Now we can Call Martian Regolith “Soil” Thanks to Phoenix

This is just one of those niggles I’ve felt ever since I started working on Mars projects and articles. How can “Mars soil” be an accurate description of the stuff that sits on the surface of Mars? You see it written everywhere, from NASA to New Scientist, writers have referred to Martian regolith as soil. Why is this? Is regolith and soil that much different? Perhaps I was just getting my knickers in a twist for no reason; perhaps they were the same thing after all. So back to basics, I grabbed for my trusty old dusty dictionary and stopped leafing through the pages at “S”… there, soil. Now for “R”… got R but no regolith (wasn’t that a word in 1980?), just regorge (that isn’t pretty). So I get online and do my research 21st Century style: Google.

I found my answer, but it turns out recent data from the Phoenix Mars lander has complicated matters… apparently the writers at NASA and New Scientist were right all along (even though they didn’t realise it)…

OK, so what are the best possible ways to describe a soil? I found a couple of definitions:

• The material on the surface of the ground in which plants grow; earth – Cambridge Dictionaries.
• The top layer of the earth’s surface, consisting of rock and mineral particles mixed with organic matter. – Answers.com

So, as far as I can tell, the key factors that constitute a soil are:

1. It contains rock particles.
2. It contains mineral particles.
3. Plants grow in it.
4. It contains organic material.

Looking up regolith online, I get these definitions:

• The layer of rocky or icy debris and dust made by meteoritic impact that forms the uppermost surface of planets, satellites and asteroids. – About.com
• The layer of loose rock, resting on the bedrock, that constitutes the surface of most land (and the surface of the moon etc.). – Allwords.com

So the key factors that constitute regolith are:

1. It contains rock, ice or dust particles.
2. It’s made by meteorite impacts on planets, asteroids or satellites.

So regolith could be viewed as inorganic, whereas soil is organic, although they do share some characteristics:

1. It contains rock particles.
2. It contains mineral particles.
3. Plants grow in it.
4. It contains organic material.

That was of course until the Phoenix Mars lander uncovered something quite amazing about Martian regolith: it is more like terrestrial soil than we previously believed. In fact, Martian “regolith” (in my view) is only one step away from being called a proper “soil” (it turns out that you could grow plants in the stuff!) – whether or not the Mars regolith actually contains organic material. For organic material to be there (and I don’t mean organic molecules), something had to be living there. For that historic discovery I think we’ll be waiting for a long time yet (if at all)…

Even so, I have started calling Mars regolith “Mars soil” – three out of four characteristics isn’t that bad!

For more on the recent Phoenix findings, check out my article “Phoenix: Mars Soil Can Support Life” at the Universe Today.