Shape of (101955) Bennu indicative of a rubble pile with internal stiffness.

The shapes of asteroids reflect interplay between their interior properties and the processes responsible for their formation and evolution as they journey through the Solar System. Prior to the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission, Earth-based radar imaging gave an overview of (101955) Bennu's shape. Here, we construct a high-resolution shape model from OSIRIS-REx images. We find that Bennu's top-like shape, considerable macroporosity, and prominent surface boulders suggest that it is a rubble pile. High-standing, north-south ridges that extend from pole to pole, many long grooves, and surface mass wasting indicate some low levels of internal friction and/or cohesion. Our shape model indicates that, similar to other top-shaped asteroids, Bennu formed by reaccumulation and underwent past periods of fast spin leading to its current shape. Today, Bennu might follow a different evolutionary pathway, with interior stiffness permitting surface cracking and mass wasting.

In-flight characterization of the lunar orbiter laser altimeter instrument pointing and far-field pattern.

The Lunar Orbiter Laser Altimeter (LOLA) aboard the Lunar Reconnaissance Orbiter (LRO) has collected nearly seven billion measurements of surface height on the Moon with an absolute accuracy of ∼1 m and a precision of ∼10 cm. Converting time-of-flight laser altimeter measurements to topographic elevations requires accurate knowledge of the laser pointing with respect to the spacecraft body-fixed coordinate system. To that end, we have utilized altimetric crossovers from LOLA, as well as bidirectional observations of the LOLA laser and receiver boresight via an Earth-based laser tracking ground station. Based on a sample of ∼780,000 globally distributed crossovers from the circular-orbit phase of LRO's mission (∼27 months), we derive corrections to the LOLA laser boresight. These corrections improve the cross-track and along-track agreement of the crossovers by 24% and 33%, respectively, yielding RMS residuals of ∼10 m. Since early in the LRO mission, the bidirectional laser tracking experiments have confirmed a pointing anomaly when the LOLA instrument is facing toward deep space or the night side of the Moon and have allowed the reconstruction of the laser far-field pattern and receiver telescope pointing. By conducting such experiments shortly after launch and nearly eight years later, we have directly measured changes in the laser characteristics and obtained critical data to understand the laser behavior and refine the instrument pointing model. The methods and results presented here are also relevant to the design, fabrication, and operation of future planetary laser altimeters and their long-term behavior in the space environment.

Multiplex KRASG12/G13 mutation testing of unamplified cell-free DNA from the plasma of patients with advanced cancers using droplet digital polymerase chain reaction.

Background: Cell-free DNA (cfDNA) from plasma offers easily obtainable material for KRAS mutation analysis. Novel, multiplex, and accurate diagnostic systems using small amounts of DNA are needed to further the use of plasma cfDNA testing in personalized therapy. Patients and methods: Samples of 16 ng of unamplified plasma cfDNA from 121 patients with diverse progressing advanced cancers were tested with a KRASG12/G13 multiplex assay to detect the seven most common mutations in the hotspot of exon 2 using droplet digital polymerase chain reaction (ddPCR). The results were retrospectively compared to mutation analysis of archival primary or metastatic tumor tissue obtained at different points of clinical care. Results: Eighty-eight patients (73%) had KRASG12/G13 mutations in archival tumor specimens collected on average 18.5 months before plasma analysis, and 78 patients (64%) had KRASG12/G13 mutations in plasma cfDNA samples. The two methods had initial overall agreement in 103 (85%) patients (kappa, 0.66; ddPCR sensitivity, 84%; ddPCR specificity, 88%). Of the 18 discordant cases, 12 (67%) were resolved by increasing the amount of cfDNA, using mutation-specific probes, or re-testing the tumor tissue, yielding overall agreement in 115 patients (95%; kappa 0.87; ddPCR sensitivity, 96%; ddPCR specificity, 94%). The presence of ≥ 6.2% of KRASG12/G13 cfDNA in the wild-type background was associated with shorter survival (P = 0.001). Conclusion(s): Multiplex detection of KRASG12/G13 mutations in a small amount of unamplified plasma cfDNA using ddPCR has good sensitivity and specificity and good concordance with conventional clinical mutation testing of archival specimens. A higher percentage of mutant KRASG12/G13 in cfDNA corresponded with shorter survival.

