Recurring slope lineae in equatorial regions of Mars

Transcription

1 Alfred S. McEwen, Colin M. Dundas, Sarah S. Mattson, Anthony D. Toigo, Lujendra Ojha, James J. Wray, Matthew Chojnacki, Shane Byrne, Scott L. Murchie, Nicolas Thomas This supplementary material includes a large table of information about confirmed and partially confirmed RSL in places other than the southern mid-latitudes (Table S1). Tables S2 and S3 list THEMIS brightness temperatures for two craters on the floor of VM. Figures S1 to S4 illustrate features discussed in the main paper. Table S4 provides CRISM column abundances of H 2 O at representative RSL sites; data from two sites are plotted in Figures S5 and S6. In addition, a set of animated GIFs are available at to illustrate the motion of some of the well-monitored RSL sites where we have produced DTMs and orthorectified images. See for similar animated GIFs of mid-latitude RSL from reference 3. We also include a GIF of active dune slopes in Nili Patera to show how these streaks differ in behavior from RSL each streak appears suddenly with no evidence for incremental growth over time periods of weeks. Supplementary Discussion SUPPLEMENTARY INFORMATION DOI: /NGEO2014 Recurring slope lineae in equatorial regions of Mars A rigorous definition of RSL is necessary because many different processes create dark lines on steep slopes. Our three categories and some discussion follow: Fully confirmed RSL: We have observed the full temporal sequence: Incremental growth of many ( 10) flows on a slope, fading when inactive, and recurrence in multiple Mars years. The fading may not lead to complete disappearance. This category includes seeing the full sequence in one year, and a single image showing RSL candidates on the same slope in another year, that are different from the other year (demonstrating that they are not just due to incomplete fading). On some east- and west-facing slopes in VM the RSL may be intermittently active year-round, in which case we consider >1 year observation of activity to be the equivalent of recurrence. Partially confirmed RSL: We have seen many ( 10) flows on a slope plus either incremental growth or recurrence. Apparent fading may be deceptive so only this observation is not sufficient for partial confirmation. There are some cases where either recurrence or incremental growth may have occurred, and we cannot tell which is the case. There are some sites in this category in Table S1 where slope lineae grow incrementally but rarely fade or recur, such as in Aram Chaos, which may need a different classification. Candidate RSL: Any place with many ( 10) relatively dark lines on a slope that resemble RSL but with insufficient temporal observation for even partial confirmation to be possible (until we get more data). True RSL fading can be difficult to confirm because it may be incomplete, or difficult to observe due to topographic shading and because RSL follow channels, or because they may appear faded when the atmospheric dust opacity is high. Also, we now realize that some active RSL actually NATURE GEOSCIENCE 1

2 have a very low contrast with the background in HiRISE images (they look faded, but are growing from image to image). There are also partially confirmed RSL in equatorial regions that do not appear to fade at all, such as in Aram Chaos, or only their tips fade and regrow, as in Hydrae Chasma. The RSL fans sometimes exhibit rapid changes in brightness. Figure S2 illustrates fans darkening for at most a few weeks in Coprates Chasma, and Figure S3 shows more extensive darkened fans in the Eos/Capri Chasma site. Elorza crater (see animations in shows additional examples of fans darkening. We can resolve that the darkening is due to many lineae in some fans in Elorza crater, so unresolved lineae may explain the darkening of other fans. As with RSL, the darkening might be due to a small degree of water in the soil or to changes in surface grain size or texture 19. The darkening illustrated in Figure S2 was associated with a period of time in which dust storm activity was especially high in VM, as discussed in the main text. Atmospheric dust will lower daytime temperatures, thus keeping relative humidities higher, conceivably prolonging the early morning conditions that allow metastability of deliquescent liquids 28. However, the darkening in Figure S3 was not associated with dusty air, and may have persisted longer. Future monitoring will improve our understanding of fans that sometimes darken. RSL appear to interact with 3 basic types of surfaces: (1) Bedrock, where the RSL may temporarily darken the surface or where the darkening may be difficult to see under some lighting conditions (perhaps hidden in cracks and shadows). RSL often appear to emanate from just below such bedrock, but they could flow over or through the bedrock from higher levels, yet not be clearly detected in HiRISE images. (2) Slopes with fine-grained soils that are slowly permeable, where RSL flow may gradually erode small gullies. Some gully floors have the same relative IRB color as the fans. RSL might not fade away completely in such locations, if the gullies have exposed a darker substrate or the RSL have deposited material with a distinctive color. (3) Slopes covered by permeable materials such as windblown sand, where water might infiltrate and wick to the surface in discontinuous lines. The fans where RSL terminate often have distinctive grooves that are elongated down the slope (Figure S4), similar to the avalanche slopes of active Martian dunes. The groove spacing is a few meters, similar to the ubiquitous ripples in sand deposits, and sometimes they transition into what are clearly aeolian ripples. These are also the locations where we often see discontinuous RSL (Figure S4). Further analysis is needed to understand the relations between RSL and saltating sand. Several RSL attributes are not yet understood: (1) the relation between apparent RSL activity and dustiness of the atmosphere; (2) variability in RSL activity from year to year (perhaps related to dustiness of the air); (3) salt composition and concentration; (4) seasonal control of activity on north-facing equatorial slopes in spite of little change in temperature; and (5) temporal changes in the color properties of fans where RSL terminate. Continued orbital monitoring and data analysis, laboratory experiments, modeling, and future orbital and landed exploration with new measurement types are needed. 2

