Sciency Words: Recurring Slope Linea (RSL)

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:


By now, I’m sure you’ve heard the news. In fact, this is kind of old news. There’s water on Mars. Liquid water. On the planet’s surface. It was discovered through spectral analysis of something called a recurring slope linea or RSL (plural: recurring slope lineae or RSLs).

I suppose recurring slope linea is really three words, so lets examine each word individually:

  • Recurring: these things go away and come back, apparently due to the changing of the Martian seasons.
  • Slope: they appear on sloped terrain, usually with inclines between 25º and 40º.
  • Linea: this is a fancy Latin word meaning straight line. In both geology and astronomy, lineae are lines on the surfaces of planets or moons, like the criss-crossing pattern of lines on Europa.

So RSLs are straight lines, only a few feet wide but often many miles long, that appear and disappear on sloped surfaces on Mars in correlation with seasonal temperature changes. It seems Mars has an embarrassing problem. The Red Planet just can’t keep itself from…

Ja04 Mars Interrupts

Anyway, this raises a big question, something for scientists and science fiction writers alike to ponder. Where is all that water coming from?

The obvious answer is that RSLs are just the tip of the iceberg (pun intended). Vast quantities of water must be trapped beneath the planet’s surface. Much of this water—though perhaps not all of it—is frozen, and during warmer seasons the top most layer of ice starts melting.

As much as I like that explanation and what it implies about Mars’s habitability, there’s a problem. If RSL water is coming from underground, we should expect it to first appear at lower elevations. Instead, RSLs tend to originate uphill and slowly trickle downward.

An alternative explanation is that the water has an atmospheric origin. Perchlorate salts are common in Martian soil, and these salts have a way of sucking water vapor out of the air.  While this would mean that water is present uphill, downhill, and everywhere in between, significant water flow might only be noticeable on sloped surfaces.

But there’s a problem with that explanation too. I mean, have you seen Mars? Do you really think the Martian atmosphere contains that much water vapor? It seems unlikely, but some scientists say it’s not completely impossible.

So we’re left with an enigma. We now know Mars has liquid water, at least seasonally. But where the heck does it come from?

Correction: I previously stated that water accumulates in Martian soil due to the condensation of atmospheric water vapor into frost, overlooking the role perchlorate salts play.  Condensation is not currently believed to be an important factor in the formation of RSLs.

7 thoughts on “Sciency Words: Recurring Slope Linea (RSL)

  1. Interesting. I never realized the origin of the water was a mystery. I just took it for granted that it was coming from underground. It seem though if that were that much humidity in the atmosphere, that one or more of the rovers would have detected it.


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