Geoscience Reference
In-Depth Information
Fig. 16.19 The capture magnet
on the Spirit rover, showing a
range of particle sizes adhering to
the rings of different magnetic
strengths (rather stronger than a
typical refrigerator magnet).
NASA/JPL
indicates they must have been saltated. Some surfaces may
be specifically designed to observe the accumulation of
material—among these are surfaces with magnets of vari-
ous strength (Fig. 16.19 ) mounted behind them so that the
amounts of certain minerals such as magnetite can be
assessed.
In addition to manipulation with an arm, another inter-
action of a vehicle with the surface can be studied, namely
that of the wheels on a rover. Usually, the motor current
required to drive the wheel is measured, as well as the
number of turns a wheel makes. This in turn can be com-
pared with the forward motion measured from camera
imagery—usually the wheel rotation corresponds to rather
more than the distance moved, owing to slippage of the
wheel in soft ground (see Chap. 22 ). This slippage and the
energy required to move a given distance are diagnostic of
the surface properties. The textures in the track (Fig. 16.20 )
also reveal the cohesive properties and friction angles of the
regolith; a detailed study of those from the Mars Exploration
Rovers is given in Sullivan et al. (2011).
Because tracks or other marks made by rovers are cre-
ated at known times, their subsequent erasure can be used to
estimate rates of processes. Spirit and Opportunity tracks
have been given close follow-up observations by orbital
imaging (Geissler et al. 2010) to determine that erasure by
erosion and deposition by surface winds takes about one
Martian year.
While the manipulations of the Martian environment by
rovers and landers over long-planned hours and days are
hopelessly clumsy in comparison with what a human could
perform in minutes, these vehicles are often equipped with
formidable analytical instrumentation. The most sophisti-
cated of this is devoted to looking for evidence of life, but
many mineralogical and other investigations can be per-
formed. Sometimes these are contact measurements made
with instruments on an arm; more generally, material must
be ingested into an instrument, a relatively straightforward
process for sand but requiring grinding for rocks. A com-
mon technique has been APXS (Alpha Particle X-Ray
Spectroscopy), wherein a small radioactive source induces
 
Search WWH ::




Custom Search