3 Table S1. Observations and notes about confirmed and partially confirmed RSL north of 30S latitude. (SL = slope lineae, not necessarily RSL) Eos/Capri Chasma Lat Lon Stereo: DTM: Elev. (km) N N -2.7 Image MY Ls ( ) SL? Aspect Emis. Illum Phase Fully confirmed ESP_029331_ y NW, N Many long narrow gullies, some with faint SL (bin2 image) ESP_030109_ n Maybe very faded SL ESP_030610_ n Maybe very faded SL ESP_032100_ y NW, NE Huge numbers of new SL, large fan areas darkened within and below SL ESP_032667_ y NW, NE Significant growth, very active. Fans still dark. Slopes in E Lat Lon Stereo: DTM: Elev. (km) Coprates Y Y -2.1 Image MY Ls SL? Aspect Emis. Illum Phase Partially confirmed (incremental growth + fading) ESP_025441_ y W, SW Striated fan merges with wind-rippled deposits; dashed SL permeable? Faint SL on dunes S end. Can t see SE-facing sites in shadows. ESP_027973_ y W, SW, SE Incremental growth. Some SL may be faded, extreme west edge. Faint SL on dunes S end. ESP_029318_ y W, SW, SE More incremental growth, new SL on dunes, all slope aspects ESP_029674_ y W, SW Mostly fading, some growth (W, SW facing slopes) ESP_029885_ y W, SW Hazy image, maybe fading ESP_030241_ y W, SW Hazy image, but whole fans darkened and changed color! See figure ESP_030452_ y W, SW Fans re-brightened, similar to and prior images ESP_031019_ y W, SE, SW, S Subtle differences (new streaks + fading) N half dark surfaces ESP_031876_ y W, SW, S, SE Some growth (W, S facing slopes), new RSL on SWfacing slope and SL on W-facing dunes ESP_032298_ y W, SW, S, SE Growth of RSL on SE, S, and SW-slopes. Very active large SW-slope RSL. Lower elevation RSL appear ESP_032865_ y W, SW, S, SE faded. W-facing SL hard to identify in earlier images Continued growth. (No significant N-facing slopes in scene.) Eastern Coprates Lat Lon Stereo: DTM: Elev. (km) Y Y -1.3 Image MY Ls SL? Aspect Emis. Illum Phase Fully confirmed PSP_010277_ y NW, N Faint SL, large striated fans PSP_010699_ y NW, N Faint SL, but clear incremental growth. Dunes to N with faint SL ESP_032087_ y NW Similar faint SL on NW slopes as in MY29 but with new patterns (recurrence), SL in dunes ESP_032430_ y NW Low-phase view, difficult to compare to with opposite large rolls. RSL more prominent. ESP_032654_ y NW Maybe low-contrast changes from (similar geometry) N-facing slopes in Lat Lon Stereo: DTM: Elev. (km) Coprates ridge Y N -3.8 Image MY Ls SL? Aspect Emis. Illum Phase Partially confirmed (incremental growth, maybe fading) ESP_026786_ y N Most fans S end of image (higher elevation) ESP_027063_ y N Small changes between images incremental growth ESP_030030_ n SL faded, but somewhat hazy image Crater on Coprates Lat Lon Stereo DTM: Elev. (km) floor Y Y -3.1 Image MY Ls SL? Aspect Emis. Illum Phase Fully confirmed ESP_018123_ y N,W Crater with steep slopes on floor of Coprates, striated fans on N,W,S slopes with rel. greenish IRB color, clear gullies on C, E-facing slopes. Slopes on S. Side: upper gullies 27-55, SL 37 +/-5, fans 23 (variable) 3

