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Advances in Space Research 41 (2008) 1853–1860
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Surface temperatures at the nearside of the Moon as a
record of the radiation budget of Earth’s climate system
Shaopeng Huang
Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA
Received 30 October 2006; received in revised form 11 April 2007; accepted 30 April 2007
Abstract
Understanding the balance between incoming radiation from the Sun and outgoing radiation from Earth is of critical importance in
the study of climate change on Earth. As the only natural satellite of Earth, the Moon is a unique platform for the study of the disk-wide
radiation budget of Earth. There are no complications from atmosphere, hydrosphere, or biosphere on the Moon. The nearside of the
Moon allows for a focus on the solar radiation during its daytime, and on terrestrial radiation during its nighttime. Additionally, lunar
regolith temperature is an amplifier of the terrestrial radiation signal because lunar temperature is proportional to the fourth square root
of radiation as such is much more sensitive to the weak terrestrial radiation in nighttime than the strong solar radiation in daytime.
Indeed, the long-term lunar surface temperature time series obtained inadvertently by the Heat Flow Experiment at the Apollo 15 landing site three decades ago may be the first important observation from deep space of both incoming and outgoing radiation of the terrestrial climate system. A revisit of the lunar surface temperature time series reveals distinct characteristics in lunar surface daytime and
nighttime temperature variations, governed respectively by solar and terrestrial radiation.
Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Climate system; Radiation budget; Lunar surface temperature; Apollo program
1. Introduction
The climate system of Earth is driven by the balance
between incoming energy from the Sun and outgoing
energy from Earth. The incoming energy comes in the form
of solar radiation. The outgoing energy takes two forms:
reflected solar radiation and infrared radiation. Understanding this radiation/energy budget is of fundamental
importance to our ability to predict future climate (IPCC,
2001). However, detecting changes in this energy budget
has been a challenging task with existing technologies.
Ground-based radiation budget observation is obstructed
by the atmosphere. Over the past three decades, several
satellite missions have been launched for the purpose of
studying this radiation budget. Results from those spacecraft missions have improved our understanding of the cli-
E-mail address: [email protected]
mate system of Earth, although the interpretation of the
satellite measurements are difficult and sometimes controversial due to the lack of long term continuous monitoring
from a certain platform (Willson and Mordvinov, 2003;
Allan et al., 2004; Lean, 2005; Pinker et al., 2005; Wielicki
et al., 2005; Bender et al., 2006).
The nearside of the Moon is a unique platform for
studying the radiation budget of the Earth’s climate system. There is little complication from an atmosphere on
the Moon. Solar and terrestrial radiation are separated
by nature during its daytime and nighttime due to the synchronous rotation of the Earth–Moon system. An observatory on the Moon’s nearside has a disk-wide coverage of
Earth. Additionally, lunar regolith temperature is an
amplifier of the terrestrial radiation signal.
Indeed, the first important observation from deep space
of both incoming and outgoing radiation might have been
made inadvertently by the Apollo 15 mission three decades
ago. As a bonus of the Heat Flow Experiment at the
0273-1177/$34.00 Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.asr.2007.04.093
1854
S. Huang / Advances in Space Research 41 (2008) 1853–1860
landing site of the Apollo 15 mission, in situ lunar surface
temperature was recorded over a 41-month period from six
thermometers (Mission Evaluation Team, 1971). The
Apollo time series demonstrated distinct patterns in the
daytime and nighttime lunar surface temperatures, apparently governed by solar and terrestrial radiations,
respectively.
This study is not focused on quantitative detection of
radiation signals from the Apollo lunar surface temperature time series. Rather, the objective of this study is to
show that lunar surface temperature analysis represents a
novel approach to studying the radiation budget of Earth’s
climate system.
2. Energy budget and climate change of Earth
Solar radiation is the ultimate energy source of the climate system of Earth. The intensity of solar radiation is
conventionally measured by solar irradiance, or the rate
of solar energy arriving at the top of Earth’s atmosphere.
