Gamma-ray Research Articles

Determination of mutagenic effectiveness and efficiency of gamma rays and EMS in Coriander

Coriander is a dual purpose vegetable used as leaf as well as a spice, along with medicinal properties. Owing to its compound umbel and difficulty in crossing programme, mutation breeding is the available source to create variability. Mutagenesis is a well-recognized tool to expand genetic variability. The present investigation involves mutagenesis in coriander cv. Sindhu with physical (gamma rays) and chemical mutagens (EMS) and their combinations. The mutation frequency, mutagenic effectiveness and mutagenic efficiency were calculated for the gamma rays, EMS and their combination doses. In gamma rays, a drop in mutagenic effectiveness was observed at 10Kr followed by increase with higher concentration. Mutagenic effectiveness due to gamma rays, ranged from 6.5 to 8.51 due to EMS ranged from 42.09 to 49.55 and due to combination treatments ranged from 12.17 to 18.80 (5kr+0.2% EMS) in coriander variety, Sindhu. The mutagenic efficiency based on survival ranged from 1.78 to 4.0, injury ranged from 3.83 to 8.2, sterility ranged from 2.37 to 5.18 indicating differential response to different mutagens. The combination of gamma rays and EMS at lower and moderate doses expressed its worth to create higher mutation frequency by manifesting higher efficiency than the individual mutagens single-handed.

γ-Cascade V4: A semi-analytical code for modeling cosmological gamma-ray propagation

Since the universe is not transparent to gamma rays with energies above around one hundred GeV, it is necessary to account for the interaction of high-energy photons with intergalactic radiation fields in order to model gamma-ray propagation. Here, we present a public numerical software for the modeling of gamma-ray observables. This code computes the effects on gamma-ray spectra from the development of electromagnetic cascades and cosmological redshifting. The code introduced here is based on the original γ-Cascade, and builds on it by improving its performance at high redshifts, introducing new propagation modules, and adding many more extragalactic radiation field models, which enables the ability to estimate the uncertainties inherent to EBL modeling. We compare the results of this new code to existing Monte Carlo electromagnetic transport models, finding good agreement within EBL uncertainties.

Diacerein-loaded surface modified iron oxide microparticles (SMIOMPs): an emerging magnetic system for management of osteoarthritis via intra-articular injection

IntroductionOsteoarthritis (OA) is regarded as one of the most prevealent irreversible joint degenerative disorder worldwide. Recently, considerable interest in utilizing intra-articular (IA) injections for managing OA has been raised.MethodsIn this study, IA injectable surface modified iron oxide microparticles (SMIOMPs) loaded with Diacerein (DCN) were developed. The effects of formulation parameters on particle size, entrapment efficiency, and zeta potential were explored using factorial design. The optimized formulation was characterized regarding morphology and in vitro release. Differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) were done to assess interactions. Further, sterilization and in vivo performance in rats with induced arthritis has been performed for the optimized formulation.Results and DiscussionThe selected optimized system included 2M FeCL3 and 1% chitosan as a surface modifier achieved high drug entrapment of 85.25% with a PS of 1.54 µm and sustained DCN release. Morphological examination of the optimized formulation revealed spherical particles with chitosan coat. DSC and FTIR results indicated the absence of undesired interactions between DCN and the used components. No significant change in the measured parameters was observed following sterilization using gamma radiation. In vivo assessment revealed superior performance for the optimized formulation in reducing cartilage inflammation and degradation. Plasma levels of tumor necrosis factor α and Interleukin-1 beta, as well as knee diameter, were significantly reduced in the treated groups compared to the untreated ones.ConclusionOverall, the results suggest that the proposed DCN-loaded SMIOMPs represent a promising advancement in the arena of cartilage regeneration.

