Up until about 2 billion years ago, the sun was 30 percent dimmer than it is now. ██ ███ ███ ████ ████ ███ ████ ███ ██████ █████ ██ ██████████ ███████ █████████ ██ ██████ █████████ ████████ ████ ███ ██████ █████ ████ ████ ███████ ██ █████ ██ ███ ███████ █████ ████
Why was liquid water present back when the sun was so dim?
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.
Which one of the following, ██ █████ ████ █████ ██ ███████ ███ ████████ ███████████ █████████ ██████
Our atmosphere currently █████ ██ █████████████ ████ ████ ████ ██ ███ ███ ███████ █████ ████
The liquid water ███████ ███ ███████ █████ ███ █████ ██████ ████ ██ ████ █████ █████ █ ███████ █████ ████
A significant source ██ ████ █████ ████ ███ ███ ███████████ ██ ███ ███████ ██ ███ ██████ █████████████ █ ███████ █████ ████
Evidence suggests that ███████ ███████ ██ █████ ████████ ██████ █████ ████ ████ ████████ ████ █ ███████ █████ ████
When large portions ██ ███ █████ ███ ████████████ ████ ██ ███ █████ ████ ██ █████████ ███ ███ ████████ ██ ███ █████ ████ ████ ████ ███ █████ ███ ████████████