Complex sentence: The Mars Terahertz Microsatellite is a planned spacecraft that will be carrying a terahertz sensor to the surface of Mars to measure the oxygen isotope ratios of various molecules in the Martian atmosphere.
Split into simpler sentences: The Mars Terahertz Microsatellite is a planned spacecraft that will be carrying a terahertz sensor to the surface of Mars. The terahertz sensor will observe the oxygen isotope ratios of various molecules in the Martian atmosphere.