Gamma irradiation green synthesis of (polyacrylamide/chitosan/silver nanoparticles) hydrogel nanocomposites and their using as antifungal against Candida albicans and anti-cancer modulator

Silver nanoparticles-loaded hydrogel nanocomposites are exploited for medicinal and pharmaceutical applications. Hydrogel nanocomposites were prepared from acrylamide (Am), chitosan (CS) and AgNO3 utilizing gamma rays. Diverse variables were applied in preparation of silver nanoparticles-laoded hydrogel nanocomposites of (PAm/CS)-AgNPs such as influence of radiation dose and influnece of CS concentration. Diverse techniques were utilized to characterize hydrogel nanocomposites; Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), X-ray diffraction (XRD), Transmission electron microscopy (TEM), energy dispersive X-ray (EDX) and scanning electron microscopy (SEM). Results confirmed formation of silver nanoparticles-loaded hydrogel nanocomposites of (PAm/CS)-AgNPs. Antifungal activity of (PAm/CS)-AgNPs hydrogel nanocomposites on viability of C. albicans was esitmated. Results displayed the efficient microbial inhibition activity of treatment against C. albicans compared to control. Furthermore, (PAm/CS)-AgNPs hydrogel nanocomposite against cervical cancer HeLa cell line was investigated. Cytotoxicity of (PAm/CS)-AgNPs hydrogel nanocomposites on prior cancer cell line empolyed to prohibition of cell growth assesssed by MTT test. HeLa cancer cell is treated by (PAm/CS)-AgNPs for 48 h exposed a potential apoptotic activity by noticeable up-regulation of p53 gene expression. Moreover, anticancer activity was investigated by down-regulation of platelet-based growth variable receptor beta (PDGFR-β), Bcl2, Cathepsine, and MMP-2 gene expression. antioxidant activity was investigated and results showed antioxidant activity of (PAm/CS) hydrogel and (PAm/CS)-AgNPs hydrogel nanocomposite are 87.8% and 62.9%, respectively.

Lumped Parameter Analysis of Autogenous Propellant Pressurization System for Nuclear Thermal Rocket

This work proposes a new propellant management configuration for an ammonia-fueled nuclear thermal propulsion system. The suggested configuration maximizes the advantage deriving from the autogenous pressurization of ammonia by exploiting the thermal power lost by the nuclear reactor toward the vacuum space due to leaking radiation. In this layout, a tank containing ammonia in saturated conditions is placed near the nuclear reactor and receives an input thermal power proportional to the dose of gamma rays and neutrons absorbed by the tank walls and the ammonia itself. Such a thermal power accelerates the vaporization process of the saturated ammonia, thus increasing the pressure in the tank. A pressure regulator valve exploits this overpressure to pressurize the ammonia propellant contained in a run tank to the level required by the mission by connecting the two ammonia volumes. The pressure achieved inside the run tank pushes the propellant with an adequate mass flow rate inside the nuclear reactor. The developed lumped parameter analysis shows how this propellant management system can provide a constant mass flow to the nuclear reactor without using a turbopump assembly. Moreover, it is shown how the proposed concept allows for a reduction in the radiation shield mass.

Exogenous Brassinolide Ameliorates the Adverse Effects of Gamma Radiation Stress and Increases the Survival Rate of Rice Seedlings by Modulating Antioxidant Metabolism

Gamma irradiation-based mutant creation is one of the most important methods for rice plant mutagenesis breeding and molecular biology research. Although median lethal dose irradiation severely damages rice seedlings, applying brassinolide (BR) can increase the survival rate of irradiated seedlings. In this study, we investigated the effects of soaking seeds in solutions containing different BR concentrations (0.001, 0.01, 0.1, 1.0, and 5.0 μmol/L) and then spraying the resulting seedlings twice with 0.1 μmol/L BR. The combined BR treatments markedly decreased the superoxide anion (O2•−), hydrogen peroxide (H2O2), and malondialdehyde contents but increased the chlorophyll content. An appropriate BR treatment of gamma-irradiated samples substantially increased the activities of the antioxidant enzymes superoxide dismutase, peroxidase, and ascorbate peroxidase as well as the proline, ascorbic acid, and glutathione contents in rice seedling shoots. The BR treatment also promoted the growth of seedlings derived from irradiated seeds and increased the shoot and root fresh and dry weights. Most notably, soaking seeds in 0.01 or 0.1 μmol/L BR solutions and then spraying seedlings twice with 0.1 μmol/L BR significantly increased the final seedling survival rate and decreased mutant loss. The study results suggest that exogenous BR treatments can protect rice seedlings from gamma irradiation stress by enhancing antioxidant metabolism.

