Far Side Of The Moon As Never Seen Before: LRO Camera Team Releases High Resolution Global Topographic Map Of Moon

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The science team that oversees the imaging system on board NASA’s Lunar Reconnaissance Orbiter (LRO) has released the highest resolution near-global topographic map of the moon ever created.

This new topographic map, from Arizona State University in Tempe, shows the surface shape and features over nearly the entire moon with a pixel scale close to 100 meters (328 feet). A single measure of elevation (one pixel) is about the size of two football fields placed side-by-side.

LROC WAC color shaded relief of the lunar farside. (Credit: NASA’s Goddard Space Flight Center/DLR/ASU)

Although the moon is our closest neighbor, knowledge of its morphology is still limited. Due to instrumental limitations of previous missions, a global map of the moon’s topography at high resolution has not existed until now. With the LRO Wide Angle Camera and the Lunar Orbiter Laser Altimeter (LOLA) instrument, scientists can now accurately portray the shape of the entire moon at high resolution.

“Our new topographic view of the moon provides the dataset that lunar scientists have waited for since the Apollo era,” says Mark Robinson, Principal Investigator of the Lunar Reconnaissance Orbiter Camera (LROC) from Arizona State University in Tempe. “We can now determine slopes of all major geologic terrains on the moon at 100 meter scale. Determine how the crust has deformed, better understand impact crater mechanics, investigate the nature of volcanic features, and better plan future robotic and human missions to the moon.”

Called the Global Lunar DTM 100 m topographic model (GLD100), this map was created based on data acquired by LRO’s WAC, which is part of the LROC imaging system. The LROC imaging system consists of two Narrow Angle Cameras (NACs) to provide high-resolution images, and the WAC to provide 100-meter resolution images in seven color bands over a 57-kilometer (35-mile) swath.

The WAC is a relatively small instrument, easily fitting into the palm of one’s hand; however, despite its diminutive size it maps nearly the entire moon every month. Each month the moon’s lighting has changed so the WAC is continuously building up a record of how different rocks reflect light under different conditions, and adding to the LROC library of stereo observations.

The LROC (WAC) has a pixel scale of about 75 meters (246 feet), and at the average altitude of 50 km (31 miles) a WAC image swath is 70 km (43 miles) wide across the ground-track. Since the equatorial distance between orbits is about 30 km (18 miles) there is complete overlap all the way around the moon in one month. The orbit-to-orbit WAC overlap provides a strong stereo effect. Using digital photogrammetric techniques, a terrain model can be computed from the stereo overlap.

The near-global topographic map was constructed from 69,000 WAC stereo models and covers the latitude range 79°S to 79°N, 98.2% of the entire lunar surface. Due to persistent shadows near the poles it is not possible to create a complete stereo based map at the highest latitudes. However, another instrument onboard LRO called LOLA excels at mapping topography at the poles. Since LOLA ranges to the surface with its own lasers, and the LRO orbits converge at the poles, a very high resolution topographic model is possible, and can be used to fill in the WAC “hole at the pole.” The WAC topography was produced by LROC team members at the German Aerospace Center.

“Collecting the data and creating the new topographic map was a huge collaborative effort between the LRO project, the LOLA team, the LROC team at ASU and in Germany at the DLR,” says Robinson. “I could not be more pleased with the quality of the map – it’s phenomenal! The richness of detail should inspire lunar geologists around the world for years to come.”

Shaded relief images can be created from the GLD100 by illuminating the “surface” (in this case the shape model) from a given Sun direction and elevation above the horizon. To convey an absolute sense of height the resulting grayscale pixels are painted with colors that represent the altitude. Visualizations like these allow scientists to view the surface from very different perspectives, providing a powerful tool for interpreting the geologic processes that have shaped the moon.

The LRO spacecraft is managed by NASA’s Goddard Space Flight Center in Greenbelt, Md., for NASA’s Science Mission Directorate in Washington.

Future versions

The current model incorporates the first year of stereo imaging; there is another year of data that can be added to the solution. These additional stereo images will not only improve the sharpness (resolution) of the model but also fill in very small gaps that exist in the current map. Also the LROC team has made small improvements to the camera distortion model and the LOLA team has improved our knowledge of the spacecraft position over time. These next generation steps will further improve the accuracy of the next version of the LROC GLD100 topographic model of the moon.

Global topography

Global topography — a boon to lunar scientists and explorers around the world! Today the LROC team releases Version 1 of the Wide Angle Camera (WAC) topographic map of the Moon. This amazing map shows you the ups and downs over nearly the entire Moon, at a scale of 100 meters across the surface, and 20 meters or better vertically. Despite the diminutive size of the WAC (it fits in the palm of one’s hand), it images nearly the entire Moon every month. Every month? Yes! Redundant data? No! Each month the Moon’s lighting changes, so the WAC methodically builds up a record of how different rocks reflect light under different conditions, and adds to the LROC library of stereo observations. The WAC really is the little camera that could! It was built by Malin Space Science Systems (MSSS) in San Diego CA, and is very similar to another MSSS camera (MARCI) that is now in orbit around Mars.

