JA Grier - Academia.edu (original) (raw)

Papers by JA Grier

ABSTRACT A host of resources and tools have been developed by the NASA SMD Planetary Sciences For... more ABSTRACT A host of resources and tools have been developed by the NASA SMD Planetary Sciences Forum and its partners to facilitate scientist involvement in E/PO efforts.

We present resources for higher education faculty provided by the NASA SMD Forums including colle... more We present resources for higher education faculty provided by the NASA SMD Forums including collections of instructional resources and professional development.

The Balloon Science Program is an ideal venue for development of robust research programs as well... more The Balloon Science Program is an ideal venue for development of robust research programs as well as learning opportunities. We are facilitating partnerships, old and new, between parties interested in the Balloon Science and suborbital programs.

We report on the activities of the NASA Science Mission Directorate Education Support Network Pre... more We report on the activities of the NASA Science Mission Directorate Education Support Network Pre-Service Education Working Group.

The relative ages of large lunar craters can be inferred from spectral estimates ejecta maturity.... more The relative ages of large lunar craters can be inferred from spectral estimates ejecta maturity. The radiometric ages of Tycho, Copernicus, and Autolycus help constrain the relative ages of other large craters, such as Lichtenberg.

One of the best ways to improve any astronomy education effort is to know your audience -- before... more One of the best ways to improve any astronomy education effort is to know your audience -- before, during, and after they participate in particular learning experiences. We describe a variety of tools we have used for gauging the astronomical knowledge and understandings held by middle-school, high-school, and adult learners, including qualitative surveys, performance assessments, clinical interviews, as well as

We have developed a targeted set of activities, presentations, and assessments that immerse teach... more We have developed a targeted set of activities, presentations, and assessments that immerse teachers in learning about two key themes from the National Science Education Standards: origin and evolution of the universe, and the unifying concept of models, evidence, and explanation in science. Students of all ages come to the astronomy classroom with their own ideas and internal models of

Meteoritics & Planetary Science, 2000

— We analyzed noble gases from 18 samples of weathering products (“iddingsite”) from the Lafayett... more — We analyzed noble gases from 18 samples of weathering products (“iddingsite”) from the Lafayette meteorite. Potassium‐argon ages of 12 samples range from near zero to 670 ± 91 Ma. These ages confirm the martian origin of the iddingsite, but it is not clear whether any or all of the ages represent iddingsite formation as opposed to later alteration or incorporation of martian atmospheric 40Ar. In any case, because iddingsite formation requires liquid water, this data requires the presence of liquid water near the surface of Mars at least as recently as 1300 Ma ago, and probably as recently as 650 Ma ago. Krypton and Xe analysis of a single 34 μg sample indicates the presence of fractionated martian atmosphere within the iddingsite. This also confirms the martian origin of the iddingsite. The mechanism of incorporation could either be through interaction with liquid water during iddingsite formation or a result of shock implantation of adsorbed atmospheric gas.Our strongest conclusion is that the iddingsite in Lafayette formed on Mars, in agreement with the microstratigraphic arguments of Gooding et al. (1991) and Treiman et al. (1993). A preterrestrial origin of the iddingsite is required both by the many non‐zero K‐Ar ages and by the presence of Xe that is isotopically distinct from any terrestrial Xe.The Xe is accompanied by Kr, but the Kr and Xe have been fractionated if they are derived from the present martian atmosphere. This is presumably the result of either incorporation via interaction with liquid water (Drake et al., 1994; Bogard and Garrison, 1998) or by adsorption from the martian atmosphere, perhaps accompanied by shock (see also Gilmour et al., 1998, 1999). Although the iddingsite is enriched in Kr and Xe compared to whole‐rock analyses, it is not clear whether iddingsite is the dominant carrier of the atmospheric‐derived gas (Drake et al., 1994) or merely a minor carrier (Gilmour et al., 1999).Our 40Ar‐39Ar experiment was disappointing, in that it mostly served to confirm that the iddingsite, which contains fine‐grained clays, is susceptible to recoil loss of 39Ar during irradiation. Only one sample of five gave a clear signal of radiogenic or extraterrestrial 40Ar, and that was only by 3°.Potassium‐argon ages of the second set of samples were more successful, ranging from near 0 to 670 ± 91 Ma. It is not clear whether any or all of the ages represent iddingsite formation, as opposed to later alteration. The fact that a Rb‐Sr experiment (Shih et al., 1998) gave an apparent age for iddingsite of 679 ± 66 Ma (2a) suggests that perhaps formation of iddingsite occurred (or began) ∼650 Ma ago and that some samples either formed, or were thermally altered, later. The ages could be even younger than 650 Ma, if the samples have incorporated martian atmospheric 40Ar.This means that liquid water was certainly present on Mars in the last 1300 Ma (the formation age of Lafayette), and probably within the last 650 Ma.

Contrary to Viking-era expectations, impact crater populations extend to diameters 11 m or less. ... more Contrary to Viking-era expectations, impact crater populations extend to diameters 11 m or less. The size distribution of craters (and impacting planetesimals) derived from the moon and transferred to Mars by Neukum and Ivanov (1994, in Hazards Due to Comets & Asteroids, pp. 359-416) and by Hartmann (1999, Meteor. Planet. Sci. 34, 167-177) appear to apply to undisturbed Martian plains. Applying this fact, we can interpret ages (factor 4 uncertainty) and obliterative processes on Mars. Many broad lava plains on Mars, and the Arsia Mons, have model ages of a few hundred years (Hartmann et al. Nature 397, 586-589). Youngest flows on Elysium Planitia, 7-40 m thick, have model ages around 10 My or less (Hartmann and Berman, in preparation). These results are consistent with SNC basaltic meteorite ages, and establish fairly robustly that volcanism has continued on Mars into modern geologic history. At least two processes, subaerial deposition of dust and flooding by thin lava flows, tend to remove small craters from the size distribution on older surfaces (even as new ones are produced). This enables us to estimate "net deposition" or obliteration rates in different areas. Deposition is enhanced by a factor 10-100 in the polar areas, where 11 m-scale craters have survival times of less than 1 My. In many lower latitude areas, crater saturation equilibrium has set in at diameters of tens of m, producing regolith and dust, and inhibiting location of ancient outcrops. Permafrost layers, hundreds of m deep, may be a key to morphology in older areas (ages > 1-2 Gy). Episodes of early melting of such layers explain many features, such as the softening and infill of large, old craters; chaotic terrain; runoff channels; groundwater sapping; and a class of what appear to be isostatically adjusted, "melted-down" craters, of which we have discovered several examples (Hartmann and Esquerdo, 1999, Meteor. Planet. Sci. 34, 159-166).

Meteoritics & Planetary Science, 2000