An Overview of Science Results

Mosaic of the south polar region of the Moon, mapped for the first time by Clementine.

For 71 days in lunar orbit, Clementine systematically mapped the 38 million square kilometers of the Moon in eleven colors in the visible and near infrared parts of the spectrum. In addition, the tiny spacecraft took tens of thousands of high resolution and mid-infrared thermal images, mapped the topography of the Moon with a laser ranging experiment, improved knowledge of the surface gravity field of the Moon through radio tracking, and carried a charged particle telescope to observe the solar and magnetospheric energetic particle environment.

All the innovative lightweight sensors on the spacecraft, which were tested in deep space fopr the first time, met or exceeded expectations. They have provided the first view of the global color of the Moon, identifying major compositional provinces, and focussed on several complex regions, mapping their geology and composition in detail. In addition, they measured the topography of large, ancient impact features, including the largest (2500 km diameter), deepest (more than 12 km) impact basin known in the Solar System, and deciphered the gravity structure of a young basin on the limb of the Moon, where a huge plug of the lunar mantle may be uplifted below its surface.

The images from Clementine constitute the first-ever global digital data set for the Moon. The Science Team advised the project on the selection of color filters for the two principal mapping cameras: the UV- VIS camera (sensitive to light in the visible part of the spectrum, from about 0.3 to 1 micrometers) and the near-IR camera (which collects light in the near-infrared spectrum, from about 1 to 2.8 micrometers). The color of the Moon in the visible to near-infrared part of the spectrum is sensitive to variations in both the composition of surface material and the amount of time material has been exposed to space. The Clementine filters were selected to characterize the broad lunar continuum and to sample parts of the spectrum that are known to contain absorption bands diagnostic of iron-bearing minerals. By combining information obtained through several filters, multi-spectral image data are being used to map the distribution of rock and soil types on the Moon.

Clementine was successful in systematically mapping the Moon in these 11 colors at an average surface resolution of about 200 meters per picture element. The Mission Science Team has completed a preliminary look at the Clementine data on a global basis by reducing the resolution by a factor of several hundred, allowing the immense data volume to be easily manipulated. Several major compositional provinces are evident, including the volcanic lavas of the maria (the dark regions of the Moon), young and fresh craters, and the immense South Pole-Aitken basin, a compositional anomaly on the far side of the Moon. Preliminary studies of areas of already-known geological complexity, including the Aristarchus crater and plateau, the Copernicus crater (on the western near side), and the crater Giordano Bruno (on the eastern far side), have allowed the Team to identify and map the diversity within and between geological units, which have both impact and volcanic origins.

In addition to compositional data from the images, Clementine has allowed scientists to see either previously unknown regions of the Moon or known areas from a different and unique perspective, in both cases yielding new insights into lunar evolution. The Science Team has completed a mosaic of the South Polar region of the Moon, using over 1500 images obtained during the systematic mapping of the poles during the first month orbiting the Moon. This mosaic shows a previously unmapped portion of the Moon near the pole, south of the Orientale basin, in detail. A striking result from this mosaic, evidenced by an extensive region of shadow, is the discovery of a large depression centered very near the south pole, almost certainly an ancient impact basin about 300 km in diameter. Its significance lies in its geographic position: because the rotation axis of the Moon is nearly perpendicular to its plane of orbit around the Sun (axis inclination 1.5 degrees), this dark region near the pole may never receive any sunlight. If so, it is very cold in these regions, possibly only about 40 Celsius degrees above absolute zero (-273 degrees C). It has been suggested that water molecules, added to the Moon from impacting comets, may find their way into these cold traps and over billions of years, accumulate in significant amounts. To investigate further, Clementine beamed radio waves into the polar areas and the scattered radio signals were received on the large antennas of NASA’s Deep Space Network. This “bistatic radar” experiment was designed to look for echoes that are diagnostic of water ice deposits. These data continue to be analyzed; no conclusive results have been reported yet.

The laser ranging data from Clementine have revealed the large scale topography of the lunar surface on a nearly global basis. A striking result from this experiment is the confirmation of the existence of a population of very ancient, nearly obliterated impact basins, randomly distributed across the Moon. These basins had been postulated on the basis of obscure circular patterns on poor quality photographs; Clementine laser ranging has provided dramatic confirmation of their existence, including their surprising depth, ranging from 5 to 7 kilometers, even for the most degraded features. Gravity data obtained from radio tracking of Clementine indicates that these great holes in the Moon’s crust are compensated by plugs of dense rocks far below the surface; such dense rocks are probably caused by structural uplift of the mantle (the iron- and magnesium-rich layer below the low density, aluminum-rich crust) beneath these impact basins. Finally, Clementine laser ranging data have given the dimensions of the largest confirmed basin on the Moon, the 2500 km-diameter South Pole-Aitken basin: this feature averages over 12 kilometers deep, making it the largest, deepest impact crater known in the Solar System.

The Charged Particle Telescope on Clementine observed a large burst of particles from the Sun from February 20-24. It also monitored several additional low-energy particle bursts of magnetospheric origin over the course of the mission. These data will be combined with observations from many of the other spacecraft now operating in Earth-Moon space to observe and characterize the plasma environment from different vantage points.

Clementine star tracker image of the lunar surface lit by Earthshine.

The scientific significance of the lunar data set from Clementine is immense. For the first time, scientists have global, multi-spectral image data (of consistent viewing geometry, resolution, and lighting conditions) for an entire body of planetary dimensions. From the Apollo and Luna programs, they also have lunar rock and soil samples of known geological context. Such a data set does not exist for any other object in the Solar System, including the Earth. With Clementine data for the Moon, a new era begins in the exploration of the geology of the planets using global multi-variable data sets. On the basis of an initial look at the Clementine data, such a powerful analytical technique will give us new insights into how the Moon has evolved over its protracted and complex history.