Detection of biosignatures in Terrestrial analogs of Martian regions:Strategical and technical assessments

For decades, the search for potential signs of Martian life has attracted strong international interest and has led to significant planning and scientific implementation. Clearly, in order to detect potential life signals beyond Earth, fundamental questions, such as how to define such terms as “life” and “biosignature”, have been given considerable attention. Due to the high costs of direct exploration of Mars, Mars-like regions on Earth have been invaluable targets for astrobiological research, places where scientists could practice the search for “biosignatures” and refine ways to detect them. This review summarizes scientific instrumental techniques that have resulted from this work. Instruments must necessarily be our “eyes” and “hands” as we attempt to identify and quantify biosignatures on Mars.Scientific devices that can be applied in astrobiology include mass spectrometers and electromagnetic-spectrum-based spectrometers,redox potential indicators, circular dichroism polarimeters, in situ nucleic acid sequencers, life isolation/cultivation systems, and imagers.These devices and how to interpret the data they collect have been tested in Mars-analog extreme environments on Earth to validate their practicality on Mars. To anticipate the challenges of instrumental detection of biosignatures through the full evolutionary history of Mars, Terrestrial Mars analogs are divided into four major categories according to their similarities to different Martian geological periods(the Early-Middle Noachian Period, the Late Noachian-Early Hesperian Period, the Late Hesperian-Early Amazonian Period, and the Middle-Late Amazonian Period). Future missions are suggested that would focus more intensively on Mars’ Southern Hemisphere, once landing issues there are solved by advances in spacecraft engineering, since exploration of these early terrains will permit investigations covering a wider continuum of the shifting habitability of Mars through its geological history. Finally, this paper reviews practical applications of the range of scientific instruments listed above, based on the four categories of Mars analogs here on Earth. We review the selection of instruments suitable for autonomous robotic rover tests in these Mars analogs. From considerations of engineering efficiency,a Mars rover ought to be equipped with as few instrument assemblies as possible. Therefore, once candidate landing regions on Mars are defined, portable suites of instruments should be smartly devised on the basis of the known geological, geochemical, geomorphological,and chronological characteristics of each Martian landing region. Of course, if Mars sample-return missions are successful, such samples will allow experiments in laboratories on Earth that can be far more comprehensive and affordable than is likely to be practicable on Mars.To exclude false positive and false negative conclusions in the search for extraterrestrial life, multiple diverse and complementary analytical techniques must be combined, replicated, and carefully interpreted. The question of whether signatures of life can be detected on Mars is of the greatest importance. Answering that question is extremely challenging but appears to have become manageable.

Response of atmospheric carbon dioxide to the secular variation of weakening geomagnetic field in whole atmosphere simulations

Responses of atmospheric carbon dioxide(CO_(2))density to geomagnetic secular variation are investigated using the Whole Atmosphere Community Climate Model-eXtended(WACCM-X).Our ensemble simulations show that CO_(2) volume mixing ratios(VMRs)increase at high latitudes and decrease at mid and low latitudes by several ppmv in response to a 50%weakening of the geomagnetic field.Statistically significant changes in CO_(2) are mainly found above~90 km altitude and primarily redetermine the energy budget at~100-110 km.Our analysis of transformed Eulerian mean(TEM)circulation found that CO_(2) change is caused by enhanced upwelling at high latitudes and downwelling at mid and low latitudes as a result of increased Joule heating.We further analyzed the atmospheric CO_(2) response to realistic geomagnetic weakening between 1978 and 2013,and found increasing(decreasing)CO_(2) VMRs at high latitudes(mid and low latitudes)accordingly.For the first time,our simulation results demonstrate that the impact of geomagnetic variation on atmospheric CO_(2) distribution is noticeable on a time scale of decades.

MamZ protein plays an essential role in magnetosome maturation process of Magnetospirillum gryphiswaldense MSR-1