Hydrogen mapping of the lunar south pole using the LRO neutron detector experiment LEND.

Hydrogen has been inferred to occur in enhanced concentrations within permanently shadowed regions and, hence, the coldest areas of the lunar poles. The Lunar Crater Observation and Sensing Satellite (LCROSS) mission was designed to detect hydrogen-bearing volatiles directly. Neutron flux measurements of the Moon's south polar region from the Lunar Exploration Neutron Detector (LEND) on the Lunar Reconnaissance Orbiter (LRO) spacecraft were used to select the optimal impact site for LCROSS. LEND data show several regions where the epithermal neutron flux from the surface is suppressed, which is indicative of enhanced hydrogen content. These regions are not spatially coincident with permanently shadowed regions of the Moon. The LCROSS impact site inside the Cabeus crater demonstrates the highest hydrogen concentration in the lunar south polar region, corresponding to an estimated content of 0.5 to 4.0% water ice by weight, depending on the thickness of any overlying dry regolith layer. The distribution of hydrogen across the region is consistent with buried water ice from cometary impacts, hydrogen implantation from the solar wind, and/or other as yet unknown sources.

Seasonal variations of snow depth on Mars.

Using topography collected over one martian year from the Mars Orbiter Laser Altimeter on the Mars Global Surveyor (MGS) spacecraft, we have measured temporal changes in the elevation of the martian surface that correlate with the seasonal cycle of carbon dioxide exchange between the surface and atmosphere. The greatest elevation change (1.5 to 2 meters) occurs at high latitudes ( above 80 degrees ), whereas the bulk of the mass exchange occurs at lower latitudes (below 75 degrees N and below 73 degrees S). An unexpected period of sublimation was observed during northern hemisphere autumn, coincident with dust storms in the southern hemisphere. Analysis of MGS Doppler tracking residuals revealed temporal variations in the flattening of Mars that correlate with elevation changes. The combined changes in gravity and elevation constrain the average density of seasonally deposited carbon dioxide to be 910 +/- 230 kilograms per cubic meter, which is considerably denser than terrestrial snow.

Laser altimetry of small-scale features on 433 Eros from NEAR-Shoemaker.

During the Near Earth Asteroid Rendezvous (NEAR)-Shoemaker's low-altitude flyover of asteroid 433 Eros, observations by the NEAR Laser Rangefinder (NLR) have helped to characterize small-scale surface features. On scales from meters to hundreds of meters, the surface has a fractal structure with roughness dominated by blocks, structural features, and walls of small craters. This fractal structure suggests that a single process, possibly impacts, dominates surface morphology on these scales.

Internal structure and early thermal evolution of Mars from Mars Global Surveyor topography and gravity.

Topography and gravity measured by the Mars Global Surveyor have enabled determination of the global crust and upper mantle structure of Mars. The planet displays two distinct crustal zones that do not correlate globally with the geologic dichotomy: a region of crust that thins progressively from south to north and encompasses much of the southern highlands and Tharsis province and a region of approximately uniform crustal thickness that includes the northern lowlands and Arabia Terra. The strength of the lithosphere beneath the ancient southern highlands suggests that the northern hemisphere was a locus of high heat flow early in martian history. The thickness of the elastic lithosphere increases with time of loading in the northern plains and Tharsis. The northern lowlands contain structures interpreted as large buried channels that are consistent with northward transport of water and sediment to the lowlands before the end of northern hemisphere resurfacing.

The global topography of Mars and implications for surface evolution.

