New Views of the Moon (geophysics)
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Enter your email address below and we will send you your username. Gamma-ray spectroscopy can measure the elemental concentrations of many major and trace elements, such as iron, titanium, silicon, calcium, aluminum, thorium, potassium, and uranium [ 58 ]. Neutron spectroscopy is complementary to gamma-ray spectroscopy and is highly sensitive to the concentrations hydrogen and neutron absorbing elements. The most important neutron absorbing elements in lunar soils are the major elements iron and titanium and the trace elements gadolinium and samarium.
Gadolinium and samarium are important for understanding many details of lunar differentiation and evolution, and titanium is an important indicator of the presence of solar wind implanted gases, such as 3 He. The vast majority of these meteoroids is small, and burn up in the atmosphere, but a smaller number of larger objects pass through the atmosphere to make an impact crater on the surface. Such events can be catastrophic to life, either locally or globally. Two primary objectives of investigating impact hazards are to determine the size-frequency distribution of these objects, and to determine the probability that they will collide with the Earth.
The Moon is illuminated only by Earthshine. Image from [ 51 ]. These observations constrain the impact rates on the Earth and Moon, and predict significant temporal variations in their impact flux, either as a time-of-day phenomenon for the Earth or solar phase for the Moon [ 35 , 36 , 39 ]. Though predicted by theory, the magnitude of these temporal and spatial variations is difficult to measure in practice.
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Satellites in orbit about the Earth have recorded the frequency of light bursts that occur during the atmospheric entry of meteoroids that are less than 10 meters in diameter, and these can be converted into masses using the luminous efficiency parameter, which empirically relates the light emitted in the visible to the kinetic energy of the meteoroid [ 3 , 52 , 76 ]. Video observations have confirmed that these lunar meteoroid impacts are observable from Earth via the light they emit during impact [ 51 ], and impact-monitoring programs [ 66 ] have since detected more than impact flashes.
The radiant distribution at Earth calculated from near-Earth object orbital-element models is in broad agreement with the radar-inferred distribution from meteors [ 39 ]. Presuming that the orbits of the smaller meteoroids mimic those of the well characterized km-sized objects, it is possible to estimate the impact flux on the Moon at all sizes. These estimates, however, appear to be three times smaller than those derived from lunar impact flash data [ 51 ]. Seismic modeling of meteoroid impacts with the Moon and the Apollo seismic data [ 39 ], in contrast, appear to favor the high impact rates derived from impact flashes.
Finally, estimated impact rates can be compared to the size-frequency distribution of small lunar craters using impact crater scaling laws. The meteoroid flux estimated from the detonation of bolides in the terrestrial atmosphere [ 9 ] is predicted to produce craters on the Moon that are 10— meters in diameter, and the measured number of craters favors the lower impact rate derived from orbital-element models instead of the higher estimates from the impact flash data.
Secondary craters are formed from the material ejected from a primary crater and are a significant source of controversy in the crater chronology literature.
Observed and estimated lunar impact cratering rates as a function of crater diameter. Image redrawn after [ 26 ]. What is the present impact flux of bolides in the size range of centimeters to meters? What is the size-frequency distribution of small objects colliding with the Earth and Moon? And what is their velocity distribution?
Combined with the proximity of L2 to the lunar surface, these observations would allow for the detection of much smaller impacts than could be accomplished from a comparable terrestrial observatory. From these measurements, a more precise picture of impact processes in the Earth—Moon neighborhood will be obtained. Considering the expected detections of several hundreds of events per year, the main goals of the impact monitoring program are to: 1 determine seismic source locations and times for the seismology experiment, 2 obtain the average impact rate on the Moon and map any spatial and temporal variations that might exist, and 3 determine the optical-magnitude impact-frequency relationship.
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Combining the measured optical magnitudes with simultaneous data from the seismology experiment on the surface offers a unique opportunity to explore the partitioning of impactor kinetic energy into seismic and thermal energy at scales that are impossible to reach in laboratory experiments [ 47 ]. Monitoring the temporal variations of the impact flux over the lunar month would provide constraints on the meteoroid sources, since these variations are directly related to the radiant distribution of impactors entering the Earth—Moon system.
Comparison with estimates based on orbital-element models and encounter probability calculations would improve our understanding of the fate of meteoroids once they separate and evolve from their parent bodies. Multispectral observations are essential in order to monitor the thermal evolution of the impact site and ejecta cloud. Ideally, at least two bands would be utilized, one in the visible from 0. In combination with simultaneously acquired seismic data, repeated monitoring of the thermal evolution of impact flashes will help refine the relationships between emitted light, impactor mass, and impact velocity.
This work will stimulate progress in impact cratering mechanics by offering natural experiments at scales far larger than those achievable in the laboratory.
Farside Explorer would monitor lunar impact flashes on the farside of the Moon, and a complete view of the lunar impact rate would be obtained by using ground-based observations of the nearside from Earth. These observations are currently being conducted with a small number of mid-sized telescopes, and an international network for the detection of impact flashes ILIAD is under construction with an expected completion date in In addition to being a stand-alone mission that would address first-rate scientific questions, Farside Explorer would also complement past and future missions by providing much needed ground truth measurements from the surface of the Moon.