Based on analysis of gene structure of mamXY operon in Magnetospirillum gryphiswaldense strain MSR-1,we constructed a mamZ deletion mutant strain(ΔmamZ)and four complemented strains with different mamZ fragment lengths.Various cell phenotypic and physiological parameters were evaluated and compared among the wild-type(WT),mutant,and complemented strains.Cell growth rates were not notably different;however,magnetic response(Cmag)and iron uptake ability were significantly lower inΔmamZ.High-resolution transmission electron microscopy(HR-TEM)showed that magnetosomes inΔmamZ were small and irregular,and rock magnetic measurements suggested that they contained immature particles.In comparison to WT of MSR-1,intracellular iron content ofΔmamZ and the complemented strains cultured with 20mmol/L iron source was similar or slightly higher.The complemented strains were unable to synthesize mature or normal amounts of magnetosomes,apparently because of abnormal expression of the transmembrane domain of MamZ protein.Real-time reverse transcription polymerase chain reaction(RTqPCR)analysis showed that relative transcription levels of mamX and ftsZ-like genes inΔmamZ were higher at 18 h than at 12 h,suggesting that MamXY proteins play cooperative functional roles in the magnetosome maturation process.Transcription level of mms6 was significantly upregulated inΔmamZ(incubated at 12 h)and the complemented strains(incubated at 12 and 18 h),refl ecting possible interaction between MamXY and Mms6 proteins during magnetosome biosynthesis.These findings,taken together,demonstrate the essential role of MamZ in the magnetosome maturation process in MSR-1.

Determination of the heating efficiency of magnetotactic bacteria in alternating magnetic field

Magnetotactic bacteria(MTB)intact cells have been applied in magnetic hyperthermia therapy of tumor,showing great efficiency in heating for tumor cell inhibition.However,the detailed magnetic hyperthermia properties and optimum heat production conditions of MTB cells are still poorly understood due to lack of standard measuring equipment.The specific absorption rate(SAR)of MTB cells is often measured by home-made equipment at a limited frequency and magnetic field amplitude.In this study,we have used a commercial standard system to implement a comprehensive study of the hyperthermic response of Magnetospirillum gryphiswaldense MSR-1 strain under 7 frequencies of 144-764 kHz,and 8 field amplitudes between 10 and 45 kA/m.The measurement results prove that the SAR of MTB cells increases with magnetic field frequency and amplitude within a certain range.In combination with the magnetic measurements,it is determined that the magnetic hyperthermia mechanism of MTB mainly follows the principle of hysteresis loss,and the heat efficiency of MTB cells in alternating magnetic field are mainly aff ected by three parameters of hysteresis loop,saturation magnetisation,saturation remanent magnetisation,and coercivity.Thus when we culture MTB in LA-2 medium containing sodium nitrate as source of nitrogen,the SAR of MTB LA-2 cells with magnetosomes arranged in chains can be as high as 4925.6 W/g(in this work,all SARs are calculated with iron mass)under 764 kHz and 30 kA/m,which is 7.5 times than current commercial magnetic particles within similar size range.

A planetary perspective on Earth's space environment evolution

The planet Earth is an integrated system, in which its multi-spheres are coupled, from the space to the inner core. Whether the space environment in short to long terms has been controlled by the earth's interior process is contentious. In the past several decades,space weather and space climate have been extensively studied based on either observation data measured directly by man-made instruments or ancient data inferred indirectly from some historical medium of past thousands of years. The acquired knowledge greatly helps us to understand the dynamic processes in the space environment of modern Earth, which has a strong magnetic dipole and an oxygen-rich atmosphere. However, no data is available for ancient space weather and climate(>5 ka). Here, we propose to take the advantage of " space-diversity" to build a " generalized planetary space family", to reconcile the ancient space environment evolution of planet Earth from modern observations of other planets in our solar system. Such a method could also in turn give us a valuable insight into other planets' evolution.

Perchlorate-tolerant bacterial strains isolated from the Mars-analog Qaidam Basin soils exposed to Earth's nearspace

Impactstatement Earth's lower near space of 20-40 km above sea level with polyextreme conditions serves as a unique Mars analog for astrobiological research to investigate the limits of life on Earth and planetary protection considerations for Mars exploration.In this study,we exposed Mars-like desert regolith to near space at a float altitude of~35 km and isolated four bacterial strains after exposure.

Moss surviving Mars-like extreme conditions

As Mars is one of the most Earth-like terrestrial planets,numerous ambitious but challenging investigations for Mars terraforming and sustainable settlements have received plenty of attention.A newly published paper,“The extremotolerant desert moss Syntrichia caninervis is a promising pioneer plant for colonizing extraterrestrial environments”by Li and colleagues,1 reports a desert moss,Syntrichia caninervis,to be a candidate pioneer plant on Mars through investigation of its extremotolerant ability to resist several harsh conditions fatal to most known organisms.They found,for the first time,that the desert moss S.caninervis was capable of enduring 98%water loss and regenerating after being frozen at80C for 5 years or in liquid nitrogen at196C for a month and enduring high doses of gamma radiation up to 5,000 Gy.Moreover,they examined the survivability of whole plants in a simulated Martian environment and found that the moss remained regeneratable after exposure for several days.These results indicate that the desert moss S.caninervis is one of the known toughest plants on Earth.The exciting findings are of great interest for furthering the research on potential settlements on Mars in the future.

