New homework

I posted a new homework (due Thursday Feb 28th) on the class webpage. One of the questions there was first posed to me over dinner (a long time ago when I lived in Toronto) by the well-known astronomer Scott Tremaine. He asked "if human life on Earth were to end, for how long would aliens visiting the Earth be able to find evidence that our civilization once existed?". It led to a lively discussion! There's no single right answer, so I'm hoping people will think about and discuss this over the next week... it will be interesting to see what the class comes up with!