Wyatt et al., Global geologic context for rock types and surface alteration on Mars, Geology; August 2004; v. 32; no. 8; p. 645–648; doi: 10.1130/G20527.1

Rebecca Greenberger’s summary

Mars Global Surveyor Thermal Emission Spectrometer has found 2 spectral end members

·  Surface type 1 (ST1)-basalt

·  Surface type 2 (ST2)-either andesite or partly altered basalt

Purpose of paper: look at geographic distribution of ST1 and ST2 and near-surface ice/ice-rich mantles to suggest that ST2 is partly altered basalt and that this alteration occurred in a cold and episodically wet environment

The locations of ST1 and ST2 correlate with latitude but not crustal thickness, age, or crustal dichotomy

ST1

·  Near equator in the southern highlands and Syrtis Major, locally in northern plains

·  Noachian and Hesperian age

·  Where there is no ice

ST2

·  Northern lowlands and circumpolar sand seas; southern mid- to high-latitudes (mixed with ST1)

o  Noachian to Amazonian age

o  Northern Hemisphere-in Vastitis Borealis Formation (thought to be altered sediment)

o  Southern Hemisphere-more ST2 as go toward pole

·  Located in same regions as ice-rich mantle

o  Near-surface water ice and continuous mantle 60° to poles (both hemispheres)

o  Partly degraded mantle 30-60° (both hemispheres)

o  Southern hemisphere-region with ST2 starts where ice-rich mantle starts

o  Northern hemisphere-ST1 to ST2 transition at 20° (not where ice-rich mantle starts but continues through ice-rich mantle)

Why ST2 is not andesite

·  Andesites on Earth form in subduction zones, and the crust is thicker in these regions

o  But ST2 occurs where crust is thin in the Northern Hemisphere

·  If form andesites through fractionation, should produce basalts and andesites that have similar ages

o  Southern hemisphere basalts are much older than ST2 in north

·  Could form andesites through partial melting of basalt crust

o  Would need heat sources only at certain latitudes over a long period of time, so unlikely

·  Martian meteorites are not andesitic

Role of ice/volatiles and evidence for ST2 being altered basalt

·  ST1 is not altered and is located where there is no ice-rich mantle or surface ice

·  Southern hemisphere: gradual transition from ST1 to ST2

o  Paper argues that chemical weathering of basalts increases as go toward pole because of interaction with ice-rich mantles (at high obliquity)

·  Northern hemisphere: ST2 in Vastitis Borealis

o  Ice-rich mantles

o  Fluvially transported sediments

o  Could have had temporary standing water and ice (more alteration)

Analogs

·  Antarctic Dry Valleys: cold, hyperarid, stable permafrost, ground ice

·  Mauna Kea, Hawaii summit: cold, arid

·  Iceland: volcanoes + ground ice

·  Basalts in all of these are plagioclase and pyroxene with small amounts of alteration phases

From analogs, do not need much liquid water to alter basalts

Figure 2 from Wyatt et al. paper. Green=ST1, red=ST2, blue=dust. Shows correlations of ST2 with ice-rich mantles and near-surface ice.