Link between the geomagnetic polarity reversal and global-geology events

The origin of the earth’s magnetic field is one of the five most important unsolved problems in physics, especially geomagnetic polarity reversal remains one of Nature’s most enigmatic phenomena. It is proposed that the geomagnetic polarity reversal links with some global-geology events, although it is difficult to clarify the mechanism of this correlation. The history of continental structures in eastern China would be also linked with some global-geology events.

Survival of the magnetotactic bacterium Magnetospirillum gryphiswaldense exposed to Earth’s lower near space

地球临近空间指海平面之上20~100 km高度区域,具有高辐射、低温、干燥、低气压等极端条件,是一类典型的类火星环境.研究生命在临近空间的存活机制有助于深化对火星宜居性和生命探测的认识.本研究利用自主研发的临近空间天体生物学载荷和飞行实验平台,将趋磁细菌Magnetospirillum gryphiswaldense MSR-1带入23 km高度区域开展生物暴露实验.结果发现合成磁小体的趋磁细菌经过7小时16分钟的暴露后仍有部分细胞存活,而不合成磁小体的菌株经过暴露后没有检测到存活细胞;临近空间暴露后存活的样品与实验室对照组相比,其磁小体的数量、尺寸和岩石磁学性质等均发生变化.这些结果表明细胞内的磁小体纳米颗粒有助于趋磁细菌抵御临近空间的高辐射等极端环境,为趋磁细菌的起源和早期演化、生命适应类火星极端环境的机制等提供了新认识.鉴于趋磁细菌具有在类火星环境中生存的潜力,这些感磁微生物可以作为天体生物学研究的重要生物类群.

The solar wind plasma upstream of Mars observed by Tianwen-1:Comparison with Mars Express and MAVEN

On the great journey to Mars,China’s first planetary exploration mission,the Tianwen-1 came within 26 million kilometers of Mars from 31 October 2020 to 25 January 2021 and was getting closer to its destination,the red planet,in search of answers to the cataclysmic climate change that occurred in Martian history.Both the escape of the Martian atmosphere and the loss of surface water were firmly influenced by solar activities.Tianwen-1 provided a unique chance to depict the solar wind streams between Earth and Mars during the minimum of Solar Cycle 25.During the three-month cruise phase of Tianwen-1,the solar wind flows were successively observed at Earth,Tianwen-1,and Mars.After the field of view correction and noise reduction,the solar wind velocity and density measured by Tianwen-1 show good agreement with those at Earth and Mars.The results indicate that the performance of the ion analyzer onboard the Tianwen-1 orbiter is reliable and stable.It is worth looking forward to the joint observations of ion escape with other Mars probes in the following Martian years.

Mineralogical evidence of water activity in the northern lowlands of Mars based on inflight-calibrated spectra from the Zhurong rover

Hydrated minerals provide direct indications of the early Martian water environment. The various clay minerals and evaporite salts found to date are mainly exposed in the ancient southern highlands of Mars. While it is believed that the area of the northern lowlands could have hosted a global ocean in the past, its surface is covered with a layer of spectrally nonabsorbing materials, and orbital hyperspectral remote sensing has found only sparse evidence for the presence of water. China's first Mars exploration mission, Tianwen-1, landed in the putative ancient shoreline zone of the northern lowlands of Mars. Its rover,Zhurong, conducted high-spatial-resolution reconnaissance to reveal the geological and environmental evolution of this region.Owing to frequent dust storms, not only is the Martian surface covered with dust, but also the spectral calibration panels on the rover are affected by dust coverage. Data from the shortwave infrared(SWIR) spectrometer onboard the rover are also affected by other environmental factors such as instrumental temperature changes and atmospheric carbon dioxide absorption. This study recalibrated the in situ SWIR data to minimize the impact of the above factors. The recalibrated spectral data show that the reflectance of the landing area falls within the normal range of Martian soil, and confirm the presence of several important absorption peaks. The absorption characteristics indicate the presence of polyhydrated sulfates, gypsum, and hydrated silicates in the surface materials along the rover track, with sulfate content of <27–39 wt.%. Our results further confirm past activity of liquid water in the Zhurong rover landing area, providing mineralogical evidence supporting hypotheses related to underground water or an ancient shoreline.

