"Another etymology, proposed in Böhtlingk's Dictionary, which derives Marut from a root mar, to shine, labours under two disadvantages; first, that there is no such root in Sanskrit; secondly, that the lurid splendour of the lightning is but a subordinate feature in the character of the Maruts."None of these facts have changed. The verb mṛṇā́ti 'crushes, grinds' is always available to the Sanskrit etymologist but a verb root paralleling Greek marmáirein 'to shine' is absent. Does a 19th-century scholar still have a point? Have Indo-Europeanists gotten ahead of themselves attributing a PIE root behind every relationship blindly? Skepticism concerning this root, in regards to another meaning given to it which strives to explain Greek words relating to 'portion' and 'fate', is echoed more recently by Peter Schrijver in Indo-European *(s)mer- in Greek and Celtic published in Indo-European Perspectives (2004): "Yet the other cornerstone of IE reconstruction beside archaic morphology, viz. comparative evidence from other IE languages, would seem to be almost completely lacking."
If मरुत marút may be so etymologized, such that these storm gods 'crush' and 'pummel' with thunder rather than 'shine' through lightning, then surely so may Sanskrit márīci- 'mote or speck in the air' or 'particle of light' be likewise attributed to the homophonous root referring to crushing, grinding and wearing things away. Latin merus 'pure, unmixed, unadulterated' can also make better sense this way too (ie. 'worn away' → 'mere' → 'unadulterated', 'pure', 'bare'). Nothing here requires a source from 'to shine' and the issue seems even to become burdened by extra assumptions when we do. So it really begs the question whether it existed at all in PIE. Perhaps we should wonder from where Greek obtained marmáirein and related words pertaining to 'shining' if not from PIE and resist a biased tendency to see Indo-European in everything beyond what's sensible.