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.

• high resolution images of Mars from the HiRISE camera;
• chocolate cake; and
• all the visiting team members, press, and other guests participating in the excitement.

The images that start arriving today should be 10 times higher resolution than the test images we took in March. The chocolate cake we get on special occasions is better than any other desert except cheesecake. Finally, it is great to see face to face our remote team members, and it is nice to have our building fill up with visitors.

Now it is time for a bagel, the fourth thing I was looking forward to today.

What are we doing?

• Uplink people have planned the images in detail. We know exactly what spots on Mars we’ll be aiming the camera at. That involved a lot of work with our planning tools, using predictions of exactly where the spacecraft will be at any given time in the orbit. We wrote and delivered our command files earlier this week, so there would be time for testing. The commands were actually radiated to the spacecraft this morning, and are on board MRO right now!

• Downlink people are getting everything ready to receive and process the data. Our hardware and software needs to be configured and tested, so that tomorrow when the images start coming down, hopefully everything will go smoothly.

We’re all reviewing the test runs of the imaging as well. There is a testbed we call the “OTB” (Orbiter Test Bed). We’re able to simulate our command sequences on it in order to make sure everything is correct. We even get fake data (usually all zeros) back from it, so that we can make sure the “images” are the right size, etc.

We expect a lot of visitors at HiROC starting tomorrow. A number of our scientific co-investigators will be visiting for a few days, to see the images first-hand and help us prepare public releases. Members of the press also want to join us to capture the excitement of seeing the first images come down (and hopefully we’ll be able to convey that on this blog, too!).

So we’re all very busy, but we’ll try to keep you updated on what’s going on here!