Around 30% of the solar energy incident on Earth is
directly reflected back to space by atmosphere, cloud,
oceans, and land surface. The rest of about 70% of the
solar energy coming to Earth is absorbed and then retransmitted to space in the form of infrared radiation by various
climate system components.
Earth’s climate system tends to maintain a balance
between the energy that reaches Earth from the Sun and
the energy going back into space from Earth. The energy
balance that Earth climate system continuously tries to
achieve can be described by the following equation:
I ¼ ðAe IÞ þ 4 e r T 4e
ð1Þ
where I is solar irradiance; Ae is the albedo of Earth; e is the
atmospheric constant (greenhouse effect) (Kutzbach, 1996);
r is the Stefan–Boltzmann constant (5.67 108 W m2
K4); and Te is surface temperature of Earth in Kelvin.
The first term on the right hand side of the equation is
the reflected radiation, and the second term the infrared
radiation. From Eq. (1) we have
1=4
ð1 Ae Þ I
Te ¼
:
ð2Þ
4er
Eq. (2) indicates that the temperature of Earth is controlled
in principle by the solar irradiance, terrestrial albedo, and
the greenhouse effect.
Global warming over the past 150 years is well documented in the instrumental record (Jones et al., 2003) and
various proxies including borehole data (Crowley and
Lowery, 2000; Esper et al., 2002; Huang, 2004; Huang
et al., 2000; Overpeck et al., 2006). Between 1850 and
1999 the global-mean temperature at the surface of Earth
warmed by approximately 0.6 °C (Jones et al., 2003). During the same period, the amount of carbon dioxide measured in Earth’s atmosphere increased by about 30
percent, as a consequence of our ever-increasing use of fos-
sil fuels (IPCC, 2001). Meanwhile, solar activity also rose
generally, along with an 11-year periodical fluctuation.
There is a statistical association between solar radiation
variability and the change in the global mean surface temperature. However, model simulations show that the
observed change in total solar irradiance has a much smaller impact on the observed temperature change than does
the anthropogenic greenhouse effect (Bender et al., 2006;
Hansen et al., 2002; Lean, 2005; Palmer et al., 2004). No
comprehensive mechanism has been proposed to adequately explain the causal link between solar irradiance
and climate change (Crowley and Lowery, 2000; Lean,
2001; Frohlich, 2002; Hansen et al., 2002; Willson and
Mordvinov, 2003; Foukal et al., 2006). The roles of the
changes in the solar irradiance, the albedo of Earth, and
the anthropogenic greenhouse effect are still under heated
debate. The difficulty in isolating their climate impacts lies
in the fact that there is no efficient way of detecting those
changes based on observations on Earth.
A basic step toward a better understanding of solar
effect on the recent terrestrial warming is to quantify the
variations in the solar irradiance and infrared terrestrial
radiation. Accurate measurement of total solar irradiance
(TSI) was not possible until high precision and self-calibrating solar probes were lofted into orbit by spacecraft.
Satellite TSI monitoring started in late1978 by the National
Oceanic and Atmospheric Administration’s Nimbus 7
Earth Radiation Budget experiment (NIMBUS7/ERB).
The Earth Radiation Budget Experiment (ERBE) was the
first NASA mission to monitor bidirectional radiation to
and from Earth. The Experiment flew instruments on three
satellites. On each of the ERBE satellite, there were instruments measuring the energy flux from the Sun as well as
reflected sunlight and radiant heat energy emitted by Earth.
