Crawford, Fagents, Joy: Moon exploration
Abstract
The Moon is a promising sciencetarget, made a priority in recent space
exploration plans. So far, polar landingsites have been preferred, but many
promising scientific objectives lie
elsewhere. Here we summarize thepotential value of one such scientifictarget, northern Oceanus Procellarum,
which includes basalts of a wide rangeof ages. Studying these would allow
refinement of the lunar stratigraphy andchronology, and a better understandingof lunar mantle evolution. We consider
how exploration of such areas might
be achieved in the context of lunar
exploration plans.

In January 2004 the US Administration
announced a redirection of NASA’s humanspace-flight activities away from Earth orbit
and towards the Moon and Mars, with a manned
return to the Moon possibly as early as 2018.
This in turn has stimulated the development ofa Global Space Exploration strategy, as otherspace-faring nations have begun to assess how
they could contribute to, and benefit from, an
expanded programme of lunar exploration (seediscussion by Ball and Crawford 2006).
One of the principal scientific reasons for wanting to resume in situ exploration of the lunar
surface is the record it contains of the early geological evolution of a rocky planet, and of earlysolar system history more generally (e.g. bombardment history, interplanetary dust density andcomposition, solar evolution, and the near-Earthradiation environment; Spudis 1996, Crawford
2004, NRC 2006). Some of these objectives are
astrobiological in nature as they will enhance ourunderstanding of the cosmic conditions underwhich life first arose on Earth (Crawford 2006).
While the current US exploration architecture ispredicated on the establishment of a permanent
lunar base at the south pole (NASA 2006), here
we draw attention to the fact that many of thesescientific objectives will require geological investigations at other locations on the Moon.

Oceanus Procellarum
A specific example, albeit only one of many,of an interesting target for exploration awayfrom the poles is provided by the relatively
young basaltic lava flows in northern Oceanus Procellarum (figure 1). This area consistsof a patchwork of discrete lava flows with estimated individual ages ranging from about 3.5
to 1.2 Gyr (e.g. Wilhelms 1987, Hiesinger et al.
2003; figure 2). This is a far greater range of ages
than any basalt samples collected by the Apollo
and Luna missions, which occupy the narrow
age range 3.8 to 3.1 Gyr, and while the currently
3.18

Full Moon
exploration
Ian Crawford, Sarah Fagents and Katherine Joy make the case for
exploring the basaltic lava flows of Oceanus Procellarum: valuable(non-polar) lunar science facilitated by a return to the Moon.
identified basaltic lunar meteorites have a somewhat wider range of crystallization ages (3.9 to
2.4 Gyr; Korotev 2005, Fernandes et al. 2007)
their provenance, and thus geological context,
is currently unknown. Collecting samples fromone or more of the Procellarum lava flows, andreturning them to Earth for radiometric dating
and geochemical analyses, would immediately
yield two significant scientific benefits:● it would greatly improve the calibration of
the lunar cratering rate for the last three billion
years; and● it would yield information on the evolutionof the lunar mantle to more recent times than ispossible with the Apollo and Luna samples.
In addition, as the younger lava flows aresuperimposed on older ones, a third possibility
suggests itself:● layers of ancient regoliths (“palaeoregoliths”)
sandwiched between lava flows may preserve
records of the early inner solar system environment not otherwise available (e.g. Spudis 1996).
We now briefly consider the scientific importance of all three of these records that are potentially preserved in the Procellarum lavas, before
turning to the implications for the conduct of
future lunar exploration.

The lunar cratering rate
The current calibration of the cratering rate, used
to convert crater areal densities to absolute ages,
is based on the Apollo and Luna sample collection, and is neither as complete, nor as reliable,
as it is often made out to be (see Stöffler et al.
2006 for a review). Figure 3, taken from Stöffler
et al., shows the number of craters per square
km larger than 1 km in diameter as a function of“The lunar cratering-rate curveis used, with assumptions, to datecratered surfaces throughout thesolar system”

