Thermal Fatigue as a Driving Mechanism for Activity on Asteroid Bennu.

Many boulders on (101955) Bennu, a near-Earth rubble pile asteroid, show signs of in situ disaggregation and exfoliation, indicating that thermal fatigue plays an important role in its landscape evolution. Observations of particle ejections from its surface also show it to be an active asteroid, though the driving mechanism of these events is yet to be determined. Exfoliation has been shown to mobilize disaggregated particles in terrestrial environments, suggesting that it may be capable of ejecting material from Bennu's surface. We investigate the nature of thermal fatigue on the asteroid, and the efficacy of fatigue-driven exfoliation as a mechanism for generating asteroid activity, by performing finite element modeling of stress fields induced in boulders from diurnal cycling. We develop a model to predict the spacing of exfoliation fractures and the number and speed of particles that may be ejected during exfoliation events. We find that crack spacing ranges from ~1 mm to 10 cm and disaggregated particles have ejection speeds up to ~2 m/s. Exfoliation events are most likely to occur in the late afternoon. These predictions are consistent with observed ejection events at Bennu and indicate that thermal fatigue is a viable mechanism for driving asteroid activity. Crack propagation rates and ejection speeds are greatest at perihelion when the diurnal temperature variation is largest, suggesting that events should be more energetic and more frequent when closer to the Sun. Annual thermal stresses that arise in large boulders may influence the spacing of exfoliation cracks or frequency of ejection events.

Asteroid (101955) Bennu's weak boulders and thermally anomalous equator.

Thermal inertia and surface roughness are proxies for the physical characteristics of planetary surfaces. Global maps of these two properties distinguish the boulder population on near-Earth asteroid (NEA) (101955) Bennu into two types that differ in strength, and both have lower thermal inertia than expected for boulders and meteorites. Neither has strongly temperature-dependent thermal properties. The weaker boulder type probably would not survive atmospheric entry and thus may not be represented in the meteorite collection. The maps also show a high-thermal inertia band at Bennu's equator, which might be explained by processes such as compaction or strength sorting during mass movement, but these explanations are not wholly consistent with other data. Our findings imply that other C-complex NEAs likely have boulders similar to those on Bennu rather than finer-particulate regoliths. A tentative correlation between albedo and thermal inertia of C-complex NEAs may be due to relative abundances of boulder types.

In situ evidence of thermally induced rock breakdown widespread on Bennu's surface.

Rock breakdown due to diurnal thermal cycling has been hypothesized to drive boulder degradation and regolith production on airless bodies. Numerous studies have invoked its importance in driving landscape evolution, yet morphological features produced by thermal fracture processes have never been definitively observed on an airless body, or any surface where other weathering mechanisms may be ruled out. The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission provides an opportunity to search for evidence of thermal breakdown and assess its significance on asteroid surfaces. Here we show boulder morphologies observed on Bennu that are consistent with terrestrial observations and models of fatigue-driven exfoliation and demonstrate how crack propagation via thermal stress can lead to their development. The rate and expression of this process will vary with asteroid composition and location, influencing how different bodies evolve and their apparent relative surface ages from space weathering and cratering records.

Evidence for widespread hydrated minerals on asteroid (101955) Bennu.

Early spectral data from the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission reveal evidence for abundant hydrated minerals on the surface of near-Earth asteroid (101955) Bennu in the form of a near-infrared absorption near 2.7 µm and thermal infrared spectral features that are most similar to those of aqueously altered CM carbonaceous chondrites. We observe these spectral features across the surface of Bennu, and there is no evidence of substantial rotational variability at the spatial scales of tens to hundreds of meters observed to date. In the visible and near-infrared (0.4 to 2.4 µm) Bennu's spectrum appears featureless and with a blue (negative) slope, confirming previous ground-based observations. Bennu may represent a class of objects that could have brought volatiles and organic chemistry to Earth.

Responses of people with coronary artery disease to common lawn-care tasks.

