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X-641-72-198
PREPRINT
APOLLO
16
GEOCHEMICAL
X-RAY
FLUORESCENCE
EXPERIMENT:
PRELIMINARY
REPORT
I.
Adler
J.
Trombka
J.
Gerard*
P.
Lowman
R.
Schmadebeck
H.
Blodget
E.
Eller
L.
Yin
R.
Lamothe
G.
Osswald**
NASA/Goddard
Space
Flight Center
Greenbelt,
Maryland
20771
P.
Gorenstein
P.
Bjorkholm
H.
Gursky
B.
Harris
American
Science
and
Engineering
June
1972
*National
Academy
of
Sciences-National
Research
Council
Associate
**University
of
Cincinnnati
(Co-op
Student)
APOLLO
16
GEOCHEMICAL
X-RAY FLUORESCENCE
EXPERIMENT:
PRELIMINARY
REPORT
Abstract:
The
lunar
surface
was
mapped
with
respect
to
Mg,
Al
and
Si
as
Al/Si
and
Mg/Si
ratios
along
the
projected
ground
tracks
swept
out
by
the
orbit-
ing
Apollo
16
spacecraft.
The
results
confirm
the
observations
made
during
the
Apollo
15
flight
and
provide
new
data
for
a
number
of
features
not
covered
before.
The
data
are
consistent
with
the
idea
that
the
moon
has
a
widespread
differentiated
crust
(the
highlands).
The
Al/Si
and
Mg/Si
chemical
ratios
cor-
respond
to
that
for
anorthositic
gabbro through
gabbroic
anorthosites
or
feld-
spathic
basalts.
The
X-ray
results
suggest
the
occurrence
of
this
premare
crust
or
material
similar
to
it
as
the
Descartes
landing
site.
An
integrated
geochemical
package
was
carried
in
the
Command
and
Ser-
vice
Module
during
the
Apollo
16
flight
to the
Descartes
highland
area.
This
package
which was
identical
to
the
one
carried
aboard
Apollo
15
included
the
X-ray,
gamma-ray
and
alpha
particle
spectrometers.
These
experiments
were
flown
to extend
our
observations
to
larger
areas
of
the
moon
and
to
allow
us
to
extrapolate
from
the
data
obtained
on
the
surface
to
the
rest
of
the
moon.
Thus,
the
purpose
of
the
orbital
mapping
experiment
and
in
particular
the
X-ray
fluo-
rescence
experiment
on
which
we
are
reporting
here
was
to
tie
together
the
in-
formation
obtained
from
the
analysis
of
the
returned
lunar samples
from
the
various
sites
to
the global
geochemical
picture.
1
There
was
some
overlap
of
orbital
coverage
between
the
two
missions
so
that
reproducibility
of
our
results
between
the
two
missions
could
be
studied
and
thus
tied
together.
The
total
coverage
for
these
two
missions
is
greater
than
20
percent
of
the
total
moon's
surface.
The
Apollo
16
mission
provided
data
for
a
number
of
features
not
previously
covered
for
example
Mare Cognitum,
Mare
Nubium,
Ptolemaeus,
Descartes
area
and
Mendeleev
as
well
as
a
number
of
other
areas.
One
fact
of
considerable
interest
was
the
fact
that
the
X-ray
experiment
was
able
to
obtain
a
large
number
of
data
points
over
the
Descartes
landing
site
(see
Fig.
1
and
Table
1)
while
the
astronauts
were
gathering
sam-
ples
on
the
surface.
Our
results
will
hopefully
show
how
representative
these
were
of
the
Descartes
area.
Unlike
the
high
inclination
orbit
of
Apollo
15,
the
Apollo
16
flight
path
was
nearly
equatorial
(9
deg.
inclination)
so
that
the
projected
areas
covered
were
somewhat
smaller
than during
the
15
flight.
Although
the
original
flight
plan
called
for
a
plane
change
the
exigencies
of
the
mission
did
not
permit
this,
con-
sequently
some
of
the
ground
coverage
was
lost.
The
details
of
the
X-ray
experiment
have
already
been
described
(1,
2,
3).
The
solar
monitor
on
the
Apollo
16
had
an
additional
beryllium
filter
in
front
of
the
detector
window
to
enable
us
to
look
at
the
high
sun
X-ray
fluxes
without
ob-
serving
the
detector
gainshifts
experienced
on
the
15
flight.
The
X-ray
experiment
was
turned
on
initially
at
80
hours
into
the
mission
and
operated
for
about
12
hours
in
elliptical
orbit
(approx.
60
x
10
nautical
2
miles).
It
was
turned
on
again
at
106
hours
into
the
mission
with
the
spacecraft
then
flying
in
a
circular
orbit
of
about
60
nautical
miles
above
the
lunar
surface.
As
in
the
Apollo
15
flight,
the
estimated
field
of
view
for
each
data
point
used
in
this
report
is
about
60
x
80
nautical
miles.
The
data
were reduced
during the
mission
in
the
manner
described
in
the
Apollo
15
report.
Thus
it
was
possible
to
draw
conclusions
about
the
Descartes
site
and
to
report
these
to
the
crew
while
they
were
on
the
surface.
As
indicated
above,
there
was
a
region
of
overlap
between
the
Apollo
15
and
16
tracks.
This
was
mainly between
50
and
100
east
longitude
and
covered
such
areas
as
Mare
Fecunditatis,
Mare
Smythii,
Langrenus,
the
highlands
west
of
Smythii,
etc.
It
is
encouraging
that for
these
areas,
the
Al/Si
and
Mg/Si
chemical
ratios
for
both
flights
agreed
to
better
than
10
percent.
This
agree-
ment
makes
it
very
encouraging
to
draw
comparisons
between the
two
flights.
It
also
demonstrates
that
the
sun's
X-ray
spectral
distribution
which
produces
the
lunar
fluorescent
X-rays,
was
about
the same
on
both
missions.
This
in
fact
has been
confirmed
by
examination
of
the
Solrad
data
available
for
those
periods.
Figure
1
shows
the
variation
of
Al/Si
and
Mg/Si
intensity
ratios
plotted
along
the
projected
Apollo
16
ground
tracks.
These
tracks
have
been
divided
into
areas
based
in
part
on
obvious
geologic
featured
and
in
part
on
intensity
contours.
Because
of
the
relatively
low
inclination
of
the
orbit
and
the
repetitive
ground
tracks
there
is
a
high
density
of
data
points plotted
along
the
ground
3