Experimental and theoretical study on the physico-mechanical characteristics and radiation shielding capability of hardened alumina sludge waste-cement pastes containing MnFe2O4-nanoparticles

This research investigates the impact of incorporating low-cost MnFe2O4 spinel nanoparticles (MF-NPs) at varying concentrations (0.5, 1, and 2 mass%) on the mechanical and physical properties of blended cement pastes. These pastes were produced by replacing different proportions (5, 10, and 15 mass%) of ordinary Portland cement (OPC) with activated alumina sludge waste (AAS), to promote sustainability. Also, the research examined the gamma radiation shielding effectiveness of certain hardened composites against a 137Cs gamma radiation source with an energy of 661.64 keV using a NaI (Tl) detector (Oxford Model) with 3″ × 3″, amplifier and 16 k multi-channel analyzer. The linear attenuation coefficient (LAC) of all the studded samples were calculated theoretically using a Monte Carlo code MCNP-5 code. The gamma radiation shielding properties were analyzed in depth using a Monte Carlo code MCNP-5 simulation model. The theoretical and experimental results for LAC were found to be in complete agreement. Phy-X/PSD software was applied to estimate the mass attenuation coefficient (MAC) for gamma radiation at various energies, as well as the effective atomic number (Zeff), mean free path (MFP), half-value layer (HVL), and tenth-value layer (TVL). The findings demonstrated that the addition of 0.5% MnFe2O4 nanoparticles (MF-NPs) to blended cement pastes exhibited the best physical and mechanical characteristics, as well as the most effective gamma radiation shielding.

Agronomic traits of kalanamak cultivar of rice (Oryza sativa L. ) M4 induced by gamma radiation

Agronomic traits of kalanamak cultivar of rice (Oryza sativa L. ) M4 induced by gamma radiation

Gamma ray registration efficiency simulation of a HPGe detector with PHITS

Gamma ray registration efficiency simulation of a HPGe detector with PHITS

Measurement of gamma field inside the biological concrete shielding of VVER-1000 Mock-Up at the LR-0 reactor

The long-term operation of existing nuclear power reactors is a crucial concern due to the complexities and expenses associated with replacing key components, such as the reactor pressure vessel and reactor internals. Gamma radiation, a byproduct of nuclear reactions and radioactive decay, significantly influences the lifetime of these components. This radiation is responsible for various degradation pathways leading to void swelling in steel reactor components and cracking or other radiation damage in concrete structures.A study conducted at a full-scale mock-up of the VVER-1000 reactor at the LR-0 zero-power reactor employed HPGe and stilbene measurements to analyze gamma spectra behind the reactor pressure vessel and within concrete biological shielding. While simulations behind the reactor pressure vessel aligned with measurements, notably, amarked overestimation of stilbene spectrum calculations occurred deep in concrete, suggesting potential inaccuracies in radiation predictions for power plant structures.

ESR spectroscopic analysis of fructose as a dosimeter for gamma radiation

In this research, fructose was used as a dosimeter, and ESR examined the influence of radiation dose on the dosimeter’s properties. Microwave power, g-value, and decay of response tests were carried out at doses of 1, 5, and 10 kGy. The results show that fructose possesses linearity at doses of 5 × 10−2–30 kGy. The radiation detection sensitivity of fructose is better than sucrose’s but less than alanine’s over the entire dose range. The irradiation dose influences microwave saturation on ESR intensity, occurring at 5 and 10 kGy. The microwave power for irradiated fructose ranges from 1.00 to 2.46 mW. The ESR signal intensity increases with higher radiation doses, primarily due to the formation of more free radicals. Additionally, higher radiation doses lead to faster decay of the fructose dosimeter’s response. The fructose ESR intensity stability compared to alanine and sucrose at 1, 5, and 10 kGy are alanine > sucrose > fructose, alanine > fructose > sucrose, and alanine > fructose ≈ sucrose, respectively.