Elevations measured by the Mars Orbiter Laser Altimeter have yielded a high-accuracy global map of the topography of Mars. Dominant features include the low northern hemisphere, the Tharsis province, and the Hellas impact basin. The northern hemisphere depression is primarily a long-wavelength effect that has been shaped by an internal mechanism. The topography of Tharsis consists of two broad rises. Material excavated from Hellas contributes to the high elevation of the southern hemisphere and to the scarp along the hemispheric boundary. The present topography has three major drainage centers, with the northern lowlands being the largest. The two polar cap volumes yield an upper limit of the present surface water inventory of 3.2 to 4.7 million cubic kilometers.

Observations of the north polar region of Mars from the Mars orbiter laser altimeter.

Elevations from the Mars Orbiter Laser Altimeter (MOLA) have been used to construct a precise topographic map of the martian north polar region. The northern ice cap has a maximum elevation of 3 kilometers above its surroundings but lies within a 5-kilometer-deep hemispheric depression that is contiguous with the area into which most outflow channels emptied. Polar cap topography displays evidence of modification by ablation, flow, and wind and is consistent with a primarily H2O composition. Correlation of topography with images suggests that the cap was more spatially extensive in the past. The cap volume of 1.2 x 10(6) to 1.7 x 10(6) cubic kilometers is about half that of the Greenland ice cap. Clouds observed over the polar cap are likely composed of CO2 that condensed out of the atmosphere during northern hemisphere winter. Many clouds exhibit dynamical structure likely caused by the interaction of propagating wave fronts with surface topography.

The shape and internal structure of the moon from the clementine mission.

Global topographic and gravitational field models derived from data collected by the Clementine spacecraft reveal a new picture of the shape and internal structure of the moon. The moon exhibits a 16-kilometer range of elevation, with the greatest topographic excursions occurring on the far side. Lunar highlands are in a state of near-isostatic compensation, whereas impact basins display a wide range of compensation states that do not correlate simply with basin size or age. A global crustal thickness map reveals crustal thinning under all resolvable lunar basins. The results indicate that the structure and thermal history of the moon are more complex than was previously believed.

An Arabidopsis mutant with a reduced level of cab140 RNA is a result of cosuppression.

We analyzed a mutant of Arabidopsis with a severely reduced level of cab140 RNA. This mutant, named lct for low level of cab140 transcript, was obtained during a selection for phytochrome signal transduction mutants. The selection was based on reduced expression of the tumor morphology shoots gene (tms2), an introduced counter-selectable marker under the control of the cab140 promoter. Expression of the introduced cab140::tms2 gene was also greatly reduced in lct, but surprisingly, expression of other phytochrome-regulated genes was not comparably affected. Furthermore, the lct phenotype could not be separated genetically from the T-DNA insert; thus, we suggest that this phenotype was caused by cosuppression of the introduced construct and the endogenous cab140 gene, and that the mutation causing the cosuppression was located on the T-DNA insert. In vitro nuclear transcription experiments demonstrated that the suppression was occurring at the level of transcription. We also found that the suppressed cab140 genes were not significantly more methylated than the nonsuppressed cab140 genes.

Phytochrome control of the tms2 gene in transgenic Arabidopsis: a strategy for selecting mutants in the signal transduction pathway.

Introduction of the tms2 gene from Agrobacterium tumefaciens into Arabidopsis thaliana yields transgenic seedlings with a new selectable phenotype: the seedlings are strongly growth inhibited on micromolar concentrations of auxin amide substrates that do not significantly affect wild-type seedlings. The tms2 gene encodes an amidohydrolase that catalyzes the conversion of biologically inactive auxin amides into active auxins, which are toxic to plants at elevated concentrations. In the absence of exogenous substrate, tms2+ transgenic seedlings grow normally and are fertile. When grown on auxin amides, both etiolated and green tms2+ seedlings exhibit a variety of dose-dependent auxin toxicity effects. tms2 mRNA and the encoded amidohydrolase activity are both detectable in transgenic but not in wild-type seedlings, demonstrating that a cognate activity is lacking in wild-type Arabidopsis. Furthermore, when the introduced tms2 gene is fused to the Arabidopsis cab140 promoter, the tms2 RNA and its encoded amidohydrolase activity and, thus, the conditional lethal phenotype can be modulated by phytochrome action. The tms2 gene can, therefore, serve as a regulatable selectable marker in Arabidopsis that should be useful in isolation of trans-regulatory mutants that have lost the imposed regulation of tms2 gene activity.

