A three‐axis short‐period seismometer has been operating on the surface of Mars in the Utopia Planitia region since September 4, 1976. During the first 5 months of operation, approximately 640 hours of high‐quality data, uncontaminated by lander or wind noise, have been obtained. The detection threshold is estimated to be magnitude 3 to about 200 km and about 6.5 for the planet as a whole. No large events have been seen during this period, a result indicating that Mars is less seismically active than earth. Wind is the major source of noise during the day, although the noise level was at or below the sensitivity threshold of the seismometer for most of the night during the early part of the mission. Winds and therefore the seismic background started to intrude into the nighttime hours starting on sol 119 (a sol is a Martian day). The seismic background correlates well with wind velocity and is proportional to the square of the wind velocity, as is appropriate for turbulent flow. The seismic envelope power spectral density is proportional to frequency to the −0.66 to −0.90 power during windy periods. A possible local seismic event was detected on sol 80. No wind data were obtained at the time, so a wind disturbance cannot be ruled out. However, this event has some unusual characteristics and is similar to local events recorded on earth through a Viking seismometer system. If it is interpreted as a natural seismic event, it has a magnitude of 3 and a distance of 110 km. Preliminary interpretation of later arrivals in the signal suggest a crustal thickness of 15 km at the Utopia Planitia site which is within the range of crustal models derived from the gravity field. More events must be recorded before a firm interpretation can be made of seismicity or crustal structure. One firm conclusion is that the natural background noise on Mars is low and that the wind is the prime noise source. It will be possible to reduce this noise by a factor of 103 on future missions by removing the seismometer from the lander, operation of an extremely sensitive seismometer thus being possible on the surface.

The results of paleomagnetic, petrographic, and radiometric studies of the Eastern Caroline Islands in the western Pacific indicate that the islands were formed by a hot spot located near the paleoequator between 1 and 11 Ma. The islands show a linear progression of mean ages from 1 Ma in the east (Kusaie) to 11 Ma in the west (Truk). The results of volumetric measurements and geochemical studies suggest that the hot spot source is waning and perhaps was slowly dying during the time Truk, Ponape, and Kusaie were being formed. The dominant shield‐building magmas in the Caroline Islands are part of a differentiated alkalic series. The posterosional lavas are highly silica undersaturated and trace element enriched nephelinites. The latter were erupted subsequent to the cessation of the main shield phase of volcanism. The petrography and geochemical evolution of Truk are strongly reminiscent of that of the Hawaiian chain; however, the shield‐building lavas are compositionally similar to the alkalic lavas that typically form only thin, late‐stage caps on many Hawaiian volcanos. No tholeiitic rocks were found despite sampling deep within the eroded volcanic structure of the islands. This absence of tholeiitic lavas and dominance of alkalic lavas stand in contrast with Hawaii, where tholeiitic volcanism dominates and alkalic lavas form only a minor component of the exposed lavas. The absence of tholeiitic lavas in the main shield‐building phase of construction, however, is not unique to the Caroline Islands. Dominant alkalic volcanism appears characteristic of other seamounts in the Pacific, including the Samoan, Austral‐Cook, and Line Islands.

We tested Newton's inverse-square law of gravitation by comparing gravity measured on a 600-m tower with gravity calculated from ground measurements. A significant departure from the law was detected, approaching (-500 ±35)xlO~^ms~^at the top of the tower and suggestive of a rapidly attenuating non-Newtonian attractive force. These results are marginally consistent with a one-term Yukawa-type attractive force, but they are fully consistent with two Yukawa-type forces, attractive and repulsive, and then also with Airy and Cavendish experiments.

A three-axis short-period seismometer is now operating on Mars in the Utopia Planitia region. The noise background correlates well with wind gusts. Although no quakes have been detected in the first 60 days of observation, it is premature to draw any conclusions about the seismicity of Mars. The instrument is expected to return data for at least 2 years.

During the Viking mission to Mars, the seismometer on Lander II collected approximately 0.24 Earth years of observational data, excluding periods of time dominated by wind‐induced Lander vibration. The “quiet‐time” data set contains no confirmed seismic events. A proper assessment of the significance of this fact requires quantitative estimates of the expected detection rate of the Viking seismometer. The first step is to calculate the minimum magnitude event detectable at a given distance, including the effects of geometric spreading, anelastic attenuation, seismic signal duration, seismometer frequency response, and possible poor ground coupling. Assuming various numerical quantities and a Martian seismic activity comparable to that of intraplate earthquakes, the appropriate integral gives an expected annual detection rate of 10 events, nearly all of which are local. Thus only two to three events would be expected in the observational period presently on hand and the lack of observed events is not in gross contradiction to reasonable expectations. Given the same assumptions, a seismometer 20 times more sensitive than the present instrument would be expected to detect about 120 events annually.

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Matched‐filter techniques are described for the detection of characteristic geoid undulation signatures of seamounts in SEASAT radar altimeter data. The technique requires models of the expected undulation signatures and a statistical description of the background (seamount‐free) data. Examples are given of the detection of 11 known seamounts on four distinct tracks. Further extensions and applications of the matched‐filter methods are discussed.

An experiment was performed to test Newton's inverse square law of gravitation. The test compared accurately measured gravity values along a 600‐m tower with upward continued gravity estimates calculated from ground measurements using algorithms based on Newtonian potential theory. Gravity measurements with an internal consistency of better than 20 μGal (1 μGal = 10−8 m/s2) were made on the tower at intervals of about 90 m using a single LaCoste‐Romberg model G gravimeter. Field measurements of gravity (using the same instrument) and corresponding geodetic positions were performed at 77 points within a radius of 5 km from the tower. These and other gravimetric data within a radius of about 220 km were used to perform the upward continuation using two independent computational techniques. One is least squares collocation, which is based on a statistical interpretation of the local gravity field, and the other determines a local harmonic expansion of the field from the given data. The upward continuation from both methods gave almost identical results and is estimated to have an uncertainty comparable to the in situ measurements. The results of the experiment show a substantial departure from the inverse square law, asymptotically approaching about 500 μGal at the top of the tower, with a standard deviation of better than 40 μGal. This indicates that there is a non‐Newtonian attractive force which falls off rapidly with elevation.