Source Altitude of Energetic In‐Cloud Pulses Inside Thunderstorms and Implication for the Intrinsic Brightness of Terrestrial Gamma‐Ray Flashes

AbstractUpward Terrestrial Gamma‐Ray Flashes (TGFs) are mainly produced during the upward propagating negative leaders inside thunderclouds. The exact source position of TGFs, which is crucial to understanding TGF source properties, is still unclear. The link between positive energetic in‐cloud pulses (+EIPs) and TGFs provides us with a potential target to aim at. In this study, the low‐frequency radio emissions of 75 +EIPs are analyzed to retrieve the source altitudes with an improved ray theory model. Furthermore, the meteorology contexts of +EIPs derived from the ground‐based weather radars and satellite‐based infrared cloud top temperature measurements are investigated. +EIPs are produced at 8.8–13.7 km, with an average of 11.3 km inside thunderclouds, and at an average of ∼2.5 km below cloud tops. These altitudes indicate that a total number of 1.7 × 1016 to 2.6 × 1018 gamma ray photons with energy greater than 1 MeV are required for an EIP‐TGF to be measured by spaceborne detectors.

Sensitivity improvement of a deuterium-deuterium neutron generator based in vivo neutron activation analysis (IVNAA) system.

Our lab has been developing a deuterium-deuterium (DD) neutron generator-based neutron activation analysis (NAA) system to quantify metals and elements in the human body in vivo. The system has been used to quantify metals such as manganese, aluminum, sodium in bones of a living human. The technology provides a useful way to assess metal exposure and to estimate elemental deposition, storage and biokinetics. It has great potential to be applied in the occupational and environmental health fields to study the association of metal exposure and various health outcomes, as well as in the nutrition field to study the intake of essential elements and human health. However, the relatively low sensitivity of the system has greatly limited its applications. Neutron moderation plays an important role in designing an IVNAA facility, as it affects thermal neutron flux in irradiation cave and radiation exposure to the human subject. This study aims to develop a novel thermal neutron enhancement method to improve the sensitivity of the in vivo neutron activation analysis (IVNAA) system for elemental measurement but still maintain radiation dose. Utilizing a compact DD neutron source, we propose a new and practical moderator design that combines high density polyethylene with heavy water to enhance thermal neutrons by reducing thermal neutron absorption. All material dimensions are calculated by PHITS, a general-purpose Monte Carlo simulation program. The improvement of the new design predicted by the Monte Carlo simulation for the quantification of one of the elements, manganese was verified by experimental irradiation of manganese-doped bone equivalent phantoms. For the same radiation dose, a 67.9% thermal neutron flux enhancement is reached. With only 4.2% increase of radiation dose, the simulated thermal neutron flux and activation can be further increased by 84.2%. A 100% thermal neutron enhancement ratio is also achievable with a 20% dose increase. The experimental results clearly show higher manganese activation gamma ray counts for each specific phantom, with a significantly reduced minimum detection limit. Additionally, the photon dose was suppressed. The thermal neutron enhancement method can increase the number of useful neutrons significantly but maintain the radiation dose. This greatly decreased the detection limit of the system for elemental quantification at an acceptable dose, which will broadly expand the application of the technology in research and clinical use. The method can also be applied to other neutron medical applications, including neutron imaging and radiotherapy.

Effects of acute gamma irradiation on the morphology of Curcuma alismatifolia ‘Siam Shadow’ and C. alismatifolia ‘Siam Scarlet’

Curcuma alismatifolia (Pathumma in Thailand), also known as Siam Tulip, is an economically valuable cut flower in Thailand and is continuously bred to enhance its captivating array of colors. This study aimed to investigate the effects of acute gamma irradiation on 2 cultivars of C. alismatifolia ‘Siam Shadow’ and C. alismatifolia ‘Siam Scarlet’ for in vitro propagation and determination of the lethal dose 50 (LD50). The irradiation was conducted at doses of 0, 20, 30, 40 and 80 Gy using young shoots tissue culture. The results of the experiment revealed that the LD50 for ‘Siam Shadow’ could not be determined while ‘Siam Scarlet’ was 29.06 Gy. ‘Siam Shadow’ exhibited chimera leaf formation when exposed to a gamma irradiated dose of 40 Gy, while ‘Siam Scarlet’ exhibited chimera leaf formation when gamma irradiated at doses of 20 Gy in vitro. After planting in a greenhouse, ‘Siam Shadow’ was irradiated with 30 Gy of gamma irradiated, had reduced bract numbers and the bracts became smaller. Conversely, ‘Siam Scarlet’ irradiated with 20 Gy of gamma irradiated, produced smaller inflorescences with only 2 bracts, while irradiation with 30 Gy resulted in 2 lobes and spotted leaves at the edges of the leaves. The results indicated that both the C. alismatifolia cultivars and gamma irradiation rates had a significant influence on survival rate, number of shoots, plant height, chlorophyll mutants and morphology. As the outcome, it was determined that gamma irradiation with optimum doses of gamma rays could induce new characteristics in C. alismatifolia.