Expression of Light-Harvesting Chlorophyll a/b-Protein Genes Is Phytochrome-Regulated in Etiolated Arabidopsis thaliana Seedlings.

Phytochrome action results in a large and rapid increase in the light-harvesting chlorophyll a/b-protein (LHCP) mRNA level in etiolated seedlings of Arabidopsis thaliana: the RNA increase is detectable within 1 hour after 1 minute red illumination, reaches a maximum 30-fold higher than the dark level at ca. 2 hours, and decays back to dark levels by about 8 hours after the brief red illumination. S1 nuclease analysis distinguishes two kinds of mRNAs transcribed from the three members of the LHCP gene family previously characterized for Arabidopsis (LS Leutwiler, EM Meyerowitz, EM Tobin, 1986 Nucleic Acids Res 14: 4051-4064). One of these arises from the AB140 gene, while the other represents the product(s) of the AB165 and/or AB180 gene(s) (AB165/AB180 mRNA). In mature, white light-grown plants, the two kinds of mRNAs are present in nearly equal amounts. In contrast, in etiolated seedlings, 1 minute red light causes a sixfold greater increase in the level of AB140 mRNA than in the level of AB165/AB180 mRNA, although both levels are regulated by phytochrome action. The kinetics of the responses to 1 minute red light are similar for both kinds of transcripts. Additional evidence suggests that this differential expression is developmentally regulated. Because the AB140 gene offers an attractive target for further analysis of phytochrome-regulated gene expression in Arabidopsis, we have further characterized this gene by mapping its 5' and 3' transcript termini.

Transit peptides of nuclear-encoded chloroplast proteins share a common amino acid framework.

We have identified three major blocks of amino acid homology shared by the transit peptides of two nuclear-encoded chloroplast proteins, the light-harvesting chlorophyll a/b-protein (LHCP) II of the thylakoid membrane and the small subunit (SSU) of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) of the stroma. These previously unrecognized homology blocks lie at the beginning, middle and end of both transit sequences, and are separated by differing lengths of unshared (interblock) sequence in the two proteins. These interblocks may be dispensible or they might confer a specific property on the individual proteins, such as facilitating proper compartmentalization within the chloroplast. We propose that these three shared sequence elements form a common framework in transit-bearing chloroplast precursors which mediates the common functions performed by each transit peptide. Ferredoxin, the only other such nuclear-encoded protein for which a published transit sequence exists, conforms to the predictions of this hypothesis. These findings stand in contrast to mitochondrial leader sequences and the well-studied signal peptides of secretory and certain integral membrane proteins in which no such framework has been observed.

A chlorophyll a/b-protein encoded by a gene containing an intron with characteristics of a transposable element.

We have sequenced a genomic subclone (pLg AB19/H5c) of Lemna gibba nuclear DNA containing a complete chlorophyll a/b protein coding region and 5' and 3' flanking nucleotides. The coding region contains an intron of 84 nucleotides that has features characteristic of a transposable element. Evidence from S1 nuclease mapping experiments is consistent with correct transcription and splicing of the AB19 or another closely related intron-containing gene. The encoded precursor polypeptide of 264 amino acid residues has a predicted Mr of 28,327. Approximately 35 N-terminal residues are cleaved from this protein to form the mature apoprotein. We have used theoretical considerations of protein structure to propose an experimentally testable model of the structure of this protein in thylakoid membranes.