The United States planetary science decadal survey has identified a South Pole—Aitken sample return mission as one of its top priorities in the next decade [ 13 ], and the in situ surface and subsurface measurements made in this basin by one of the Farside Explorer landers would provide critical measurements for constraining the origin and evolution of these samples. Farside Explorer would also provide valuable radio-astronomy measurements that would extend the frequency range of those being made from terrestrial low-frequency arrays.
These terrestrial observations are currently being conducted by several groups, such as through the International Lunar Impact Astronomical Detection network ILIAD , and by combining the orbital and terrestrial datasets, a complete view of the lunar impact rate would be obtained. A NASA-led geophysical network would provide no more than 4 geophysical stations, with most, if not all, being situated on the lunar nearside hemisphere.
In combination with an ESA-led mission to the farside, international cooperation would allow for a unique opportunity by offering a much more robust geophysical network than could be afforded by any single agency. Other countries have expressed interest in placing landers on the Moon, and if these contained geophysical instrumentation such as is planned for the Japanese SELENE-2 mission concept , these stations would contribute to an already operating geophysical network [ 75 ]. The Farside Explorer space segment includes two spacecraft to land on the farside of the Moon, an instrumented relay satellite, and the launcher either a Soyuz—Fregat or Ariane 5 shared commercial launch.
The proposed mission concept is innovative by using a halo orbit about the Earth—Moon L2 Lagrange point LL2 to provide a communications relay to the farside landers while simultaneously enabling the impact flash monitoring program. Soyuz—Fregat left and Ariane 5 right spacecraft accommodations. Image courtesy of Astrium Satellites. Each spacecraft includes the propellant for the rest of the mission transfer to EL1, LL2 and landing for the probes; transfer to EL1 and LL2 for the relay satellite.
The upper lander and relay satellite are bound together during the Earth—Moon cruise as a composite baseline. After LL2 halo orbit injection, the satellite and lander are separated. After insertion in a geosynchronous transfer orbit, the two spacecraft are injected into a day WSB trajectory along the Earth—Sun L1 gravity manifold.
This trajectory returns to the vicinity of the Moon, allowing insertion into the Earth—Moon L2 halo orbit. Modified after [ 53 ]. Mission syntheses for Soyuz—Fregat and Ariane V launches. Open image in new window. From GTO, the lunar probes are injected one after another into a day ballistic trajectory along the EL1 gravity manifold through two or three perigee burns. The proposed trajectory uses the instability of the manifold next to EL1 to return to the vicinity of the Moon. The LL2 halo orbit will be the quasi-final orbit for the relay satellite, but only a waiting orbit for the two landers.
The spacecraft LL2 waiting orbit allows the access to any location on the lunar surface with a single large maneuver the braking burn.
The landing strategy is to follow the unstable WSB down to the surface and to set to zero the relative speed with respect to the landing site. Ground control would monitor constantly the spacecraft landing through the relay satellite. As a result of the waiting orbit strategy, the control of only one probe at a time is required for the critical landing phase. Operational halo orbit. This resonant pseudo-orbit has a day period and has altitudes that vary from 30, to 60, km above the lunar surface. Axes are scaled by the mean Earth—Moon separation. In order to simplify the probe guidance, navigation and control GNC , no precision landing capabilities are foreseen.
The American and Soviet space programs have demonstrated that precision landing and hazard avoidance capabilities are not required to safely land a robotic spacecraft on the Moon. Whereas all previous robotic landings were performed with little detailed information about the lunar surface, site selection for Farside Explorer will mitigate against potential surface hazards by using existing high-resolution images of the surface, slope maps from high precision laser altimetry and stereo images, and rock abundances from orbital radar and radiometer data.
A reduced number of stations would be baselined during the ballistic transfer to LL2, while for critical phases, the full network might be reserved.
Exploring from orbit
Due to the high data rates, X band is preferred for the telecommunication link between the relay satellite and Earth. For the radio astronomy experiment, the lack of a second station would preclude source localization techniques. New deep moonquake nests could not be located with a single station, and the known deep moonquake nests might not be uniquely identified. Although core detection could be accomplished, the core size would not be determined. Crustal thickness beneath the lander would be obtained, but tomography of the upper mantle would be difficult.
For the heat flow experiment, only one lunar terrane would be investigated instead of two. For the electromagnetic sounding experiment, it would not be possible to use the geomagnetic depth sounding technique to investigate the lower mantle. Nearside landing sites would not be protected from terrestrial radio-frequency interference, but the geophysical investigations would be able to achieve many of the International Lunar Network objectives [ 71 ].
The Farside Explorer mission consists of two essential components: an instrumented relay satellite to be inserted into a halo orbit about the Earth—Moon L2 Lagrange point, and two identical spacecraft that make soft landings on the lunar surface. The two landers would contain a suite of state-of-the-art instrumentation: a radio astronomy receiver, long- and short-period seismometers, a heat flow probe, an electromagnetic sounder, surface cameras, and a geochemical experiment. A robotic arm is included on each lander for deployment of the seismometer and heat flow experiment, but lighter alternatives with fewer degrees of freedom are also available.
The relay satellite would contain an impact flash camera, and a magnetometer.