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.
13.
Which one of the following, ██ █████ ████ █████ ██ ███████ ███ ████████ ███████████ █████████ ██████
a
Our atmosphere currently █████ ██ █████████████ ████ ████ ████ ██ ███ ███ ███████ █████ ████
This explains the presence of liquid water 3.8 billion years ago. The atmosphere held more heat back then, so the oceans were not frozen despite the dimmer sun.
b
The liquid water ███████ ███ ███████ █████ ███ █████ ██████ ████ ██ ████ █████ █████ █ ███████ █████ ████
This does not explain how liquid water was present in the first place. The sun only brightened about 2 billion years ago, so the presence of water 3.8 billion years ago remains a mystery.
c
A significant source ██ ████ █████ ████ ███ ███ ███████████ ██ ███ ███████ ██ ███ ██████ █████████████ █ ███████ █████ ████
This widens the discrepancy by suggesting the oceans were frozen until about 2 billion years ago. It does not explain the presence of liquid water 3.8 billion years ago.
d
Evidence suggests that ███████ ███████ ██ █████ ████████ ██████ █████ ████ ████ ████████ ████ █ ███████ █████ ████
This does not address the presence of liquid water 3.8 billion years ago. It implies that parts of Earth were colder than expected 2 billion years ago, but makes no mention of Earth 3.8 billion years ago.
e
When large portions ██ ███ █████ ███ ████████████ ████ ██ ███ █████ ████ ██ █████████ ███ ███ ████████ ██ ███ █████ ████ ████ ████ ███ █████ ███ ████████████
This implies frozen oceans are likely to remain frozen and liquid oceans are likely to remain liquid, without addressing Earth 3.8 billion years ago. If the oceans were liquid back then, it remains unexplained why they were liquid.