A SEARCH FOR NITRATES IN MARTIAN METEORITES

Martian atmospheric nitrogen is highly enriched in N-15; nitrates formed by interaction of the atmosphere with the Martian regolith should therefore also be characterized by an elevated delta(15)N value. A search has been made for nitrates in two Martian meteorites, in order to determine the extent of possible regolith-atmosphere interaction. Shock-produced glass from the Elephant Moraine (EET) A79001 shergottite (E1,149) and a water-soluble extract from Nakhla were analyzed by Fourier transform infrared (FTIR) spectroscopy and stepped combustion-stable isotope mass spectrometry. FTIR of both meteorites had features at 1375 cm(-1) and 1630 cm(-1), consistent with nitrates. On account of their low thermal stability, nitrates break down at temperatures below 600 degrees C; in this temperature range, E1,149 yielded similar to 1250 ppb nitrogen with delta(15)N -8+/-5 parts per thousand. If this nitrogen is from a nitrate, then it cannot be distinguished from terrestrial salts by its isotopic composition. The water-soluble extract from Nakhla also released nitrogen at low temperatures, approximately 17 ppb with delta(15)N approximately -11+/-4 parts per thousand. Since Nakhla is an observed ''fall'', this is unlikely to be a terrestrial weathering product. Nitrates apparently occur in E1,149 and Nakhla, but in very low abundance, and their origin is unclear. The isotopic composition of the salts, which is within the range of that proposed for Martian magmatic volatiles, is far removed from that of nitrogen in the present-day Martian atmosphere. If the nitrates are Martian in origin, they did not form in recent times from reactions involving atmospheric gases. Rather, the nitrates could be the result of an earlier episode of atmospheric interaction with the regolith, or with implantation of magmatic volatiles introduced during degassing.