ET v. Shadow Life
Why is this important? Well, because they are neighbours in the periodic table, arsenic shares many chemical properties as phosphorous, and phosphorous is an essential element for life as we know it: amongst other things, it is at the heart of molecules that store and transfer energy, and helps to form the backbone of RNA and DNA. Arsenic is a poison to many organisms because it interferes with phosphorous biochemistry, but although the bonds it forms are weaker, it could also substitute for phosphorous; in other words, there may be organisms with biochemistries based on arsenic rather than phosphorous, forming a shadow biosphere in parallel with our own. Several of the scientists mentioned in NASA's note have been searching for signatures of that shadow biosphere like that, in places like Mono Lake, California, which have higher than average concentrations of arsenic. If they've found evidence for it, there are all kinds of implications, not least that life may have evolved more than once on Earth. And that's genuinely exciting.
By the way, I published a short story about searching for a shadow biosphere last year. 'Shadow Life' is still online, at Discover magazine's site. Read it now, before science overtakes it tomorrow!