4 ESP_026905_ y N, W Smaller SL than prior image (recurrence), faint on W- face slope ESP_027182_ y N,W A little growth from previous image (incremental) ESP_027327_ y N.W No obvious change (much less active than MY 30) ESP_027749_ y NW Growth on W slope, not N. Slopes on W. Side: upper gullies 22-60, SL 32 +/-9, fans 25 (variable) ESP_028382_ n RSL faded N and W slopes, prob not haze (active Ls ) ESP_028593_ n Same as above ESP_029582_ y S,W New RSL and growth on W and S-facing slopes; N- facing RSL faded ESP_030294_ n RSL faded (maybe dusty air hides them) ESP_031006_ y SW Growth of RSL on SW-facing slope, but not as extensive as ESP_031995_ n Only covers E edge of the crater. Many SL on dune slip faces. ESP_032351_ n Covers center and western side of crater. Fading (?) on SW-slopes from Dark upper (short) SL on N-facing slopes near sand sheet as earlier images but little or no growth. ESP_032707_ n Same as previous image ESP_032852_ n More fading of RSL on SW wall. Coprates slope and Lat Lon Stereo? DTM? Elev. (km) Dunes Y Y -1.8 Image MY Ls SL? Aspect Emis. Illum Phase Fully confirmed PSP_006480_ y NSEW Big SL on S-facing slope, shorter ones on NW-facing slopes. Many SL candidates all over dune slip faces ESP_028026_ n All previous SL faded, on S and NW facing rocky slopes; maybe some faint new SL rather than faded?; faded on dunes ESP_028804_ n Same as above. Why not active on NW-facing slopes? (hazy image) ESP_029938_ y W, S New small SL, recurrence, growth of dark SL over bright streak at 7500, or RED nomap. ESP_030505_ y W, S Growth in small RSL, not the big ones seen in early MY29, SL on dunes new? ESP_030861_ y W, S, N,E More growth (NE corner), maybe some fading in places. Large-scale darkening uphill of RSL? (also lighting changes). Growth on dunes all aspects ESP_031217_ y NSEW More growth on dunes, growth and fading on bedrock slopes ESP_031929_ y NSEW New RSL on dunes, possible growth of some dune RSL, fading. Bedrock RSL N end of image, S-facing slopes ESP_031995_ y NSEW Not great coverage of N Wall, few new SL on dunes, little evidence for incremental growth ESP_032351_ y NSEW Bedrock RSL NE corner, SW-facing slope doesn t overlap previous images; dune SL Ridges in north Lat Lon Stereo? DTM? Elev. (km) Coprates Y N -1.4 Image MY Ls SL? Aspect Emis. Illum Phase Partially confirmed (recurrence). Spurs with RSL extended down to striated fans with rel green (IRB) color ESP_022303_ y NSEW Major SL and slopes are NW and NE facing. ESP_032562_ y NSEW Clear recurrence of RSL in same locations as Ridges in north Lat Lon Stereo? DTM? Elev. (km) Coprates Y Y -1.4 Image MY Ls SL? Aspect Emis. Illum Phase Fully confirmed. Spurs with RSL extended down to striated fans with rel green (IRB) color ESP_024993_ y N, W, E More RSL on N end of image, not covered in subsequent images except 25340_1670, N-facing slopes ESP_025349_ y N, W, E No new growth; appears faded but poorer image ESP_027815_ y N, W, E New growth (incremental) on NW-facing slopes (see fig.) 4

