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The Forefront of Space Science

Exploring the Interior of the Moon and Mars with Seismometers
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Quake observation: a powerful method to explore the interior of celestial bodies

It is known that the earth consists of four layers: (from the surface inward), the crust, mantle, outer core (liquid) and inner core (solid). This fact was revealed by observation of seismic waves. When large earthquakes occur, seismic waves propagate within the earth and in some cases circumnavigate the earth several times. Such events are observable. When layers exist in the ground, reflection and refraction are caused. By measuring the time lags of seismic waves appearing in the ground after multiplying along various routes, we can estimate how these layers exist. At the same time, hypocenters (seismic sources) are located in relatively shallow ground. Plotting the epicenters on a map, we can observe that they are distributed in strings. This suggests that the earth’s surface is divided into several plates and that quakes occur on the boundaries of the plates. Thus, earthquake observation has been a powerful tool to study the earth’s interior.

Seismic observations were performed on the Moon and Mars in the past. In the Apollo program in the 1970s, astronauts placed seismometers at a total of five locations on the Moon. The observations discovered that earthquakes (moonquakes) occur on the Moon as well, but some of them are from very deep sources. This was interpreted as indicating that, on the Moon, the brittle parts extend to its depths. Several models have been proposed for the lunar interior and analysis using advanced techniques is still being performed on the moonquake observation data retrieved by the Apollo program.

On the other hand, a seismometer was installed on the Mars landing probe Viking. Mars has atmosphere (under 0.01atm). Even though it is thinner than the earth’s, the wind on Mars is strong (maximum speed of more than 100m/s), so the vibration caused by the wind disturbs seismic observation. Thus, no clear earthquake data have been obtained. Lately, many Mars explorers sent by Europe and U.S. have enabled the retrieval of detailed images of the surface, and landing probes have delivered excellent geological data. Since volcano- or fault-like terrains are visible, some seismic activity seems likely on Mars, but a clear answer has not yet been obtained.

The Moon and Mars share certain traits: there is no large-scale seismic activity similar to that on the earth and their interiors are dormant. Since quakes on earth are caused by convective activity in the mantle as an energy source, it is guessed that the interiors of the Moon and Mars are cool. If seismic activity is estimated by seismic observations, we can infer the state of their interiors. In addition, we can calculate their internal structure by the above-mentioned method based on seismic waves. For example, if we can know the size and state (liquid or solid) of the cores of the Moon and Mars, which are currently thought to exist in their centers and composed of mainly iron, we can guess the history and nature of their formation. For the Moon, since no seismometers have been laid on its far side (opposite side of the earth), little information about far-side moonquake sources has been obtained. The surface conditions of the lunar far and near sides are very different (lunar dichotomy). If we can observe a moonquake on the far side, we can come closer to the cause of the dichotomy based on the internal structure. Thus, there is a strong desire for precise seismic observations on the Moon and Mars, since a great deal of important data are expected for solar-system research.

Required performance of seismometer

Various seismometers are used on the earth. They include “highly-sensitiveEseismometers to measure small vibrations and “strong-motionEseismometers capable of observing even large swings. Since large quakes do not occur on the Moon and Mars, we need a seismometer to sense very small vibrations.

The quake period is also important. When an earthquake occurs, vibrations of various periods are generated. Short-period seismic waves cannot propagate far since they tend to attenuate. On the other hand, when a large quake occurs in the distance, slow swings having a period of several seconds can sometimes be felt. This is because long-period seismic waves generated at the source region hardly attenuate so they propagate in the distance. To investigate the structure deep underground, we use a “broadbandEseismometer capable of observing long-period seismic waves.

To identify the location and depth of the seismic source, we need to perform simultaneous measurements at more than two sites. It is very difficult, however, to put seismometers at several sites on the Moon or Mars. We are considering a few methods. One idea is to install seismometers into warhead-like “penetratorsEand drop them into several places on the Moon or Mars. Another idea is international cooperation, whereby several countries or projects cooperate to place multiple seismometers and measure jointly. If the location of the meteorite impact is identified, we can obtain information about the underground structure by a single seismometer, since seismic waves would be caused in the impact site.

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