Cosmic-Ray Radiation Effects on Ibuprofen Tablet Formulation Inside and Outside of the International Space Station.

In extreme environments people will have different needs for medicine(s), making it crucial to understand how such environments affect drug efficacy. Ibuprofen, commonly used in tablet formulation on Earth, could fail in space despite standard pharmaceutical packaging. We introduce the concept of 'space medicines', where solid-dosage forms protect the pharmaceutical from accelerated degradation in spaceflight. We simulate dose(s) in International Space Station (ISS) through radionuclide and photon experiments, and establish the impact of alpha, beta and gamma rays. We demonstrate that tablet formulation protects from impact of alpha and beta rays; however, gamma rays decompose ibuprofen even when 'masked'. We systematically analyse 19 tablet compositions inside and outside the ISS to determine the effect of compositional changes in the tablet matrix. We confirm that the iron oxide-shielded tablets show minimal degradation (〈10%) inside the ISS, compared to moderate reductions (〉10%) for other formulations, with one exception. The tablets exhibited significantly greater ibuprofen degradation (〉 30-50%) outside ISS, due to harsh conditions. Significantly, we found that flavour have shielding potential by scavenging free radicals. We conclude that ibuprofen efficacy is adversely affected in space, and these effects are expected to worsen on missions to deeper space destinations.

Comparative study of volatile compounds of cold-pressed oils extracted from three different oilseeds after gamma irradiation.

To investigate the effect of gamma irradiation on the volatile compounds of edible oils. Three types of oilseeds, including peanut, sesame, and flaxseed, were subjected to 8 kGy gamma irradiation, followed by cold pressing to extract their oils. The volatile compounds of the oils were isolated by simultaneous distillation extraction and analyzed by gas chromatography-mass spectrometry. A total of 91 volatile compounds were identified, which can be grouped into eight categories: hydrocarbons, aldehydes, ketones, alcohols, acids, esters, furans, and benzene derivatives. Irradiation treatment resulted in a significant increase in the levels of hydrocarbons, aldehydes, and ketones in all oil samples (p<0.05), with the greatest increase observed in hydrocarbons (4-14 times). In contrast, changes in alcohols, acids, esters, furans, and benzene derivatives were related to oilseed type. The increased hydrocarbons mainly originated from the degradation of palmitic, stearic, oleic, and linoleic acids. The irradiation resistance of the three oilseeds varied considerably, in the order: flaxseed>sesame>peanut. Based on the odor activity value, 11 key aroma compounds were selected, and (E)-2-decenal (tallow, oily, and orange), 1-octanol (soapy and oily), and 1-nonanol (floral and soapy) were only detected in the irradiated samples. Principal component analysis revealed that the oil samples of the three oilseeds could be well classified based on their key aroma compounds, and that the irradiation treatment had no remarkable effect on their intrinsic aroma.

Biological effects in normal human fibroblasts following chronic and acute irradiation with both low- and high-LET radiation

IntroductionRadiobiological studies at low dose rates allow us to improve our knowledge of the mechanisms by which radiation exerts its effects on biological systems following chronic exposures. Moreover, these studies can complement available epidemiological data on the biological effects of low doses and dose rates of ionizing radiation. Very few studies have simultaneously compared the biological effects of low- and high-LET radiations at the same dose rate for chronic irradiation.MethodsWe compared, for the first time in the same experiment, the effects of chronic (dose rates as low as ~18 and 5 mGy/h) and acute irradiations on clonogenicity and micronucleus formation in AG1522 normal human skin fibroblasts in the confluent state exposed to doses of low- and high-LET radiation (gamma rays and alpha particles) to investigate any differences due to the different radiation quality and different dose rate (in the dose range 0.006–0.9 Gy for alpha particles and 0.4–2.3 Gy for gamma rays).ResultsAs expected, alpha particles were more effective than gamma rays at inducing cytogenetic damage and reduced clonogenic cell survival. For gamma rays, the cytogenetic damage and the reduction of clonogenic cell survival were greater when the dose was delivered acutely instead of chronically. Instead, for the alpha particles, at the same dose, we found equal cytogenetic damage and reduction of clonogenic cell survival for both chronic and acute exposure (except for the highest doses of 0.4 and 0.9 Gy, where cytogenetic damage is greater at a low dose rate).ConclusionThe results of this study may have an impact on space and terrestrial radioprotection of humans at low doses and low dose rates, on biodosimetry, and on the use of ionizing radiation in medicine. These results also provide insights into understanding damage induction and cell reaction mechanisms following chronic exposure (at dose rates as low as 18 and 5 mGy/h) to low- and high-LET radiation.

