The Moon's Largest Impact Basin

The South Pole-Aitken (SPA) basin is the largest and oldest recognized impact basin on the Moon. It's diameter is roughly 2,500 km or 1,550 miles. The Moon's circumference is just under 11,000 km, meaning the basin stretches across nearly a quarter of the Moon. In the LROC WAC mosaic below, which is centered on the middle of the basin, you can see SPA as an area of relatively low reflectance extending from the crater Aitken in the north and all the way down to the South Pole. Topographic data from LOLA can also help to give a sense of the enormous effect the SPA impact had on the Moon the basin is more than 8 km .

Stratigraphic relationships show that SPA is the oldest impact basin on the Moon, but scientists are intensely interested in just how old it is. Lunar samples suggest that most of the major basins on the Moon formed around 3.9 billion years ago in a period called the late heavy bombardment. By this time most of the large debris within the solar system should have already accreted to form the planets, so such a large number of big impacts occurring at nearly the same time may have been due to unusual gravitational dynamics in the early Solar System.

Was the impact that caused the SPA basin also a part of some cataclysmic event that occurred 3.9 billion years ago? If so, that impact is strong evidence for an extreme event that would have affected all of the terrestrial planets, including Earth at a time when life was just beginning. If the basin is much older, that may suggest that instead of a spike in the impact rate at 3.9 billion years, the number of impacts simply trailed off from a peak earlier on. How can we find out just how old the SPA impact basin is? The best way would be to sample materials from the interior of the basin and use radiometric age dating techniques to determine when they were last molten, as heat from the impact would have melted a large volume of material, resetting radiometric clocks.

Reiner Gamma Region of Interest

First identified by early astronomers during the Renaissance, the Reiner Gamma formation has been a subject of intense scientific study for almost five decades and is one of the highest-priority targets for future human lunar exploration. Reiner Gamma is one of the most distinctive natural features on the Moon. This striking, tadpole-shaped swirl has a significantly higher reflectance than the surrounding mare basalts.

Several LROC Featured Images have shown spectacular new images of the swirls near Mare Ingenii which are similar to the swirls of Reiner Gamma. Reiner Gamma, however, is the "prototypical" lunar swirl. A concept that comes up frequently in lunar science is "space weathering." This term is used to describe a suite of natural processes that can alter the spectral properties of lunar surface materials. Since the reflectance of the lunar surface within the Reiner swirl is so different from the surrounding mare, some process may have altered the space weathering susceptibility of the swirl materials.

There are several theories to account for the presence of the Reiner Gamma swirl. Results from previous lunar missions have indicated that the swirl region has an elevated magnetic field, so it's possible that an event hundreds of millions of years ago modified the magnetic properties of the surface materials, deflecting the solar wind and changing how the reflectance is modified by space weathering. Some investigators have proposed that the coma of a comet - streaking in just above the surface - interacted with the lunar surface, changing the surface properties to the degree where the Reiner Gamma swirl could persist for millions of years.

The Earth From The Moon

The Earth as seen from the Moon! LROC NAC mosaic of images snapped on 12 June 2010 during a calibration sequence.All cameras are susceptible to scattered light. You may have seen scattered light in pictures you have taken looking towards the Sun. Sunlight reflects off the optics and sometimes off the structure of the lens, and often appears as a gradient of brightness across the image.Attaching a baffle to your camera, like we did with the LROC Wide and Narrow Angle Cameras, can minimize this effect.

More subtle effects are often present but usually you simply just don't notice artifacts because of strong color contrasts in the scene. Since the Moon has only very small color contrasts, the LROC team must characterize even subtle scattered light effects within the 7-color Wide Angle Camera (WAC) images. Changes in composition result in subtle differences of color, typically about 10% or less. For scientists to make accurate interpretations of WAC color maps, the amount of scattered light must be quantified.

One way of measuring scattered light is imaging a bright object against a dark background.From the Moon, the Earth serves that function well. While a series of WAC calibration images of the Earth were being acquired, the Narrow Angle Camera (NAC) was shuttered to capture this spectacular Earth view. The bottom of the Earth was clipped because the prediction of the exact time when the cameras' fields of view would cross the Earth was off by a few seconds.