Left: LROC Wide Angle Camera attached to a test setup shortly before mounting on the spacecraft. Right: WAC being handed up to engineers for integration with LRO

 (photos M. Robinson).

The WAC has a pixel scale of about 75 meters, and with an average altitude of 50 km, a WAC image swath is 70 km wide across the ground-track. Because the equatorial distance between orbits is about 30 km, there is nearly complete orbit-to-orbit stereo overlap all the way around the Moon, every month. Using digital photogrammetric techniques, a terrain model was computed from this stereo overlap. The new topographic model was constructed from 69,000 WAC stereo models. Due to persistent shadows near the poles it is not possible to create a complete WAC stereo map at the very highest latitudes. Fortunately, the LRO Lunar Orbiter Laser Altimeter (LOLA) excels at characterizing the topography of the poles. Since the LRO orbits converge at the poles and LOLA ranges to the surface with its own lasers, LOLA provides a very high resolution topographic model of the poles. This LOLA map can fill in the WAC “hole at the pole”.

Detail from WAC topography model, shades of gray represent elevation. Darker values are lower terrain, while brighter values are higher points. Image center is near 19.1°N, 175.3°E

(NASA/GSFC/DLR/Arizona State University).

How is a digital topographic map created from stereo images? The WAC stereo images were compared one against another by pattern-matching a moving box of pixels until the best fit was found between two images with different viewing angles. Best fit pixel positions are combined with the LRO orbit position and the WAC viewing angles to define two 3D rays (lines of sight). The intersection point of these rays defines the location and the elevation of the point on the surface. Since the correlation box is bigger than 100 meters, surface details at the 100-meter scale are not fully resolved in a single stereo pair. However, each 100 meter square has an average of 26 stereo points within it (for a planet-wide total of 100 billion points), which helps to sharpen the elevation estimate. The resolution, in a formal sense, is probably close to 300 meters, and the accuracy of the elevations is estimated to be about 10 to 20 meters. This new map is called the Global Lunar DTM 100 m topographic model, or “GLD100”, and covers 79°S to 79°N latitudes, 98.2% of the entire lunar surface. The WAC topography was produced by LROC team members at the German Aerospace Center (DLR).

Color shaded relief detail corresponding to same area shown in figure above

(NASA/GSFC/DLR/Arizona State University).

Shaded relief images can be created from the GLD100 by illuminating the surface from a given Sun direction and elevation above the horizon, and to convey an absolute sense of height the resulting grayscale pixels are painted with colors that represent the altitude. Visualizations like these allow scientists to view the surface from very different perspectives, providing a powerful tool for interpreting the geologic processes that have shaped the Moon.

WAC mosaic corresponding to same area shown above

(NASA/GSFC/DLR/Arizona State University).

The large irregularly shaped Buys-Ballot crater, seen in the three images above, is about 47 km by 62 km. The WAC topography shows the flat floor to lie some 3100 meters below the western rim and 4600 m below the eastern rim. The central peak rises about 800 m above the floor. For comparison the width and depth of this crater are larger than those of the Grand Canyon, AZ.

Topographic profile (highly exaggerated) across Buys-Ballot crater, note the flat floor

(NASA/GSFC/DLR/Arizona State University).

Why is the floor of the crater so flat? The WAC mosaic holds an important clue — note how dark the floor is. On the Moon, dark (low albedo) material is typically basalt, which is dark because it is relatively rich in iron (mafic minerals). On the Moon, basalts are erupted as very hot lavas, making them very fluid, thus they tend to spread out and flood local topographic lows. From the topography and WAC image data, one can make a confident interpretation that the floor of this unusual crater is most likely flooded with basalt. But why such a small eruption? Most flood basalts on the nearside cover vast expanses of lunar terrain, but why not on the farside? Another mystery for future lunar explorers to unravel!

The area shown here is only a tiny portion of the Moon: see if you can find Buys-Ballot in the full resolution hemispheric view.

What improvements can be made over this first release of the LROC WAC GLD100? The current model incorporates the first year of stereo imaging, and there is another year of data that can be added to the solution. These additional stereo images will not only improve the sharpness (resolution) of the model but also fill in very small gaps that exist in the current map. The LROC team has made small improvements to the camera distortion model, and the LOLA team has improved our knowledge of the spacecraft position over time. These next generation steps will further improve the accuracy of Version 2 of the LROC GLD100 topographic model of the Moon.

Explore the new LROC GLD100!

Direct links to color shaded relief visualizations:
Orthographic projection centered at 0° longitude and 0° latitude.
Orthographic projection centered at 60° longitude and 0° latitude.
Orthographic projection centered at 120° longitude and 0° latitude.
Orthographic projection centered at 180° longitude and 0° latitude.
Orthographic projection centered at 240° longitude and 0° latitude.
Orthographic projection centered at 300° longitude and 0° latitude.
Orthographic projection centerd at 0° longitude and 90° latitude.
Orthographic projection centered at 0° longitude and -90° latitude.

NOTE: OS X users may have a problem opening the large TIFF files directly from the browser. Please right click and choose “Save As”.

Color scale legend for LROC Color Shaded Relief

 (NASA/ GSFC/ DLR/ Arizona State University).

Contacts and sources:
NASA
Arizona State University
Mark Robinson