Tuesday, November 9, 2010

Kepler Crater Ejecta

Large boulder ejected from Kepler crater, a small depression from the boulder's impact is just visible. LROC Narrow Angle Camera (NAC) observation M140155410L, LRO orbit 5788, September 26, 2010; above field of view is 320 meters, original LROC featured image (here) 800 meters [NASA/GFSC/Arizona State University].

Drew Enns
LROC News System

Kepler is a Copernican aged crater (32 km diameter, 8.1°N, 322.0°E) named for the German Astronomer Johannes Kepler, famous for his three laws of planetary motion. The impact event that created Kepler crater was energetic enough to eject this 100 m boulder out onto its continuous ejecta blanket. Impact events excavate material from great depth (approximately 1/3 the transient crater diameter) and distribute the material around the crater as ejecta. The material at the top of the impacted surface is ejected the furthest, while the deepest material has just enough energy to land on the crater rim. This distance to depth relation creates a natural core sample for astronauts to collect as they explore.

LROC Wide Angle Camera (WAC) context image of Kepler showing the location of the boulder field north of the crater on the downward slope of its ejecta blanket [NASA/GFSC/Arizona State University].

Kepler when the Moon is full, or how the optical immaturity of it's surroundings betray its relative youth in this spectacular photograph by P. Van de Haar of the Netherlands. This is how this familiar near side crater appears through modest telescopes at local "high noon."

In November 1969 the crew of Apollo 12 had a landing transfer orbit with a perilune further west than any other of the Apollo surface expeditions, and as such apparently captured the best images of Kepler prior to LRO, forty years later. The view of Kepler, 557 km northeast of the Apollo 12/Survey 3 landing site, had to have been captured late in the mission [NASA/LPI].

A center slice of a wider wallpaper-sized view of Kepler, looking south from a virtual vantage over the featured boulders, peeking over the north rim across Kepler to the south rim 40 km beyond [NASA/GSFC/Arizona State University/Google Earth].

The lunar mare were formed as old impact basins filled by massive eruptions of very fluid basalt. Its easy to measure the area of these mare basalts, but how thick are they? Are there multiple basalt flows that form the mare? If the mare is thin enough the Kepler impact may have excavated both mare and the underlying highland material. Samples from this boulder, and others like it out to the edge of ejecta could answer these questions. An astronaut would start sampling at the far edge of the ejecta blanket and work towards the rim. During this traverse the intrepid geologist would in effect be traveling down the inside of the crater, without doing all the work of climbing in and out! The last sample on the rim would be from near the bottom of the crater. With this suite of samples the history of the emplacement of the basalts at this spot could be unraveled.

Search for other ejecta boulders in the NAC image!

Related Posts:
Ejecta from Van de Graaf Crater