Life on Mars ( LoMars ): A bibliometric study of current research and future perspectives

A quest for life on Mars ( LoMars ) started in the early 1960s when several research articles were published by the most prestigious scientific journals. The rise in annual literature production started in the late 1990s, most likely associated with the launch of the National Aeronautics and Space Administration’s (NASA) first Sojourner Rover in 1996. This trend predicts that the articles on Mars would further increase in the future given the fact that the recent launch and landing of the Mars 2020 Perseverance Rover is of utmost importance to find and understand the present or past life on the planet. So far, USA dominated the citations and collaborations with the rest of the world on Mars research given its facilities equipped with relevant infrastructure and researchers’ capacity to explore the Solar System beyond Earth. Majority of these frequently cited papers report results of observational and theoretical research; however, the highly cited paper (MCKAY D. S. et al. 1996) is based on analytical studies of a unique Martian meteorite (i.e., Allan Hills 84001) found in the Antarctica. It is expected that the sample return mission associated with the Perseverance Rover could possibly increase the LoMars research exponentially in the coming decades if Martian samples are successfully brought back to Earth. Based on the total number of publications on LoMars , highly influential institute, author, and journal represent Caltech, McKay C. P., and Icarus, respectively. Both the institute and the author are directly affiliated with the NASA, suggestive of the leadership offered by the organization in LoMars research.

program was used to construct the network analysis between citation, country, and authors' collaborations (PERIANES-RODRIGUES et al. 2016).

Overall results
Eleven search strings have been used to find the 660 documents on LoMars (Table 1). A total number of documents and the average citations per document reveal that the LoMars research has been cited for 22,242 times. In the earlyto mid-1990s, the annual number of articles has been increasing until the beginning of the 21 st century, from where onwards the publication output remained below 30 publications per annum with the exception of 2003 (35), 2011 (33), and 2019 (34). The rise in the number of publications in the 1990s could possibly be due to the launching of the first Rover, Sojourner (4 December, 1996). The spikes in the number of articles ( Figure 1) may have been linked to the launching of NASA Rovers such as Spirit (10 June, 2003), Opportunity (8 July, 2003), and Curiosity (26 November, 2011). Presumably, the exceptional features of the Perseverance Rover compared to the earlier rovers would trigger the higher publication throughput in the future, and the spike in 2019 could be the beginning of yet another surge in LoMars research as previously observed in the 1990s. Over 20 publications per year began in 1998 (i.e., 23, Figure 1; Table 2) with only two articles cited for more than a hundred times (JAKOSKY & SHOCK, 1998;SLEEP & ZAHNLE, 1998), showing mean total citations per article (i.e., MTCpA) and mean total citations per year (i.e., MTCpY) equal to 32.9 and 1.5, respectively. A list of years with over 20 publications per year (Table   2) shows that the highest MTCpA (74.9) and MTCpY (6.3) values are associated with 2000 and 2015, respectively.
Four of the total twenty publications in the year 2000 have been cited for more than 150 times (YEN et al. 2000;SKIDMORE et al. 2000;BENNER et al. 2000;WYNN-WILLIAMS & EDWARDS, 2000), a potential factor of higher MTCpA, while a paper about hydrated salts on Mars (OJHA et al. 2015) has been cited for more than 300 times making the year 2015 with the highest MTCpY value ( Table 2). The same paper also represents one of the highly-cited publications among the Top 10 papers regarding their average per year citations (APYC; Table 3).

Influential Authors and Countries
The authors' productivity in LoMars research has been evaluated using H-, G-, and M-index based on the number of citations ( Table 4). Six of the Top 10 most influential authors belong to the USA (majority represent NASA; Table   4), and the other four authors are affiliated with institutions in European countries including UK, Germany, Spain, and Italy (i.e., G7 countries). McKay C. P. is strongly ranked at first position among the Top 10 authors in total number of publications, total citations, H-, and G-Index (Table 4). However, M-Index (e.g., H-Index divided by the number of years the scientist has been active) is relatively lower because the author has been active for 32 years  (Table 2). Furthermore, USA also tops the list of highly-cited countries on LoMars research (Fig. 3b).

Collaborative Research
Despite a substantial difference between the number of documents as multiple country publication (i.e., co-authors' affiliations are in various countries; MCP) and single country publication (i.e., all authors' affiliations are in the same country; SCP) for the USA, it still leads the list of the Top 10 countries ranked in terms of the affiliation of the corresponding author (Fig. 3a). If we consider MCP as a proxy for the measure of collaborative research, then the USA would be the most collaborative country in LoMars research (Fig. 4). Most of the other top-ranking countries belong to G7 nations, supporting the notion that scientific collaborations are directly related to the scientific infrastructure, basic research, and collaborative publication output.
For instance, China is leading the G7 countries in terms of publications since 1996 (HE, 2009) even though earlier it was not very prominent in LoMars research (Fig. 4).     LoMars. So far, meteorites have been investigated for this purpose, but due to their lengthy and severe journey, most of the sedimentary materials might have been lost as well as followed by possible terrestrial contamination. Thus, it is expected that pristine samples returning from the surface and subsurface of Mars in the future will most likely affect the trend of selecting keywords such as surface, mineralogy, carbonates, habitability, and climate (Fig. 8).