surface age as calibrated by samples returned by
the Apollo and Luna missions. It will be immediately apparent that there are no points on thiscurve between 1000 and 3000 million years
ago. Actually, the situation is even worse thanis implied by figure 3, because neither of the craters Copernicus or Tycho, which are indicated ashaving ages of 0.8 and 0.1 Ga, respectively, haveactually been directly measured. The former, inparticular, is actually quite uncertain (e.g. Stöffler et al. 2006), and would itself be an important
target for a future lunar sample return mission.
It is important to realize that this uncertainty inthe calibration of the lunar cratering rate for thelast 3 billion years is not merely a limitation for
constraining the Moon’s later geological history.
Of even greater significance for planetary scienceis the fact that the lunar cratering-rate curve isused, with assumptions, to date cratered surfaces
throughout the solar system from Mercury to
the icy moons of Jupiter and Saturn. Indeed, it
is precisely the essentially uncalibrated part ofthe curve that is used to estimate dates for someof the most important dates in relatively recent(“Amazonian”) martian history (e.g. Hartmann
and Neukum 2001).
Radiometric dating of one or more of therelatively young Oceanus Procellarum basalt
units with well-defined crater densities, such as
those mapped by Hiesinger et al. (2003; figure2), would place additional points in this criticalpart of the curve, with major benefits for thewhole of solar system chronology.

Lunar mantle evolution
Basalts are derived from the partial melting ofplanetary mantles, and as such provide information of the geochemical and thermal evolution ofplanetary interiors not otherwise available. Our
knowledge of lunar mantle evolution is derived
mostly from geochemical and mineralogicalstudies of basalt and picritic glass bead samples returned by the Apollo and Luna missions
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Crawford, Fagents, Joy: Moon exploration
1(a): The near side(a)of the Moon. The
anorthositic highland
crust has a relatively
high albedo, while thebasaltic lava flows
are dark. Oceanus
Procellarum (OP) is thelarge expanse of basalt
near the north-westernlimb. Apollo and Lunasample-return landingsites are marked.
(USGS) (b) In this falsecolour image blue andorange areas within themaria indicate different
chemical compositions.
Note the diversity
within Procellarum.
(NASA-JPL)(e.g. Shearer et al. 2006). However, as noted
above, these samples all date from the period
3.1 to 3.8 Ga, which was the most active periodof lunar volcanism. The youngest mare basalt
samples to have been dated so far are lunar
meteorites, including: LAP 02205 (2.95 Ga;
Nyquist et al. 2005), NWA 773 (2.87 Ga; Borg
et al. 2004) and the recently collected NEA 003(2.38 Ga; Fernandes et al. 2007). However, as
their provenance is (at least as yet) unknown,
they lack the geological context required to usefully constrain models of lunar mantle evolution, and in any case do not extend to ages as
young as are inferred for many of the Procellarum lava flows.
As discussed above, on the basis of their cratercounts many of the Procellarum basalts date tomuch more recent times (Hiesinger et al. 2003;
figure 2). Sampling these basalts would therefore
allow us to build up a picture of lunar mantle
evolution throughout the currently unsampledperiod extending from 2.9 Ga to perhaps as
recently as 1 Ga. Among other things, this would
help answer the question of whether the elevatedconcentrations of radioactive heat producing elements in the Procellarum KREEP Terrain (Jolliff et al. 2000) are responsible for magmatic
activity continuing to relatively recent times inthis region of the Moon (see, for example, thediscussion by Wieczorek and Phillips 2000, andRankenburg et al. 2007).

Palaeoregoliths
Studies of Apollo samples show that solar windparticles are efficiently implanted in the lunar
regolith, which may therefore contain a recordof the composition and evolution of the solar
atmosphere (e.g. Spudis 1996, Wieler et al. 1996,
Levine et al. 2007). Galactic cosmic-ray particles
may similarly be implanted, potentially leaving
a record of high-energy galactic events such as
nearby supernova explosions (e.g. Spudis 1996).
It has also been suggested that samples of the
A&G • June 2007 • Vol. 48 (b)