So far the TSI monitoring has been continued by several
other space missions: NASA’s Active Cavity Radiometer
Irradiance Monitor on Solar Maximum Mission
(ACRIM1, 1980–1989), ACRIM2 on the Upper Atmosphere Research Satellite (1991–2001), and ACRIM3 on
the ACRIMSAT satellite (since 2000), NASA’s Earth
Radiation Budget Satellite (ERBS, 1984–1994), the European Space Agency’s Variability of Solar Irradiance and
Gravity Oscillations (SOHO/VIRGO, 1995–1998), and
NASA’s Solar Radiation and Climate Experiment (SORCE, since 2003). Although TSI data from all experiments
are of sufficient precision, their absolute calibration is not
satisfactory. For example, TSI values from the ERB radiometer on Nimbus 7 differ by about 7 Wm2 from the
ERBS experiment over their overlapping period (Frohlich
and Lean, 2004).
Reliable determination of true solar irradiance variability requires the construction of a composite record utilizing
overlapping data for cross calibration of measurements
from different radiometers (Frohlich and Lean, 1998; Willson and Mordvinov, 2003; Foukal et al., 2006). But the
lack of long term continuous monitoring from a permanent
observatory has made the task difficult and sometimes
S. Huang / Advances in Space Research 41 (2008) 1853–1860
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controversial. Different calibration/composition techniques
may lead to different conclusions about the trend in solar
forcing. Willson and Mordvinov (2003) reported a 0.05
percent per decade upward trend in solar activity minima
from their composite TSI series, whereas Frohlich and
Lean (1998) derived a composite TSI series showing almost
identical minima over the last two decades. Nevertheless, a
positive correlation between TSI and Sunspot Number
during solar cycles 21–23 is common in the composite satellite TSI series by both Frohlich and Lean (1998) and Willson and Mordvinov (2003).
3. Are terrestrial radiation signals detectable from lunar
surface temperatures?
Lunar surface temperature on the nearside of the Moon
is determined by solar and terrestrial radiation. The radiation the Moon receives from the Sun is several orders
greater than that from Earth. At first glance, it might seem
impossible to detect any terrestrial signal from lunar surface temperature change. But two important factors need
to be considered:
During its nighttime when the Moon is between the Sun
and Earth, a given site on the nearside of the Moon is
blocked from solar radiation and receives only terrestrial radiation.
Lunar surface temperature change on the nearside is far
more sensitive to radiation during its nighttime when
radiation is weak than during its daytime when radiation is strong.
Fig. 1. A scheme showing the difference between the nearside and farside
of the Moon with respect to solar and terrestrial radiations. Synthesized
from NASA photos.
space, the ‘‘Total Earth (Terrestrial) Irradiance’’ TEI at
the distance of the Moon would be
Locked in synchronous rotation, the Moon always hides
its farside away from Earth and faces Earth only with its
nearside. In contrast to its farside where no terrestrial radiation can reach, the nearside of the Moon constantly
receives radiation from Earth. During its daytime when
the Moon is at the far end of the Sun–Earth–Moon system,
the nearside receives most sunshine with some infrared
radiation from the dark side of Earth. While during its
nighttime when the Moon is in between the Sun and Earth
(Fig. 1), there is no solar radiation but terrestrial radiation
from the bright side of Earth on the nearside. Nearside
nighttime surface temperature is driven by the total terrestrial irradiance (both reflected and infrared), with no direct
interference from solar radiation.
The following highly simplified back-of-the-envelope
calculation illustrates the significance of both solar and terrestrial radiation on lunar surface temperature at a site on
the nearside equator of the Moon.
