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Behind the Water Ice Decision

by Keri Bean

June 23, 2008 -
I can’t believe I’m approaching my last week here at the SOC. I have to go back to Texas A&M to take some summer classes. I leave Tucson Saturday and start classes Tuesday. I’m certainly going to miss it here.

So I run the Phoenix Lander Facebook page (and the Mars Rovers page, go add them both!) and it’s really good to reach out and be able to answer the questions people have.  I recently posted a blog for Phoenix on there about how the water ice decision was made, and I’ll go ahead and repost it here, so please excuse the anthropomorphic nature in the blog.

So a lot of people have been asking me really good questions like "How can you be sure it's water ice and not dry ice?" or "Why didn't TEGA find water?" I thought I'd take a few minutes to explain how the process has gone and answer a few of the questions I've gotten.

The first indication of water ice was when the robotic arm camera (RAC) looked under me and saw a huge patch of something. The only thing we could tell is that it was white and really bright in sunlight. Some of the team immediately jumped to the conclusion it was ice, most wanted more evidence.

The second major discovery was when I started digging with my robotic arm (RA) and in the trench, a white streak was evident. Some more team members immediately jumped to the conclusion it was ice, more still wanted more evidence. I dug another trench next to the one I had just dug, and more of the white stuff showed up. I dug again and even more showed up! The team commanded me to do was to expose more of the white stuff, so I combined all 3 trenches and dug a bit deeper. More of the white stuff was exposed. By this time, half the team said it was ice, half said it was some sort of salt. A lot were skeptical because they couldn't believe my entire mission was going right.

The next thing I did was take what's called a multispectral spot. In front of my CCD cameras, I have a filter wheel. There are many filters that can see in all sorts of wavelengths in the visible and infrared range. When you combine all these filters in an image, you can make a spectrum that can tell you what light is being reflected from that certain spot in the image. I took one of the white spot, and to make sure we're calibrated right, we do the multispectral spot on a calibration target, or caltarget for short. These are pre-tested before launch and we know what we should see from them and can adjust the other images accordingly based on that data. When we took the spot on the white stuff, it was very similar to the spectrum of water. It was off the charts in the blue part of the spectrum. This was even better evidence of the white stuff being water ice.

The final nail in the coffin on the salt theory was when we looked in the trench after a few days and some of the chunks were gone. They must have sublimated, or gone from a solid to gaseous state without becoming liquid, to have disappeared. Salts do not do that.

Some ask "Why didn't TEGA see ice in its bake?" Because of the TEGA problem of filling the oven, the sample sat out in the sun for a few sols. Since it was exposed, the water ice would sublimate away before the doors could be closed. Engineers are currently testing new methods with the test model of me back in Tucson of sample delivery to try and get some of the water ice into TEGA.

Some ask "Why isn't it dry ice?" First off, dry ice can't exist in this region quite yet because it simply isn't cold enough. Dry ice needs to be below -78.5 degrees Celcius (-109 degrees Fahrenheit) on Earth to be a solid. This is even much lower for on Mars because of the lack of atmospheric pressure. I simply haven't seen the sorts of temperatures necessary for dry ice so far.

Some ask "Why can't it be some chemical we've never seen before that acts like that?" While it very well could be some chemical we've never seen before, the evidence is strongly against this hypothesis. If it looks and acts like water ice, then it probably is water ice.

Well it's about bedtime for me, so see ya'll next time!