MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 1 Gale Crater MSL Candidate Landing Site in Context by K. Edgett April 2010

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 2 What do I mean by “Context”? How will the things we can learn at this landing site inform us about the rest of Mars? How do things we can observe with MSL fit into the bigger picture? My focus here is on key aspects of Mars geology and geologic history. Granted, there are many minor areas that could be discussed, as well as some non-geological topics.

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 3 What aspects of Gale connect with the rest of Mars? Liquid water Layered “fill” in an impact crater The so-called “Medusae Fossae Formation” Mineralogy remote sensing ground truth Eolian dunes Relative age

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 4 Liquid Water (or other liquid) Liquid cut channel on northwest wall and deposited sediment in the proposed MSL landing ellipse. Liquid cut gorges into the layered rocks of the lower part of the Gale mound. Some sediments from they might still be present. Materials date back to a time when liquid was flowing at the surface of Mars. Materials present opportunity to study clasts transported by flowing liquid.

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 5 Liquid Water (or other liquid) channel “fill” material MOC M , looking westward Channel cutting rocks of lower Gale mound in an area accessible to MSL

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 6 Layered “fill” in Impact Craters Thousands of Mars craters of sizes from meters to hundreds of kilometers in diameter have been filled or partially filled with material. Some were filled and then buried. Some were exhumed or partially exhumed. Many remain filled and buried and are either completely unknown to us, or only vaguely recognized (or speculated to be there) by the presence of a circular depression. Gale is of the broad family of craters that have exposures of layered fill materials (usually lithified) in them— these include examples where the fill is expressed today as an eroded mound, but there are other expressions, too. The processes that deposit the fill, lithify it, and erode it, may be similar all over Mars— what we learn at Gale applies to all. If some of the fill is lake sediment (or not!) then we learn a lot about these fill materials all across the planet. Were there lakes in any of the martian craters? Gale is a large, deep crater. If there was never a lake, that is as major a result as if there was a lake.

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 7 Layered “fill” in Impact Craters Schiaparelli Gale Lasswitz Wien Knobel Pollack Henry Capen Tuscaloosa note this “fill” has broken-up Gale is part of a family of craters with interior layered mounds or other interior layered “fill”. Gale ~155 km diameter mound

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 8 Layered “fill” in Impact Craters Asimov & neighbors Henry Crater Gale Crater Becquerel Crater The scale is the same in these Viking image mosaics.

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 9 Layered “fill” in Impact Craters Even Gale’s “little buddy” has layered fill material (and a fan). fan landing ellipse HiRISE ESP_013329_1745

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 10 So-Called “Medusae Fossae Formation” Unit named in global Viking-era “geologic” mapping by Scott and Tanaka (1986). Yardang-forming material, generally superimposed on cratered terrain and lava flow plains. Outcrops across 25% of Mars equator between Tharsis and Gale Interpretations vary; most “popular” is volcaniclastic, particularly airfall tephra and ignimbrites; could be eolian dust or something else altogether. A few investigators have suggested the Gale mound, or some part of the mound (e.g., upper materials) are remnants of a formerly more extensive “blanket” of “Medusae Fossae Formation” materials (e.g., Scott and Chapman 1995; Zimbelman et al. 2009). Others disagree. Scott and Tanaka (1986) USGS Map I-1802-A Scott and Chapman (1995) USGS Map I-2397 Zimbelman et al. (2009) GSA Annual Meeting Abstract #95-1

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 11 So-Called “Medusae Fossae Formation” Zimbelman (2010) LPSC abstract #1157 Zimbelman (2010) map — This is new/preliminary work. He suggests there are Medusae Fossae Formation remnants/outliers near and in Gale. he says the light blue and purple units are “MFF” “MFF” GALE these “MFF” units have a plethora of fluvial landforms, mainly in the form of inverted stream channels. this map was “blown up” from a tiny figure in the LPSC abstract; the unit labels are not readable in the original. The “fluorescent green” markings are indicators that he interprets the presence of layered material.

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 12 Mineralogy Remote Sensing Ground Truth Investigators working with TES, THEMIS, OMEGA, CRISM, and other orbiter data have made interpretations regarding the presence of various minerals in Gale Crater. Layered phyllosilicate-bearing rock in the mound. Layered sulfate-bearing rock in the mound. Pyroxene-bearing materials in or near the mound. Olivine-bearing mafic materials in the dark eolian dunes. –(called “olivine basalt” by the THEMIS investigators that report this, but I prefer to avoid saying “basalt” because this implies the genesis of the materials is known to be extrusive igneous or very shallow intrusive igneous) Like all of the MSL landing sites, Gale offers “ground truth” to check our mineralogical results interpreted from orbiter data. This includes an opportunity to explore the “abundance” of these minerals—for example, when someone says they see “sulfates” in CRISM spectra, how much of the material is not sulfate? This is important.

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 13 Eolian Dunes Gale offers something not available at all 4 landing sites: a look at low-albedo eolian dunes. These are located in and between the landing ellipse and the mound. Similar dunes occur all over Mars and we can compare the properties of these dunes with all the others observed from orbit to understand how similar or different they are. And then we can use the Gale dunes to “ground truth” our understanding of both particle size and composition as inferred from remote sensing data (particularly thermal- and near-infrared). CTX stereo anaglyph, Gale dunes, images B06_011984_1754_XN_04S222W & B07_012340_1750_XN_05S222W

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 14 Relative Age Independent of whether certain materials in Gale can be interpreted as being Noachian, Hesperian, or Amazonian — and independent of whether these 3 epochs can be tied to an absolute date via models of past cratering rate — we can say some key things about where Gale falls in the sense relative timing of events on Mars. Water. Materials in the landing ellipse and on the lower mound date back to a time when a liquid could flow across the surface of Mars, cut channels, and transport sediment. Sedimentary rock. The layered rocks of the lower Gale mound were deposited, lithified, then eroded by fluvial (and probably other) processes. The sedimentation, then, was a long time ago and pre-dates the gorges cut into the lower mound. Crater “fill”. The layered rocks of the Gale mound, furthermore, date back to a time when conditions prevailed over all (or much of) Mars such that large craters could become filled—even buried—by materials. This does not seem to be a process that has acted in more recent times. Erosion. The rocks that make up the Gale mound not only lithified a long time ago, they were eroded a long time ago (and might still be eroding today). Layers formerly extending to the crater walls were broken-up and removed such that the mound is all that is left today. This happened so long ago that there is no clear indication as to “how did the stuff get out of the hole?” This is a huge mystery, one that Gale shares with many other craters on Mars that have these mounds and interior layered rock outcrops.

MSL Science Team Landing Sites Discussions — Gale CraterEdgett, p. 15 Take-Home Points Each of the 4 MSL candidate landing sites has things we’ll learn via Curiosity’s field study that inform our larger view of Mars. Gale could inform us about… –the liquid that carved channels, etc., on Mars –the general nature of light-toned, layered, “sedimentary rock” crater-filling material –were there lakes in these craters at some point? –ground truth for remotely-sensed mineralogy –general nature of eolian dunes –what was Mars like back at this time when liquid “water” could flow across the surface and “stuff” was filling the craters?