Timothy and his family come to Mars with the pretext that they're just off for a holiday jaunt. Surprise: they're not. They're resetting, and Timothy is either in for an amazing adventure or a lifetime of loss and soul-killing isolation. He could end up reconquering Mars (it's okay this time, since there are no Martians), or his whole family could die from an unknown disease.
Timothy's age is key to the openness of this ending (hey, unless it's actually a beginning!). He's somewhere between the supposedly carefree days of childhood and the anxious responsibilities of adulthood.
As he's trying to figure out what exactly his family is doing on Mars, he thinks that "Just behind the veil of the vacation was not a soft face of laughter, but something hard and bony and perhaps terrifying. Timothy could not lift the veil, and the two other boys were busy being ten and eight years old, respectively" (43). In other words, he's too young to figure out what's going on—but too old not to know that something is definitely up.
We have to say, this is kind of a nice way to end. The book has focused a lot on the past (see "Memory and the Past" under "Themes"), but—despite the nuclear war that's just wiped out most of Earth—we close with a sense of hope and adventure.