Denoted E for the total solar radiation intercepted by
Earth, we would have
With a mean TSI of 1366 W/m2 (Foukal et al., 2006), TEI
would be around 0.095 W/m2. The Moon is covered by a
layer of dark soil (regolith) with an albedo of 12% (Salisbur, 1970). For a first order estimate of lunar surface temperature responses to the changes in solar and terrestrial
radiation, I further assume that the surface of the Moon
is a black-body to the radiation it actually receives, i.e.,
the radiation after reflection. For a black body, Stefan–
Boltzmann law states that the intensity of radiative energy
is proportional to the fourth power of its absolute temperature T. For a given site on the nearside of the Moon, we
have
E ¼ TSI p R2 ;
ð1 0:12Þ ðTSI þ TEIÞ ¼ r T 4 :
ð3Þ
where R is the radius of Earth. Assumed that E is emitted
(via albedo and infrared emission) isotropically into deep
2
TEI ¼ E=ð4 p D2 Þ ¼ 0:25 ðR=DÞ TSI:
ð4Þ
The Earth–Moon distance D is about 60 times the Earth
radius R. Therefore,
TSI 14 400TEI:
Therefore,
ð5Þ
ð6Þ
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S. Huang / Advances in Space Research 41 (2008) 1853–1860
1=4
0:88
T¼
ðTSI þ TEIÞ
62:766 ðTSI þ TEIÞ1=4 ;
5:67 108
ð7Þ
where T is lunar surface temperature in units of Kelvin,
and TSI and TEI in W/m2. During the lunar daytime,
TEI from the dark side of Earth is trivial and can be neglected. Whereas during lunar nighttime, TSI is completely
absent. Therefore, we have
1=4
during lunar daytime;
ð8Þ
1=4
during lunar nighttime:
ð9Þ
T 62:766 ðTSIÞ
T 62:766 ðTEIÞ
It is well known that TSI is relatively stable. Lean (2001)
shows that the magnitude of the TSI variation over the past
two centuries would not exceed 4 W/m2. An improved TSI
reconstruction (Wang et al., 2005) further reduces the
change in TSI since the Maunder minimum to about
1 W/m2. In contrast, TEI could be very volatile, subject
to changes in the spatial and temporal characteristics of
aerosols, clouds, greenhouse gases, and other factors.
Although the ranges of the terrestrial albedo and emitted
terrestrial radiation have not been well quantified (Raschke
et al., 2005), various studies (Palle et al., 2004; Anderson
and Cahalan, 2005; Casadio et al., 2005; Charlson et al.,
2005; Pinker et al., 2005; Wielicki et al., 2005; Wild et al.,
2005) suggest that TEI can change substantially and rapidly. For a first order comparison of the possible TSI effect
on the daytime temperature and TEI effect on the nighttime
temperature on the nearside of the Moon, a change of
±2 W/m2 is assumed for TSI and a change of ±0.002 W/
m2 for TEI. The following are the estimates derived from
Eqs. (8) and (9).
A 4 W/m2 TSI change from 1364 to 1368 W/m2 would
lead to a 0.28 K change in lunar surface daytime
temperature.
A 0.004 W/m2 change in TEI from 0.093 to 0.097 W/m2
would lead to an about 0.37 K change in lunar surface
nighttime temperature.
When projected back to Earth, the hypothetic 0.004 W/
m2 TEI variation on the nearside of the Moon corresponds
to an about 15 W/m2 radiation imbalance on Earth.
Changes in the terrestrial radiation of this magnitude have
been reported in ground based and earthshine observations
(e.g., Palle et al., 2004; Wild et al., 2005), although manmade satellite measurements tend to suggest smaller
changes (Wielicki et al., 2005). Based on the new high quality data from the Baseline Surface Radiation Network
(BSRN) of the World Climate Research Program, Wild
et al. (2005) show that annual mean surface solar radiation
at all-sky conditions can change abruptly. A change of up
to 22 W/m2 between the 1997 and 1998 annual means of
observed surface solar radiation at all-sky conditions has
been recorded by the BSRN Kwajalein site in the Marshal
Islands (Fig. 2A in Wild et al., 2005). Although the change
in clear-sky conditions is much smoother (Fig. 2B in Wild
et al., 2005), terrestrial radiation associated with all-sky
conditions is likely what could be observed on the nearside
of the Moon.
