TEXTURAL AND GEOCHEMICAL CHARACTERISTICS OF SOIL TARGETS OF GALE CRATER, MARS

Unconsolidated sediments, commonly referred to as soils, have been investigated by the Curiosity rover in Gale crater, Mars. Eleven soil targets have been imaged so far by the Mars Hand Lens Imager (MAHLI) and analyzed by the Alpha Particle X-Ray Spectrometer (APXS). MAHLI observations and APXS data are utilized to study the relationship between texture and geochemical compositions of the soil targets in Gale crater.

Some soil targets are unconsolidated surface sediments or regoliths. These include the following targets: Maya (Sol 373), King (Sol 523), Johnny Cake (Sol 558), Lagrange (Sol 605), and LaBrea (Sol 955). Others are aeolian bedforms such as ripples and dunes and includes targets such as Portage (Sol 89), Baker (Sol 530), Argyle (Sol 531), Sourdough (Sol 673), Kelso (Sol 801), and Dumont (Sol 802).

Regolithic soil targets are poorly sorted, consisting of gravel, sand, and grains too small to be resolved by MAHLI (silt and clay). Observed particles are well-rounded to sub-angular. Grains not covered by dust are black in color and lack any internal features. Bulk elemental composition of the five regolithic soil targets are very similar; average values (in weight percent) are shown in the table below.

Targets	Na2O	MgO	Al2O3	SiO2	K2O	CaO	TiO2	Cr2O3	MnO	FeO
Regolith	2.65	8.26	9.25	43.31	0.51	7.09	1.13	0.45	0.41	19.66
Eolian BF	2.85	8.17	9.46	43.65	0.65	6.95	1.03	0.43	0.37	18.30

Aeolian bedform soil targets show a high abundance of small grains. Some bedform soils are unsorted ranging in grain size from coarse sand to silt and clay; others appear fine-grained and moderately sorted. Grains in aeolian bedform targets are well rounded to subangular, and are mostly black in color but grains of other colors also occur. Similar to regoliths, bulk elemental composition of aeolain bedform targets vary little among the six targets investigated. Their weight percent averages are shown in the above table.

It appears that, regardless of their origin, soil targets display large variations in texture such as grain size and grain sorting. But their bulk elemental compositions are nearly identical within the 1.7 cm diameter field of view of the APXS instrument.