Elder offspring: Since soccer season is over, you should take us someplace fun on Saturday.
Younger offspring: Aww, do you have to go to commencement?
Dr. Free-Ride: Yes, I have to. This year I’m a commencement marshal.
Younger offspring: A commencement Martian?! Oh no!
Elder offspring: Do you get to carry a ray-gun?
Younger offspring: Why does that Martian in the Looney Tunes cartoons wear a skirt?
Elder offspring: And why is his helmet so big?
Dr. Free-Ride: I don’t know. Why do you suppose creatures like him are what people imagine when they think of life on Mars?
Younger offspring: Because people watch a lot of cartoons.
Dr. Free-Ride: Seriously, though, what do you think Martian critters would be like if there were life on Mars?
Elder offspring: It would depend on the creature.
Dr. Free-Ride: Sure, but there are some pretty big differences between the conditions on Mars and the conditions on Earth. You’d figure those might make a big difference to what kind of features a creature would need to survive on Mars.
Younger offspring: A Martian would need dogs and cats and horses.
Dr. Free-Ride: What??
Younger offspring: To get milk.
Elder offspring: Wouldn’t that be cows?
Dr. Free-Ride: Back up a minute, kiddo. Before we worry about whether a Martian can get a glass of milk, I think there are some bigger issues to reckon with. How is Mars different from Earth?
Younger offspring: No oxygen.
Elder offspring: And because it doesn’t have an atmosphere like Earth’s, it can get much hotter and much colder, all in the same day.
Dr. Free-Ride: Yep. So, I’m trying to imagine a life form that can deal with big temperature extremes and that doesn’t need oxygen. We have anaerobic bacteria, but most of the critters we’re familiar with here on Earth depends on oxygen to keep them alive.
Elder offspring: Martian critters would have to use something else the way we use oxygen.
Younger offspring: Oxygen has to do with why we have blood, right?
Dr. Free-Ride: Uh huh. Remember you asked me last time you needed a bandage why we have blood?
Younger offspring: Yeah. It’s because we need the blood to take oxygen to all the parts of our body.
Elder offspring: Otherwise, each cell would have to find some way to get its own oxygen.
Dr. Free-Ride: It would sort of take the fun out of being multicellular, wouldn’t it?
Younger offspring: But why do our cells need oxygen?
Dr. Free-Ride: Well, they use oxygen in the chemical reactions they do to that keep us alive. I’m pretty sure the reactions that turn our food into energy to move our muscles, for example, all require oxygen.
Younger offspring: That’s why we’d need spacesuits on Mars.
Dr. Free-Ride: Now, maybe there are some other ways you could make a multicellular critter that don’t depend on reactions with oxygen, but as an Earthling, I have a hard time thinking what they’d be like.
Younger offspring: Also, what would they eat on Mars?
Elder offspring: Maybe Martian critters would eat stardust.
Dr. Free-Ride: OK, stardust might be in more ready supply to a Martian than strawberries or soybeans or cow’s milk, but I have no idea what a life form would need to be like in order to “metabolize” stardust.
Elder offspring: The universe is a big and mysterious place. It makes life forms go weird.