The primary purpose of the present study was to determine oxygen uptake (VO2) and heart rate (HR) responses of patients with coronary artery disease (CAD) to common lawn-care activities. The study was conducted in three phases. In phase I, 8 men with CAD performed 30 min of push motorized lawn mowing at a self-paced rate. In phase II, 9 men with CAD performed push (no power) mowing, trimming (power and manual), and raking for 8 min each. In phase III, age-matched men and women with and without CAD (9-11 per group) performed self-propelled motorized mowing and push motorized mowing. In phase I, VO2 averaged 17.3 (SEM 3.8) ml.kg-1.min-1 during 30 min of mowing. Relative effort was 68 (SEM 1) and 76 (SEM 4)% of treadmill maximal VO2 (VO2max) and HR, respectively. In phase II, mean VO2 ranged from 8.6 (SEM 0.4) with grass trimming to 22.2 (SEM 1.6) ml.kg-1.min-1 with push manual mowing. With self-propelled mowing at three speeds in phase III, mean VO2 of the CAD groups ranged from 9.5 (SEM 0.3) to 13.8 (SEM 1.4) ml.kg-1.min-1 and represented 37%-62% VO2max. The results indicated that lawn mowing is often performed at an exercise intensity recommended for aerobic exercise training; patients who achieve a treadmill peak capacity of 4 times resting metabolic rate (4 METs) should be able to perform self-propelled motorized lawn mowing (slow speed) and grass trimming at less than 80% peak VO2; and VO2 demands of lawn mowing can be adjusted by equipment selection and/or pace.

Energy expenditure during household tasks in women with coronary artery disease.

The energy expenditure for and heart rate responses to common household tasks were determined in 26 older (mean age 62 +/- 2 years) women with coronary artery disease (CAD). Each activity was performed at a self-determined pace for 6 or 8 minutes. The average oxygen uptake (ml/kg/min) for each task evaluated was 6.5 for washing dishes, 6.8 for ironing, 7.2 for scrubbing pans, 8.6 for unpacking groceries, 9.5 for vacuuming, 9.8 for sweeping, 10.1 for mopping, 12.0 for changing bed linens, and 12.4 for washing the floor (hands and knees). None of the subjects reported angina. Mean relative oxygen uptake (i.e., percentage of peak response with treadmill testing) ranged from 31 +/- 2% for washing dishes to 62 +/- 3% for changing the bed linens and washing the floor. Percentage of peak treadmill heart rate ranged from 62 +/- 2% for washing dishes to 73 +/- 2% for washing the floor. In 4 of the more physically demanding household activities (i.e., vacuuming, mopping, washing the floor, and changing bed linens), the responses of 10 age-matched normal women were evaluated. The absolute and relative demands of the tasks were similar between the CAD and normal groups. Results indicate that the mean energy expenditure rate of common household tasks evaluated in this study range from 2 to 4 METs, suggesting that most women with CAD who are able to achieve > or = 5 METs during a treadmill exercise test without adverse signs or symptoms should be able to resume these activities.

Effect of enhanced preload with head-out water immersion on exercise response in men with healed myocardial infarction.

Head-out water immersion shifts venous blood to the central vasculature and heart and subsequently increases cardiac preload. In healthy men, cardiac output and stroke volume are greater during upright leg cycle exercise in water than on land. Heart rate is similar during work loads < 50% of peak oxygen consumption but is decreased in water at higher work intensities. To determine if men with myocardial infarction (MI) show a similar response, 15 men with a documented MI exercised upright on a leg cycle ergometer on land and immersed in water (31 +/- 1 degree C) to the level of the shoulders. Heart rate, cardiac output (carbon dioxide rebreathing procedure) and oxygen consumption were measured at rest and at work loads corresponding to approximately 40, 60 and 75% of peak oxygen consumption in both environments. At rest, cardiac output and stroke volume were elevated (p < 0.05) in water. During exercise, heart rate, cardiac output and stroke volume did not differ between water and land. When subjects were given beta-blocking medications (n = 8) and subjects with exercise-induced ST-segment depression (n = 5) were separately excluded from the analysis, water immersion still did not significantly change exercise responses. These results suggest that MI alters the normal cardiac response to increased preload during exercise. The alteration may involve reduced myocardial compliance or near-complete use of the Frank-Starling reserve, or both, during land exercise.