Across
1
The Red Planet
3
Biological catalyst
6
Gas essential for breathing
7
Molten rock beneath Earth surface
9
Force that attracts objects together
13
Basic unit of life
Down
2
Second lightest element
4
Nerve cell in the brain
5
Smallest unit of an element
8
Tectonic landmass section
10
Path of a planet around the sun
11
Center of an atom or cell
12
Measure of matter in an object