This simple calculation shows that the thermal signature
of possible TEI variation could be more significant than the
thermal signature of possible TSI variation. In this calculation, terrestrial radiation (including reflection and infrared
emission) is assumed to be isotropic. In reality, the outgoing energy of Earth is mostly radiated by its sunlit side
which faces the nearside during lunar nighttime. Therefore,
the terrestrial signal is expected to be further amplified in
the nighttime temperature time series from a nearside location such as the Apollo 15 landing site. Terrestrial signal is
detectable from lunar nearside surface temperature
measurements.
4. Apollo 15 lunar surface temperature record
Apollo 15 was the first in a series of missions designed to
explore the Moon over longer periods, greater ranges, and
with more instruments for scientific data acquisition (Mission Evaluation Team, 1971). The Heat Flow Experiment
at the Apollo 15 landing site was an important component
of the Apollo Lunar Surface Experiments Package
(ALSEP). The objective was to measure the rate of heat
flowing from the interior of the Moon, to study its evolution and deep structure (Langseth et al., 1972).
Methodologically, heat flow is determined as the product of subsurface temperature gradient and the thermal
conductivity of the medium in which the temperature gradient is measured. For the lunar Heat Flow Experiment,
two boreholes were drilled by astronauts into the lunar regolith at the Apollo 15 landing site. Each borehole received
a heat flow probe (Langseth et al., 1970; Langseth et al.,
1972) designed for measuring subsurface temperature gradient and thermal conductivity.
Additional subsurface temperature measurements were
expected to be obtained from eight thermocouple junctions, with an absolute accuracy of 0.7 K, mounted on
the cables connecting the heat flow probes to the electronic
box where raw data were initially processed before being
transmitted to Earth. The thermocouple junctions were calibrated at 90, 200, 250, and 350 K (Lauderdale and Eichelman, 1974). Preflight long-term testing showed that the
Apollo Heat Flow Experiment thermal sensors were of
high stability. The drift of the reference platinum bridge
of the thermal couples over a three-year testing period
was 0.0005 K (Lauderdale and Eichelman, 1974).
The depths of the boreholes were designed to be about
3.0 m and all the temperature sensors including the thermocouple junctions were expected to be lowered into the boreholes below the surface. However, the deployments of the
heat flow probes were much shallower than originally
designed (Langseth et al., 1970; Langseth et al., 1972).
The two boreholes at the Apollo 15 site were actually
drilled to a depth of 1.72 m (Mission Evaluation Team,
S. Huang / Advances in Space Research 41 (2008) 1853–1860
1971). Consequentially, six of the eight thermocouple sensors, three associated with each borehole, were deployed
on or above the regolith surface. These thermometers
designed for measuring subsurface temperature ended up
measuring surface temperature instead.
The Apollo 15 ALSEP system including the Heat Flow
Experiment was powered by a radioisotope thermoelectric
generator (Mission Evaluation Team, 1971). The operation
of the experiment was controlled via the ALSEP central
station by commands transmitted from the Johnson Space
Center in Houston. Temperature data collected from the
sensors were converted into a telemetry format and transmitted to Earth. The amplified 13-bit digital outputs of
the thermal sensors were clocked into a shift register along
with a 7-bit identification and binary-measurement code. A
resulting 20-bit number was transferred as two 10-bit
words into the ALSEP data stream (Lauderdale and Eichelman, 1974). Temperatures of all sensors were sampled in
a time interval varying from several minutes to several
hours over the observation period. Six long-term lunar surface temperature time series, which span 43 lunations or
lunar days, were therefore archived (Fig. 2).
Both the diurnal and annual variations are well documented in the Apollo 15 lunar surface temperature time
series, although the measured temperatures from the six
sensors are different from each other, likely due to the specific settings such as sensor orientation, detail configuration
of the regolith surface, and the position relative to the other
ALSEP instruments. All those factors can affect the radiation imposed on a specific sensor, and therefore, the temperature of the sensor.
