NASA’s top officials and the Mars rover missions’ top scientists will announce “a major scientific finding” at 2 p.m. ET Tuesday, the space agency announced today. The last time a Mars news briefing was announced just a day in advance, it was to report geological hints that the Opportunity rover’s landing site was “drenched” with water many millions or even billions of years ago. Those were considered “significant findings.” So how does a major finding compare? One could argue that “major” outranks “significant,” because NASA Administrator Sean O’Keefe is due to give opening remarks this time around.