5 ESP_028303_ y N,W,E Appears faded, but poorer image ESP_028659_ y N,W.E Partial image, some new growth and apparent fading (? Image quality) on NW, NE slope ESP_029226_ y N,W,E Changes not obvious, fading confirmed on NE-, NWface slope ESP_032127_ y N, W, E Numerous SL apparent at low phase, some new (recurrence v ), some lengthening from previous image. New (?) RSL on east-side of ridge resolvable with low phase angle, some darkening? Light-toned SL on sand. Coprates landslide Lat Lon Stereo? DTM? Elev. (km) scarp Y N +0.1 Image MY Ls SL? Aspect Emis. Illum Phase Partially confirmed (incremental growth) ESP_020879_ y SW Big SL, can see in CTX, but changes difficult to assess ESP_021657_ y SW Some incremental growth, like tip of biggest SL (west side) ESP_024347_ y SW Some growth(?). Not fading. ESP_029503_ y W No change to big SL, but RSL growth on extreme west middle outcrop. (moved target a bit) ESP_030070_ y W No change to big SL, but growth on west middle outcrop. Big SL not fading when inactive like RSL. ESP_032549_ y W Minor tip growth of a few big SL. ESP_032694_ y W No obvious changes from previous image ESP_032839_ y No obvious changes, missed the most active site Melas Chasma Lat Lon Stereo? DTM? Elev. (km) slope N N -3.0 Image MY Ls SL? Aspect Emis. Illum Phase Partially confirmed (recurrence, maybe fading) PSP_004054_ y NW, SW A few tens of SL, sulfate bedrock erosion morphology, BD1900 in CRISM 61F3 ESP_030703_ Some SL on both slopes faded. Probably new SL in some of the same places but not as long as in 4054, or fading of distal ends. No distinct SL fans. S of large impact in Lat Lon Stereo? DTM? Elev. (km) Coprates Y N -3.1 Image MY Ls SL? Aspect Emis. Illum Phase Fully confirmed ESP_018189_ y N, W Good candidate RSL, bin-2 image ESP_025982_ y N, W, S? New RSL (recurrence) (overlaps SE corner) 25626_1675 (Ls 57) misses active area; other RSL seen again in (N-facing) not obviously changed ESP_029292_ y W RSL incremental growth, esp. NW corner. Moved target footprint. ESP_029859_ y W Mostly fading, but some growth from prior image ESP_030360_ y SW Mostly fading, but appear to be a few new small SL on SW-facing slopes ESP_030571_ y SW Active RSL SW-facing. Also E-facing SL on dunes (poor SNR) ESP_030927_ y S, SW, W, NW? Very active RSL on S and SW-facing slopes-middle and SE corner. SL on SE, NE dune slopes. Maybe a bit on NW slope, mid bottom image. ESP_031784_ ? Hazy image, but RSL appear largely faded? ESP_031850_ y S, SW, W, NW? RSL distinct, but many shorter than in faded or covered by dust then new formation? Nice highcontrast image probably due to low phase angle. Dark SL on N-facing slope of and are bright, and IRB greenish. Dune SL ESP_032061_ y S, SW, W Lengthening of RSL on SW wall, few new SL, some no change, most activity on W-facing wall, new SL on dune (E and N facing) with some recurrence? ESP_032206_ y S, SW, W, NW? Fading on S wall, SL becomes relatively darker to the S but with no growth. ESP_032272_ y S, SW, W, NW? RSL on S- SW-slopes remain relatively dark, but no clear growth. Possible new SL on dark dune slip faces. Some growth of faint SL on SW-facing slope near bottom of image. 5

