NASA has started using these press conferences for what would be considered minor discoveries by most astronomy departments worldwide. They're doing this because they're afraid of losing funding and relevance, and think that by hyping their discoveries they can postpone the inevitable attrition of their funding and projects.
Look back at their recent press conferences. It's like the boy who cried 'wolf'. When you embargo the announcement of extremely minor scientific advancements and make a big deal out of them, you contribute to the abrogation of scientific discoveries which actually matter. NASA is becoming toxic to scientific advancement and federal funding of scientific programs.
http://www.nasa.gov/mission_pages/cassini/whycassini/cassini...
Oxygen and CO2 on Rhea; suggesting an easy route to complex chemistry near gas giants (which are so much easier to find outside our solar system).
Don't get me wrong, I still think that, on their own, these discoveries are amazing and the fact that we do have a bunch of robots roaming Mars is quite encouraging. But perfectly good science is not as exciting as the news media would like it to be.
The idea behind the arsenic theory is that arenate, which has 4 O atoms, could provide act as a suitable substrate during Oxygen Evolving Complex to provide the same protons and electron break down.
There is also a parallel chain of thought that arsenic is similar in its makeup to phosphorus, which is needed to create DNA and other genetic material, and so it could help cell division. However, I'm doubtful as arsenic is toxic in any real quantity and thus would be counterproductive to that.
However, with the problems arsenic causes blocking PDH enzyme, causing H2O2 in the cells and a huge list else like IIRC thiamine deficiency.
Then again, life exists in such a broad range of conditions that I find it hard to believe that life couldn't adapt to life with arsenic for one main reason. If it is using arsenic as a DNA base, it means it is sequestering the arsenic rather than leaving it in the cells to do damage.
If you are at all a bio geek, you really must check it out - it's a low-oxygen, alkali-rich simple ecosystem that supports a clearly visible food chain of about just 5-6 spices. The place is weird, creepy and totally amazing.
What was supposed to be a romantic road trip planned by my girlfriend for my birthday became an unexpected biology field trip.
[1] http://www.newscientist.com/article/dn14537-arseniceating-ba..., [2] http://en.wikipedia.org/wiki/Mono_Lake
