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The Forefront of Space Science

Exploring the Interior of the Moon and Mars with Seismometers
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Another method is to measure the “free oscillationEcaused by seismic activity. When a large quake occurs, resonance oscillation arises across the whole earth. This technique is similar to tapping a watermelon to guess its ripeness. In the case of a watermelon, we hear pitch and tone. For seismic vibration, we simply measure the oscillation period with a seismometer. We can guess the internal structure with a single seismometer. Figure 1 shows free-oscillation periods of the earth and Mars. In a large quake, a longest oscillation of approx. 54-minute in period occurs on the earth. Since the radius of Mars is about half that of the earth, its oscillation period is shorter. Figure 1 indicates that the free-oscillation period varies depending on the size and state (liquid or solid) of the core.

Figure 1
Figure 1. Period (minute) of free oscillation of the earth and Mars
For Mars, theoretical values are indicated per variations in size (radius) and state (liquid/solid) of the core (according to calculations by Yasuhiro Nishikawa).

We need to note, however, that we can observe free oscillation only when a large quake occurs. In the case of the earth, this corresponds to approx. M8 magnitude. For seismic activity on the Moon and Mars, unfortunately, we cannot expect to observe free oscillation. Nonetheless, we think that on Mars it is possible to measure “background free oscillation,Ean event where a celestial body is continuously shaken by its atmosphere. On earth, this event was discovered by a Japanese research group and has been studied. Though its atmosphere is thin, Mars is a small planet, having fast-moving atmosphere with a very large rough surface, and is likely to be easily shaken by its atmosphere. Some preliminary calculation suggests that MarsEbackground free oscillation has almost the same amplitude as the Earth’s. By using the highest sensitivity seismometer, we may be able to catch MarsEbackground free oscillation.

Performance of prototype seismometer Emoving forward step by step

Our target for lunar and planetary exploration is to develop a seismometer with performance capable of measuring MarsEbackground free oscillation. A seismometer for the Moon and Mars needs to endure vibration (several tens of G) at the time of launch and landing, temperature variation (-50 deg. C to +50 deg. C) and cosmic radiation. Further, there are severe constraints in size, weight, and power consumption. We cannot use conventional seismometers, but must create a new one to meet the requirements. Figure 2 shows prototypes produced until now. The first model, Fig. 2(a), was designed to focus on detection performance to ascertain its ability to measure fine, long-period vibrations. The broadband seismometer’s mechanism features a sensor to measure pendulum motion and a pendulum control to offset its movement; the control signal provides us with the ground motion. A long-period pendulum is required to sense long-period vibration. We adopted a laser interferometer as a sensor because it can measure fine pendulum motion while being robust to temperature variation and cosmic radiation. Detection performance of the model is almost as expected.

Figure 2
Figure 2. Prototype seismometers

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