6 Crater on Melas Lat Lon Stereo? DTM? Elev. (km) Chasm floor Y Y -5.2 Image MY Ls SL? Aspect Emis. Illum Phase Fully confirmed CTX-7034_ n No SL detected in CTX CTX-8181_ n No SL detected in CTX ESP_027802_ y N, W, E Large, extensive RSL on N-facing slope, can see in simultaneous CTX (but not prior CTX images). Many small gullies. Slopes (all sides): upper gullies (variable), RSL 30 +/-2, fans 25 (variable) ESP_028501_ y N, W, E Significant RSL growth (incremental) from prior image ESP_029213_ y N, W, E, S RSL faded on N-facing slopes; new RSL on S-facing slopes ESP_029780_ y S Much RSL growth on S-facing slopes ESP_030136_ y S No (or very little) new growth! Maybe fading, but dusty air ESP_030347_ y s fading ESP_030769_ y s New growth, dark tips ESP_031059_ y s More growth large RSL ESP_031771_ n RSL faded. Hazy image, but preferential fading of formerly dark RSL is distinct compare to other features. ESP_032048_ y N,W,S,E A few new flows on SW-, E- N- and W-facing slopes, very small and few ESP_032615_ y N Faded on E slope, slight growth on N slope? ESP_032905_ y N Minor growth N slope (recurrence) ESP_033116_ y N Minor growth N slope Big little gullies in Lat Lon Stereo? DTM? Elev. (km) Coprates Y N -4.1 Image MY Ls SL? Aspect Emis. Illum Phase Fully confirmed (but still weak incremental growth). Close match SL size to gullies. CRISM: BD530 and SH600 bright on SL slope PSP_008616_ y N Slope with many long gullies; a few dark SL. PSP_009051_ y N A few changes incremental growth ESP_028791_ y N SL faded ESP_030281_ n No change ESP_030426_ n No change ESP_031204_ n Nice clear image but no clear SL. Only active Ls 78-93? ESP_032338_ y N Incremental growth of small SL on N-slope, same slope as in MY 29 (recurrence) W Melas Chasma Lat Lon Stereo? DTM? Elev. (km) Y N -1.2 Image MY Ls SL? Aspect Emis. Illum Phase Fully confirmed PSP_007522_ y NW Distinct SL ESP_025389_ y? NW Bin2 RED; color-only strip ESP_025666_ y NW Fewer SL, some new (recurrence), very oblique S2 ESP_030123_ ? Faded RSL? (hazy image) ESP_031257_ y E, SE Maybe new activity, or maybe a better image (lower phase angle) ESP_032892_ y E, SE A little growth on E-facing slopes Crater in Terra Lat Lon Stereo? DTM? Elev. (km) Sabaea Y N +2.1 Image MY Ls SL? Aspect Emis. Illum Phase Partially confirmed (incremental growth). Highest elevation candidate RSL in equatorial region ESP_025240_ y N, W Bin2 image, SL on N-facing slope and N and W slopes of smaller crater to south ESP_025728_ y N, W Bin2, same as above ESP_027007_ y N, W SL fans of southern crater brightened relative to and 25728, can t tell if SL lengthened ESP_028339_ y N, W Incremental growth, SL fans S crater darkened again? ESP_029618_ y N, W Looks the same but hazy image ESP_030897_ y N, W New RSL, fans on larger crater darkened in places ESP_032453_ y N, W RSL prominent at low phase and maybe clear atmosphere, probably some changes NE Melas Lat Lon Stereo? DTM? Elev. (km) Y Y -3.6 Image MY Ls SL? Aspect Emis. Illum Phase Partially confirmed (incremental growth) ESP_028857_ y W SL over small area, middle right. Small gullies and 6

