The HiRISE team met up this summer in Whitefish, Montana. In between meetings, we were also able to take several geologic field trips and hikes. Glacier National Park has many cool (haha) glacial features, of course, and we also learned about some interesting sedimentology that occurred in the ancient geologic past. The patterns we saw in the sedimentary rocks are similar to those discovered by the Mars Opportunity Rover – cross-bedding and festooned ripples that form when sand is laid down under a body of water. The shape and direction of the ripples can tell you how much water was present, how fast it was flowing, and whether it was a river, a lake, or an ocean. These are important questions we’d like to answer about the history of water on Mars.

The park also has wonderful examples of glacial geology. HiRISE has taken images of many features thought to be related to glaciers, so it’s important to understand the terrestrial analogs that lead scientists to think these are evidence of flowing ice on Mars. For example, we hiked along a moraine composed of jumbled rocks the Grinnell Glacier left behind as it flowed downhill. In addition to the remains of the (rapidly disappearing) glacier itself, we also saw typical glacial erosional structures such as U-shaped valleys, hanging valleys, and cirques. For a HiRISE image of cirque-like features, see PSP_005730_1405.

On one of our field trips, we were accompanied by reporter Michael Jamison of The Missoulian. This story was on the front page of the paper the following day:

I thought the story was really good – a quirky (but so are we!) description of why we would want to stare at the rocks in such a magnificent setting, and their relevance to our mission to Mars. We all thought it was funny when he called Alfred McEwen, our Principle Investigator, a “Marsman”!

HiRISE Team in Glacier National Park, in front of a classic U-shaped valley carved by glacial erosion.

Today a press release went out about a forthcoming paper in the journal Geology (click here for full text online or here to download a PDF). John Grant, a Co-Investigator on the HiRISE science team, is the lead author, and most of the co-authors are also on our science team.

What is a megabreccia? A breccia is a jumbled-up mixture of broken rocks, cemented together by a finer-grained material. We see them in impact craters and volcanoes on the Earth, places where there was a lot of violent energy to break up rocks. A megabreccia is just a larger version of that – something we can see with HiRISE resolution, as opposed to something you’d have to pick up in your hand to identify. The megabreccia in Holden formed when the explosion that opened the crater shattered rocks, mixed them up, and then the fragmented ejecta collapsed back down into the crater. Before HiRISE, we didn’t have the resolution to detect these textures.

This is a cutout of an image taken in Holden Crater, showing the megabreccia texture, in false color as usual. A context map is shown to the right, showing where in the crater rim this image is located (click these images to enlarge). The blocks here are mostly darker, and they’re embedded in a lighter-toned material. The dark chunks are kind of “scooped out,” which means they’re more easily eroded than the surrounding light-colored rock. Scientists think this may be because they’re sedimentary rocks, formed at the bottom of a lake or river. The stripey dark blobs on top are sand dunes that are slowly covering up the area again.

This megabreccia is located in an area scientists find fascinating for other reasons, too: there are clays that were laid down over a long period of time when it had to be wet. This implies there was once a lake in this crater – perhaps more than once over its history. At one point, the lake broke through the rim of the crater, releasing a huge flood of liquid water. You can see the channel this formed in the context map above. This flood eroded away material that was covering the megabreccia, exposing it for HiRISE to see.

The HiRISE image PSP_003077_1530 shows another part of Holden Crater, and the caption includes more information about the geologic history of the area.