Radio astronomy with the Lunar Lander: opening up the last unexplored frequency regime. Marc Klein Wolta,b,∗, Amin Aminaeia, Philippe Zarkac, Jan-Rutger Schraderd, Albert-Jan Boonstrae, Heino Falckea aAstronomical Institute, Radboud University Nijmegen, Heijendaalseweg 135, 6525 AJ Nijmegen, The Netherlands bScience & Technology, Olof Palmestraat 14, 2616 LR Delft, The Netherlands cLaboratoire d'´etudesSpatiales et d'Instrumentation en Astrophysique (LESIA) Observatoire de Paris, Section de Meudon 5, Place Jules Janssen, 92195 Meudon Cedex France dNetherlands Institute for Space Research (SRON), Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands eNetherlands Institute for Radio Astronomy (Astron), Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands Abstract The moon is a unique location in our solar system and provides important information regarding the exposure to free space that is essential for future human space exploration to mars and beyond. The active broadband (1 kHz−100 MHz) tripole antenna now envisaged to be placed on the European Lunar Lander located at the Lunar South Pole allows for sensitive measurements of the exosphere and ionosphere, and their interaction with the Earths magnetosphere, solar particles, wind and CMEs and studies of radio communication on the moon, that are essential for future lunar human and science exploration. In addition, the lunar South pole provides an excellent opportunity for radio astronomy. Placing a single radio antenna in an eternally dark crater or behind a mountain at the south (or north) pole would potentially provide perfect shielding from man-made radio interference (RFI), absence of ionospheric distortions, and high temperature and antenna gain stability that allows detection of the 21 cm wave emission from pristine hydrogen formed after the big bang and into the period where the first stars formed. A detection of the 21 cm line from the moon at these frequencies would allow for the first time a clue on the distribution and evolution on mass in the early universe between the Epoch of Recombination and Epoch of Reionization (EoR). Next to providing a cosmological breakthrough, a single lunar radio antenna would allow for studies of the effect of solar flares and Coronal Mass Ejections (CMEs) on the solar wind at distances close to earth (space weather) and would open up the study of low frequency radio events (flares and pulses) from planets such as Jupiter and Saturn, which are known to emit bright (kJy−MJy) radio emission below 30 MHz (Jester and Falcke, 2009). Finally, a single radio antenna on the lunar lander would pave the way for a future large lunar radio interferometer; not only will it demonstrate the possibilities for lunar radio science and open up the last unexplored radio regime, arXiv:1209.3033v1 [astro-ph.IM] 13 Sep 2012 but it will also allow a determination of the limitations of lunar radio science by measuring the local radio background noise. Keywords: Lunar Exploration, Lunar ionosphere, Radio Astronomy, Cosmology Preprint submitted to Planetary Space Science, accepted for publication May 21, 2018 1. Introduction only from the lunar exploration perspective but also highlighting the unique opportunities such The European Lunar Lander ELL mission an radio antenna offer for low frequency radio is an exploratory ESA mission that aims at in- astronomy and cosmology. For a more detailed vestigating the conditions on the Lunar surface description of the science cases that support a for future human missions and science projects. space-based or lunar low frequency radio an- The primary objective of the Lunar Lander is tenna we refer to (Jester and Falcke, 2009). to prove ESA's capabilities of performing a soft precision landing in the moon, and once on the moon the Lunar Lander will perform a number 2. Opportunities provided by the Lunar of scientific studies. The L-DEPP (Lunar Dust Lander Environment and Plasma Package) package is one of the proposed instrument packages to be The moon is a unique and still relatively lit- placed on the Lunar Lander and it consists of tle studied place in our solar system. From three instruments: a radio antenna (Lunar Ra- ESA's exploratory perspective the moon is the dio eXperiment - LRX), dust camera and Lang- next step after the ISS for pursuing exploration muir probe to measure the lunar electromag- of the universe, and provides the ideal location netic fields and waves, lunar dust and plasma, to test human habitation and scientific explo- respectively. The radio antenna was also a con- ration in preparation for a future mars mis- sequence of our early proposal for ESA's Explo- sion. However, the extreme conditions on the ration architecture studies to build a lunar low- moon have to be taken into account for any frequency radio array (Lunar LOFAR) where it mission to the surface. The moon is known for was intended as a path-finder instrument prov- having a significant dust layer and, in combi- ing the capabilities of low-frequency astronomy nation with the absence of a significant atmo- (below ∼ 30 MHz). sphere, provides a