The extended daytime high and nighttime low temperature diagram (Fig. 3) shows that the seasonal variability is
1857
much greater and clearer in lunar daytime than in lunar
nighttime. Interestingly, the seasonality of the Apollo 15
site is equivalent to that of the southern hemisphere of
Earth, with November to January as its summer and
May to July as its winter. While the summer maximum
temperature increased slightly, the winter maximum temperature declined substantially. More importantly, there
is a warming trend in the nighttime temperature over the
entire observation period. The fluctuation of the nighttime
temperature did not follow the same pattern of the daytime
temperature. The peaks and troughs are offset in the daytime and nighttime temperatures.
5. Discussion
The back-of-the-envelope calculation in Section 3 shows
that lunar surface temperature can potentially record
changes in the radiation budget of Earth’s climate system.
Section 4 demonstrates that the lunar surface temperature
time series from the Apollo 15 mission embrace distinct
daytime and nighttime characteristics apparently controlled by solar and terrestrial radiations, respectively.
However, the long-term lunar surface temperature monitoring was not part of the original intention of the ALSEP.
Direct exposure to intensive solar radiation might lead to a
decay of the temperature sensors. Any lunar dust deposition on the sensors could affect the surface temperature
measurements. The signals of the radiation budget of the
Earth’s climate system could be submerged by various
noises.
For the purpose of data verification, daytime surface
temperatures are estimated for the Apollo 15 landing site
based on the solar distance and elevation calculated from
Fig. 2. Lunar surface temperature time series from the landing site of the Apollo 15 mission. The series are part of the historical data archive PSPG-00093
of the US National Space Science Data Center. The color-coding of the temperature series used in this diagram is also used in Figs. 3 and 4. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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S. Huang / Advances in Space Research 41 (2008) 1853–1860
Fig. 3. Variations of the daytime high and nighttime low temperatures at the Apollo 15 landing site. This is a diagram zooming in the top and bottom
portions of the lunar surface temperature time series shown in Fig. 2, to allow for a closer examination of the daytime high and nighttime low temperatures
over the observation period. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the Jet Propulsion Lab (JPL) Horizons Ephemeris System
(Standish, 2002), which takes into accounts orbit eccentricities and rotation axis inclinations of the Moon and Earth
as well as other astrometry factors.
The total incident solar energy at the Apollo 15 site Ia15
is determined by its distance from the Sun d in units AU
and the elevation of the Sun h
I a15 ¼
TSI
sin h:
d2
ð10Þ
In parallel to Eq. (7), daytime surface temperature at the
Apollo 15 site Ta15 would be
1=4
ð1 Am Þ
T a15 ¼
TSI sin h
:
ð11Þ
r d2
A theoretical lunar surface temperature time series is estimated by assuming a solar constant TSI of 1366 W/m2
and a constant regolith albedo Am of 12%.
Despite certain discrepancies in details including a small
time shift, the estimated lunar surface temperatures from
this highly simplified radiation model are consistent in general with the daytime surface temperature time series from
the Apollo 15 landing site (Fig. 4). Both the increasing
trend in the summer daytime peak temperature and the
decreasing trend in the winter daytime trough temperature
documented in the Apollo time series have been well reproduced by the simple radiation model prediction derived
from the JPL Ephemeris System. The reproduction of the
trends of the daytime surface temperatures proves that
Fig. 4. Comparison of the calculated maximum daytime temperature
(dashed) to that of a sample Apollo 15 lunar surface temperature time
series shown in Fig. 2. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
the Apollo 15 time series are of sufficient quality and retain
valid radiation information.
The reproducible daytime temperature trends of the
Apollo 15 time series provide an independent confirmation
of a stable radiation incoming from the Sun. In contrast to
the lunar daytime maximum temperature variation, there is
an upward trend in the nighttime minimum temperature.
More intriguingly, the fluctuation in the lunar nighttime
temperature does not have the same rhythm as the fluctuation
S. Huang / Advances in Space Research 41 (2008) 1853–1860
in the daytime temperature. To a great extent, the nighttime temperature variation is independent of daytime temperature. This is evidence that daytime and nighttime
temperatures at the lunar surface are governed by two different radiation processes, namely the solar radiation and
terrestrial radiation.
