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P. J. Helm

Publications and source records attributed to P. J. Helm.

5 recordsLinked to original sources
USGS2012

Global map of eolian features on Mars

Ten basic categories of eolian features on Mars were identified from a survey of Mariner 9 and Viking orbiter images. The ten features mapped are (1) light streaks (including frost streaks), (2) dark streaks, (3) sand sheets or splotches, (4) barchari dunes, (5) transverse dunes, (6) crescentic dunes, (7) anomalous dunes, (8) yardangs, (9) wind grooves, and (10) deflation pits. The features were mapped in groups, not as individual landforms, and recorded according to their geographic positions and orientations on maps of 1:12.5 million or 1:25 million scale. In the north polar region, light and dark streaks indicate winds from the west and northwest. Frost streaks show northeast and northwest winds. Barchan dunes show southwest and west winds. Transverse dunes show east and west winds. Local erosional features show winds from the northwest or southeast and northeast or southwest. In the middle and low northern latitudes, streaks show northeast winds; yardangs and deflation pits show mostly easterly and northeasterly winds. In the low southern latitudes, light streaks and dunes record northwest and northeast winds; dark streaks record southeast winds. In high southern latitudes, most streaks and dunes record southeast and east winds. In the south polar region, light and frost streaks record southwest and northwest winds, whereas dark streaks and transverse dunes show southeast winds. The patterns recorded for ephemeral features conform to global conditions of strong southern spring and summer (northern fall and winter) wind circulation. Erosional features in bedrock indicate long-term and perhaps ancient wind trends, whereas depositional features may record relatively more recent winds. Wind directions indicated by yardangs, pits, and other erosional features seldom correspond to those shown by streaks. These erosional features indicating winds of different directions than those that formed the streaks may have been carved when the effective regional winds on Mars were different due to relative hemisphere insolation differences, which are related to obliquity variations. Alternatively, many yardangs and pits may be largely controlled by structure and carved along surface structural patterns or features that channeled both ancient and present-day winds. Deflation pits in the mantled terrain may contain the best record of ancient wind trends. These pits seem to have stratigraphically related orientations. Different stratigraphic units composing the mantle contain wind erosion pits that possibly record large-scale wind direction changes because of long-term changes in the obliquity of the Mars.

Journal of Geophysical Research Solid Earth↗
USGS2001

Supply-limited horizontal sand drift at an ephemerally crusted, unvegetated saline playa

A site at Owens Dry Lake was observed for more than 4 years. The site was a vegetation-free saline playa where the surface formed “ephemeral crusts,” crusts that form after rainfall. Sometimes these crusts were destroyed and often a layer of particles on the crust would engage in vigorous aeolian activity. Three “phases” of active sand drifting are defined as almost no movement (extreme supply limitation), loose particles on crust with some degree of sand drift (moderate supply limitation), and unlimited source movement corresponding to a destroyed surface crust (unlimited supply). These “phases” occurred 45, 49, and 6% of the time, respectively. The accumulation of loose particles on the crust was mostly the result of in situ formation. Crusted sediments with loose particles on top can exhibit mass flux rates about the same as for noncrusted sediments. Crusted sediments limit or eliminate sand drift in two conditions: for rough crusts that effect a sufficiently high threshold friction velocity (above the wind friction velocity) and for limited amounts of loose particles on the crust where particle supply is less than would be transported in normal saltation for a thick sandy surface. These “supply-limited” cases are similar to wind erosion of limited spilled material on a hard concrete surface. We quantified “supply limitation” by defining a “potential” or “supply unlimited” sand drift function Q = AG where A represents supply limitation that decreases as the particle source is depleted. Here Q is the mass of sand transported through a surface perpendicular to the ground and to the wind and having unit width during time period t , and G = ∫ u * ( u 2 * − u 2 * t ) dt for u * > u * t . G is integrated for the same time period t as for Q , u * is the friction velocity of the wind, and u * t is the threshold friction velocity of the wind. Hard crusts (usually formed in the summer) tended to show almost no change of threshold friction velocity with time and often gave total protection from wind erosion. Rough crusts provided sufficient protection expressed as high threshold friction velocities. For these high threshold friction velocities, aeolian activity was greatly reduced or practically prevented. The softest crusts, usually formed in the winter, provided much less protection and sometimes were destroyed by the wind. Following this destruction the “potential” or “supply unlimited” sand drift would be observed.

California↗
USGS1995

Geometeorological data collected by the USGS Desert Winds Project at Gold Spring, Great Basin desert, northeastern Arizona, 1979-1992

The primary purpose of the Desert Winds Project (DWP) is to obtain high-resolution meteorological data and related surface geological and vegetation data for natural (e.g., uncultivated) desert sites where wind is or has been a major erosive or depositional force. The objectives are twofold: (1) to provide the detailed field measurements needed to carry out quantitative studies of wind as an agent of surface geologic change; and (2) to establish a baseline for defining the 'normal' range of climatic conditions that can be expected to occur on a decadal time scale, in areas considered representative of the major American deserts. The Gold Spring locality was selected to represent that part of the Great Basin Desert that extends into northeastern Arizona. The long-term goal for acquiring and analyzing the Desert Winds Project data is to use them to address problems of land resource degradation by wind, whether resulting from climatic variation aridification) or human activities (desertification), or both (see techinfo.doc).

Arizona↗
USGS1990

A method of evaluating effects of antecedent precipitation on duststorms and its application to Yuma, Arizona, 1981-1988

Precipitation causes several short- and long-term effects on wind-induced surface erodibility and subsequent dust emission. Among the principal effects considered by this paper are soil moisture, soil crusts, and vegetation. A quantitative method is developed to assess these effects using differences between the potential and the actual amounts of dust emitted from dust sources as inferred from surface meteorological measurements obtained downwind from those sources. The results of this assessment must be interpreted with caution, however, when the size and location of dust sources are unknown. Using meteorological data recorded near Yuma, Arizona at the Yuma Marine Corps Air Station (YMCAS), the method is applied to calculate the potential and actual amounts of dust emitted from upwind dust sources during the spring and fall/winter seasons between January 1, 1981 and May 31, 1988. (Spring is considered to be the period between February 1 and May 31; fall/winter, between October 1 and January 31.) Because summer precipitation is intermittent and wind patterns are localized, summer meteorological data are not used to evaluate regional correlations between precipitation and dust storms. For the period between 1981 and 1988, a correlation of -0.60 was found between fall/winter precipitation and the actual amount of dust emitted from sources upwind of YMCAS during the following spring. A particularly strong reduction in dust emission was noted during the springs of 1983 and 1984 following the start of an 'El Nino event' in fall/winter 1982. Photographs taken at a geological and meteorological data-collection (Geomet) site, located in the natural desert 25 km southeast of YMCAS, show a correspondence between increased antecedent precipitation recorded at the site and increased vegetation. Whereas the annual precipitation totals at YMCAS and the Geomet site from the beginning of 1982 through 1984 are high, their seasonal totals, especially during the fall/winter seasons, are disparate. This fall/winter precipitation disparity may account for evidence suggesting that significant vegetation growth occurred at dust sources upwind of YMCAS by spring 1983, but that such growth did not occur at the Geomet site until fall/ winter of 1983. Spatial inhomogeneity in fall/winter precipitation probably contributed to the relatively low correlation (-0.60) between fall/winter precipitation recorded at YMCAS and the actual amount of dust emitted from upwind sources during the following spring. ?? 1990 Kluwer Academic Publishers.

Climatic Change↗