In-situ preservation of nitrogen-bearing organics in Noachian Martian carbonates.

Understanding the origin of organic material on Mars is a major issue in modern planetary science. Recent robotic exploration of Martian sedimentary rocks and laboratory analyses of Martian meteorites have both reported plausible indigenous organic components. However, little is known about their origin, evolution, and preservation. Here we report that 4-billion-year-old (Ga) carbonates in Martian meteorite, Allan Hills 84001, preserve indigenous nitrogen(N)-bearing organics by developing a new technique for high-spatial resolution in situ N-chemical speciation. The organic materials were synthesized locally and/or delivered meteoritically on Mars during Noachian age. The carbonates, alteration minerals from the Martian near-surface aqueous fluid, trapped and kept the organic materials intact over long geological times. This presence of N-bearing compounds requires abiotic or possibly biotic N-fixation and ammonia storage, suggesting that early Mars had a less oxidizing environment than today.

Below are a few minor comments/suggestions: Line 23. Delete 'ancient'. In addition, I feel that in most cases that 'ancient' is used in the MS this is actually obsolete and encourage the authors to check the MS carefully. Line 40. Delete 'have'. Line 41. Delete 'present'. This since we have no idea how old these systems really are. Line 50. Delete 'Have'. Line 54. Nitrogen is not only an essential element for terrestrial organisms but for all life on earth. Lines 59-61. Replace 'In this study, we accomplished the micrometer-scale in situ analysis of N K-edge micro X-ray absorption near-edge structure (μ-XANES) on the 4 Ga ALH carbonates for the first time' with 'In this study, for the first time, micrometer-scale in situ analysis of N K-edge micro X-ray absorption near-edge structure (μ-XANES) on the 4 Ga ALH carbonates were accomplished.'. Lines 66-67. Replace 'We conducted their N µ-XANES measurements along with various N-bearing reference compounds at the SPring-8 synchrotron facility (Hyogo, Japan). Our XANES spectra…' with 'N µ-XANES measurements were conducted along with various N-bearing reference compounds at the SPring-8 synchrotron facility (Hyogo, Japan). The XANES spectra…' Line 92. Replace 'In this study, we minimized the possibility of laboratory contamination by..' with 'In this study, the possibility of laboratory contamination is minimized by …' Line 131. Delete 'then'. Line 172. Replace 'We expect that additional…' with 'It is expected that additional….'. Lines 272-73. Replace 'We obtained a rock fragment from the interior of Allan Hills 84001, sub-sample 248, donated to us by the ….' with ' A rock fragment from the interior of Allan Hills 84001, sub-sample 248, was donated by the….' Lines 278-279. Replace '…, we did not use any organic materials normally utilized for the preparation of a…' with '…, ion, none of the organic materials normally utilized were used for the preparation of a …'. Line 280-281. Replace '…., we collected small grains of ALH silicate (orthopyroxene grains)…' with '…, small grains of ALH silicate (orthopyroxene grains) were collected….' Line 285. Replace 'We then observed the plucked carbonates and silicate grains using…' with 'The plucked carbonates and silicate grains were observed using….'. Lines 305-306. Replace '….we obtained the reference XANES spectra of several N can survive at ~10 cm depth, and it is even possible for complex molecules to survive at greater depths for long periods of time. It is most likely that ALH carbonates preserved the N-bearing organic molecules in the Martian subterranean for a long period, shielding them from the severe cosmic ray irradiation (and from strong oxidants and UV irradiation as well).

Reviewer #2 (Remarks to the Author):
The manuscript by Koike et al. is

Response:
We This work reports that the mm-sized orthopyroxene clasts of ALH 84001, separated along the grain boundaries, pass a paleomagnetic conglomerate test. Their results demonstrate that the host rock was not heated high enough to re-magnetize the grains after they were put in place.
Bulk reheating of ALH 84001 high enough to form the magnetite would make all of the magnetic directions to point in the same direction, which would cause the paleomagnetic conglomerate test to fail.
This work with scanning (SQUID) magnetic microscopy puts a numerical value on the peak (re)heating temperature at < 40 ˚C for the entire time since ALH carbonates formed, ejected from Mars, landed in Antarctica and even until the meteorite was brought to the Johnson Spaceflight Center in Houston. These magnetic studies confirm that the subsequent processes of ALH 84001 (e.g., impact, ejection from Mars, and arrival to Earth) did not heat ALH carbonates above 40˚C. Moreover, the following isotopic studies support also their low temperature formation and preservation processes: This study demonstrates that the clumped isotopic signature in the carbonates would be easily reset if they were briefly (re)heated above 450 °C.
All of the data from ALH84001 confirm that ALH carbonates did not experience high temperature heating since their crystallizations, although the host rock may have been suffered impact deformation event(s). The characteristic zoning of the carbonates is most likely the primary feature, i.e., this zoning formed in the fluctuations of the co-existing fluidal chemistry.
Consequently, the post-crystallization process is not enough to effect the nitrogen speciation in ALH carbonates.
Although an age of 4.1 Gyr is cited in the paper, the chronometer should be given and a slightly fuller description of the complex series of events recorded by the radiometric dating.