One of NASA’s leading planetary scientists and a renowned expert on the possibility of life on Mars, Dr. Christopher P. McKay, will present a lecture at the University of Arkansas at 4 p.m. Tuesday in Giffels Auditorium. Titled “Life on Mars: Past, Present and Future,” McKay’s presentation will discuss evidence that, early in Mars’ history, the planet had liquid water, more active volcanism and thicker atmosphere — conditions remarkably similar to those of Earth. In fact, Mars exhibited these conditions about 3.5 billion years ago, about the same time that life appeared on our planet.