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Spectral evidence for weathered basalt as an alternative to andesite in the northern lowlands of Mars
MICHAEL B. WYATT AND HARRY Y. MCSWEEN JR
doi:10.1038/417263a
First paragraph
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v417/n6886/abs/417263a_fs.html

Supplementary information
http://www.nature.com/nature/journal/v417/n6886/suppinfo/417266a.html

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A News and Views piece on the Wyatt and McSween paper:

Planetary science: A wet and altered Mars
JOHN F. MUSTARD
Has Mars always been cold and dry or was it once warm and wet? Reanalysis of spacecraft data reveals a signature from the surface rocks that indicates their composition has been altered in the presence of water.
Full text
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v417/n6886/full/417234a_fs.html

Alex R. Blackwell
University of Hawaii