Interstellar ${}^{60}\mathrm{Fe}$ on the Surface of the Moon

A dying massive star ends in a supernova explosion ejecting a large fraction of its mass into the interstellar medium. If this happens nearby, part of the ejecta might end on Solar System bodies and, in fact, radioactive ${}^{60}\mathrm{Fe}$ has been detected on the Pacific ocean floor in about 2 Ma old layers. Here, we report on the detection of this isotope also in lunar samples, originating presumably from the same event. The concentration of the cosmic ray produced isotope ${}^{53}\mathrm{Mn}$, measured in the same samples, proves the supernova origin of the ${}^{60}\mathrm{Fe}$. From the ${}^{60}\mathrm{Fe}$ concentrations found we deduce a reliable value for the local interstellar fluence in the range of $1\times {10}^8\mathrm{at}/{\mathrm{cm}}^2$. Thus, we obtain constraints on the recent and nearby supernova(e).

Figure 1

This graph shows, as a function of depth, on a logarithmic scale, (a) the concentration of ${}^{60}\mathrm{Fe}/\mathrm{Fe}$ compared with the blank level (dashed line) during these measurements and (b) disintegrations per minute per kilogram of Ni [$\mathrm{dpm}/(\mathrm{kg}\mathrm{Ni})$], compared with the average activity measured in meteorites [thin shaded region at $0.5\mathrm{dpm}/(\mathrm{kg}\mathrm{Ni})$], published up to this date (Refs. [21, 22, 23, 24]; see Ref. [18] for a summary of these). The labeling of the data points is specified in Tables I and II in Ref. [18]. The empty triangles and squares show the calculation of the expected contribution for the SCR spallation production of ${}^{60}\mathrm{Fe}$ in the positions of the Apollo 12 and 16 landings, respectively. The error bars indicate the $1\sigma$ confidence level [25].

Figure 2

Measured activities of ${}^{60}\mathrm{Fe}$ versus ${}^{53}\mathrm{Mn}$ in meteoritic and lunar samples. Units are disintegrations per minute per kilogram of Fe and Ni, for ${}^{53}\mathrm{Mn}$ and ${}^{60}\mathrm{Fe}$, respectively. Samples 1 through 11 (filled points) are lunar samples; the other values (error bars only) are for iron meteorites. The labeling of the data points is specified in Table I in Ref. [18]. The shaded bar indicates the error band for cosmogenically produced ${}^{53}\mathrm{Mn}$ and ${}^{60}\mathrm{Fe}$ activities in meteorites. The error bars indicate the $1\sigma$ confidence level [25].

Figure 3

Estimation of the local fluence of ${}^{60}\mathrm{Fe}$ on the Moon’s surface. The dashed curves represent two different integration scenarios. They symbolize a lower and an upper limit. The labeling of the data points is detailed in Table I in Ref. [18]. The error bars indicate the $1\sigma$ confidence level [25].