Across
1
Substance with pH below 7
3
Form of thermal energy
6
Compound containing oxygen
8
Nerve cell in the brain
10
Ball of burning gas in space
12
Second lightest element
13
The Red Planet
Down
2
Our home planet
4
Blocking of light by a celestial body
5
Curved glass that refracts light
7
Natural satellite of Earth
9
Mass per unit volume
11
Absolute temperature scale unit