Biochemical and biological studies of irradiated and non-irradiated extracts of Solanum aculeastrum Dunal fruit

This study explores the impact of γ-irradiation on ethanolic extracts of Solanum aculeastrum Dunal. The anti-cancer and antimicrobial properties were investigated. The obtained results revealed that total phenol (TP) and total flavonoid (TF) of total ethanol extract (100%) (FTE) were higher than 70% ethanol extract (SE), and these contents increased after gamma radiation with 5 kGy. The results of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the Solanum aculeastrum extracts suggested that FTE and 5 kGy-irradiated FTE can be used to control and prevent skin infections caused by MRSA and endocarditis, urinary tract infections, and prostatitis caused by Enterococcus faecalis. The FTE sample irradiated at 5 kGy showed cytotoxicity for A431 and Hct-116 cell lines similar to the control sample and higher than the toxicity revealed by the samples irradiated at 10 kGy. In normal cells (Bj-1), the toxicity was decreased after irradiation (IC50 = 31 μg/ml) compared to the non-irradiated extract (IC50 = 26.1 μg/ml). Molecular docking suggested Sortase A to play a role in chlorogenic acid antibacterial activity towards Staphylococcus aureus. In conclusion, γ-irradiation can be used to enhance the phytoconstituents of Solanum aculeastrum fruit extracts and, consequently, its biological properties.

Determination of the ambient dose equivalent H*(10) at high-altitude cities using a NaI(Tl) scintillator.

The ambient dose equivalent rate H*(10) was determined at various locations in the metropolitan area of the cities of La Paz and El Alto in Bolivia. A NaI(Tl) scintillation detector was employed to infer the gamma-ray fluence, and the fluence was transformed in dose rates by means of the appropriate coefficients. In this paper, a full methodology for the estimation of dose rates associated with gamma rays emitted from the ground (terrestrial radiation) and the atmosphere is developed from measurements made with the scintillator instrument. The energy calibration and resolution of the apparatus are presented, followed by the estimation of its response and efficiency by means of Monte Carlo simulations. Lastly, the definition of H*(10) is used to determine the conversion coefficients needed to transform gamma-ray fluence in dose rates. With this methodology, rates for H*(10) around 123 ± 8 nSv/h were estimated using data collected in three city districts: Mallasa (3320ma.s.l.), San Antonio (3620ma.s.l.), and Zona Sur (3400ma.s.l.). The dose rates are believed to be representative of the terrestrial radiation fields encountered in the highlands of Bolivia.

Rapid Destruction of Lipid Biomarkers Under Simulated Cosmic Radiation.

Understanding how organics degrade under galactic cosmic rays (GCRs) is critical as we search for traces of ancient life on Mars. Even if the planet harbored life early in its history, its surface rocks have been exposed to ionizing radiation for about four billion years, potentially destroying the vast majority of biosignatures. In this study, we investigated for the first time the impact of simulated GCRs (using gamma rays) on several types of lipid biosignatures (including hopane C30, sterane C27, alkanes, and fatty acids [FAs]) in both the presence and absence of salts (NaCl, KCl, and MgCl2). We measured that the lipids degraded 6-20 times faster than amino acids in similar conditions; moreover, when irradiated in the presence of a salt substrate, degradation was at least 4-6 times faster than without salt, which suggests that salty environments that are often preferred targets for astrobiology warrant caution. We detected radiolytic by-products only for FAs-in the form of alkanes and aldehydes. These results expand our understanding of the degradation of organic molecules in Mars analog environments and underscore the urgent need to direct rover missions to sampling sites protected from GCRs, for example, sites on Mars that have been recently exposed by a wind scarp retreat or meteoritic impact.