Why Is Mars Red But Earth Is Blue?

The grad students’ Crazy Theory Contest event at Al’s coffee shop is breaking up.  Amanda’s flaunting the Ceremonial Broom she won with her ‘Spock and the horseshoe crabs‘ theory.  Suddenly a voice from behind me outroars the uproar.  “Hey, Mars guy, I got questions.”

Jim looks up and I look around.  Sure enough, it’s Mr Richard Feder.  I start with the introductions but he barrels right along.  “People call Mars the Red Planet, but I seen NASA pictures and it’s brown, right?  All different kinds of brown, with splotches.  There’s even one picture with every color in the rainbow.  What’s with that and what color is Mars really?”

Jim’s a newly-fledged grad student so I step in to give him a chance to think.  “That rainbow picture, Mr Feder, did it have a circular purple spot about a third of the way up from the bottom and was it mostly blue along the top?”

“Yeah, sounds about right.”

“That’s a NASA topographic map, color-coded for relative elevations, purple for low areas to red high-up.  The blue area is the Northern Lowlands surrounding the North Pole, and that purple spot is Hellas Basin, a huge meteor crater billions of years old.  It’s about 5 miles deep which is why they did it in purple.  The map colors have nothing to do with the color of the planet.”

“About your question, Mr …. Feder is it?”

“Yeah, kid, Richard Feder, Fort Lee, New Jersey.”

“Good to meet you, sir.  The answer to your question is, ‘It depends.’  Are you looking down from space or looking around on the surface?  And where are you looking?  Come to think of it, when are you looking?”

“All I’m asking is, is it red or not?  Why make it so complicated?”

“Because it is complicated.  A few months ago Mars had a huge dust storm that covered the whole planet.  At the surface it was far darker than a cloudy moonless night on Earth.  From space it was a uniform butterscotch color, no markings at all.”

“OK, say there’s no dust in the air.”

“Take away all the floating dust and it almost wouldn’t be Mars any more.  The atmosphere’s only 1% of Earth’s and most of that is CO2 — clear and colorless.”

“So what would we see looking down at the surface?”

“Uh … you’re from New Jersey, right?  What color is New Jersey’s surface?”

<a little defensively> “We got a lot of trees and farms, once you get away from all the buildings along the coast and the Interstates, so it’s green.”

“Mars doesn’t have trees, farms, buildings or roads.  What color is New Jersey underneath all that?”

“The farmland soil’s black of course, and the Palisades cliffs near me are, too.  Down-state to the south we got sand-colored sand on the beaches and clay-colored clay.”

“Mars has clay, the Curiosity rover confirmed that, and it’s got basalt like your cliffs, but it has no soil.”

“Huh? How could it not have soil?  That’s just ground-up rocks, right, and Mars has rocks.”

“Soil’s way more then that, Mr Feder.  If all you have is ground-up rocks, it’s regolith.  The difference is the organic material that soil has and regolith doesn’t — rotted vegetable matter, old roots, fungus, microorganisms.  All that makes the soil black and helps it hold moisture and generally be hospitable to growing things.  So far as we know, Mars has none of that.  We’ve found igneous, sedimentary and metamorphic rocks just like on Earth; we’ve found clays, hematites and gypsum that had to have been formed in a watery environment.  But so far no limestone — no fossilized shelly material like that would indicate life.”

“What you’re saying is that Mars colors look like Earth colors except no plants.  So why do astronomers call Earth a ‘pale blue marble’ but Mars is ‘the red planet’?”

“Earth looks pale blue from space.  The blue is the dominant color reflected from the 70% of Earth’s surface that’s ocean-covered.  It’s pale because of white light reflected from our clouds of water vapor.  Mars lacks both.  What Mars does have is finely-divided iron oxide dust, always afloat above the surface.”

“Mars looks red ’cause it’s atmosphere is rusty?”

“Yessir.”Earth and Mars

~~ Rich Olcott

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