The observed lunar nighttime warming from mid 1972
to the end of 1975 appears to be consistent with the global
dimming of Earth prior to the late 1980s (Stanhill and
Cohen, 2001; Pinker et al., 2005; Wild et al., 2005). Global
dimming is resulted from a general decrease of sunlight
over land surfaces. Widespread ground-based radiation
records show that the solar radiation reaching the Earth’s
surface decreased by about 5% between 1960 and 1990
(Stanhill and Cohen, 2001). Given a constant total solar
irradiance, the less sunlight reaches the Earth’s ground surface, the more solar radiation is reflected to the deep space,
and the higher lunar nighttime temperature would be.
To what an extent the observed nighttime lunar warming in the early 1970s can be explained by the reported global dimming on Earth remains yet to be investigated. There
could be other contributing factors to the lunar warming as
well. Although quantitative isolation of signals of solar
radiation and terrestrial radiation from the Apollo 15 temperature time series is not the focus of this study, it is a goal
worthy of pursuing. Such an effort could at least extend
space-based observation of the Earth’s energy budget to
1972. As the first records from a different platform with a
focus on disk-wide sunlit-side terrestrial radiation, the
Apollo temperature time series may bear importance clues
to deepening our understanding of the operation of the terrestrial climate system.
6. Summary and conclusion
Understanding the balance between incoming energy
from the Sun and outgoing energy from Earth is of fundamental importance to our ability to predict future climate. Ground-based observation of this radiation
budget is obstructed by the atmosphere. Over the past
three decades, several satellite missions have been
launched for obtaining measurements from space. Satellite data have improved our understanding of the climate
system of Earth. However, the interpretation of the satellite measurements is difficult and sometimes controversial
due to the lack of long term continuous monitoring from
a stable platform.
As the sole natural satellite of Earth, the Moon is an
enduring platform without complications from atmosphere, hydrosphere, or biosphere. Because the Moon
keeps the same hemisphere towards Earth, solar radiation
and terrestrial radiation are naturally separated during
daytime and nighttime on the nearside of the Moon. The
distance from Earth and the synchronic rotation make it
possible for an observatory on the nearside of the Moon
to have a disk-wide observation angle and a focus on the
sunlit side radiation of Earth. Therefore, lunar records of
1859
the radiation budget of Earth are complementary to
ground-based and man-made satellite records.
The accidentally obtained Apollo 15 lunar surface temperature time series reveal distinct characteristics in the
lunar daytime and nighttime surface temperatures. Superimposed on the diurnal and seasonal variations was an
inter-annual daytime cooling trend over the observation
period. In contrast, there was an upward trend in the lunar
nighttime temperature which supposedly is controlled by
the radiation from Earth. Although the data analysis of
this study is preliminary, both the simple radiation model
calculation and the long-term lunar surface temperature
time series from the Apollo 15 landing site show that signals of the radiation budget of the Earth’s climate system
is detectable on the Moon.
Global climate change on Earth is among the most profound scientific, social, economical, and political challenges
of our time. This study shows that the Moon is a plausible
platform for monitoring the terrestrial climate system, and
lunar surface temperature data comprise information on
the radiation budget of Earth. The author calls for international effort to develop a network of temperature and radiation observatories on the Moon for the study of terrestrial
climate change.
Acknowledgements
This study is supported in part by the US National Science Foundation Grant ATM-0317572 and Michigan
Space Grant Consortium Research Seed Grant. Lunar surface temperature data is provided by the US National
Space Science Data Center. The author thank Henry Pollack, Yosio Nakamura, Meyle Standish, Po-Yu Shen, Robert Cahalan, Mark Wieczorek, Chongyin Li, and Jianping
Li for their discussions and/or computational assistance;
and three anonymous reviewers for their constructive
comments.
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