Detection of visible-wavelength aurora on Mars

Abstract

Mars hosts various auroral processes despite the planet's tenuous atmosphere and lack of a global magnetic field. To date, all aurora observations have been at ultraviolet wavelengths from orbit. We describe the discovery of green visible-wavelength aurora, originating from the atomic oxygen line at 557.7 nanometers, detected with the SuperCam and Mastcam-Z instruments on the Mars 2020 Perseverance rover. Near-real-time simulations of a Mars-directed coronal mass ejection (CME) provided sufficient lead-time to schedule an observation with the rover. The emission was observed 3 days after the CME eruption, suggesting that the aurora was induced by particles accelerated by the moving shock front. To our knowledge, detection of aurora from a planetary surface other than Earth has never been reported, nor has visible aurora been observed at Mars. This detection demonstrates that auroral forecasting at Mars is possible, and that during events with higher particle precipitation, or under less dusty atmospheric conditions, aurorae will be visible to future astronauts.

Fig. 1.. Simulation of the solar wind plasma and IMF enhancement at Mars during the ICME event.

The figure is adapted from simulation results generated by the NASA CCMC using the WSA-ENLIL + cone model. The left figure shows the solar wind ion density and velocity in the top and the IMF magnitude and plasma temperature in the bottom, with horizontal dotted lines marking the corresponding solar event selection criteria as described in the “Solar storm selection” section. The right figure shows a time snapshot of the modeled solar wind ion density (scaled by solar distance squared) in the ecliptic plane from 0.1 to 2 AU at the time of peak ion density at Mars, coinciding with the maximum compression of the solar wind at 1.5 AU by the ICME structure. The full simulation and solar activity details are available at https://kauai.ccmc.gsfc.nasa.gov/DONKI/view/WSA-ENLIL/29602/1.

Fig. 2.. Energetic solar particles affecting Mars during the 18 March event.

Left: MEx memory error time series. The orange dotted line represents the number of errors per day. Right: MAVEN/SEP electron and ion flux timeline. The color bars represent ion (top) and electron (bottom) fluxes. For all figures, the black dashed line indicates the time of the rover observations.

Fig. 3.. SuperCam average spectrum and best-fit model.

The average of 75 × 2 spectra from the sol 1094 aurora detection observation shown in black, with the corresponding best-fit model for the continuum and the 557.7 nm emission in green. Observed spectra from nondetections on sols 790, 900, and 1107 are shown for comparison in gray, with the signal levels offset so that their averages between 559 and 566 nm are matched to that of sol 1094.

Fig. 4.. Evidence of excess green signal in the Martian sky on March 18th.

Profiles of excess green signal for all four aurora detection attempts (left), and images of the martian sky with aurora on sol 1094 (middle) and on a reference sol without aurora (right). Both images, which have identical color stretches, aerosol-scattered Phobos-light removal, and 2-pixel Gaussian smoothing, show the martian night sky contrasted against nearby topography seen at the bottom edge. See the Supplementary Materials for further image processing details. In each image, there are three squares which show how different noise-free uniform signals would appear if measured by Mastcam-Z and presented in the color stretch used for the images. The top squares represent the 557.7-nm aurora with no other sky illumination. The middle squares show the observed average signal in the center of each image after Phobos signal removal. The bottom squares show the average signals in the center of the images, with the excess green aurora signal removed for the aurora image and added for the aurora-free image. Thus, the bottom squares illustrate the colors that an aurora-free sky would have had on sol 1094 for the left image and that an aurora-illuminated sky would have had on sol 1108 for the right image. Left: The average excess green signal (calculated as described in the text) in the left image as a function of elevation angle. Mastcam-Z profiles and the model are shown as lines, while the SuperCam radiance measurements are indicated by diamonds. The colors represent different mission sols. Only sol 1094 (solid green) yielded a positive detection. The shaded green and gray areas represent the Mastcam-Z instrumental uncertainty of the best fit and the 95% confidence interval including uncertainties due to corrections for scattered Phobos light, respectively. The orange dashed line shows an auroral line radiative transfer model calculation fit to the sol 1094 SuperCam measurement.