“Solar wind and galactic cosmicray particles should be fully
preserved within palaeoregoliths
at depths greater than 0.1 to 1 m”
Earth’s early atmosphere may be preserved in
the lunar regolith (Ozima et al. 2005), as well
as samples of its early crust blasted off in large
meteorite impacts (Armstrong et al. 2002).
Clearly, this record provides a potentially very
valuable window into the history of the earlysolar system. However, as the present surficial
regolith has been subject to comminution andoverturning (“gardening”) by meteorite impacts
for the last three to four billion years, the record
it contains will be an average over most of solar
system history, weighted towards relatively
recent times. From the point of view of accessing ancient solar system history, it will be desirable to find palaeoregoliths that were formed
and buried (and thus protected from more recent
geological processes), billions of years ago (figure4). The lava flows of Oceanus Procellarum may
provide us with just such an opportunity.
A regolith will form when a fresh surface is
exposed to the flux of micrometeorites that constantly impinges on the lunar surface. The contemporary regolith formation rate is very low,of the order of 1 mm per million years (Horz etal. 1991). However, regolith is expected to haveformed more quickly in the past, because impact
rates used to be higher and because a thickening regolith shields the underlying bedrock andthus slows its own formation. For example, theregolith at the Apollo 11 landing site is thought
to have accumulated at the rate of 5 mm per
million years when the underlying basalts werefirst emplaced at about 3.6 to 3.8 Ga (Horz etal. 1991). Older lava flows are likely to haveinitially accumulated regolith at an even greater
rate. As solar wind and galactic cosmic-ray particles are implanted within the top few micronsof exposed mineral grains, regoliths as thin as afew millimetres ought to be sufficient to retain arecord of these, although thicker layers will be
required to ensure survival.
A worthwhile geochemical record will only
be preserved within a palaeoregolith layer if
it survives the thermal consequences of burial
by the overlying lava flow. In particular, solar
wind-implanted ions are degassed from regolithgrains if the latter are heated to temperatures ofabout 700 °C (Haskin and Warren 1991). We
have recently developed a numerical solution tothe one-dimensional heat conduction equation
to describe the heat transfer from a hot, initiallymolten lava flow to the underlying particulate
regolith (Crawford et al. 2007, Fagents et al.
in preparation). Figure 5 shows the results fora 1 m thick lava flow initially at a temperatureof 1200 °C, in which case the 700 °C isotherm
never penetrates deeper than about 7 cm into
the underlying regolith. For a 10 m thick flow,
comparable to the thicknesses of individual flows
mapped from orbit (e.g. Hiesinger and Head2006), the 700 °C penetration depth would be
about 80 cm. However, evidence for metre-scalelayering within lava flows exposed in the wall ofHadley Rille at the Apollo 15 landing site (figure6) suggests that the thicker flows mapped fromorbit are themselves likely to be built up fromthinner flows. If so, as far as heating the under­
lying regolith is concerned, the results for the
1 m thick case may be most appropriate for these
also. These results suggest that solar wind andgalactic cosmic-ray particles should be fully preserved within palaeoregoliths at depths greaterthan 0.1 to 1 m, depending on the thickness ofthe overlying lava flow. Palaeoregoliths of thelatter thickness may also be sufficient to preserve
ancient terrestrial meteorites which will therefore also be protected from thermal alterationby the overlying lava.
Given the regolith accumulation rates estimated for early lunar history (Horz et al. 1991),
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Crawford, Fagents, Joy: Moon exploration
2: Estimated ages of lava flows
in Oceanus Procellarum (inbillions of years) based on
crater counts, as mapped by
Hiesinger et al. (2003). (Image
courtesy Dr H Hiesinger, ©AGU)

1.0

1.5

2.0

2.5

3.0

3.5

4.0
age (Gyr)

0.012

0.010

A17
A11 (“old”)

0.008
N(1) (km–2)

3: The lunar crater density (number ofcraters larger than 1 km in diameter
per square km) as a function of
surface age as calibrated by Apollo(A) and Luna (L) samples (Stöffler
et al. 2006). Note the lack of points
between 3 and 1 billion years ago;
dating one or more of the Procellarumbasalt flows will provide data points
in this region of the curve. Note thatthe dates for the craters Copernicus
and Tycho are also quite uncertain, as
neither has been visited by a samplereturn mission – as drawn here theformer lies off the preferred curve by
500 million years! (Image reproducedwith the kind permission of theMineralogical Society of America.
©MSA)

A11 (“young”)

0.006

0.004

A12
A15
L24

L16
0.002

Copernicus
Tycho

0
4

4: Schematic
representation of
the formation of apalaeoregolith layer: (a) a
new lava flow is emplaced
and meteorite impacts
immediately begin todevelop a surficial regolith;(b) solar wind particles,
galactic cosmic-ray(a)particles and “exotic”
material derived from
elsewhere on the Moon
(and perhaps elsewhere)
are implanted; (c) the
regolith layer, with its
embedded historical
record, is buried by a more
recent lava flow, forminga palaeoregolith; (d) theprocess begins again on
the upper surface.
(c)

3.20

3

2
age (Gyr)10

implantation of GCR
and solar wind

micrometeorite
impacts

impact introductionof exotic clasts
regolith
formation
lava flow 1

lava flow 1(b)implantation of GCR
and solar wind

micrometeorite
impacts

lava flow 2

lava flow 2
regolithtrappedimpact introductionof exotic clasts
regolith
formation
palaeoregolithlava flow 1

lava flow 1(d)individual lava flows would have to remain
exposed for between 20 and 200 Myr to accumulate regoliths in the required thickness range. The
ages of individual basalt flows mapped by Hiesinger et al. (2003) indicate that this is likely to havebeen a common occurrence, and that suitablepalaeoregolith deposits likely await discovery in
Oceanus Procellarum (and doubtless elsewhere,
albeit not spanning such a large fraction of lunar
history as in Procellarum). The archival valueof such palaeoregoliths will be enhanced by thefact that both the under- and overlying basalt
layers will lend themselves to radiometric dating,thereby precisely defining the age of the material
and the geological record they contain.

