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



ST2


Near equator in the southern highlands and Syrtis Major, locally in northern plains
Noachian and Hesperian age
Where there is no ice
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.