Update from JAXA
KAGUYA (SELENE) Observations with Laser Altimeter (LALT)
and Lunar Radar Sounder (LRS) Sounder Mode
January 10, 2008 (JST)
Japan Aerospace Exploration Agency (JAXA)
The Japan Aerospace Exploration Agency (JAXA) carried out observations using two onboard sensors of the lunar explorer KAGUYA -- the Laser Altimeter
(LALT) and sounder mode of the
Lunar Radar Sounder (LRS).
The LRS system transmits RF pulses with frequency of 5 MHz which is able to penetrate deep into the Moon's subsurface region. When the radio waves
meet with the discontinuity of subsurface material, it makes a subsurface echo signal. Observation of subsurface signal makes it possible to see
subsurface structure of the Moon.
Initial results on the lunar subsurface structure were obtained using the LRS sounder mode observation data collected on November 20 and 21, 2007. The
received radar echo was as expected through computer simulation. The extraction of radar echoes reflected by subsurface structures was demonstrated to
be satisfactory.
In addition to the conventional sounding technique that tests echo trace in the plots like Figures 1 and 2, a new method that uses not only the
amplitudes of the echoes but also their phases was proved feasible. This method utilizes the synthetic aperture radar (SAR) technique with foci of
variable depths and ensures robust detection of radar echoes from subsurface structures.
1 - Simulated radar echoes
Simulated radar echoes to be observed by the KAGUYA (left) as it travels over the model surface structure (right) (Kobayashi et al. 2002). In the
diagram, vertical and horizontal axes correspond to the apparent depth and positional offset, respectively, and coded in color is the amplitude of
radar echo. The apparent depth is the delay timing divided by the speed of light. In the 2-D diagram, echoes from large craters appear as hyperbolic
coherent features, while surface clatters from numerous small craters appear as random noise near the surface. The targeted radar echoes from
subsurface structures appear as continuous features with nearly constant depths. Subsurface echoes can be detected in this manner.
2 - Observed radar echoes taken with the LRS near the Poisson crater (30.4S, 10.6E) on November 20, 2007, in a 20 second period from
18:22:50 to 18:23:10.
( Full size image )
These agree with the simulated echoes in the previous figure in the following aspects:
- Trace and amplitude variation of the surface crater echoes in the 2-D diagram.
- Behavior of the surface clatters (unwanted radar echoes caused by adjacent craters that interfere with the detection of subsurface echoes,
particularly in highland regions).
These ensure validity of the new data analysis method established with the computer simulation results. Here both crater echoes and surface clatters
are classified as interference due to surface reflection, and we refer to the coherent echoes as "crater echoes" and the incoherent ones as
"surface clatters".
3 - The synthetic aperture radar (SAR) image and strata identification of the northeastern part of the Mare Imbrium near the Kirch
crater (39.2N, 5.6W, 11 km dia.) retrieved from the LRS sounder mode observation data on November 21, 2007, from 22:13 to 22:15.
(Full size image)
Shown in the upper left panel are results of the SAR detection of reflective subsurface structures down to 500 m by focusing on relatively shallow
regions. The depth refers to the 1737.4 km sphere centered at the center of the mass of the Moon. Red lines in the lower left panel show evident
reflecting interfaces in the section, which is 180 km long. True dips of the interfaces are very small, as those sections are vertically exaggerated
by a factor of ~30. This means that the mare is underlain by horizontally lying strata as thick as 500 m, probably composed of lava, volcanic ashe and
ejecta blankets. This experiment verified the performance of the LRS sounder mode observation, and at the same time provided direct evidence for the
existence of depositional units underneath the Mare Imbrium.
Source:
Japan Aerospace Exploration Agency
(quote tags)
[edit on 4-7-2008 by Jbird]
