Recurring Slope Lineae

  • Kirby RunyonEmail author
  • Lujendra Ojha
Living reference work entry


Recurrent Slope Lineae (RSL) Equator-facing Slopes Southern Midlatitudes Mars Year Deliquescence Relative Humidity 
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Recurring slope lineae (RSL) are elongate, low albedo (dark) features with anastomosing and/or digitate flow-like morphology downslope and often originating at the base of outcrops (McEwen et al. 2011c; Runyon et al. 2012) that exhibit both incremental growth and recurrence during multiple Mars years (MY).



RSL are narrow (up to 5 m wide), dark surface markings (up to 40 % darker than their surroundings) that extend incrementally downslope on steep (25–40°) rocky slopes of Mars (McEwen et al. 2011b). They are active (formation and incremental growth) during warm seasons and fade and often completely disappear during colder seasons but recur over multiple MY.

To date, they are mostly found on equator-facing slopes of southern midlatitude of Mars (McEwen et al. 2011c; Ojha et al. 2013, 2014) but have been recently discovered in the equatorial regions as well (McEwen et al. 2014). In southern midlatitude, RSL exhibit incremental growth during Ls 250–340 (Ls is the areocentric longitude of the sun, a measure of season on Mars. Southern summer begins at Ls 270.), when temperature ranges between 240 and 290 K in equator and west- and east-facing slopes (Ojha et al. 2014). In equatorial regions, RSL activity has been documented during Ls 50–288 (McEwen et al. 2014).

In some cases they occur in small numbers (tens of lineae), but in other locations hundreds or thousands of them may form. They are observed to divert around obstacles, and individual lineae may split or merge. They extend downslope from the base of outcrops usually in clusters and terminate often at the steep slope before its end, suggesting their length could be controlled by the volume of mobile material (McEwen et al. 2011a).


They are 0.5–5 m wide and up to hundreds of meters long.


RSL are a type of slope lineae that are observed to extend incrementally downslope on steep equator-facing rocky slopes on Mars and recur in multiple Mars years (McEwen et al. 2011c; Ojha et al. 2013). If a slope lineae shows either incremental growth or recurrence but not both, they are classified as partially confirmed RSL. When slope lineae have the morphology and setting of RSL, but neither fading/incremental growth nor recurrence has been observed, they are classified as candidate RSL.


Their strong seasonal (warm season), latitudinal (midlatitude and equatorial), and slope aspect (mostly equator-facing slope in southern midlatitude) distributions suggest that these features require relatively warm temperature to form (McEwen et al. 2011c; Ojha et al. 2011). Several dry and wet hypotheses have been considered to explain the formation mechanism of these features (McEwen et al. 2011c):
  1. (1)

    Dry mass wasting: Dry mass wasting triggered by seasonally high winds is a possible mechanism for RSL formation (McEwen et al. 2011c); however, it does not explain the lack of RSL in the northern hemisphere nor incremental growth nor their slope aspect preference.

  2. (2)

    CO2 sublimation: CO2 sublimation drives many other dynamic phenomenon on Mars (e.g., Hansen et al. 2011; Diniega et al. 2010; Dundas et al. 2010), but the times and places when RSL are active is too warm for CO2 frost to persist.

  3. (3)

    Subsurface melting of brines: The strong association of RSL with temperature greater than 240 K suggests a formation mechanism involving briny water. Pure water is not stable on the surface of Mars, but salts can lower the freezing point by up to 70 K and lower the evaporation rates by a factor of 10 or more, making salty water more stable than pure water. However, the source and the recharge mechanism of water are not understood.

  4. (4)

    Deliquescence: Deliquescence is a phase transition from crystalline to aqueous state when the relative humidity is equal to or greater than deliquescence relative humidity. Besides forming brines at low temperature, salts on the Mars’ surface can readily absorb water vapor from the atmosphere and deliquesce, providing a possible source of water for RSL formation. However, it is unclear whether this mechanism can supply significant volumes of water for RSL formation.



Recent features, forming currently, under the present climate.


Many fade over time, especially during winter (McEwen et al. 2011c).

Studied Locations

Confirmed recurring slope lineae occur in the middle latitude band at the southern hemisphere of Mars between 32 and 52°S latitude mostly on equator-facing slopes where the temperature reaches 250–300 K in local summer during the hottest part of the day. There are 13 confirmed (e.g., at Palikir [Fig. 1] and Horowitz [Fig. 2] craters), 20 partially confirmed, and 49 candidate RSL sites (Ojha et al. 2013). Additionally, five confirmed RSL and 14 partially confirmed RSL sites have been discovered in the equatorial region, in Valles Marineris (McEwen et al. 2014). Transient slope lineae have been seen on equator-facing slopes in the northern equatorial regions of the Cerberus Fossae (10°N and 158°E) (Runyon et al. 2012).
Fig. 1

Recurring slope lineae in Palikir crater at 41.6°S 202.3°E (Ojha et al. 2013). HiRISE images: (a and d) MY 30 Ls = 302° ESP_022689_1380, (b) MY 28 Ls = 340.9° PSP_005943_1380, (c) MY 29 Ls = 184.7° ESP_011428_1380, (e) MY 30 Ls = 318° ESP_023045_1380, (f) MY 31 Ls = 272.6° ESP_030891_1380 (NASA/JPL/UA)

Fig. 2

Recurring slope lineae on the NE slopes of a mound in Horowitz crater (McEwen et al. 2011b) at 32°S, 140.8°E at Ls = 334°, summer. PSP_005787_1475 (NASA/JPL/UA)


RSL have preferential seasonal, latitude, and slope aspect distributions (Ojha et al. 2011; Fig. 3).
Fig. 3

Distribution of confirmed (black dots, Ojha et al. 2014; additional white dots, McEwen et al. 2014), partially confirmed (blue dots), and candidate (red dots) recurring slope lineae from southern midlatitude (Ojha et al. 2014). Confirmed RSL are observed to grow incrementally and recur in multiple MY; partially confirmed slope lineae are either observed to grow incrementally or recur in multiple years, but not both; and candidate RSL are observed on slopes but without verification for incremental growth or recurrence (Ojha et al. 2014)

Astrobiological Significance

If formed by liquid brine, their water content might help chemical or even biological processes.

Terrestrial Analog

Antarctic water tracks (Levy et al. 2011; Dickson and Head 2012) (water track (Antarctica)).

History of Investigation

RSL was termed transient slope lineae (TSL) in early publications (McEwen et al. 2011a; Ojha et al. 2011). The term “transient” is also used for those slope lineae whose recurrence has not been confirmed (slope lineae).

See Also Similar Features

Recurring slope lineae are darker, an order of magnitude thinner than slope streaks and, unlike slope streaks, are associated with rock outcrops (Runyon et al. 2012). They are also different from dry crater wall flow-like features (Moon, asteroids) and triangular scars (Mars).


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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  1. 1.Johns Hopkins UniversityBaltimoreUSA
  2. 2.Lunar and Planetory LaboratoryUniversity of ArizonaTucsonUSA