7 ESP_028857_ y W SL over small area, middle right. Small gullies and IRB green fans are extensive. Slopes on W. side: upper gullies 32-55, RSL 34 +/-8, fans 30 (variable) ESP_028923_ y w Incremental growth of SL Elorza Crater Lat Lon Stereo? DTM? Elev. (km) central pit Y Y -0.2 Image MY Ls SL? Aspect Emis. Illum Phase Fully confirmed PSP_005649_ y N,W,S,E Small SL, all slope aspects, dust-storm year, more RSL than any subsequent image ESP_012479_ n Faded SL but subtle changes ESP_021551_ n Faded SL, brightened fans ESP_021828_ n Subtle changes ESP_021973_ ? small SL ESP_023107_ n Subtle changes ESP_024874_ y N Small, new SL, darkened fans ESP_025652_ Same as previous image ESP_027907_ y N Incremental growth on several slopes, whole fans darkened ESP_029819_ y N, SW A little more growth, maybe some fading, very small features ESP_031098_ y W RSL in previous image faded, some small ones still present. See Elorza figure ESP_032509_ y W RSL prominent at low phase, lots of small changes as in previous images Central Ius Chasma Lat Lon Stereo? DTM? Elev. (km) N N +0.1 Image MY Ls SL? Aspect Emis. Illum Phase Partially confirmed (incremental growth or recurrence). CRISM BD530 bright over RSL slopes PSP_002103_ y N, W A few SL in SW corner (or topo shading?) PSP_006639_ y N,W,NE Many more RSL in overlap with 2103 probably either incremental growth or recurrence. Much activity after dust-storm summer. ESP_029741_ n No obvious RSL, but worth monitoring ESP_030664_ n No obvious changes ESP_032365_ n No obvious changes Hydrae Chasma Lat Lon Stereo? DTM? Elev. (km) Y N -1.6 Image MY Ls SL? Aspect Emis. Illum Phase Partially confirmed (minor recurrence and incremental growth, minor fading) ESP_013033_ y W, S SL on west edge (Science Fig S6); N wall (bin2) ESP_013745_ Y W,S Slight extension of a few SL (bin2) ESP_019336_ Y W Only covers the W-facing slope (bin2), no obvious changes ESP_019969_ y W Slight growth (bin2) ESP_021815_ y W A few changes from previous new and retreated SL (bin2). Definite changes from (~same Ls and photometric angles) ESP_022804_ y W,S Some new growth? (bin1) changes not obvious W or S slopes ESP_028738_ y W, S RSL often different (recurrence), but some very similar in whole set (bin1) ESP_029516_ y W, S Some growth compared to 28738, but also a sharper image. In general SL are very stable here compared to typical RSL. 7

8 Juventae Chasma Lat Lon Stereo? DTM? Elev. (km) Y N -4.2 Image MY Ls SL? Aspect Emis. Illum Phase Fully confirmed PSP_006203_ y NESW Large SL on all slope aspects ESP_030373_ y SW New RSL N hill ESP_030795_ y SW Some new/growing RSL on N hill, and some fading ESP_030940_ y SW Big, new RSL on SW-facing slope of N hill, and some fading, very active ESP_031863_ y ESW New long RSL on SW-, S-, and SE-facing slope of N hill, new RSL on W-face of S hill ESP_032074_ y ESW New growth, especially SE-facing on N hill. Also some fading ESP_032219_ y NESW New growth, especially SE-facing on N hill. Also fading. New activity NE-facing slopes N hill, and 1 flow on N side of S hill. Overall activity comparable to MY28 image. ESP_032496_ y NESW Minor growth, mostly fading (? low phase angle), including NE-facing slopes of N hill and N side of S hill. ESP_032997_ y NESW Fading on all slopes Aram Chaos Lat Lon Stereo? DTM? Elev. (km) Y N -3.3 Image MY Ls SL? Aspect Emis. Illum Phase Partially confirmed (minor incremental growth, minor fading). Nearby CRISM with strong BD530 and SINDEX along scarp to the SE (7FA4) ESP_025954_ y SW Big SL all along scarp. Color like sand; distinct from background ESP_029343_ y SW Growth in a few SL, stunted like Hydrae Chasma? ESP_030121_ y SW No changes are obvious (bin-2 image), but broad darkening of a slope region (3467, 10317) ESP_031756_ y SW Mostly fading of thin RSL, some minor growth ESP_031967_ y SW No change or minor fading Crater in S Acidalia Lat Lon Stereo? DTM? Elev. (km) Planitia N N -4.4 Image MY Ls SL? Aspect Emis. Illum Phase Fully confirmed Low-albedo and rocky crater so temperatures high for this latitude ESP_027840_ y W Prominent SL on W-facing slope, faded SL on S- facing slope? ESP_028684_ y W Largely unchanged, but slight incremental growth on a few flows. Did we catch the very end of the active season? ESP_028829_ y W unchanged ESP_029607_ y W Fading SL (but hazy image) ESP_030319_ y W More fading ESP_030530_ y W Mostly faded ESP_032020_ y W Mostly faded ESP_033088_ y W New RSL (recurrence) 8

