Test the origins of Mars’s moons

Martian Moons eXploration, or MMX, is Japan’s mission to investigate Phobos and Deimos and return material from Phobos. Its origin question has two leading explanations: Mars captured bodies formed elsewhere, or the moons assembled from debris following a large impact on Mars. Dark, asteroid-like spectra motivate comparisons with primitive small bodies; orbital geometry also has to fit any proposed history. A persuasive explanation must address both composition and dynamics.

Imagine two detectives examining the same object. One studies what it is made of; the other reconstructs how it arrived. Agreement between those investigations is more valuable than an attractive explanation that answers only one. Before collecting evidence, write a prediction for each hypothesis and specify what observation could weaken it. A question becomes testable when an unfavorable result is allowed.

The photograph here was acquired by the Mars Reconnaissance Orbiter’s HiRISE camera on March 23, 2008. Its blue-green, red and near-infrared channels produce enhanced color. Grooves, craters and regional contrasts are observations worth investigating. The picture is target context for MMX; it is neither an MMX image nor proof of a particular formation history.

For an original reasoning exercise, suppose a laboratory finds two kinds of grains in a returned sample: a common dark material and a few grains with a different chemical signature. What additional information would help? Their proportions, textures, isotopic measurements and collection context could test different histories. Merely noticing that the sample is mixed does not tell you when or how the mixing occurred.

Phobos may accumulate material ejected from Mars by later impacts. Consequently, a grain with a Martian origin would not by itself prove that the whole moon formed from Mars. Samples can preserve multiple episodes: original assembly, impacts, surface alteration and later deposition. MMX combines remote measurements, local observations and planned laboratory analysis to connect these scales.

Think of a handful of sand collected beside a road. A fragment of pavement in the handful does not establish that every grain came from the road. To test the source of the whole deposit, compare many grains and their setting. The same reasoning helps separate a moon’s bulk origin from material added to its surface later. State the scale of the claim as carefully as the scale of the measurement.