Across
1
Path of a planet around the sun
4
Chemical element in table salt
5
Preserved remains of ancient life
8
Compound containing oxygen
12
Basic unit of life
Down
2
Unit of energy
3
Mass per unit volume
6
Natural satellite of Earth
7
Nerve cell in the brain
9
The Red Planet
10
Center of an atom or cell
11
Substance with pH below 7
13
Naturally occurring inorganic solid