NASA is searching for life in Congress for support of a planetary science budget, so these announcements must be taken with a big dose of sodium chloride.
Back in 1976, NASA flew the twin Viking missions to Mars, each with its own orbiter and stationary lander. All were quite successful. But at what a cost: something close to a cool billion dollars back then; that would be maybe four or five billion today. And there was another cost. To get support for the mission, NASA had to drum up expectations of finding some positive result from the life detection experiments on board and so these experiments took up most of the scientific payload at the expense of the more usual array of geophysical instruments. No life signs were found, the popular press declared a failure, and serious funding for Mars exploration dried up for nearly twenty years.
The more recent NASA probes including Pathfinder, Odyssey, Phoenix, and the twin rovers have all done extremely well and have in total produced far, far more science per dollar than did Viking. These probes have done so in part because the emphasis wasn't on life detection -- iffy at best -- but on good old geology and chemistry experiments that were guaranteed to produce lots of valuable knowledge no matter what.
Could NASA be setting itself up for another Viking-like episode with tales of possible life on Europa and Enceladus? Could life-detection instruments once again shove aside less exciting but more productive geophysical experiments? Since Congress is inhabited mostly by the scientifically illiterate, you can guess how I'll bet.