PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 91109. TELEPHONE (213) 354-5011
FOR IMMEDIATE RELEASE

       The accompanying images showing major geologic features on the ocean floor were recently produced from data collected by JPL's Seasat oceanographic satellite, flown by NASA in 1978. Each image represents global "snapshot" of sea floor topography never before available, and at least one previously unknown feature is revealed.

       The images of the ocean floor were made by measuring the topography of the ocean surface with the satellite's altimeter. The global images are each comprised of more than 50 million physical measurements including 10 corrections for atmospheric and other interferences.

       The images were produced from the same data, but each was processed differently to emphasize unique set of features. They supply new, detailed bathymetric and geologic information for wide areas of the world's seas, especially in the southern oceans which are poorly surveyed. Map "B," for example, shows variety of sea floor geologic features, including one previously unknown major feature: existing bathymetric charts in this region show the Louisville Ridge as discontinuous chain of seamounts running southeast of the Tonga-Kermadec Trench. The image, however, clearly shows nearly continous chain of features.

       JPL oceanographer Dr. Michael Parke produced the data base used in constructing the maps from the Seasat altimeter data. The various images were produced by Parke, JPL marine geologist Dr. Timothy Dixon, and Kevin Hussey of JPL's Image Processing Laboratory. Scientific analysis of the images was conducted by Parke and Dixon.

       Mapping the sea floor by measuring the sea surface topography is possible because of the relationship between gravity, the sea floor, and the ocean. Gravity over the Earth is not constant. It varies, depending on the local thickness, density, age and geology of the Earth's crust. The ocean conforms to variations in this uneven gravity field because it is fluid. Sea surface topography (called the geoid) dominantly conforms to the sea floor topography beneath. For example, mountainous formation on the sea floor causes peak on the ocean surface detectable by satellite altimeter.

       Seasat collected 70 days of oceanographic data over l00-day period. The radar altimeter measured the distance from the spacecraft to the ocean surface. By calculating the satellite's position, and correcting for passage of the radar beam through the atmosphere, the height of the ocean surface was determined.

       The resulting maps, which show the ocean surface at onehalf degree resolution, were created through computer processing of the Seasat altimeter data. They are designed to emphasize features on the ocean floor ranging in size from 50 to 500 kilometers (30 to 300 miles).

       A proposed follow-on mission called TOPEX (Ocean Topography Experiment) to study the ocean's circulation with highresolution satellite altimeter is being planned by JPL for NASA.

       Seasat was managed for NASA by JPL. Production and analysis of the altimeter maps was sponsored by the Ocean Processes Branch in NASA's Office of Space Science and Applications (OSSA). 10/25/82

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#1006MBM

PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 91109. TELEPHONE (213) 354-5011
PHOTO CAPTION

       MAP

       This topographic relief map of the world's ocean surface was recently produced from altimeter data from JPL's Seasat oceanographic satellite, flown by NASA in 1978. The image is dominated by features indicative of sea floor spreading: the mid-ocean ridges, trenches, fracture zones and seamount chains.

       The data were processed to emphasize steep, small scale features (white areas) that tilt toward or away from the northwest. Most apparent are the mid-Atlantic Ridge and associated fracture zone systems. In addition, the trenches along the west and northwest margins of the Pacific plate are clearly visible.

       The volcanic Hawaiian Islands and Emperor Seamount Chain dominate the center of the Pacific. These volcanoes were created as the Pacific plate moved northwestward over hot spot in the mantle beneath Hawaii; new volcanoes are continually created as older ones to the northwest become extinct. This chain of seamounts provides fossil record of the past movement of the Pacific plate: the bend in the chain marks major shift in the direction of plate motion that occured about 40 million years ago.

       The extent to which seamount affects the local gravity field depends on the age of the underlying crust when the seamount was formed. If the seamount formed on young, flexible crust, the crust deforms around the seamount, compensating the effect of the seamount's extra mass. Seamounts formed on older, more rigid crust are not compensated in this way, resulting in gravitational signature that is more apparent at the sea surface. The HawaiianEmperor Seamount chain in the Pacific and large "seamount province" southeast of Japan are areas where young seamounts have been created on old crust.

       This is one of several images produced from Seasat altimeter data by JPL scientists Dr. Michael Parke, Dr. Timothy Dixon and Kevin Hussey of JPL's Image Processing Laboratory. Each map was processed to emphasize unique set of sea floor, gravity or other features, and is the result of more than 50 million physical measurements of the ocean surface. The effort was conducted by JPL for the Ocean Processes Branch of NASA's Office of Space Science and Applications (OSSA).

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10/25/82

PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 91109. TELEPHONE (213) 354-5011
PHOTO CAPTION

       MAP

       This computer-processed map of region of the southwest Pacific shows variety of sea floor geologic features, including at least one previously unknown major feature: existing bathymetric charts in this poorly surveyed region show the Louisville Ridge as discontinuous chain of seamounts running southeast of the Tonga-Kermadec Trench. However, this image clearly indicates nearly continuous chain of features.

       The image was produced from altimeter data from JPL's Seasat oceanographic satellite, flown by NASA in 1978. It is dominated by the gravitational effect of the Tonga-Kermadec Trench (blue, indicating low area) and the corresponding Tonga-Kermadec Ridge just to the west (red, indicating high area). This trench arc system marks the location where old, westward-moving Pacific sea floor is consumed (subducted). After subduction, this oceanic crust undergoes or induces partial melting at great depth, causing formation of the island arc volcanoes comprising the Tonga-Kermadec Ridge. To the northwest is the New Hebrides arc-trench. The direction ofd subduction of each arc-trench system can be determined just from this image of the sea surface. swell occurs seaward of the trench, as the oceanic crust buckles upward prior to subduction. However, much stronger topographic high occurs on the other side of the trench due to the formation of the arc ridge.

       It is apparent in this image that the New Hebrides system subducts in different direction (from southwest to northeast) relative to the Tonga-Kermadec. The New Caledonia Basin represents topographic low southwest of the New Hebrides trench. Its depth and "nested" shape relative to the present trench to the northeast suggest that it is fossil (ancient) trench, which may mark former position of the New Hebrides subduction zone.

       The geologic youth of the Fiji Plateau can also be inferred from the image. This recently created triangular patch of sea floor is the product of small local spreading center, common feature behind active subduction zones. This region is warmer and less dense than surrounding sea floor and so stands higher -- direct result of recent creation.

       East of New Zealand, large continental shelf occurs, known as the Chatham Rise and Campbell Plateau, which are separated by the Bounty Channel. Bollon's "Tablemount" can be seen immediately east of Campbell Plateau. The Pacific-Antarctic Rise, an active spreading center which is the southwest extension of the East Pacific Rise, is visible near the southern edge of the picture.

       Four prominent fracture zones which offset this ridge are also apparent: The Udintsev, Balleny, Tasman, and George V.

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10/25/82
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