This post is part of a slowly unfolding saga of my experience at the Meteor Crater Field Camp that was held from October 17-23, 2010. The field camp was run under the NASA Lunar Science Institute and headed by Dr. David Kring of the Lunar and Planetary Institute.
This post also doubles as my entry into Accretionary Wedge #49: Out of This World, which focuses on extraterrestrial geology and terrestrial analogues. Thanks to Dana at En Tequila Es Verdad for hosting this month’s Wedge!
Tuesday, October 19, 2010.
From above, our deluge of sun hats would appear to run into and froth against the tourist rope corral for a moment before spilling over and around into the area of No Trespassing. Rapidly arriving at a flattened part of the rim, we diffuse and come to rest, idly shifting for something in our backpacks and maybe a better view into the crater. David starts speaking of the plans for the day, and it takes us a few moments to realize we’re looking in the wrong direction. We turn around and he points at a small hill near the side of an access road. Pondering aloud, he wonders What do you suppose that boulder is doing on top of that hill?
The plains surrounding Meteor Crater are afflicted with an excess of flatness. Aside from the crater itself, the only relief is from scattered blocks, mounds and low rises of Coconino Sandstone and Kaibab Dolostone. They are blemishes on the otherwise flat patchwork terrain surrounding the crater. Like the boulder on the hill, many large coherent blocks of ejecta excavated during impact were thrown out of the crater and now rest upwards of 300 feet above where they ought to. Three days in, we were no longer tourists; It was time for science. We started work to answer a few relatively simple questions: Where in the crater did those blocks originate? How big are they? What would it take to launch them tens and hundreds of meters to their current position?
To answer those questions, the group split in two. One team examined the ejecta blocks, recording their dimensions and lithology. The other team measured the distance from crater to ejecta, using a physical measuring tape and recording map position and GPS coordinates to cross-check calculations. It was a rather straightforward assignment that also got us thinking in terms of cratering processes. The furthest blocks we studied were a solid five minute walk from the crater. It is easy to lose sight of something that is no longer present, but after the initial impact we would have been scrambling over several additional meters of ejecta the whole trip.
Team Tape and Team Lithology knocked out six profiles over the course of the day, including an assessment of the famous three-story-tall House Rock (a.k.a. Monument Rock). Six blocks is a small sample set to be sure, but one that lays the groundwork for a more thorough and complete ejecta study to be conducted over a number of Meteor Crater Field Camps. With a few simplifying assumptions – radial path, ballistic trajectory, 45 degree angle of ejection† – our results indicated flight times for these boulders of three to fifteen seconds at ~60-360 km/hr [~15-100 m/s]. These velocities get well above hurricane force winds (though they pale in comparison to the ~12 km/s impact velocity). And we’re not talking about shingles and trees flying around – these are multi-ton boulders getting hucked out in all directions. Given enough force, ejecta can go anywhere. House Rocks might not get very far, but there are loads of examples of ejecta traveling hundreds of kilometers, into the atmosphere, or even off-planet. We only have fragments of Mars as a result of a few impacts into the martian surface sending material into space and eventually to Earth. Simon Wellings (@metageologist) wrote a bit more about the evidence of impacts in his contribution to the Wedge, What came from outer space.
The Apollo Era really brought to light the importance of impact processes on the evolution of planetary surfaces. Apollo missions also proved a challenge to geologists. No lunar material was collected in-situ, which means the provenance of many samples is uncertain. The provenance of regolith (soil) and impact breccia fragments is still the subject of intense debate. Many of these fragments likely have origins in basin forming events (e.g., the Sea of Tranquility). Boulders like those surrounding Camelot Crater in the above photo, are a bit easier to reconcile with their source. Mapping the distribution of ejecta lithology around terrestrial and lunar craters is the ground-truth to theoretical distribution models. Gravity and atmospheric conditions may differ between the Earth and Moon, but the results of an impact are similar across the solar system.
†Learn more about impact cratering processes with the Lunar & Planetary Institute Impact Cratering Lab