Review of Radio Observations of the Moon
O. V. Tvorun
Vinnytsa Pedagogical University, Ostroz'ky street, 32, 21000, Vinnitsa, Ukraine
[email protected]
Review of radio observations of the Moon during meteor showers and spacecraft impacts is given. The
upper limit of the intensity of lunar radio emission of seismic origin at several wavelengths is discussed.
Introduction
There are several sources of lunar radio emission: thermal radiation, reected solar radiation, reected
radiation of the Galaxy and radiation of the seismic origin. The main component of the lunar radio emission
is the thermal radiation. The lunar thermal radiation has been rather well investigated and it allowed us to
estimate the density of the porous lunar regolith, as well as the temperature mode of the surface layers. The
prediction of earthquakes is an important problem, which is still far from a nal solution. One of the possible
methods for recognition of precursors of earthquakes is the investigation of the electromagnetic radiation of
seismic origin.
Massive impactors may signicantly increase seismic activity on the Moon. Duration of such moonquakes
may reach several hours. For this reason radio observations of the Moon after Lunar Prospector and SMART1 impacts were provided.
The main source of the lunar radio emission at centimeter wavelengths is the thermal emission with
brightness temperature equal to 250 K. The intensity of the thermal radio emission changes slowly with a
period of one month. However after strong impacts of meteorites or spacecrafts with the Moon's surface, the
quick changes of the intensity of the lunar radio emission in various frequency ranges were detected. The
nature of this emission is connected with radiation of impact-produced tracks in the silicates (see e.g. [2]).
Radio emission of impact-produced tracks at 2 and 22 GHz is already detected during impact experiments
(see e.g. [4]).
Our knowledge about the properties of impactor and target is maximal for impact experiments and
minimal for sporadic meteoroids, while we know something about collisions of spacecrafts and meteoroid
showers with the Moon (see Table 1). In this paper we present the review of the searches for lunar radio
emission caused by the meteoroid bombardment and spacecraft impacts on the Moon.
Observations
The direct impact of the American spacecraft Lunar Prospector with the Moon in the south polar region
occurred in July 31, 1999. This collision was accompanied by seismic eects comparable to a big moonquake.
Radio seismic radiation during moonquake was detected with 64-m radio telescopes at Kaliazin at 13 and
21 cm and at Medvezhi Ozera at 90 cm wavelengthes (see e.g. [3]). The south polar region, seismically active
and seismically passive regions of the Moon were observed.
The observations of the Moon at the frequency of 8.2 GHz during Leonid meteor shower in 1999 were
carried out with 32 m radio telescope of the Kashima Space Research Center, Japan (36 N, 141 E) on
November 15, 16, and 18. The bandwidth of the receiver was 800 MHz, the output time constant was
0.021 ms. Averaging of the data was performed with a new output time constant equal to 1 s. The calibration
sources and sky were not observed in Kashima. That is why the additional observations of the Moon at the
frequency of 8.415 GHz were carried out on May 7, 2003 at RT-22 in Crimea (44 N, 35 E). The bandwidth
of the receiver is 2 MHz, the output time constant is 1 s.
During April 17−21, 2001 study of lunar radio emission was continued with RT-22, 22 m radio telescope of
the Pushchino Radioastronomy Observatory, Moscow region, Russia (see e.g. [1]). The predicted maximum
of Lyrid stream on the Earth was at 4 UT on April 22, 2001 with zenith hour rate (ZHR) of about 15. So the
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O. V. Tvorun
Table 1: Our knowledge about parameters of impactors. + means quantative estimate, ± qualitative estimate,
and + no information is available about this value.
The parameters
of impact
place
time
speed
mass
chemical composition
angle
Experiment
LP, SMART-1
+
+
+
+
+
+
±
+
+
+
±
±
Meteoroids from main
meteor showers
−
±
+
−
+
±
Sporadic meteoroids
−
−
±
−
−
−
days of observations coincided with the initial stage of the Lyrid meteoroid stream for study of variability
of lunar radio ux at quiet conditions. The observations were carried out at 13.5 mm, 6 cm and 18 cm
wavelengthes. Signicant quasi-periodic variations of the lunar brightness radio temperature with amplitudes
from 1 to 10 K, similar to the observed in the previous experiments, had been registered at all the three
wavelengths. The most contrasting results with valley-to-peak amplitude variations up to 10 K were obtained
observing at 13.5 mm wavelength with the beam-switching technique.
During November 8 − 9, 2003 the simultaneous observations of the Moon were carried out in Pushchino
(55 N, 38 E) and in Simeiz (44 N, 35 E). The frequency and the bandwidth of the receiver in Simeiz are
1.666 GHz and 2 MHz, respectively. The frequency and the bandwidth of the receiver in Pushchino are
22.283 GHz and 6 MHz, respectively. The diameter of both radio telescopes is 22 m; the output time
constant is 1 s. The antenna feed was left circular polarized in both cases. The antenna was guided to the
center of the lunar disc.
European spacecraft SMART-1 collided with the Moon at grazing angle of about one degree at 5:42:22
UT on September 3, 2006. The mass and the velocity of the spacecraft were equal to 285 kg and 2 km/s at
the time of impact. The duration of uninterrupted observations of impact region (34 S, 43 W) of the Moon
was equal to 30 minutes, and then seismic passive region (30 S, 30 E) was observed during 20 minutes.
