No matter in which time period a book is set, a good story about kids having adventures, solving problems independently, and realizing that adults are human is always welcome. The Lion of Mars takes place 70 years in the future, a time in which different countries have set up colonies on Mars. Something has taken place in the US that has resulted in the existence of a state called "New California," but there's no background on whether the colonies are for purely scientific exploration or because of problems on Earth. But whatever issue brought the colonists to Mars, it doesn't loom large in their lives.
We see life in the colony through the eyes of Bell, an 11-year-old boy. The personalities of and relationships between the children are familiar and comfortable. Kids will be able to see themselves in at least one of the characters. The relationships between the children and adults is less familiar, however, in that the children do not have a parental relationship with any particular adults. They follow the philosophy of "it takes a village," or in this case, "it takes a colony." In fact, I'm not sure if there is even a biological relationship between any of the adults and children.
Most of the story in Life on Mars revolves around a crisis within the Earth colony. The kids have to use bravery, smarts, and a willingness to break the rules to solve the problem. Books in which kids are the heroes, acting autonomously to help adults and/or clean up messes created by adults, are usually quite popular with children. Especially if the adults are presented as whole and realistic characters (i.e., not one-dimensional dunderheads). The kids are also realistically drawn, with diverse personalities. There's even a goth girl (apparently that's still a thing in 2092!). Although the kids are presented in an overall rosy light, they aren't perfect, which keeps them interesting.
An interesting feature of the characterization is that the issue of race does not really arise. Bell describes himself as being in the middle of a range of skin tones. That's pretty much the only mention of race or ethnic heritage. I have to say I was suprised that the characters all appeared to be cisgender. There were no characters specifically described as binary or transgender. Also the romantic relationships were all heterosexual. And the characters pretty much remained within traditional gender roles. There is a female captain in one of the other colonies, but that's not so unusual these days; and she has a much softer personality than the male captain. I would have thought that 2092 would have more LGBTQ representation and more diversity in gender roles than 2021. I suspect that this may have more to do with the targeted age range (8-12 yrs) than the author's actual vision of the future.
The story ticks a lot of genre boxes. It takes place in the future, so we have science fiction. There are elements of conflict and adventure when the kids have to solve the problem, and there's some mystery surrounding the relationship of the US colony to the other colonies. The mystery and the story are tidily -- and rather quickly and easily -- wrapped up. Jennifer Holm is an always reliable and popular author, and Lion of Mars does not disappoint.