Secular variations in geomagnetic Field caused by the fluctuations in the fluid flow in the outer-core

The theoretical spatial power spectrum of a dipole located at （ r0, θ0, φ0） can be fitted by a straight line in logarithmic scale when n is larger than 2. Based on the spherical harmonic coefficients of geomagnetic field during 1900—1995, the depth（r0） of source-layer of every 5a is calculated. The results show that r0 decreased from 1900 to 1960; abruptly changed from 1945 to 1950 related to some kind of disturbance; decreased again from 1960 to 1975; increased from 1975 to 1985; and kept stable after 1985. Then the mean energy density （MED） of each year is induced to its corresponding r0. We find that MED of dipole field kept nearly unchanged from 1900 to 1960. While, MED of non-dipole field increased. The change of r0 coinciding with the geomagnetic secular variation, impulse and length-of-day happened around 1970, suggesting that the change of r0 may be related to the impulse. The variation in the fluid flow in the outer-core caused by the core-mantle coupling is a plausible candidate

In vitro assembly of the bacterial actin protein MamK from ＇Candidatus Magnetobacterium casensis＇ in the phylum Nitrospirae

Magnetotactic bacteria （MTB）, a group of phylogeneti- cally diverse organisms that use their unique intracellular magnetosome organelles to swim along the Earth＇s magnetic field, play important roles in the biogeochemical cycles of iron and sulfur. Previous studies have revealed that the bacterial actin protein MamK plays essential roles in the linear arrangement of magnetosomes in MTB cells belonging to the Proteobacteria phylum. However, the molecular mechanisms of multi- ple-magnetosome-chain arrangements in MTB remain largely unknown. Here, we report that the MamK filaments from the uncultivated ＇Candidatus Magnetobacterium casensis＇ （Mcas） within the phylum Nitrospirae polymerized in the presence of ATP alone and were stable without obvious ATP hydrolysis-mediated disassembly. MamK in Mcas can convert NTP to NDP and NDP to NMP, showing the highest preference to ATP. Unlike its Magnetospirillum counterparts, which form a single magnetosome chain, or other bacterial actins such as MreB and ParM, the polymerized MamK from Mcas is independent of metal ions and nucleotides except for ATP, and is assembled into well-ordered filamentous bundles consisted of multiple filaments. Our results suggest a dynamically stable assembly of MamK from the uncultivated Nitrospirae MTB that synthesizes multiple magnetosome chains per cell, These findings further improve the current knowledge of biomineralization and organelle biogenesis in prokaryotic systems.

A new aggregation-induced emission-based fluorescent probe for effective detection of Hg^(2+)and its multiple applications

Detection of mercury ions(Hg^(2+))in actual samples is of significant importance due to the toxicity of Hg^(2+)to human health.In this work,a simple tetraphenylethene(TPE)derived fluorescent probe TPE-Hg based on aggregation-induced emission(AIE)mechanism was synthesized.TPE-Hg can visually recognize Hg^(2+)in THF/HEPES(1:9,v/v,HEPES 20 mmol/L,pH 7.3)system with rapid response,strong anti-interference ability,large Stokes shift(203 nm),and low detection limit(7.548×10^(-7)mol/L).The results show that Hg^(2+)triggered elimination of TPE-Hg lead to releasing of an AIE-active compound 2 is responsible to the sensing mechanism.TPE-Hg is applicable to detect Hg^(2+)in actual water samples and image Hg^(2+)in living MCF-7 cells.In addition,TPE-Hg is suitable to assay the Hg^(2+)level in seafood and tea samples,and it is alsoapplicable intest strips.

Magnetic susceptibility variation and AMS exchange related to thermal treatment of siderite

Siderite is one of the common iron-bearing carbonates in deposits. Its magnetic properties are usually neglected due to its paramagnetic property at room temperature. It was shown that the magnetic susceptibility changes with temperature variation. Also it was revealed that the maximum axis (K1) of AMS ellipsoid exchange position gradually with the minimum axis (K3) during thermal treatment, and thus, AMS ellipsoids of samples change from initial prolate to oblate. It was concluded that such changes resulted from the decomposition and oxidation of siderite during heating in which siderite was transformed into magnetite, maghemite, and finally to hematite.

A recording phase lag between ocean and continent climate changes: constrained by the Matuyama/Brunhes polarity boundary

The Matuyama/Brunhes polarity boundary (MBPB) is recorded in Chinese loess L8, which is associated with the glacial period. It is proved that loess L8 is capable of recording earth’s secular variation, and the lock-in depth in Chinese loess is no more than that of marine sediments. Comparison with the location of MBPB in Chinese loess and marine sediments, it is put forward that there is a recording phase lag between ocean and continent climate changes.