOORS at the Moon's poles appear

to hold plentiful water ice - but
they're not the only places to look. The
Stratospheric Observatory for Infrared
Astronomy (SOFIA) has found evidence
of water molecules sheltering in the
Moon's sunlit Southern Highlands. And
an analysis of lunar topography has
revealed enough micro cold traps - small
areas where ice collects - to affect the
Moon's potential water content. However, more work is needed to understand
what the discoveries' effects will be on
lunar exploration.
The ﬁrst result, reported by Casey
Honniball (now at NASA Goddard) and
colleagues on October 26th in Nature
Astronomy, follows on previous studies that had found spectral ﬁngerprints
of hydrogen on the sunlit part of the
Moon. However, while those features
could come from water, they could also
indicate hydroxyl-containing minerals.
While the Sun's harsh ultraviolet radiation ought to break up water molecules,
it would not split hydroxyl groups.
Honniball's team used the airborne
SOFIA telescope to investigate two
regions for a particular spectral feature
at 6 microns that's unique to water:
one around 60°S near Clavius Crater,
where high hydrogen levels had previously been measured, and a reference

site at lower northern latitudes in Mare
Serenitatis, where hydrogen levels were
low. The researchers not only found
water molecules, they also report that
the Clavius site hosts concentrations
100 to 400 parts per million higher
than the reference site.
But how have the water molecules
survived? Comparing spectra of the
Clavius site with those of meteorites
and Earth-based basalts showing signs
of water interaction, Honniball's team
suggested that water could be trapped in
impact glasses formed after micrometeorites smash onto the lunar surface.
Separately, but in the same issue of
Nature Astronomy, Paul Hayne (University of Colorado, Boulder) and
colleagues analyzed the topography in
5,250 images taken by NASA's Lunar
Reconnaissance Orbiter. Using photos taken when sunlight struck the
surface at steep angles, they identiﬁed
the distribution of shadows and modeled the surrounding landscape. They
report large numbers of permanently
shadowed cold traps as small as one
centimeter across, increasing the total
cold-trap area by 20%, to about 40,000
square kilometers (15,000 square
miles), or 0.15% of the lunar surface.
Both discoveries open several additional questions, and more observations
are coming soon on both fronts: from
SOFIA and from in situ investigations
aboard NASA's polar lander, due to
launch in 2022.
 JEFF HECHT

(Very) Small Chance of
Apophis Impact in 2068
Sunlight's subtle effect may turn near-Earth
asteroid 99942 Apophis toward Earth by
April 2068, but chances for impact remain
small. At the virtual meeting of the Division
for Planetary Sciences (DPS), David Tholen
(University of Hawai'i) announced that the
asteroid's orbit is slowly tightening. Discovered in 2004, the roughly 350-meter-wide
rock was originally thought to be on a
collision course with Earth in 2029 or 2036,
but more precise observations reduced the
chance of impact in those cases to zero.
However, the Yarkovsky effect, the subtle
net force of sunlight on a small rotating
body, tweaks Apophis's orbit over longer
periods of time. Tholen and colleagues
gauged this effect by precisely measuring
the asteroid's position using the 8.3-meter
Subaru telescope on Mauna Kea, Hawai'i.
Their results indicate that the semi-major
axis of the asteroid's orbit is decreasing by
170 meters per year, so its Earth encounter in 2068 may be closer than thought.
The chance of impact remains small,
though, currently about 1 in 150,000. Much
depends on how close Apophis comes to
Earth when it passes by in 2029.
 GOVERT SCHILLING

The Nature of Psyche
Researchers say 16 Psyche, target of
NASA's eponymous Psyche mission,
shows iron on its surface. High radar
reflectivity had already suggested metals
on the main-belt asteroid, and at the virtual
meeting of the Division of Planetary Sciences, Tracy Becker (Southwest Research
Institute) presented ultraviolet Hubble
Space Telescope observations that confirm
surface iron. At least 10% of Psyche's surface would need to be iron to explain the
UV signal, Becker and colleagues report in
the December Planetary Science Journal.
But at the same meeting, Lauri Siltala
(University of Helsinki) presented a new
mass measurement for the asteroid, resulting in a density of just 3.4 grams per cubic
centimeter - much lower than expected
for a supposedly iron-nickel body. Siltala
measured Psyche's mass by its effect on a
number of small main-belt asteroids. However, even if Psyche's composition is more
of a mix, it's still more iron-rich than most
asteroids and could have experienced
ferrovolcanism - Becker might even have
detected its metallic lava flows.
 GOVERT SCHILLING
sk yandtelescope.org * FEBRUARY 2 021

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