9 Table S2. THEMIS brightness temperatures for crater on Melas Chasma floor. Ls THEMIS # TOD MY Peak T (K) on sun-facing slope 3 I (W-facing) 129 I (NW-facing) 160 I (W-facing) 230 I (SW-facing) 307 I (SW-facing) 323 I >287 (WSW-facing) Table S3. THEMIS brightness temperatures for crater on floor of Coprates Chasma. Ls THEMIS # TOD MY Peak T (K) on sun-facing slope 24 I (WNW-facing) 58 I (WNW-facing) 168 I (W-facing) 201 I (W-facing) 221 I (WSW-facing) 240 I (SW-facing) 279 I (SW-facing) 289 I (SW-facing) 319 I (WSW-facing) 339 I (W-facing) 9

10 Table S4. CRISM column abundances of H 2 O at representative RSL sites. Data are precipitable microns normalized to a reference 6.1 mbar pressure surface, binned by 5 degrees in longitude, 2 degrees in latitude, and 30 degrees in L s, and come from the first full Martian year of MRO observations 26. Blanks indicate no data were available in the spatial and temporal bins. Ls (degrees) Lat Lon

11 Figure S1. Comparison of albedo/color and size of streaks on dusty slopes of Olympus Mons (left), likely dust avalanches, to RSL in Palikir crater (middle and right). Surface and atmospheric photometric corrections were applied, but without using DTMs to correct for slopes. Instead, since the areas of interest in both images are about 30 sun-facing slopes, we normalized brightnesses to an incidence angle of 30 (~0 on these slopes). Resulting normalized albedo values at the 5 lettered locations (IR, RED, and BG bandpasses, respectively), are A: 0.38, 0.23, 0.09; B: 0.52, 0.30, 0.11; C: 0.29, 0.16, 0.07; D (RSL): 0.059, 0.059, 0.027; E (bright fan): 0.079, 0.073, This illustrates that RSL are ~3x darker and ~2x less red than slope streaks. 11

12 Figure S2. Set of three images in west Coprates Chasma (14.7 S, E) showing transient darkening of the IRB greenish fans where RSL terminate. Photometric angles are similar in each image (Table S1) but the middle image was acquired through dustier air. Red arrows point to some of these fans. Geometry is that of the raw images; north is ~7 to the right of up. Larger fans darkened and rebrightened in the larger region covered by simultaneous CTX images. Image widths ~300 meters. Figure S3 (below). Extensive darkening of fans observed in Eos/Capri Chasma site (15.4 S, E). The first 2 images (L s =234 and 269 of MY 31) show no RSL and bright fans, while the last 3 images (L s =329 and 352 of MY 31) show extensive RSL ending on darkened fans (white arrows). The white box marks a location that is enlarged at bottom. This site includes some of the longest RSL seen to date, up to 1.2 km in length. Scene width ~1.1 km, raw image geometry with north approximately up. 12

13 13

14 Figure S4. Part of ESP_025441_1650 in Coprates Chasma illustrating the lineated fans that RSL frequent. North is up and downhill is to the west (left). Arrows at A point to typical RSL, B points to discontinuous RSL, and C shows where the lineated fan transitions into wind ripples. 14

15 20 Lon = 9.537, Lat = (pr. µm) 6.1_mbar h water L S Figure S5. Plot of CRISM column abundance of atmospheric water vapor over a typical mid-latitude RSL site. Data are from L s =120 of MY 28 to L s =120 of MY 29 (Table S4). Empty regions here and in Figure S6 indicate lack of data. 15

16 20 Lon = , Lat = (pr. µm) 6.1_mbar h water L S Figure S6. Plot of CRISM column abundance of atmospheric water vapor over a typical RSL site in Valles Marineris. Data are from L s =120 of MY 28 to L s =120 of MY 29 (Table S4). The error bars are larger than in the middle latitudes (Figure S5) due to fewer observations. 16