Implications for exploration
Some of the above scientific objectives may be
met by robotic exploration. For example, geochemical analyses of individual lava flows could
be performed by in situ X-ray fluorescence spectroscopy using instruments like the X-ray Spectrometer (XRS) designed for Beagle 2 (Sims etal. 1999). However, much of the scientific interest of this area relies on dating individual lava
flows, and while in situ methods of radiometricdating may be possible in principle, there is no
doubt that the most accurate way of achieving this would be to return carefully selected
samples to a terrestrial laboratory for analysis(Taylor et al. 2006). Sample return would alsoenable much more sensitive mineralogical andgeochemical analyses than would be possible
with robotic instruments in situ. For these reasons, in response to ESA’s Call for Ideas for the
Next Exploration Science and Technology Mission (NEXT), a lunar sample-return mission to
Oceanus Procellarum has recently been proposedby a consortium led by Birkbeck College and
EADS Astrium.
On the other hand, there seems little doubtthat a really detailed geological investigation,
and associated sampling, of a wide range of
discrete lava flows in this region would be best
achieved by human exploration, and the identification and sampling of buried palaeoregolith
deposits would probably be infeasible otherwise(Crawford 2004). However, while these scientificbenefits would be facilitated by renewed human
exploration of the lunar surface, there are several implications for the exploration architecture proposed by NASA (2006). In particular,
the architecture must:● Support the ability to conduct “sortie-class”
expeditions to non-polar localities far from the
proposed base at the south pole.
● Provide adequate provision for sample collection and return capacity (roughly estimated atseveral hundred kg per sortie).
● Make provision for surface mobility – in thecase of the basalt flows discussed above (figure2) a range of order 250 km would permit accessto several different units with a wide range of
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Crawford, Fagents, Joy: Moon exploration

ages. This in turn would imply the provision ofa pressurized rover (figure 7).
● Provide the means to detect and sample palaeo­
regolith deposits. For detection, ground penetrating radar would seem to be a suitable technique(see discussion by Sharpton and Head 1982). For
access, provision of a drilling capability (perhaps
to ~100 m depths) may be required, unless suitable outcrops can be found at the boundaries
between flows.
Note that while, for all the reasons given
above, we consider the scientific case for exploring Oceanus Procellarum to be extremely strong,
there are without doubt other non-polar regionsof the lunar surface that would also benefit fromsuch a capability.

Conclusion
Geological field work in northern Oceanus
Procellarum could address the following threescientific objectives:● Better calibration of the lunar cratering rate
for the last 3 billion years (with clear benefits tothe dating of planetary surfaces throughout thesolar system).
● Better understanding of the geochemical evolution of the lunar mantle to more recent times
than possible using the Apollo samples.
● A search for buried palaeoregolith deposits
throughout the age range 3.5 to 1.2 Gyr, which
likely contain a record of the solar wind, galactic cosmic rays and, more speculatively, samplesof the Earth’s atmosphere and crust, from theseearly times.
Taken together, this would be a very rich scientific harvest, and we recommend that the developing lunar exploration architecture be designed
so as to permit such geological field activities at
distances remote from the proposed lunar baseat the south pole. ●I A Crawford (i.crawford@ucl.ac.uk) and K H Joy(k.joy@ucl.ac.uk) are in the School of Earth Sciences,
Birkbeck College, London, WC1E 7HX. S A Fagents(fagents@higp.hawaii.edu) is at the Hawaii Instituteof Geophysics and Planetology, University of
Hawaii, HI 96822, USA.
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5: Temperature
profiles throughlava and regolith
as a function oftime after flow
emplacement fora 1 m thick lava
flow at 1473 K(1200 °C). The 973 K(700 °C) isotherm
is represented bya vertical dashedline: the maximum
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depth (m)“Sample return would enablemuch more sensitive mineralogicaland geochemical analyses thanwith robotic instruments”

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