unique interface between a The ELL will land on one of the predefined dusty surface and free space. In addition, the landing sites on the Lunar South Pole. The moon is subjected to large temperature varia- South Pole was chosen based on arguments re- tions (caused by night and day variations), is lated to power (sunlight) and communication exposed to comic ray and solar wind plasma (Earth visibility) but also as it has not been and interacts with the Earth magnetotail. All explored before and offers unique opportuni- this has a large impact on the lunar ionosphere ties for future human exploration (e.g. resource and plasma and affects conditions below and mining, see Gardini, 2011). For the work pre- above the lunar surface. For example, it is ob- sented here we will assume the South Pole lo- vious that levitating dust plays a crucial and cation as a fixed parameter. The LRX offers potentially hazardous role for surface opera- unique scientific opportunities but it should be tions. However, how does the dust get there noted that these science cases are not driving and where does it go? In fact, the dusty sur- the ELL mission. face plasma will not end just a few meters Here we would like to present a concept de- above the surface. Since a few decades we sign for, and review the relevance of a single know, for example, that the moon has an ex- tripole radio antenna on board the Lunar Lan- osphere. It is generally believed that the ions, der on the a Lunar South Pole location, not which make up this exosphere, are generated at the Moon's surface by interaction with solar UV photons, plasma in the Earth's magneto- ∗Corresponding author Email address: [email protected] (Marc sphere, or micrometeorites (see Reasoner and Klein Wolt) Burke 1972, Benson et al. 1975 and references 2 therein). Apart from direct in-situ measure- in-situ capabilities of the L-DEPP package. In ments this exosphere has been seen with radio addition, the LRX will investigate the use of ra- observations, revealing a variable plasma fre- dio communication on the moon. The low fre- quency in the 100 kHz regime during daytime. quency radiation is able to penetrate through the lunar regolith or crater rims; it can be re- One of the main scientific objectives is to in- flected from the lunar surface or even the lu- vestigate the interplay between the dust, the nar exosphere. And finally, using the radio an- electromagnetic fields and lunar plasma on the tenna as passive ground-penetrating radar, the one hand and the effects of the earth's magnetic reflected radiation provides information about fields, solar light and plasma, and cosmic ray the lunar surface and sub-surfaces. and (micro-) meteorites. The synergy between The LRX radio antenna plays a crucial role the three instruments proposed to comprise L- in investigating the lunar conditions for hu- DEPP provides a unique opportunity to study man exploration, but furthermore offers unique these effects. As explained above, the solar UV opportunities for low frequency radio astron- and X ray photons are predicted to create a omy: a single antenna placed on the moon positively charged surface and a dusty plasma would be a pathfinder mission for radio as- sheet on the day side, through the combined ef- tronomy and address pending cosmological is- fects of photo electron emissions and low con- sues. Placing a broad-band and low frequency ductivity of the lunar regolith. Pick-up ions (∼kHz−100MHz) radio antenna on the moon and solar wind electrons should create a lunar would be the demonstration and site-testing ionosphere that is more extended on the night mission required to make large low-frequency side; a night side that has a negatively charged radio antenna systems in space possible. How- surface as an interface to the ionosphere that ever, LRX would also detect the Askaryan ef- gradually becomes a near perfect vacuum in- fect from cosmic rays and make a serious at- side the lunar wake (see also Fig. 1 and 2). tempt at detecting the global dark ages and The generated electric fields should often dom- EOR signal. The main limitation for the lat- inate the Lorentz force, breaking MHD plasma ter is the unknown interference situation at a equilibrium and enabling levitation of dust and polar landing site, where the Earth would set transport of dust and plasma across the ter- only occasionally during the mission. A future minator. It is far from clear, though, how all large radio synthesis antenna with many more these processes work in detail. Clearly, placing individual nodes and baselines of a few kilo- the three L-DEPP instruments on the South meters would be required to perform detailed Pole of the Moon would allow for detailed in- cosmological studies such as the EoR tomogra- situ measurements and remote sensing of the phy and the determining the power spectrum effects, which should provide significantly more of the 21-cm line at redshifts of 30 − 50, as insight into the underlying processes well as for instance the study of high redshift The radio antenna as part of the L-DEPP galaxies and quasars and fossil radio galaxies.