Results
The mean amplitude of sporadic uctuations of lunar radio ux at the frequency of 8.2 GHz was about
ve relative units on November 16 and 18, 1999. The amplitude of uctuations of lunar radio ux did not
increase at the time of predicted maximum of Leonid meteor shower on the Moon.
Observations of the Moon at 6.2 cm during 2001 Leonid meteor shower do not give support to the lunar
origin of detected variations (see e.g. [7]). Flashes of radio emission at 3.6 cm during optical ashes caused
by 1999 Leonids impacts onto the Moon (the impact speed was 72 km/s) also were not detected. Some
possible ashes were detected, but the nature of these ashes is not connected to the Moon.
According to simultaneous observations of the full lunar eclipse at the wavelengths of 18 cm and 1.35 cm
in Simeiz and Pushchino on November 8 − 9, 2003 the correlation between uctuations of lunar radio ux
at both wavelengths is absent (see e.g. [8]).
Radio observations of the Moon at 3.6 and 13 cm were performed at 22 m radio telescope in Simeiz
(Crimea, Ukraine) several hours after the SMART-1 impact with the Moon on September 3, 2006 (see e.g.
[6]). The nature of uctuations of detected lunar signal is not connected with the Moon.
Raw observational data were calibrated, and then standard deviation of variations of received signal from
the regression curve was determined. Amplitudes of variations of received signal from calibration source and
the sky were equal to 0.5 K at 2 MHz bandwidth and 0.2 K at 500 MHz bandwidth as on September 2 as
on September 3. Amplitudes of variations of received signal from calibration source, the sky, the lunar radio
ux at impact region and seismic passive region were equal to about 2 K and 0.7 K, respectively at both
bandwidths.
Correlation coecient between variations of lunar radio ux at 3.6 cm with 2 and 500 MHz bandwidth
after subtraction of polynomial t curve (caused mainly by receiver instability) is high, about 0.9. This fact
may be explained by lunar origin, Earth atmosphere noises, receiver instability, and by telescope vibrations.
However Earth atmosphere noises cannot explain higher values of amplitudes of variations of lunar radio
ux in comparison with that from the sky. Receiver instability hypothesis cannot explain low correlation
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O. V. Tvorun
coecient (about 0.1) between variations of received signal from the sky at 3.6 cm with 2 and 500 MHz
bandwidths.
Additional observations were performed during November 29 − 30, 2006 at the same moon phase for
checking possible dependence of received variations of lunar radio ux from the moon phase and the distance
of the center of the lunar disk caused by telescope vibrations. These observations show increasing of amplitude
of uctuations near the terminator and edges of the lunar disk where temperature gradients are maximal.
Thus, telescope vibrations are suitable for explanation of quasi-periodic uctuations of received lunar signal.
Discussion and conclusions
Quasi-periodicity of the lunar radio ux is stronger at SMART-1 impact region in comparison with the
center of the lunar disk. It can be explained by telescope vibrations which inuence on variations of the
received lunar ux stronger near the limb of the Moon (SMART-1 impact region) in comparison with the
vicinity of the center of the lunar disk.
Observations of the Moon at 6.2 cm during 2001 Leonid meteor shower did not give a support to the lunar
origin of detected variations because correlation between these variations at two isolated radio telescopes is
absent (see e.g. [8]). Flashes of radio emission at 3.6 cm during optical ashes caused by 1999 Leonids
impacts onto the Moon with impact speed of 72 km/s were not detected also (see e.g. [5]).
Study of dierences between values of the nearest pixels was performed for search for impact-produced
radio ashes. Based on 3σ criterion corresponding to 1 s ash intensity of about 5 Jy any ashes at 3.6 cm
and at both 2 and 500 MHz bandwidth were not detected during 10 hours of observations of the Moon.
For more accurate search for lunar impact radio ashes the receivers with quite dierent wavelengths and
with time resolution of about 0.01 s at the same telescope should be used.
Some possible ashes were detected, but the duration of such ashes was about 3 s (the expected duration
of meteoroid-induced ashes is less than 1 s) and such ashes occur only at broad bandwidth. For these
reasons the nature of these ashes is not connected with the Moon.
Small lunar brightness temperature variations at 3.6 cm during September 2 − 3, 2006 with amplitude of
about 10 K may be explained by changes of conditions of the solar illumination of the Moon.
Radio observations of the Moon during impact of Japanese KAGUYA spacecraft on June 10, 2009 (see
[9]) are highly desirable for additional study of radio emission during impact events on atmosphereless bodies
of the Solar system.
The further study of the radio emission of the Moon is necessary for better understanding of the nature
of the detected variations of lunar radio ux.
Acknowledgment
I would like to thank Alexey Berezhnoy for helpful comments and suggestions.
References
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[4] Ohnishi H., Chiba S., Soma E., et al., J. of Applied Physics, V. 101, p. 124901 (8 p.) (2007)
[5] Osaki H., Okubo H., Koyama Y., J. of the Communications Research Laboratory, V. 48, pp. 159-162 (2001)
[6] Volvach A. E., Berezhnoy A. A., Foing B. H., et al., 38th Lunar and Planetary Conference, Abstract No. 1015, League City,
Texas, USA (2007)
[7] Volvach A. E., Berezhnoy A. A., Khavroshkin O. B., et al., Kinematika i Fizika Nebesnykh Tel, V. 21, pp. 60-65 (2005).
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(2005)
[9] http://www.jaxa.jp/projects/sat/selene
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