Open data policies have been introduced by governments, funders, and publishers over the past decade. Previous research showed a growing recognition by scientists of the benefits of data-sharing and reuse, but actual practices lag and are not always compliant with new regulations. The goal of this study is to investigate motives, attitudes, and data practices of the community of earth and planetary geophysicists, a discipline believed to have accepting attitudes towards data sharing and reuse. A better understanding of the attitudes and current data-sharing practices of this scientific community could enable funders, publishers, data managers, and librarians to design systems and services that help scientists understand and adhere to mandates and to create practices, tools, and services that are scientist-focused. An online survey was distributed to the members of the American Geophysical Union (AGU), producing 1372 responses from 116 countries. The attitudes of researchers to data sharing and reuse were generally positive, but in practice scientists had concerns about sharing their own research data. These concerns include the possibility of potential data misuse and the need for assurance of proper citation and acknowledgement. Training and assistance in good data management practices are lacking in many scientific fields and might help to alleviate these doubts.