Self-study
Up until about 2 billion years ago, the sun was 30 percent dimmer than it is now. ██ ███ ███ ████ ████ ███ ████ ███ ██████ █████ ██ ██████████ ███████ █████████ ██ ██████ █████████ ████████ ████ ███ ██████ █████ ████ ████ ███████ ██ █████ ██ ███ ███████ █████ ████
"Surprising" Phenomenon
Why was liquid water present back when the sun was so dim?
Objective
A hypothesis resolving this discrepancy must explain how Earth had liquid water 3.8 billion years ago. It must either identify a source of water other than the oceans or state a difference between Earth today and Earth 3.8 billion years ago that allowed the oceans to contain liquid water before the sun brightened.
Analysis by LukeWilson
13.
Which one of the following, ██ █████ ████ █████ ██ ███████ ███ ████████ ███████████ █████████ ██████
a
Our atmosphere currently █████ ██ █████████████ ████ ████ ████ ██ ███ ███ ███████ █████ ████
b
The liquid water ███████ ███ ███████ █████ ███ █████ ██████ ████ ██ ████ █████ █████ █ ███████ █████ ████
c
A significant source ██ ████ █████ ████ ███ ███ ███████████ ██ ███ ███████ ██ ███ ██████ █████████████ █ ███████ █████ ████
d
Evidence suggests that ███████ ███████ ██ █████ ████████ ██████ █████ ████ ████ ████████ ████ █ ███████ █████ ████
e
When large portions ██ ███ █████ ███ ████████████ ████ ██ ███ █████ ████ ██ █████████ ███ ███ ████████ ██ ███ █████ ████ ████ ████ ███ █████ ███ ████████████